SELF-ASSEMBLY OF LYOTROPIC CHROMONIC LIQUID ......SELF-ASSEMBLY OF LYOTROPIC CHROMONIC LIQUID...

SELF-ASSEMBLY OF LYOTROPIC CHROMONIC LIQUID

CRYSTALS: EFFECTS OF ADDITIVES AND APPLICATIONS

A dissertation submitted

to Kent State University in partial

fulfillment of the requirements for the

degree of Doctor of Philosophy

by

Heung-Shik Park

December 2010

ii

Dissertation written by

Heung-Shik Park

B.S., Korea University, Seoul, Korea, 1996

M.S., Korea University, Seoul, Korea, 1998

Ph. D., Kent State University, 2010

Approved by

Dr. Oleg D. Lavrentovich , Chair, Doctoral Dissertation Committee

Dr. Peter Palffy-Muhoray , Members, Doctoral Dissertation Committee

Dr. Jonathan Selinger ,

Dr. Satyendra Kumar ,

Dr. Edgar Kooijman ,

Accepted by

Dr. Liang-Chy Chien , Chair, Department of Chemical Physics

Dr. John R. D. Stalvey , Dean, College of Arts and Sciences

iii

Table of contents

List of figures…………………………………………………………………………… vii

Acknowledgements………………..………………………………..…………...……… xv

Chapter 1 Lyotropic chromonic liquid crystals .................................................................. 1

1.1 Overview ................................................................................................................... 1

1.2 Motivation and objectives of the thesis .................................................................. 10

1.3 Structure of the thesis.............................................................................................. 11

1.4 Reference ................................................................................................................ 12

Chapter 2 Self-assembled lyotropic chromonic liquid crystals in water .......................... 20

2.1 Introduction ............................................................................................................. 20

2.2 Experimental techniques ......................................................................................... 23

2.2.1 Materials .......................................................................................................... 23

2.2.2 Optical studies of phase transitions .................................................................. 26

2.2.3 Synchrotron x-ray studies ................................................................................ 26

2.2.4 Dynamic Light Scattering ................................................................................ 27

2.2.5 Cryo-transmission electron microscopy (TEM) .............................................. 27

2.3 Experimental Results .............................................................................................. 28

2.3.1 Phase behavior of pure SSY in water .............................................................. 28

2.3.2 Phase behavior of pure DSCG in water ........................................................... 33

2.3.3 The length of aggregates from a dynamic light scattering (DLS) and cryo-

transmission electron microscopy (cryo-TEM) measurements. ............................... 38

iv

2.4 Discussion ............................................................................................................... 41

2.5 Conclusions ............................................................................................................. 49

2.6 References ............................................................................................................... 49

Chapter 3 The effect of ionic additives on lyotropic chromonic liquid crystal Sunset

Yellow ............................................................................................................................... 57

3.1 Introduction ............................................................................................................. 57


3.2.1 Materials .......................................................................................................... 59

3.2.2 Optical studies of phase transitions .................................................................. 61

3.2.3 Synchrotron x-ray studies ................................................................................ 64

3.3 Experimental Results .............................................................................................. 64

3.3.1 Effect of the simple salts on LCLCs ................................................................ 64

3.3.2 Effects of spermine in a salt and free base form .............................................. 70

3.3.3 Effects of monovalent pH changing agents ..................................................... 77

3.4 Discussion ............................................................................................................... 80

3.5 Conclusions ............................................................................................................. 89

3.6 References ............................................................................................................... 90

Chapter 4 Phase separation and condensation of the self-assembled lyotropic chromonic

liquid crystals in poly(ethylene glycol) solution ............................................................... 97

4.1 Introduction ............................................................................................................. 97


v

4.2.1 Materials .......................................................................................................... 99

4.2.2 Phase diagram study ........................................................................................ 99

4.2.3 Fluorescence microscopy ................................................................................. 99

4.2.4 Cryogenic transmission electron microscopy (TEM) .................................... 100

4.2.5 Density measurements ................................................................................... 100

4.2.6 Synchrotron x-ray studies .............................................................................. 100

4.3 Experimental Results ............................................................................................ 101

4.3.1 Phase diagram of ternary mixture .................................................................. 101

4.3.2 Spatial distribution of components ................................................................ 105

4.3.3 The correlation length of aggregates and the distance between the aggregates

................................................................................................................................. 108

4.3.4 The effect of non-ionic additives on LCLCs in the presence of salts ............ 112

4.4 Discussion ............................................................................................................. 114

4.5 Conclusions ........................................................................................................... 121

4.6 References ............................................................................................................. 122

Chapter 5 Chromonic materials for nano-fabrication: Side-by-side and end-to-end

assembly of Au nanorods using self-assembled chromonic stacks ................................ 126

5.1 Introduction ........................................................................................................... 126

5.2 Experimental techniques ....................................................................................... 129

5.2.1 Materials ........................................................................................................ 129

5.2.2 Synthesis of gold nanorods ............................................................................ 130

5.2.3 Ionic polymer layer deposition on the assembled structure of NRs. ............. 131

vi

5.2.4 Transmission electron microscopy (TEM) .................................................... 131

5.2.5 Dynamic Light Scattering .............................................................................. 131

5.3 Experimental Results ............................................................................................ 132

5.3.1 Side-by-side assembly ................................................................................... 132

5.3.2 Quenching gold NR assembly by polymer coating. ...................................... 139

5.3.3 Polymer composite of assembled NR. ........................................................... 141

5.3.4 End-to-end assembly ...................................................................................... 143

5.4 Discussion ............................................................................................................. 147

5.5 Conclusions ........................................................................................................... 151

5.6 References ............................................................................................................. 152

Chapter 6 ......................................................................................................................... 161

6.1 Summary ............................................................................................................... 161

6.2 References ............................................................................................................. 165

vii

List of figures

Figure 1.1. Molecular structures of LCLCs, (a) DSCG and (b) SSY. ................................ 3

Figure 1.2. Schematic diagram of LCLC aggregates in (a) I phase, (b) N phase, and (c) C

phase. .................................................................................................................................. 4

Figure 2.1. The molecular structure of two forms of SSY, (a) NH hydrazone tautomer and

(b) OH azo tautomer. The model structures on the bottom illustrate the relative electric

charge distribution: Red color corresponds to the positive charge and blue to the negative

charge. ............................................................................................................................... 21

Figure 2.2. The molecular structure of DSCG. The model structures on the right hand

side illustrate the relative electric charge distribution: Red color corresponds to the

positive charge and blue to the negative charge. .............................................................. 22

Figure 2.3. Phase diagram and polarizing micrographs of SSY water solutions. The error

bars represent the difference between the data taken on heating (upper end of the bar) and

cooling (lower end of the bar). The filled circles at the vertical line indicate the

temperatures at which the pictures were taken. ................................................................ 29

Figure 2.4. Typical x-ray patterns (a-c) of SSY water solutions at different concentrations

: (a) I phase, 0.7 mol/kg; (b) N phase, 0.9 mol/kg solution; (c) C

phase, 1.36 mol/kg. (d) Diffractographs of SSY water solutions at different

concentrations : 0.7 mol/kg (green), 0.9 mol/kg (blue), and 1.36 mol/kg (red). All

data are taken at 28.4 . .................................................................................................. 32

viii

Figure 2.5. Phase diagram and polarizing micrographs of DSCG water solutions. The

filled circles at the vertical line indicate the temperatures at which the pictures were

taken. ................................................................................................................................. 35

Figure 2.6. X-ray patterns of (a) N phase, 0.34 mol/kg and (b) C phase,

0.96 mol/kg. (c) Diffractographs of DSCG water solutions at different

concentrations : 0.34 mol/kg (black), 0.62 mol/kg (red), and 0.96 mol/kg (blue). All

data are taken at 28.4 . .................................................................................................. 37

Figure 2.7. Relaxation rate Γ versus for 0.1 mol/kg, I phase DSCG solution at 22 .

........................................................................................................................................... 39

Figure 2.8. Cryo-TEM image of a 0.344 mol/kg DSCG solution. The scale bar is 50 nm.

........................................................................................................................................... 40

Figure 2.9. Schematic models of the N phase in a LCLC: (a) a standard model with rod-

like aggregates; (b) a model with shift junctions and Y junctions and their clusters,

coexisting with the rod-like aggregates. ........................................................................... 47

Figure 3.1. The molecular structure of spermine in the neutral form (Spm0) (a) and fully

charged form (SpmH4+4) (b). ............................................................................................ 61

Figure 3.2. Schematic diagram of the temperature gradient device (a). Polarizing

micrographs allow one to determine the location of the interfaces between the I, N, and

biphasic I+N regions, and thus to determine the temperatures of corresponding phase

transitions, as illustrated for a 0.9 mol/kg SSY solution doped with various amounts

of salt MgSO4 indicated on the left hand side in the mol/kg units (b). ............................. 63

ix

Figure 3.3. Effect of monovalent salts on the temperature of transition !"#! and

# !" in 0.9 mol/kg SSY solution. ....................................................................... 65

Figure 3.4. I→(I+N) transition temperature shift ∆# !%" caused by divalent cation

salts added to a 0.9 mol/kg SSY solution, data with monovalent salt NaCl shown for

comparison. ....................................................................................................................... 66

Figure 3.5. Polarizing micrograph (a) and x-ray diffraction pattern (b) of the N phase

induced by adding &'() 0.54 mol/kg of MgSO4 to the I phase of 0.7 mol/kg

SSY solution. The diffraction patterns show the N phase well aligned by the in-situ

magnetic field; (c) diffractographs for 0.7 mol/kg and 0.9 mol/kg water

solutions of SSY, both doped with &'() 0.54 mol/kg of MgSO4. All data have been

taken at 28.4. ................................................................................................................. 68

Figure 3.6. Phase behavior of SSY solution in the presence of NaCl, !+, 0; 0.5; and 1 mol/kg. The transition temperatures were determined as the temperature

decreased. .......................................................................................................................... 70

Figure 3.7. Phase diagrams of 1.14 mol/kg SSY solution doped with the salt SpmCl4

........................................................................................................................................... 71

Figure 3.8. Phase diagrams of 1.14 mol/kg SSY solution doped with Spm free base;

the inset shows the polarizing micrograph of the I+C biphasic region corresponding to the

blue circle at the phase diagram. ....................................................................................... 72

Figure 3.9. X-ray diffraction patterns, (a) wide angle and (b) small angle range, of the

coexisting I and C phases in the 1.14 mol/kg SSY water solution doped with

x

1234 0.3 mol/kg of Spm free base; note hexagonal symmetry in part (b). All data

have been taken at 28.4. ................................................................................................ 74

Figure 3.10. Polarizing micrographs illustrating a transformation of a biphasic I+C state

of a water solution with 1.14 mol/kg SSY and 1234 0.2 mol/kg Spm free base

into a N phase upon addition of HCl in concentrations 0.008 mol/kg (a), 0.04 mol/kg (b),

and 0.4 mol/kg (c). The polarizing micrographs show (a) C phase coexisting with I phase;

(b) N droplets surrounded by I phase and (c) Schlieren texture of a homogeneous N

phase. All textures have been taken at o25 C. ................................................................... 75

Figure 3.11. X-ray diffraction patterns illustrating a transformation of a biphasic I+C state


into a N phase upon addition of acid HCl. The diffraction patterns for the mixture with no

HCl added, recorded at large (a) and small (b) angles. The diffuse (green) rings at large

and small angles and sharp reflections at small angle indicate a coexistence of I and C

phases. When 0.08 mol/kg (c) and 0.4 mol/kg (d) of HCl are added, the diffraction

patterns show the N phase well aligned by the in-situ magnetic field. The length scales

corresponding to the small angle (horizontal) and wide angle (vertical) reflections in (c)

and (d) are 22.5/24.4 Å and 3.33/3.33 Å. All data have been taken at 28.4. ................ 76

Figure 3.12. Absorption spectra of 2.3 5 1036 mol/kg SSY solution at a different pH;

pH=6 (blue) caused by adding 1 5 1037 mol/kg HCl, pH=6.7 from pure SSY solution,

pH=10.5 caused by adding 1 5 1037 mol/kg NaOH, and pH=13 caused by adding

1 mol/kg NaOH. .............................................................................................................. 78

xi

Figure 3.13. Polarizing micrographs for 0.9 mol/kg SSY solution doped with NaOH

at different concentrations: (a) !+(8 0 mol/kg, pH=6.5, homogeneous N phase; (b)

!+(8 0.04 mol/kg, pH=11.3, coexisting N and I phases; (c) !+(8 0.2 mol/kg,

pH=12.1, coexisting C and I phases. All pictures have been taken at room temperature. 79

Figure 3.14. Polarizing micrographs for 1.14 mol/kg SSY solution doped with

NaOH at different concentrations: (a) !+(8 0 mol/kg, pH=6.5, homogeneous N

phase; (b) !+(8 0.1 mol/kg, pH=11.6, coexisting N and I phases; (c) !+(8 0.2 mol/kg, pH=11.9, coexisting C and I phases; (d) !+(8 0.5 mol/kg, pH=12.5,

homogeneous I phase; (e) !+(8 2 mol/kg, pH=13.2, precipitate. All pictures have

been taken at room temperature. ....................................................................................... 80

Figure 4.1. The ternary phase diagram (a) and polarizing micrographs of SSY and PEG

water mixtures in the N phase (b), I+N phase (c), I+N+C phase (d), and I+C phase (e).

......................................................................................................................................... 103

Figure 4.2. (a) Concentration dependency of density 9 for homogeneous I and N phase

of SSY solution at 296 K, (b) density 9: of the condensed LC region vs concentration of

PEG. ................................................................................................................................ 104

Figure 4.3. The phase separation in a 29wt.% SSY water solution caused by PEGs and

FITC-PEGs. (a) Fluorescence micrograph and the fluorescence intensity profile along the

dashed line (inset), and (b) polarizing micrograph of the same area of the sample. ...... 106

Figure 4.4. Cryo-TEM image of SSY-PEG-water mixture (: <=: > 22.2: 7.8: 70). .................................................................................................................. 108

xii

Figure 4.5. X-ray diffraction patterns of 29wt.% SSY with <= 7.5wt. % (a) and

<= 20wt. % (b). Diffractographs of 29wt.% SSY in the presence of PEG with <= =

0, 7.5, and 20wt.% (c). The arrow in (a) represents the direction of the magnetic field. 110

Figure 4.6. (a) SSY-PEG-water mixture (: <=: > 23.1: 3.9: 73.0) in the presence

of NaCl !+, 0 mol/kg (first mixture from the left), !+, 0.2 mol/kg (second),

!+, 0.4 mol/kg (third), !+, 0.6 mol/kg (fourth). The polarizing micrographs of

(b) the I phase of the mixture (: <=: > 23.1: 3.9: 73.0) and (c) the N+I

coexistence induced by the addition of NaCl, !+, 0.2 mol/kg, into the mixture (b).

......................................................................................................................................... 113

Figure 4.7. Polarizing micrograph of (a) the N phase in an additive-free 33 wt. %

SSY solution, (b) C+I coexistence for : <=: > 31.3: 5.2: 63.5 mixture, and (c)

the N+I coexistence induced by the addition of 1 mol/kg NaCl to the mixture (b). ....... 114

Figure 4.8. The schematic diagram shows the overlap of the excluded volumes of the

face-to-face and side-by-side configuration: (a) for individual molecules and (b) for

elongated aggregates. (c) The overlapping excluded volumes, BC, for face-to-face and

side-by-side placement as a function of the number of SSY molecules in an aggregate.

BCD+EC and BC%FGC were calculated using H' I 2 nm, J I 1nm, KL 0.33 nm, and

M I 2 nm. ....................................................................................................................... 117

Figure 4.9. Schematic illustration of the excluded volume effect of the increasing

concentration of PEG chromonic assembly: elongation of short aggregates (a), followed

by parallel arrangement in the N phase (b) and C phase (c). .......................................... 118

xiii

Figure 5.1. Side-by-side assembly of gold NRs induced by a 0.8mM DSCG solution

mixed with ~2nM gold NR solution at a 1:1 ratio. (a) TEM image of the control sample,

no DSCG; (b), (c), (d) and (e) the assembled structures of gold NRs formed after the

addition of the DSCG solution. ....................................................................................... 134

Figure 5.2. The absorption spectra of ~2 nM Au NRs with CTAB coatings modified by

the addition of 0.5 mM DSCG, as a function of time; the longitudinal plasmon peak is

blue shifted and the transverse plasmon peak is red shifted. The inset shows the picture of

the NR solution immediately after (left) and 30 minutes after adding DSCG (right). ... 135

Figure 5.3. Absorption spectra of ~2 nM Au NRs with CTAB coatings modified by the

addition of a 0.04 mM DSCG solution (a) and 40mM DSCG solution (b), as a function of

time. ................................................................................................................................ 136


addition of 0.1 M NaCl solution (a) and 0.1 M MgSO4 solution (b). ............................. 137


addition of 0.5 mM DSCG at a 1:1 ratio; the longitudinal plasmon peak is blue shifted

and the transverse plasmon peak is red shifted, as a function of time (dot lines). 15 min

after the addition of DSCG, 8µl of 0.5M NaOH is added; the longitudinal plasmon peak

reverses its shift back to the longer wavelength and the transversal peak shifts back to the

shorter wavelength (colored solid lines). ........................................................................ 138

Figure 5.6. Absorption spectra (a) and picture (b) of the assembled NR structure solution

with the addition of 1% PSS solution at different reaction times; PSS added on NR

solution without DSCG and on NR solution 0, 5, 15, 30, 60, and 120 min after initiating

xiv

the assembly reaction by adding the DSCG solution. The plasmon peaks resulting from

the solutions quenched by PSS coating do not change even after a period of one month.

......................................................................................................................................... 139

Figure 5.7. DLS data for the NR assembled structure solution quenched at 0, 5, 15, 30,

60, and 120 min. The diffusion coefficient decrease with reaction time, while the

hydrodynamic diameters increase with reaction time. .................................................... 141

Figure 5.8. Absorption spectra (a) and picture (b) of the assembled NR structure

embedded in PVA film. The composite films were prepared with three different NR

structure solutions, an isolated NR solution and assembled structure solutions with 5 and

15 min reaction times. ..................................................................................................... 142

Figure 5.9. End-to-end assembly of Au NRs with CTAB and PAA coating, induced by

0.1M DSCG added to ~2 nM NR solution at a 1:1 ratio. TEM images of the end-to-end

chains were taken at about 2 hours (a) and (b), 5 hours (c) and (d), and 24 hours (e) and

(f) after the preparation of the mixture. .......................................................................... 144

Figure 5.10. Absorption spectra of Au NRs with PAA coatings modified by the addition

of 0.1 M DSCG, as a function of time. The inset shows the picture of the NR solution just

after (left) and 1day after adding DSCG (right). ............................................................. 145

Figure 5.11. The longitudinal plasmon peak from gold NRs with PAA coationgs modified

by the addition of 0.1M DSCG changes with time (a). The dilution of the concentration

of the DSCG quenches the assembly reaction (b). ......................................................... 146

Figure 5.12. Schematic representation of the side-by-side assembly. ............................ 148

Figure 5.13. Schematic representation of the end-to-end assembly. .............................. 151

xv

ACKNOWLEDGEMENTS

I am very grateful to my advisor Prof. Oleg D. Lavrentovich for his excellent

mentoring as a scientist and the wonderful opportunities he has given me.

It was a great experience to study at the liquid crystal institute. I want to thank

Prof. Philip J. Bos, Prof. Liang-Chy Chien, Prof. Eugene C. Gartland, Prof. Tony Jakli,

Prof. Quan Li, Prof. Peter Palffy-Muhoray, Prof. Jonathan Selinger, Prof. Robin Selinger,

Prof. Sergij Shiyanovskii, Prof. John L. West, and Prof. Deng-ke Yang for teaching

wonderful classes. Thanks to all the administrative and technical staff, classmates,

colleagues, and our group members for their kind help and friendship.

I would like to thank Prof. Peter Palffy-Muhoray, Prof. Nick Kotov, Ashish

Agarwal, Dr. Andrii Golovin, Jake Fontana, Dr. Michele Fontana, Dr. Paul Luchette, and

Jeanette Killius for helping me in the study of metamaterials and Prof. Satyendra Kumar,

Dr. Yuriy Nastyshyn, Dr. Luana Tortora, Dr. Shin-Woong Kang, Dr. Vasyl Nazarenko,

and Oleksandr Boiko for helping me in the study of chromonic liquid crystals.

Thanks to all the committee members for reviewing my dissertation and their

helpful comments.

I would like to thank Prof. Kang-Jin Kim, my advisor for the master’s degree,

who encouraged and supported me to pursue further study.

Finally I must thank my family for their endless support and belief in me: my

parents, brother, wife, and two sons.

1

Chapter 1 Lyotropic chromonic liquid crystals

1.1 Overview

Liquid crystals (LCs) are a special class of soft materials characterized by so-

called mesophases where they flow like an isotropic liquid yet possess a long-range

orientational order and a complete or partial absence of positional order of building units

which can be individual molecules or their aggregates [1]. The two main types of liquid

crystals are thermotropic LCs and lyotropic LCs. Thermotropic LCs show mesophases

depending on temperature and pressure. Their basic building units are usually individual

molecules which have a feature of pronounced shape anisotropy, such as rods, disk, etc.

Thermotropic LCs have been successfully used in display devices. Lyotropic LCs are

formed on the dissolution of lyotropic LC molecules in a solvent (usually water). A

feature of lyotropic LCs distinguishing them from thermotropic LCs is the self-assembly

of molecules into supermolecular structures that represent the basic unit, of these

mesophases [2, 3]. The most common lyotropic LC system are those formed by water and

surfactants (amphiphiles), such as soaps, synthetic detergents, and lipids. Surfactant

molecules are formed by a hydrophilic part chemically bound to a hydrophobic part.

Mixtures of these surfactant molecules with a solvent under certain conditions of

temperature and relative concentration produce several different types of mesophases [2,

3].

2

Over the last 10 years since the early work of Lydon, Attwood, Tiddy, and

coworkers [4-6] there has been a growing interest in a distinct family of lyotropic LCs,

the so-called lyotropic chromonic liquid crystals (LCLCs). LCLCs represent a broad but

not well understood class of soft matter in which the reversible self-assembled aggregates

formed by non-amphiphilic molecules show liquid crystalline phases [4, 7, 8]. The range

of materials which form chromonic liquid crystal phases includes drugs [4-6, 9], dyes

[10-15], and DNA nucleotides, such as guanosine derivatives [16, 17]. The name

“chromonic” was suggested by Lydon because it connotes both color and chromosomes

[4-6]. Chromonic molecules are different from the conventional lyotropic amphiphiles.

They have a plank-like or disk-like polyaromatic central core with two or more ionic

groups at the periphery [4, 7, 8]. Figure 1.1 shows the molecular structures of typical

LCLCs. Disodium cromoglycate (DSCG), Fig. 1.1(a), an antiasthmatic drug with the

commercial name “INTAL”, is one of the most extensively studied LCLC materials.

Figure 1.1.b represents the molecular structure of disodium salt of 6-hydroxy-5-[(4-

sulfophenyl)azo]-2-naphthalenesulfonic acid, a food dye, also known as Sunset Yellow

(SSY).

The geometry of the basic structural unit in LCLCs is different from spherical or

cylindrical micelles and bilayers formed by amphiphilic molecules in the conventional

lyotropic LCs. In water, the chromonic molecules typically stack on top of each other (the

so-called H-aggregation) leaving the ionic solubilizing groups at the aggregate-water

interface [4, 7, 8], which was proven by x-ray diffraction studies and polarized UV-vis

spectroscopic measurements for many LCLCs [10, 11, 18-20]. However in a recent

3

publication by Luk et al. [21], an alternative model of side-by-side stacking has been

proposed. The typical separation between the adjacent molecules along the stacking

direction in H-aggregates is about (0.33 - 0.34) nm as measured by x-ray diffraction [10,

11, 18]. This value of the stacking distance does not depend on the chromonic

concentration and the phase of solution, indicating that it represents a basic feature of the

chromonic aggregates. When the polar groups are fully ionized, the line density of

electric charge along the aggregate formed by LCLC molecules with two ionic groups

can be very high, e.g., ~6O/nm (O is the electron’s charge) in assumption that there is

one molecule in the cross-section of the cylindrical stack. The stacking distance and the

line charge make LCLC aggregates similar to the double-strand B-DNA molecules with

the important difference that in LCLCs, there are no chemical bonds to fix the size of the

aggregates.

Figure 1.1. Molecular structures of LCLCs, (a) DSCG and (b) SSY.

The underlying mechanism of aggregate self-assembly in LCLCs is analogous to

the process resulting in worm-like micelles formed by surfactant molecules in solutions

and the so-called “living” polymerization. The balance of energy gained by placing a

monomer inside the aggregate and the entropy term promoting a larger number of

4

aggregates, produces a polydisperse system of linear aggregates [22, 23]. As the

concentration of LCLC increases, the aggregates multiply, elongate, and align parallel to

each other and then form mesophases. The average orientation of aggregates is denoted

by a unit vector with the property PQ R SPQ called the director, Fig. 1.2. The two most

commonly met phases in LCLCs are the uniaxial nematic (N) phase and the columnar (C)

phase with aggregates forming a hexagonal lattice in the plane perpendicular to PQ [4, 7,

8], Fig. 1.2.

Figure 1.2. Schematic diagram of LCLC aggregates in (a) I phase, (b) N phase, and (c) C

phase.

5

The details about the aggregate structure are still the subject of discussion [4].

Woodard et al. [18] assumed that there is a molecule in the cross-section of the

cylindrical aggregate. They estimated the cylinder diameter to be about 1.6 nm [18].

Later, Lydon [4] proposed that the aggregate is shaped like a hollowed square formed by

four molecules linked by electrostatic salt bridges. Recently, Dickinson et al. [20]

assumed that there are two DSCG molecules in the cross-section of the aggregate. They

comprehensively analyzed both past and recent x-ray and absorption data concerning the

different chromonic liquid crystals and concluded that the aggregate structures of many

chromonic systems are very simple, consisting of one or two molecules in a cross-section

of aggregate [20]. If the charged groups of the chromonics are located on opposite sides

of the molecules, then the aggregates have one molecule in their cross-section. But if the

charged groups are on the same side of the molecules, then two molecules can be

arranged in the cross-section of the aggregate with the charged groups far away from

each other.

The molecular structure-property relationships of LCLCs present an interesting

question but are not clearly understood because most of the studies of LCLCs have been

performed on a few available materials. Recently, Tam-Chang et al. have synthesized

several derivatives of perylene dye and described the effect of the structures of the side

chain and the core ring systems on LC properties of these compounds [14, 15, 24]. They

demonstrated that replacing the side-chain with a long bulky alkyl chain destabilizes the

N phase by lowering the transition temperatures of both !#! and ! # [14, 15]. The

introduction of a sterogenic center in a side-chain results in a chiral phase, as evidenced

6

by the typical fingerprint texture [24]. They also showed that different poly-aromatic

cores, such as perylenebis(dicarboximide) and quaterrylenebis(dicarboximide), differed

dramatically in solubility and absorption spectra [14, 15].

Although the structure of aggregates and the role of concentration and

temperature are not clearly understood, many studies have shown that the transition

temperature from the I phase to the LC phase and the size of the aggregates are

influenced by additives, either charged, such as salts [9, 25-27], or non-charged, such as

neutral polymers [28]. The two main mechanisms associated with the role of additives are

(a) electrostatic interactions within and between the aggregates and (b) excluded volume

effects induced by the neutral additives. Neither of the two is well understood.

In a pioneering work, Yu and Saupe [9] explored the electrostatic effects

observing that the addition of a monovalent salt, NaCl, to DSCG increased the

temperature !#! at which the homogeneous N phase transforms into the biphasic N-

isotropic (I) coexistence region and the temperatue ! # of the complete melting.

These studies have been continued by Kostko et al. [25] who demonstrated that small

cations, Na+, K+, and Li+ increase both !#! and ! #, but some salts, such as

tetraethylammonium bromide and tetrabutylammonium bromide, destabilize the N phase.

They proposed that the small cations formed salt bridges between adjacent DSCG and

promoted the growth of the aggregates, while large cations, such as tetraalkylammonium,

were too large to fit in the aggregate and consequently suppressed its growth [25]. Prasad

et al. [26] described that neither the dimension of the aggregates nor the dynamics

7

associated with them are significantly altered by the addition of NaCl, while at the same

time attributing the observed increase of the LCLC viscosity to the salt-induced changes

in the hydrogen bonding [26]. Jones et al. [27] showed that the temperatures !#! and

! # in water solutions of SSY decrease, rather than increase, upon the addition of the

same salt NaCl.

The effects of the neutral additives on LCLCs are not much clearer. Simon et al.

[28] demonstrated that some water soluble polymers added to an isotropic DSCG

solutions cause the formation of birefringent droplets with different director

configurations. For example, polyvinyl alcohol induced LCLC droplets with normal

orientation of PQ at the interface and a radial configuration of PQ inside, while

polyacrylamide induced LCLC droplets with tangential alignment of PQ at the interface

and a bipolar director structure inside [28].

The recognition of the lyotropic chromonic liquid crystals (LCLCs) as a

fascinating and distinct class of lyotropic LCs is not widespread. As one of the pioneers

in this field, J. Lydon, writes [4], “A single large-scale commercial application of

chromonics will of course chang this picture overnight…the continuing discovery of

unique properties and versatility of these systems promises much. I would hazard the

guess that wherever nanotechnology takes us, the liquid crystalline state will never be far

away – and chromonic systems will have something vital to offer.” Recently, there have

been several studies enhancing interest in chromonic materials for potential uses as

functional materials and devices, as summarized below.

8

The first relatively well studied application for LCLCs is the fabrication of the

highly ordered thin films with anisotropic properties for optical elements [14, 29-35],

such as linear polarizers, retarders and optical compensators. The alignment of LCLCs

can be achieved by using a magnetic field [36], mechanical shearing [31, 37], or

alignment layer [19]. The first claim of the LCLC-based polarizers was made by Dreyer

in 1948 [29]; however it has not been commercialized yet, in part because the dried

LCLC films develop undesirable periodic stripe patterns. The later study [38]

demonstrated the elimination of the stripes by adding a certain block copolymer, but the

exact mechanisms behind this empirical recipe will remain unclear until we learn much

more about the viscoelasticity of LCLCs. Tam-Chang et al. showed that the broad

spectrum polarizing films can be made by mixing LCLCS with different wavelength

absorption [35]. Schneider et al. prepared the monolayer and multilayers of LCLC film

with in-plane orientational order using an electrostatic layer-by-layer deposition

technique [37, 39, 40]. It is known that the twisted nematic (TN) liquid crystal display

(LCD) has a problem known as ‘grey scale inversion,’ which can be corrected by the

addition of optical compensators. To counteract the positive birefringence of the nematic

(N) phase thermotropic LC, optical plates or films with negative optical anisotropy are

required. Lavrentovich et al. demonstrated that the twisted chiral nematic cell using

disodium cromoglycate (DSCG) solutions doped with a certain aminoacids as chiral

dopants can be used as the optical compensating plates with negative birefringence for

TN LCD [32, 33]. Matsunaga et al. demonstrated that micropatterns of anisotropic

aromatic materials can be fabricated using the photoalignment technique and self-

9

organization properties of LCLCs [41]. This technique offers the advantage of the direct

patterning of multiple orientation of dyes, ultimately providing the ability to align

individual pixels to any required orientation in a single film [41]. Tam-Chang et al. also

showed the fabrication of micropatterns of anisotropic materials using a template with

micro-scaled line features [42].

Chromonic liquid crystal phases have anisotropic optical properties, such as

birefringence, suggesting that they can be used in another fascinating application for

biosensors. LCLCs are not toxic to many microbial species [43] and antibody-antigen

binding is not altered by LCLCs [44], which is also an important condition for using them

in biological sensing. Recent reports [43, 45-47] explored the use of LCLCs in

biosensors. The idea of LCLC-based detection is as follows. Each microbe has

characteristic molecular groups-antigens at the surface, to which a corresponding

antibody can bind, thus "recognizing" and "detecting" it. Each antibody molecule has

two binding sites; thus this binding often results in the formation of an aggregate of

microbes, namely, an immune complex. The problem is to amplify this highly selective

binding. The idea of amplification resides in utilizing the elastic properties of the LCLC

surrounding the immune complex. If such a complex grows in the LCLC bulk and

becomes larger than some critical size JE~ T U⁄ , then it should cause director distortions

and optical distinction by placing the sample between two crossed polarizers. Here T is

the Frank elastic constant in a one constant approximation and U is the polar anchoring

coefficient at the LC particle interface.

10

Lydon also mentioned in his recent review paper [4] that LCLCs can be applied as

cheap, organic electrical conducting materials and elements for a viable light-harvesting

device.

1.2 Motivation and objectives of the thesis

LCLCs represent an interesting self-assembled system with orientational and

positional order that is strongly sensitive to a number of factors, which explains the

expansion of the studies of their basic properties. Recently they have also shown a

potential for new applications as described above. However, the basic properties of

LCLCs, including details of aggregation, the molecular structure-property relationship,

role of concentration, temperature, ionic content remain practically unexplored.

This thesis explores how the aggregate structure and the phase diagrams of

LCLCs in water depend on their concentration, temperature, pH of the solution, and the

presence of various additives, such as salts and neutral polymers. We also describe a

potential application of LCLCs as a functional material for nanofabrication, namely, a

controlled and reversible assembly of gold nanorods. These studies provide a basic

understanding of phase behavior and physical properties of the reversible self-assembled

chromonic materials.

11

1.3 Structure of the thesis

The plan of this thesis is to give a presentation of LCLC phase behavior in the

presence of different types of additives and to describe an application of LCLCs for nano-

fabrication. Chapter 1 presents a short overview of LCLCs and clarifies the motivation of

the thesis.

Chapter 2 explores how the phase diagrams and aggregate structures of pure SSY

and DCCG in water depend on their concentration by employing optical microscopy and

synchrotron x-ray scattering. The very existence of the nematic phase in these typical

LCLCs represents an apparent puzzle, since the correlation length associated with the

stacking measured in the x-ray measurement is too short to explain the orientational order

by the Onsager model. Here, we propose that the aggregate can be more complex than

simple rod and contain “stacking faults” such as junctions with a shift of neighboring

molecules, 3-fold junctions, etc. This conjecture received an independent proof in recent

NMR experiments by Day et al. [48].

Chapter 3 presents the effect of ionic additives on LCLC with a discussion of how

different ionic additives, such as salts of different valency and pH-altering agents, can

change the phase behavior of LCLCs. Simple salts enhance the stability of the N phase

when : is small, while they suppress the mesophases when : is large. A base, such as

NaOH destabilizes the N phase at a low concentration of NaOH and then induces the

biphasic regions I+N or I+C at higher !+(8. Spermine in tetravalent salt form

suppresses the N phase. However, spermine in base form induces the biphasic states, a

12

densely packed N phase or C phase coexisting with the I phase, by raising pH to the level

at which spermine molecules become neutral.

Chapter 4 describes the aggregate structure and phase behavior of LCLCs in the

presence of the electrically neutral polymer poly(ethylene glycol) (PEG). Three

component phase diagrams constructed for the entire composition range demonstrate that

the addition of PEG to a SSY solution leads to phase-separation into a liquid crystalline

region with a high concentration of SSY aggregates and a PEG-rich isotropic region. This

behavior can be qualitatively explained by the depletion (excluded volume) effects.

Finally, Chapter 5 shows an application of chromonic materials for nano-

fabrication. We present a simple and universal technique for the controlled non-covalent

assembly of metallic nanorods (NRs) using self-assembled stacks of lyotropic chromonic

materials. The anisotropic electrostatic interaction between the metallic NRs and

chromonic stacks allows one to achieve either side-by-side or end-to-end assembly,

depending on the surface charge of the NRs. The assembly of NRs can be controlled by a

number of factors influencing the self-assembly of chromonic materials, such as the

concentration and pH of the solution.

1.4 Reference

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Verlag, New York, 2003).

13

[2] Figueiredo Neto, A. M.; Salinas, S. R. A. The physics of lyotropic liquid crystals

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14

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[14] Tam-Change, S.-W.; Seo, W.; Iverson, I. K.; Casey, S. M. Ionic

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on the Liquid-Crystalline Properties of Ionic Perylenebis(dicarboximide)s Using UV-Vis

Spectroscopy, Polarized Light Microscopy, and NMR Spectroscopy. Langmuir 2008, 24,

2133-2139.

15

[16] Spindler, L.; Drevensek-Olenik, I.; Copic, M.; Romih, R.; Cerar, J.; Skerjanc, J.;

Mariani, P. Dynamic light scattering and 31P NMR spectroscopy study of the self-

assembly of deoxyguanosine 5’-monophosphate. Eur. Phys. J. E 2002, 7, 95-102.

[17] Mariani, P.; Spinozzi, F.; Federiconi, F.; Amenitsch, H.; Spindler, L.; Drevensek-

Olenik, I. Small angle x-ray scattering analysis of deoxyguanosine 5’-monophosphate

self-assembling in solution: nucleation and growth of G-quadruplexes. J. Phys. Chem. B

2009, 113, 7934-7944.

[18] Hartshorne, N. H.; Woodard, G. D. Mesomorphism in the system disodium

chromoglycae-water, Mol. Cryst. Liq. Cryst. 1973, 23, 343.

[19] Nastishin, Yu. A.; Liu, H.; Schneider, T.; Nazarenko, V.; Vasyuta, R.;

Shiyanovskii, S. V.; Lavrentovich, O. D. Optical characterization of the nematic lyotropic

chromonic liquid crystals: light absorption, birefringence, and scalar order parameter,

Phys. Rev. E 2005, 72, 041711.

[20] Dickinson, A. J.; LaRacuente, N. D.; Mckitterick, C. B.; Collings, P.J. Aggregate

structure and free energy changes in chromonic liquid crystals. Mol. Cryst. Liq. Cryst.

2009, 509, 9-20.

[21] Wu, L.; Lal, J.; Simon, K. A.; Burton, E. A.; Luk, Y.-Y. Nonamphiphilic

assembly in water: polymorphic nature, thread structure, and thermodynamic

incompatibility. J. Am. Chem. Soc. 2009, 131, 7430-7443.

16

[22] Gelbart, W. M.; Ben-Shaul, A. The “New” Science of “Complex Fluids.” J. Phys.

Chem. 1996, 100, 13169-13189.

[23] Cates, M. E.; Candau, S. J. Statics and dynamics in worm-like surfactant micelles.

J. Phys.: Condens. Matter 1990, 2, 6869-6892.

[24] Tam-Change, S.-W.; Seo, W. Synthesis and Studies of the Properties of a Liquid-

Crystalline Quaterrylenebis(dicarboximide) by 1H NMR and UV−vis Spectroscopies. J.

Org. Chem. 2004, 69, 2719.

[25] Kostko, A. F.; Cipriano, B. H.; Pinchuk, O. A.; Ziserman, L.; Anisimov, M. A.;

Danino, D.; Raghavan, S. R. Salt effects on the phase behavior, structure, and rheology of

chromonic liquid crystals. J. Phys. Chem. B 2005, 109, 19126.

[26] Prasad, S. K.; Nair, G. G.; Hegde, G.; V. Jayalakshmi. Evidence of wormlike

micellar behavior in chromonic liquid crystals: Rheological, X-ray, and dielectric studies.

J. Phys. Chem. B 2007, 111, 9741.

[27] Jones, J. W.; Lue, L.; Ormerod, A. P.; Tiddy, G. J. T. The influence of sodium

chloride on the self-association and chromonic mesophase formation of Edicol Sunset

Yellow. Liq. Cryst. 2010, 37, 711-722.

[28] Simon, K. A.; Sejwal, P. R.; Gerecht, B.; Luk, Y.-Y. Water-in-water emulsions

stabilized by non-amphiphilic interactions: polymer-dispersed lyotropic liquid crystals.

Langmuir 2007, 23, 1453.

17

[29] Dreyer, J. F. The fixing of molecular orientation. Phys. Colloid Chem. 1948, 52,

808.

[30] Dreyer, J. F. Dichroic light-polarizing sheet materials and the like and the

formation and use thereof. US Patent 2,544,659 (1951).

[31] Sergan, T.; Schneider, T.; Kelly, J.; Lavrentovich, O. D. Polarizing-alignment

layers for twisted nematic cell. Liq. Cryst. 2000, 27, 567-572.

[32] Lavrentovich, M.; Sergan, T.;, Kelly, J. Planar and twisted lyotropic chromonic

liquid crystal cells as optical compensators for twisted nematic displays. Liq. Cryst. 2003,

30, 851-859.

[33] Lavrentovich, M.; Sergan, T.; Kelly, J. Lyotropic chromonic liquid crystals for

optical applications: An optical retardation plate for twisted nematic cells. Mol. Cryst.

Liq. Cryst. 2004, 409, 21-28.

[34] Fujjwara, T.; Ichimura, K. Surface-assisted photoalignment control of lyotropic

liquid crystals. Part 2. Photopatterning of aqueous solutions of a water-soluble anti-

asthmatic drug as lyotropic liquid crystals. J. Mater. Chem. 2002, 12, 3387.

[35] Tam-Change, S.-W.; Seo, W.; Kyle, R.; Casey, S. M. Molecularly Designed

Chromonic Liquid Crystals for the Fabrication of Broad Spectrum Polarizing Materials.

Chem. Mater. 2004, 16, 1832.

[36] Nazarenko, V.G.; Bioko, O.P.; Park, H.-S.; Brodyn, O.M.; Omelchenko, M.M.;

Tortora, L.; Nastishin, Yu. A.; Lavrentovich, O.D. Surface alignment and anchoring

18

transitions in nematic lyotropic chromonic liquid crystal. Phys. Rev. Lett. 2010, 105,

017801.

[37] Schneider, T.; Lavrentovich, O. D. Self assembled monolayers and multilayered

stacks of lyotropic chromonic liquid crystalline dyes with in-plane orientational order.

Langmuir 2000, 16, 5227-5230.

[38] Schneider, T.; Golovin, A.; Lee, J. C.; Lavrentovich, O. D. Lyotropic chromonic

liquid crystals in aligned films for applications as polarizing coatings. J. Info. Display,

The Korean Information Display Society 2004, 5, 27-38.

[39] Schneider, T.; Artyushkova, K.; Fulghum, J. E.; Broadwater, L.; Smith, A.;

Lavrentovich, O. D. Oriented monolayers prepared from lyotropic chromonic liquid

crystal. Langmuir 2005, 21, 2300-2307.

[40] Boiko, O.; komarov, O.; Vasyuta, R., Nazarenko, V.; Slominskiy, Yu.; Schneider,

T. Nano-Architecture of Self-Assembled Monolayer and Multilayer Stacks of Lyotropic

Chromonic Liquid Crystalline Dyes. Mol. Cryst. Liq. Cryst. 2005, 434, 633.

[41] Matsunaga, D.; Tamaki, T.; Akiyama, H.; Ichimura, K. Photofabrication of micro-

patterned polarizing elements for stereoscopic displays. Adv. Mater. 2002, 14, 1477.

[42] Tam-Change, S.-W.; Jelbley, J.; Carson, T. D.; S.-W., Seo, W.; Iverson, I. K.

Template-guided organization of chromonic liquid crystals into micropatterned

anisotropic organic solids. Chem. Commun. 2006, 503.

19

[43] Woolverton, C. J.; Gustely, E.; Li, L.; Lavrentovich, O. D. Liquid crystal effects

on bacterial viability. Liquid Crystals 2005, 32, 417-423.

[44] Luk, Y. Y.; Jang, C. H.; Cheng, L. L.; Israel, B. A.; Abbott, N. L. Influence of

lyotropic liquid crystals on the ability of antibodies to bind to surface-immobilized

antigens. Chemistry of Materials 2005, 17, 4774-4782.

[45] Shiyanovskii, S. V.; Schneider, T.; Smalyukh, I. I.; Ishikawa, T.; Niehaus, G. D.;

Doane, K. J.; Woolverton, C. J.; Lavrentovich, O. D. Real-time microbe detection based

on director distortions around growing immune complexes in lyotropic chromonic liquid

crystals. Phys. Rev. E 2005, 71, 020702 (R).

[46] Shiyanovskii, S. V.; Lavrentovich, O. D.; Schneider, T.; Ishikawa, T.; Smalyukh,

I. I.; Woolverton, C. J.; Niehaus, G. D.; Doane, K. J. Lyotropic chromonic liquid crystals

for biological sensing applications. Mol. Cryst. Liq. Cryst. 2005, 434, 633.

[47] Helfinstine, S. L.; Lavrentovich, O. D.; Woolverton, C. J. Lyotropic liquid crystal

as a real-time detector of microbial immune complexes. Letters in Appl. Microbiology

2006 43, 27-32.

[48] Renshaw, M. P.; Day, I. J. NMR characterization of the aggregation state of the

azo dye Sunset Yellow in the isotropic phase. J. Phys. Chem. B 2010, 114, 10032-10038.

20

Chapter 2 Self-assembled lyotropic chromonic liquid

crystals in water

2.1 Introduction

Even in the dilute isotropic (I) solutions, chromonic molecules have a strong

tendency to stack into aggregates. The balance of energy gained by placing a chromonic

molecule inside the aggregate and the entropy term promoting a larger number of

aggregates produces a polydisperse system of linear aggregates [1, 2] that can arrange

themselves into ordered liquid crystal phases as a function of both the concentration of

LCLC and the temperature. In this chapter, we explore the phase behavior of LCLCs and

their aggregate structure using the typical LCLCs Sunset Yellow (SSY) and disodium

cromoglycate (DSCG).

SSY is a food coloring azo dye, having the chemical name disodium salt of 6-

hydroxy-5-[(4-sulfophenyl)azo]-2-naphthalenesulfonic acid, Fig. 2.1. SSY contains

central aromatic groups, phenyl and naphthyl rings linked via an azo group with two

solubilizing sulfonate groups attached to either end of the molecule. As an LCLC, SSY

has been studied by Ormerod [3], Luoma [4], and recently by Horowitz et al. [5],

Edwards et al. [6], and Chami et al. [7]. Compared with other LCLCs, such as DSCG, the

aggregate structure of SSY has been somewhat better established. Most significantly, it

has been shown that the stacks of SSY in water contain only one molecule in cross

21

section and the SSY molecules are on average perpendicular to the aggregate axis [5-7].

The average diameter J of the aggregate, according to Ref. [6], is about 1 nm, while

Louma [4] found J 1.4nm. Luoma [4] also determined the length of the SSY

aggregates to be about 2.4 nm at the temperature of the transition between the nematic

(N) and the I phase, which noted to be too short to fit the Onsager criterion for the N

phase.

Figure 2.1. The molecular structure of two forms of SSY, (a) NH hydrazone tautomer and

(b) OH azo tautomer. The model structures on the bottom illustrate the relative electric

charge distribution: Red color corresponds to the positive charge and blue to the negative

charge.

(a) NH Hydrazone tautomer: prevailing form in aqueous solutions

(b) OH Azo tautomer

22

DSCG is also known as Cromolyn and Intal, an antiasthmatic drug with the

chemical name disodium 5,5'-[(2-hydroxy-1,3-propanediyl) bis(oxy)] bis[4-oxy-4H-1-

benzopyran-2-carboxylate], Fig. 2.2. The liquid crystalline phases of DSCG were first

reported in the 1970s by Woodard et al. [8, 9], and subsequently studied more extensively

by other groups [10-18]. An x-ray study by Woodard et al. [9] suggested the N phase is

formed by aggregates of stacked DSCG molecules which are on average perpendicular to

the aggregate axis. However, the details about the aggregate structure are still the subject

of discussion; see, e.g., the reviews by Lydon [18]. Woodard et al. [9] assumed that there

is a molecule in the cross-section of the cylindrical aggregate. They estimated the

cylinder diameter to be about 1.6 nm [9]. Later, Lydon [18] proposed that the aggregate is

shaped like a hollowed square formed by four molecules linked by electrostatic salt

bridges. Recently, Dickinson et al. calculated from x-ray scattering experiments that there

are two DSCG molecules in the cross-section of the aggregate [17].

Figure 2.2. The molecular structure of DSCG. The model structures on the right hand

side illustrate the relative electric charge distribution: Red color corresponds to the

positive charge and blue to the negative charge.

23

By employing optical microscopy and synchrotron x-ray scattering measurement,

we explore how the phase diagrams and aggregate structure of LCLCs in water depend

on concentration and temperature. The very existence of the N phase in chromonic water

solutions is puzzling, as the correlation length W: measured along the stacking direction in

the x-ray experiments is too small to satisfy the Onsager criterion for orientational order

of concentrations at which the N phase is being observed [19]. We propose that

chromonic molecules might form not only the simple rod-like aggregates but also more

complex geometries with “stacking faults,” such as junctions with a shift of neighboring

molecules, three-fold junctions, etc, where the x-ray correlation is lost but a physical

connection remains. Dynamic light scattering (DLS) and cryogenic transmission electron

microscopy (cryo-TEM) measurements of LCLCs reveal that the aggregate’s size is

indeed much larger than W: determined from the x-ray measurement. This conjecture

received an independent proof in recent NMR experiments by Day et al. [20].

2.2 Experimental techniques

2.2.1 Materials

Sunset Yellow (SSY). The SSY batch used in this investigation was purchased

from Sigma Aldrich and had a purity of 95.7%, according to the HPLC test by Sigma

Aldrich. The two main types of impurities found in SSY are as follow: (a) byproducts of

the synthesis with a molecular structure close to that of SSY, such as the trisodium salt

of 3-hydroxy-4-(4-sulfophenylazo)-2,7-naphthalenedisulfonic acid and the trisodium salt

24

of 6-hydroxy-7-(4-sulfophenyl)-5-(4-sulfophenylazo)-2-naphthalenesulfonic acid [21,

22]; the latter represents a molecule in which an additional sulfophenyl group is attached

to the SSY core; (b) lower-molecular weight compounds such as inorganic salts; for

example, NaCl is used to induce dye precipitation in the last step of the SSY synthesis.

The HPLC-determined number 95.7% specifies mainly that the material contains about

4.3% impurities (a); it does not provide information on the quantity of (b). To purify SSY

from impurities (b), we followed the procedure established earlier [3-5]. Namely, SSY

was dissolved in deionised water with the subsequent addition of ethanol to cause the

precipitation of the dye. The precipitate was filtered from the solvent with impurities (b)

and dried in a vacuum; the procedure was repeated twice [3-5]. This purification

procedure makes our data comparable to that obtained earlier by Ormerod [3], Luoma

[4], Horowitz et al. [5], and Edwards et al. [6].

SSY might exist in two tautomeric forms: an NH hydrazone form, Fig. 2.1(a),

with the hydrogen residing on the distant nitrogen of the azo bond, and an OH azo-

tautomer form, with the hydroxide proton residing on the oxygen in ortho to the azo

bond, Fig. 2.1(b). For most phenylazonaphthols, the NH hydrazone form has been

reported as dominating in water solutions [23, 24]. Edwards et al. [6] demonstrated that

the NH hydrazone form, Fig. 2.1(a), prevails for both monomers and stacks of SSY.

Chami et al. [7] also calculated that the NH hydrazon form is more stable than OH azo

form for both the dianion and the protonated compound. The phase diagram is sensitive

to the purity and hydration of SSY. The phase transition temperatures in solutions

prepared from the commercially available SSY (used as purchased) are about 6 lower

25

than those for SSY purified as specified above. The degree of SSY dehydration prior to

the preparation of the mixtures is an important issue. In all our experiments, SSY was

dehydrated by placing it, after purification, in a vacuum oven for two days. We tested

how easily the dry SSY might be hydrated during storage, by monitoring the weight of

three samples (about 0.3 g each), one in an open vial, another in a vial closed with a

plastic lid, and the third in a vial closed with a plastic lid and sealed with a parafilm. The

SSY vials were placed next to an open water beaker (100 g) at room temperature. Over

the two days, the open sample of SSY added about 20 % to its weight, while SSY in the

closed vial gained about 5 %. The third sample, with a lid and a parafilm, added only 1%

over 1 month of storage. To avoid these hydration effects, we placed the purified and

dried SSY in a desiccator; the subsequent weight gain was only 0.1% over the period of 3

months. In the experiments, we used either freshly prepared samples or the samples in

well sealed vials kept in a dessicator.

Disodium cromoglycate (DSCG). DSCG was purchased from Spectrum

Chemical Mfg. Corp. (Gardena. CA) with a purity of 99 % and used without further

purification. To avoid the effects of hydration, we placed the dried DSCG in a vacuum

dessicator after dehydartion as described above.

We use molality units for LCLC concentration :, determined as the number of

moles of chromonics in 1 kg of water, and the volume fraction units,

X EYZ&YZ[\]^_`[YZ EYZ&YZ[\]^_`. Here a: is the molecular weight of LCLCs, 0.452 kg/mol for

SSY and 0.5124 kg/mol for DSCG; 9: is the density of LCLCs, 1.4 5 107 kg/m7 for

26

SSY and 1.55 5 107 kg/m7 for DSCG; and 9>+bCc 1.0 5 107 kg/m7 is the density of

water. Distilled water further purified with a Millipore water purification system

(resistivity ≥18.1 MΩ·cm ) was used for preparing all the solutions.

2.2.2 Optical studies of phase transitions

For optical observations, samples were prepared by placing a drop of the solution

between two glass plates separated by mylar spacers (thickness 6 µm and 12 µm) and

sealing the edges of the cell with an epoxy glue (Davcon) and nail polish. The phase

behavior was determined by observing the samples through a polarizing microscope,

while changing the temperature at a rate of 0.2 /min.

2.2.3 Synchrotron x-ray studies

For x-ray diffraction measurements, the samples were loaded into 1.5 mm

diameter Lindéman capillaries with 10 µm thick walls or in sandwich-like cells made

with thin (60µm) glass plates. These were placed in an oven under an in-situ magnetic

field of strength 2.5 kG, sufficient to align the LCLC. The sample was then exposed to

synchrotron x-ray radiation of wavelength 0.7653 Å at station 6-IDB of the Midwestern

Collaborative Access Team at the Advanced Photon Source of Argonne National

Laboratory. The diffraction patterns were recorded at room temperature ( 28.4 )

using a high resolution image plate area detector, MAR345, placed at a distance of 476.0

mm from the samples. The data were calibrated against a silicon standard (NIST 640C).

The intensity of the incident beam was controlled using a bank of Cu and Al attenuators.

Data accumulation times ranged between 1 and 60 s. The 2D diffraction patterns were

27

analyzed using the software package FIT2D developed by Hammersley et al. [25]. The

length scale and the correlation lengths corresponding to various peaks were calculated

from the position and width of the diffraction peaks.

2.2.4 Dynamic Light Scattering

Dynamic light scattering (DLS) was performed using ALV/LSE-5004 (Germany)

with a He-Ne laser (maximal power 25mW, power stability better than ≤1% over 24h,

and wavelength e 632.8 nm). An LCLC solution was filtered through 0.2 fm filters

and then placed into a cylindrical cuvette of the internal diameter 9 mm. The intensity

correlation functions were collected at different scattering angles between g 15° and

g 90°. The size of the aggregates was estimated through measurement of the intensity

correlation function of the scattered light that probed the dynamics of the system [26].

The translation diffusion coefficient was determined as Mbc+i% 1 j⁄ , where 6kl m sin o, m 1.33 is the refractive index of water, and j is the characteristic relaxation

time obtained from the correlation function. Assuming a spherical shape of the aggregate,

the typical size was estimated as 2pq I rst7ku^`]vw, where xy is the Boltzmann constant,

is the absolute temperature, and z is the shear viscosity of the solvent. This assumption

underestimates the largest extension of the aggregates, which conforms to our goal of

finding the lower limit of the size of the aggregates, as will be discussed later.

2.2.5 Cryo-transmission electron microscopy (TEM)

To prepare the vitrified sample for cryo-TEM, 5 µl of an LCLC solution were

dropped on a holey carbon grid (Ted Pella, Redding, CA) in a controlled environment

28

vitrification chamber (Vitrobot, FEI) in which the atmosphere surrounding the sample

grid was kept at room temperature and 100% relative humidity. The sample grid was

immediately vitrified in cryogen (50/50 ethane/propane) after blotting using Vitrobot

(FEI). The vitrified samples were examined on a FEI Technai G2 microscope operated at

200kV.

2.3 Experimental Results

2.3.1 Phase behavior of pure SSY in water

Fig. 2.3 shows the phase diagram and typical textures of the water solutions of

SSY. The phase diagram shows the I, N, and columnar (C) phases with broad coexistence

regions. The N phase produces Schlieren textures with disclinations (characterized by

two dark brushes of extinction) and point defects-boojums (with four brushes of

extinction) [27]. The columnar phase shows characteristic “developable” domains [27].

The phase diagram is in good agreement with the data from Horowitz et al. [5] (note that

the notation for concentrations in Ref. [5] should read “mol/kg” instead of “M” [28]) and

with Edwards et al. [6]. We observed the C phase at 1.2 mol/kg, Fig. 2.3, while

in Ref. [6], the C phase is observed for 1.4 mol/kg. This discrepancy might be

caused by the degree of SSY purification and dehydration, or by a different initial purity

of SSY, as discussed above.

29

Figure 2.3. Phase diagram and polarizing micrographs of SSY water solutions. The error

bars represent the difference between the data taken on heating (upper end of the bar) and

cooling (lower end of the bar). The filled circles at the vertical line indicate the

temperatures at which the pictures were taken.

30

The phase identification is confirmed by the x-ray data, Fig. 2.4 and Table I. In

the I phase, 0.7 mol/kg, the aggregates have no particular orientation, Fig. 2.4(a).

The diffraction at the large angle (2g 13.2°) is ascribed to the stacking repeat distance

(KL 0.33nm) between the SSY molecules in the aggregate. If the correlation length

W:, corresponding to the stacking repeat distance KL is inversely proportional to the full

width at half maximum (FWHM) of the scattering wave vector peak, then the correlation

length for 0.7 mol/kg SSY is W:, 2 Δ⁄ 2.4 nm, where the wave vector

is ) sin g and e is a synchrotron x-ray radiation of wavelength 0.7653 Å. If we

assume that the aggregates are rod-like, with SSY molecules stacked on top of each other,

then W:, can be taken as a measure of an average length 2.4 nm of such a rod-

like stack, and the corresponding aggregation number would be m I 7.3.

Fig. 2.4(b) shows the x-ray diffraction pattern for a 0.9 mol/kg N phase in

the magnetic field directed horizontally, as indicated by the arrow. Because SSY

molecules possess positive anisotropic magnetic susceptibility they orient parallel to the

direction of the magnetic field. Consequently, the director n , the common direction of

aggregates, aligns perpendicularly to the magnetic field; the walls of the vertical circular

capillary containing the sample, being perpendicular to the magnetic field, assist in a

uniform alignment of PQ along the axis of the capillary, i.e., vertically. The diffraction

pattern of the aligned N phase has two pairs of arcs in orthogonal directions, Fig. 2.4(b).

This feature supports the model of so-called H-aggregation [29], with the molecules

stacked on top of each other, being on average perpendicular to the aggregate axis and n .

31

One pair of arcs in the vertical direction at the large angle (2g 13.2°) ascribed to the

stacking distance (KL 0.33nm) along n remains the same as in the I phase. The

associated correlation length calculated from the FWHM of the large angle peak is

W:, 3.5 nm; the aggregation number corresponding to the correlated stacking is

m! I 10.4. In the aligned N samples, another pair of arcs in the horizontal direction is

from the small angle diffraction corresponding to a d-spacing of 2.63 nm, which is

proportional to the average distance between the SSY aggregates. Fig. 2.4(b).

We note that the x-ray pattern of the N phase, Fig. 2.4(b), has four faint

reflections visible at approximately 2g I 7.8 which may be due to the phenyl group of

these molecules being oblique to the naphthalene plane of the hydrazone form of SSY

[30], or possibly to other effects, such as the formation of chiral arrangements of dye

molecules within the aggregates [6], etc.

A further increase of leads to the C phase. The diffraction patterns of the C

phase show the same large angle peak at 2g 13.2° as do the I and N phases, which

indicates the space between chromonic molecules within an aggregate is not altered by

the concentration of SSY, Fig. 2.4(d) and Table 1. The correlation length W:,

calculated from the FWHM value of this peak is longer than the corresponding value in

the N phase, namely W:, 13.4 nm at 1.36 mol/kg. The x-ray of a 1.36 mol/kg C phase at small angle 2g 2.1° shows a strong sharp diffraction line (J)

and three faint, but sharp diffraction lines (J, J7, J6) whose diffraction spacings are the

ratio of 1: 1 √3 ⁄ : 1 √4⁄ : 1 √7⁄ , characteristic of the hexagonal structure [9, 10]. The

32

correlation length W, associated with the positional order transverse to the aggregate

axis determined from the FWHM of the peak at 2g 2.1° is much larger than the

corresponding value in the N phase, Table 1. For the C phase, the inter-aggregate axis-to-

axis distance M can be directly related to these diffraction lines,

M 2J √3 2J 4J7 √3⁄⁄ , Table 1.

Figure 2.4. Typical x-ray patterns (a-c) of SSY water solutions at different concentrations

: (a) I phase, 0.7 mol/kg; (b) N phase, 0.9 mol/kg solution; (c) C

phase, 1.36 mol/kg. (d) Diffractographs of SSY water solutions at different

concentrations : 0.7 mol/kg (green), 0.9 mol/kg (blue), and 1.36 mol/kg (red). All

data are taken at 28.4 .

33

Table 1. X-ray diffraction data for four different concentrations of SSY in water,

expressed in molal and volume fraction X units. The temperature is fixed at

28.4 .

(mol/kg) X J (nm) M(nm) W (nm) W: (nm) phase

0.7 0.184 3.11 - 4.42 2.4 I

0.9 0.225 2.63 - 5.56 3.5 N

1.14 0.269 2.34 - 8.56 4.8 N

1.36 0.305 2.07 2.39 104.72 13.4 C

J J and M 2J √3⁄ in the C phase.

W was calculated from the FWHM of a small angle peak at 2g 1.45~2.12°. W: was calculated from the FWHM of a large angle peak at 2g 13.2°.

2.3.2 Phase behavior of pure DSCG in water

Fig. 2.5 shows the phase diagram which agrees well with the data from Woodard

et al. [9]. At room temperature (23ºC), as the concentration of DSCG increases, the

aqueous solution of DSCG shows the I phase at 0.217 mol/kg 10 wt%". In

the range 0.217 mol/kg c 0.266 mol/kg 12 wt%", the solution demonstrates

a wide biphasic region of coexisting N and I phases. The N phase is observed in the

range 0.266 mol/kg c 0.43 mol/kg ~18 wt%", another biphasic region with

34

coexisting N and C phases at 0.43 mol/kg c 0.519 mol/kg 21 wt%" and then

the homogeneous C phase at 0.519 mol/kg c ~0.919 mol/kg ~32wt%".

We note that highly concentrated DSCG solutions show a different phase

transition sequence with increasing temperature compared with SSY solutions. These

highly concentrated DSCG solutions first become less ordered and then reenter the more

ordered phase as the temperature increases. For example, 0.616 mol/kg of DSCG, being

in the C phase at room temperature, goes to the N+C biphasic region, the I+N biphasic

region, and then the I+C biphase as the temperature increases, Fig. 2.5. Qualitatively this

is in close agreement with the theoretical model proposed by Hentschke et al. [31].

According to this model, as aggregation becomes weaker or more flexible, as a function

of temperature, the N phase is abbreviated and the C phase becomes stable [31].

35

Figure 2.5. Phase diagram and polarizing micrographs of DSCG water solutions. The

filled circles at the vertical line indicate the temperatures at which the pictures were

taken.

The x-ray diffraction patterns of a DSCG solution are similar to those from SSY

solution. The x-ray diffraction pattern for a 0.344 mol/kg 15wt. %" N phase

DSCG solution in the magnetic field directed horizontally shows two pairs of arcs in

36

orthogonal directions, Fig. 2.6(a). One pair of arcs in the vertical direction at the large

angle diffraction at 2g 12.8°, representing the stacking repeat distance (KL 0.34nm)

between the DSCG molecules in the aggregate, is perpendicular to the magnetic field,

and the small angle diffraction is parallel to the magnetic field, indicating that the

molecules stack on average perpendicular to the aggregate axis. The stacking repeat

distance is not altered, while the correlation length W:, calculated from the FWHM

value of the large angle peak increases, as increases, Fig. 2.6(c) and Table 2. We

note that the correlation length of the 0.344 mol/kg N phase DSCG solution is W:, 5.2 nm, longer than that of the 0.9 mol/kg N phase SSY solution.

The x-ray diffraction patterns of the C phase of DSCG is similar to the C phase of

SSY, showing a strong sharp diffraction line (J) at the small angle 2g I 1.1~1.5° and

three faint diffraction lines, which arise from the long-range hexagonal packing of the

columns in the plane perpendicular to PQ. The inter-aggregate axis-to-axis distance M

decreases as increases, Table 2. The large angle diffraction peak of the C phase

describes that the stacking distance KL 0.34 nm is not changed, but W:, is longer

than that in the N phase, Table 2.

37

Figure 2.6. X-ray patterns of (a) N phase, 0.34 mol/kg and (b) C phase,

0.96 mol/kg. (c) Diffractographs of DSCG water solutions at different

concentrations : 0.34 mol/kg (black), 0.62 mol/kg (red), and 0.96 mol/kg (blue). All

data are taken at 28.4 .

38

Table 2. X-ray diffraction data for four different concentrations of DSCG in water,

expressed in molal and volume fraction X units. The temperature is fixed at

28.4 .

(mol/kg) X J (nm) M(nm) W (nm) W: (nm) phase

0.103 0.033 3.99 - - 1.5 I

0.344 0.102 4.68 - - 5.2 N

0.616 0.169 3.63 4.19 69.8 11.2 C

0.961 0.241 3.01 3.48 74.8 12.1 C

J J and M 2J √3⁄ in the C phase.


2.3.3 The length of aggregates from a dynamic light scattering (DLS) and cryo-

transmission electron microscopy (cryo-TEM) measurements.

We estimated the size of the chromonic aggregates with two other methods: DLS

and cryo-TEM measurements. DLS measurement allows one to estimate the size of

chromonic stacks through the hydrodynamic diameter 2pq [26]. The I phase 0.1 mol/kg

DSCG solution was used to measure the aggregate size. DLS data obtained at several

scattering angles between g 15° and g 90° show a broad range of relaxation times

(j). Fig. 2.7 shows the plot of the relaxation rate Γ Γ 1 τ⁄ " versus for the large

39

value of relaxation times, indicating linear behavior with slope Mbc+i% 3.54 5103 m s⁄ . Assuming a spherical shape for the aggregate, one can estimate the

aggregate size in a 0.1 mol/kg DSCG solution, 2pq I rst7ku^`]vw I 140nm, which is

much larger than the size of a monomer or the correlation length W:, 1.5 nm from

the x-ray measurement. The lower value of relaxation time corresponds to the

hydrodynamic diameters 2pq I 1 nm which matches the diameter J of DSCG

molecules. The difference in the characteristic length scales measured in DLS and x-ray

experiments suggests that the aggregates have two scales of structural organization:

relatively short branches of size W: within which the stacking of monomers is correlated,

and a much larger scale 2pq W: characterizing the entire aggregate, comprised of many

connected W:-branches, where the x-ray correlation is lost but a physical connection

remains.

Figure 2.7. Relaxation rate Γ versus for 0.1 mol/kg, I phase DSCG solution at 22 .

40

Cryogenic transmission electron microscopy (cryo-TEM) provides the high

resolution images of the DSCG solution. Vitrification by rapid freezing (105Ks-1) ensures

the preservation of the assembled structure and phases while avoiding the risk of the

result being an artifact associated with crystallization or dehydration. A cryo-TEM image

of the homogeneous N phase 0.344 mol/kg DSCG solution shows a dense collection of

parallel aggregates, Fig. 2.8. The inspection of TEM images suggests the length of the

aggregate is at least one order of magnitude larger than the correlation length W:, I5 nm from x-ray measurement. Some aggregates seem to have “stalking faults,” such as

molecular shifts and three-fold junctions, inside dotted circles, as in Fig. 2.8.

Figure 2.8. Cryo-TEM image of a 0.344 mol/kg DSCG solution. The scale bar is 50 nm.

41

2.4 Discussion

The simplest phenomenological model able to describe the phase diagram of

LCLCs in water can be based on the enthalpy-entropy balance of the reversible self-

assembly of one-dimensional aggregates, similar to the models of worm-like micelles or

“living” polymers [1, 2]. The monomers, in this case disk-like chromonic molecules of

diameter J: I 1~2 nm and “thickness” KL I 0.33 nm, prefer to stack face-to-face in

order to minimize the areas of unfavorable contact with water. The aggregate would grow

indefinitely if it were not for the entropy that is roughly proportional to the number of

aggregates. The balance of the “end” energy of an aggregate (also called the scission

energy, i.e., the energy needed to cut an aggregate into two), and the entropy gained by

producing more “ends”, results in a broadly polydisperse system of rod-like aggregates

with the average aggregation number

m Xexp =rst (1)

determined by the volume fraction X of the solute and strongly dependent on and the

absolute temperature [1, 2].

This relationship (1) has been derived for dilute isotropic solutions and for

electrically neutral monomers; the later assumption does not apply to LCLCs. For

example, the ionic groups of SSY dissociate in water, producing electric charges at the

lateral surface of aggregates and releasing Na+ counterions. Mutual Coulomb repulsion

weakens the association of monomers and reduces the scission energy # S C. The

electrostatic correction C should depend on the spatial distribution of the counterions

42

and on X. For highly charged linear aggregates, some of the counterions are immobilized

at the surface [32-36], while others are released into the solvent. The condensed

counterions decrease the effective charge of the rod from the maximum possible jO I6O nm⁄ to jO~ O y⁄ , where y C

6krst is the Bjerrum length, i.e. the distance at which

the repulsion energy of two elementary charges O equals the thermal energy xy. For

water at room temperature, y 0.7 m and thus jO I 1.4O nm⁄ . Furthermore, C

should decrease with X: as X increases, the clouds of “free” counterions are pushed

closer to the aggregates and thus screen the charges of monomers in them more

effectively (each elementary cell comprising an aggregate with the surrounding

counterions should be electrically neutral) [37]. For moderate X 0.1, MacKintosh et al.

[37] found that the aggregation number for one-dimensional aggregates in an isotropic

solution becomes modified as:

m Xexp =3=_rst , (2)

where C yJjxy 2X . In the order of magnitude, C I 1.64xy for X I 0.18

(which corresponds to the I phase at room temperature). If one assumes the rod-like

model of SSY aggregates, then the experimental value W:, 2.4 nm in the I phase can

be used to determine S C from Eq.(2), as the aggregation number m I W:, KL⁄ I7.3 is known in this model. One estimates S C I 5.7xy and I 7.3xy, close to

the values reported by Luoma [4], Horowitz et al. [5], and Chami et al. [7] for SSY and

by Nakata et al. [38] for a similar self-assembling system of short DNA segments.

43

Correlation length and aggregate length. According to the Onsager theory [19],

when the length-to-diameter ratio J⁄ of the rod-like aggregates and X are sufficiently

high, the solution should experience an I→N phase transition. A monodisperse system of

rigid neutral rods forms the N phase when X exceeds a critical value X! I 4J ⁄ [19,

39, 40]. This condition cannot be fulfilled in our experiments if we treat the structure of

chromonic aggregates as simple rod-like aggregates of length W:. In our experiments,

SSY shows the N phase at X I 0.23 at room temperature and at this concentration,

W:, I 3.5 nm, Table 1. For DSCG, the N phase clearly exists at X I 0.10 with

W:, I 5.2 nm at room temperature, Table 2. If one identifies the correlation length W:

with and takes J I 1 nm for SSY and J I 1.6 nm for DSCG, then

X J I 0.8⁄ for SSY and X J I 0.3⁄ for DSCG are clearly too

small to fulfill the condition for the N order. Numerical simulations [39] demonstrate that

there should be no orientational order in the Onsager system at any X, if J⁄ 4.7. In

other words, from the point of view of the Onsager model, the N phase of rod-like

aggregates in our typical solutions of SSY with X I 0.23, J I 1 nm, and IW:, I 3.5 nm or of DSCG with X I 0.10, J I 1.6 nm, and IW:, I 5.2 nm should not exist at all.

In the estimates above, we used the minimum “bare” value of J:, as if the

aggregates were electrically neutral. Accounting for the effect of charges here would not

improve the situation. Mean-field theories [19, 41] predict that electric charges in rod-like

polyelectrolyte solutions cause Coulomb repulsion similarly charged rods and thus favor

44

perpendicular alignment (a “twist” effect), which amounts to destabilization of the N

phase. Taking into account that the aggregates are flexible would not be helpful either in

the resolution of the puzzle of small X:: J:⁄ , as flexible rods need an even higher

X: to produce the N phase as compared to their rigid counterparts [42-45].

The discrepancy between the experimental facts of the existence of the N phase

and the smallness of Lξ might simply indicate that the Onsager theory is not applicable to

the case of self-assembled polydisperse aggregates. Indeed, in the Onsager model, the

rods are rigid and monodisperse, the phase transitions dependent on X but not on , etc.

In LCLCs, the rods are reversibly self-assembled, polydisperse, with T-dependent size

[13]; the phase diagram is also T-dependent, Fig. 2.3 and Fig. 2.5. In a polydisperse

system, the population of longer rods might dictate the onset of the N order, while the

shorter rods remain disoriented [43]. An approach based on these ideas and thus better

fitting the behavior of LCLCs has been proposed by Taylor and Herzfeld [46]. Their

model deals with electrically neutral monomers that reversibly self-assemble into one-

dimensional rod-like aggregates. The inter-aggregate potential is taken in Ref. [46] as an

infinite hard-core repulsion surrounded by a short-ranged “soft” repulsion of a finite

amplitude and width that mimics electric double layers, hydration forces, etc. The total

free energy density contains an ideal mixing term and an intra-aggregate association term

with temperature-dependent end energy [46]. Numerical minimization [46] yields a phase

diagram that is qualitatively close to the experimental Fig. 2.3 and Fig. 2.5. Namely, the

only stable phase at low X is the I phase. Increasing X at T=const, one finds the N and C

phases. Both melt into the I phase as increases (at X const). The two transition lines

45

separating the biphasic I+N region from the homogeneous I and N phases are tilted with

respect to both and X axes. At high , there is no N phase, but the C phase is preserved.

The N-C transition is mostly concentration-driven and depends little on . All these

features are reproduced in our experiments, indicating that the excluded volume effects

and sufficiently large are the two important features responsible for the formation of

LC phases. However, there are still significant quantitative differences. The model [46]

deals with spherical monomers and predicts that the aggregation number m I 10 at the

I-N transition for X I 0.2, which translates into the aspect ratio J⁄ I 2m 3 I 7⁄ and

X J⁄ I 1.4 [46]. In our experiments, since the chromonic monomers are disc-like, the

same m I 10 implies smaller J I 0.33m I 3.3⁄ and X J I 0.7⁄ for

SSY or J I 0.21m I 2.1⁄ and X J I 0.2⁄ for DSCG, still

below the limit necessary for the formation of the N phase in Ref. [46].

The source of discrepancies between our experiments and the theoretical models,

either the classic Onsager theory [19, 39, 40] or its modification for reversibly self-

assembled monomers [45, 46] appears to be in the oversimplification of the actual shape

of aggregates and in the assumption that the correlation length W: reflects the true

dimension of the aggregates. We propose that the LCLC aggregates can form

morphologies more complex than simple rods, with two spatial scales: a short scale W:

related to correlated stacking of monomers on top of each other and a much larger scale

characterizing the entire aggregate, comprised of many connected (but uncorrelated) W:-

branches. The correlation between the Lξ -branches is lost through “stacking faults,” such

46

as molecular shifts, three-fold “Y” junctions, etc., Fig. 2.9. Note that the Y junctions have

been observed in isotropic solutions of block copolymers and surfactants [47, 48]; in dry

films formed from LCLCs, the Y junctions are seen as clearly prevailing over the open-

ended rod-like bundles [49]. Recently, molecular simulations by the Colorado group [50]

confirmed that the distances over which the correlated arrangements of molecules take

place are shorter than the length of aggregates.

The aggregates with junctions should be understood as transient rather than rigid

formations. At this stage, it is difficult to describe the detailed geometry of aggregates.

As is clear from the molecular structure of SSY and DSCG, an ideal “face-to-face”

stacking of these molecules is impossible from the electrostatic point of view. The

neighboring molecules should shift and rotate with respect to each other to overlap

effectively the electronegative and electropositive regions. For example, as proposed by

Edwards et al. [6], the two-ring parts of SSY are stacked on the top of each other but the

one-ring parts of two neighboring molecules are located at the opposite sides of the stack.

The interaction potential landscape, besides the absolute minimum corresponding to the

regular stacking, contains local minima for non-ideal configurations with a different shift,

twist, or a number of immediate neighbors. The number of possible “defects” or

“stacking faults” is increased by the existence of two tautomers of SSY, Fig. 2.1. All

these defects are expected to increase the configurational entropy of the system as already

discussed for Y-junctions [51]. Note that although the Y-junctions might be a universal

type of defects in many self-assembled systems, such as worm-like micelles, living

47

polymers, and LCLCs, the shift stacking faults, Fig. 2.9 appear to be specific to the

chromonic type of aggregation.

Figure 2.9. Schematic models of the N phase in a LCLC: (a) a standard model with rod-

like aggregates; (b) a model with shift junctions and Y junctions and their clusters,

coexisting with the rod-like aggregates.

48

To elucidate this model, we estimated the size of the chromonic aggregates with

two other different methods: a dynamic light scattering (DLS) measurement and cryo-

transmission electron microscopy (cryo-TEM) measurement. We deliberately chose the

simplest hydrodynamic model (spherical scatterers) to extract 2pq from the DLS data for

the I phase DSCG solution, as the goal was to compare 2pq to W:,; we found that that

2pq, regardless of the type of the hydrodynamic model used, is much larger than W:,.

Cryo-TEM measurements for the N phase DSCG solution also show that the length of

DSCG aggregate is at least one order of magnitude longer than the size W:, from x-

ray measurement. Some aggregates seem to have “stalking faults” such as molecular

shifts and three-fold junctions.

Complex geometry of aggregation can reconcile the observed stability of the LC

phases with the low W:. Two rod-like branches of length W: linked through a shift by, say,

J: 2⁄ , would produce a cluster with an aspect ratio 2W: 1.5J:⁄ , higher than the aspect

ratio W: J:⁄ of the individual branch. At higher levels of aggregation, with 1

generations of connected branches, one would expect the length of the aggregate to scale

as E,%bCc W: and the width as JE,%bCc √, as in one-dimensional random walk,

so that the aspect ratio increases with , :¡¢£w^_`G¡¢£w^_` √. The clusters formed by W:-

branches connected through “stalking faults” would thus be capable to satisfy the

Onsager criterion even if the correlation length W: does not.

49

2.5 Conclusions

The very formation of the orientationally ordered phases in LCLC solutions

represents a puzzle, since they are observed when the volume fraction of LCLCs and the

correlation length W: measured along the stacking direction are too low to satisfy the

conditions of the Onsager model or its variations considering rod-like aggregates. We

propose that the true structure of LCLC aggregates includes morphologies more complex

than the simple rods, with two levels of structural hierarchy, a small scale W: of correlated

stacking and a much larger scale of uncorrelated stacking, corresponding to the overall

size of the aggregates; it is the larger scale that is responsible for the formation of the

orientational order.

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57

Chapter 3 The effect of ionic additives on lyotropic

chromonic liquid crystal Sunset Yellow

3.1 Introduction

Lyotropic chromonic liquid crystal (LCLC) molecules have two or more polar

groups connected to a polyaromatic central core, typically stacking on top of each other

to form the so-called H-aggregates, leaving the polar solubilizing groups at the aggregate-

water interface [1-3]. These self-assembled aggregates act like highly charged

polyelectrolytes; when the polar groups are fully ionized, the line density of electric

charge along the aggregate can be very high, e.g., ~6O/nm (O is the electron’s charge)

for the LCLC molecules with two ionic groups. Thus, salts added to the solution can

affect the structure of the aggregate and the phase behavior of LCLCs by screening the

electrostatic repulsive forces both within the aggregates and between the aggregates.

Several studies have demonstrated that the addition of mono- or di-valent salts to

the N phase of LCLC can change the temperature of the transition between the nematic

(N) and the isotropic (I) phase. Yu and Saupe [4] discovered that the addition of NaCl to

disodium cromoglycate (DSCG) solutions increases the transition temperature, attributing

the observed salt effect to a decrease in electrostatic repulsion between the aggregates.

Kostko et al. [5] demonstrated that small cations, such as Na+ and K+, increase the

temperature of transition N # I, but some salts, such as tetraethylammonium bromide and

58

tetrabutylammonium bromide, destabilize the N phase. They proposed that the small

cations formed salt bridges between adjacent DSCG and promoted the growth of the

aggregates, while large cations, such as tetraalkylammonium, were too large to fit in the

aggregate and consequently suppressed its growth [5]. Prasad et al. [6] showed that

neither the dimension of the aggregates nor the dynamics associated with them alter

significantly with the addition of NaCl and attributed the observed increase of the LCLC

viscosity to the salt-induced changes in the hydrogen bonding. Jones et al. [7], however,

showed that a large amount of NaCl (!+, 1M) added to a highly concentrated SSY

solution resulted in a slight destabilization of the liquid crystalline ordering as evidenced

by the decrease of the temperatures of transition between the N and I phases.

In this chapter, we explore how the aggregate structure and phase diagrams of

SSY solution depend on the presence of various ionic additives such as salts of different

valency and pH-controlling agents. We have found two general trends in the additive-

induced modification of the liquid crystalline phase. In the first scenario, additives such

as simple salts enhance the stability of the N phase when the concentration of LCLCs is

low, while they suppress LC phases when the concentration of LCLC is high. This can be

qualitatively explained by the salt-induced reduction of electrostatic repulsion within the

aggregates and between the aggregates. In the second scenario, additives such as NaOH

base, spermine free base, or spermine salt destabilize the N phase. Consequently the

transition temperatures decrease and the N phase separates into a more densely packed N

phase or C phase, coexisting with a less condensed I phase. One distinct mechanism here

is brought about by pH: at a higher pH, the SSY molecules acquire a higher negative

59

charge and are less likely to aggregate. Interestingly, a subsequent addition of HCl that

reduces pH, stabilizes the N phase, thus reversing the effect of the bases. However, other

mechanisms, such as electrostatic attraction and excluded volume effects, can also

intervene when the additive is spermine, either in its salt or free base form.


3.2.1 Materials

The sulfonate groups of the SSY molecule play an important role in providing the

water solubility of SSY, as they are easily ionized throughout a broad range of pH [8, 9].

SSY in water is thus negatively charged, with the total charge close to 2e at a neutral pH.

The negative charge increases with a proportional increase in pH [10, 11], as a result of

the additional ionization of either the OH or N-NH groups in the central part of the SSY

molecules. As we recall from the description in Chapter 2, the NH hydrazon form of SSY

is more stable than the OH azo form.

SSY of a purity of 95.7% was purchased from Sigma Aldrich and purified by the

procedure described in the previous chapter. The purified SSY was dehydrated by placing

it in a vacuum oven for two days before use, because dried SSY easily becomes hydrated

during the storage.

All additives here used were of regent grade, purchased from Sigma-Aldrich, and

used without further purification. These include:

60

(a) mono- and divalent salts: sodium chloride (NaCl), lithium chloride (LiCl), ammonium

chloride (NH4Cl), sodium acetate (CH3COONa), sodium sulfate (Na2SO4), magnesium

sulfate (MgSO4);

(b) multivalent agents: spermine tetrahydrochloride salt (Spm.4HCl, abbreviated as

SpmCl4), and spermine free base (Spm);

(c) pH changing agents: sodium hydroxide (NaOH) and hydrochloric acid (HCl).

Spermine free base, a polyprotic base, has four basic sites, which can adopt to

different forms: a neutral Spm0, single-charged SpmH+, and multiple-charged SpmH2+2,

SpmH3+3, and SpmH4

+4, Fig. 3.1. Their relative concentration is a function of the pH. At a

low pH, the ammonium groups are more likely to be protonated and thus positively

charged. Using the values of the acid dissociation constants (pKa) for charged forms of

spermine from Ref.[12], we estimated their relative population in water solutions as a

function of pH. At pH=6, Spm molecules are almost completely protonated, existing in

multivalent ionic forms with more than 99% of them having a charge of 4+. As pH

increases, the dissociation of these charged protonated forms becomes more likely and

Spm becomes less charged; above pH=11, the predominant form is a neutral and fully

basic Spm0.

To avoid the occurrence of an accidentally inhomogeneous specimen, we

prepared the mixtures by adding SSY to water that already contained dissolved additives.

Finally, to avoid any non-specific effects in the mixtures with additives, we always used a

control sample and measured the additives-induced changes in temperatures of phase

transitions as a relative change with respect to the temperature I I NT → + of the I→I+N

61

transition of a control sample (with no additives), prepared with SSY obtained from the

same batch, purified and stored the same way as the SSY in the samples with additives.

This protocol also helped to mitigate the aging effects. Distilled water further purified

with a Millipore water purification system (resistivity ≥18.1 MΩ·cm ) was used for

preparing all the solutions.

Figure 3.1. The molecular structure of spermine in the neutral form (Spm0) (a) and fully

charged form (SpmH4+4) (b).

3.2.2 Optical studies of phase transitions

Two different techniques were used to prepare the samples for optical

observations: (a) placing a drop of the solution between two glass plates separated by

mylar spacers (thickness 6 µm and 12 µm) and sealing the edges of the cell with an

62

epoxy glue (Davcon) and nail polish, and (b) filling the solutions into a rectangular

capillary that is 20 µm or 40 µm thick and 200 µm or 400 µm wide, then sealing the ends

of the capillary. The phase behavior was determined by observing the samples through a

polarizing microscope, while changing the temperature at a rate of 0.2°C/min.

To explore multiple compositions, we took advantage of a technique utilizing an

array of capillaries each filled with a different composition placed into a temperature

gradient, Fig. 3.2 [13]. The temperature gradient was created by circulating hot and cold

water along two tubes separated by 10 mm distance at the opposite ends of the arrays,

Fig. 3.2. The temperature gradient was calibrated with a thermocouple and adjusted to be

practically linear, ( ) ( ) /cold hot coldT x T T T x X= + − , where ( )T x is the local temperature at

the point x along the capillary axis, coldT and hotT are the temperatures at the two ends of

the unit, separated by the distance X . We determined the transition temperatures while

decreasing the temperature of the “cold” end and using the polarizing microscope to

locate the interface between the phases, Fig. 3.2(b). The N phase is birefringent while the

I phase is not, Fig. 3.2(b).

63

Figure 3.2. Schematic diagram of the temperature gradient device (a). Polarizing

micrographs allow one to determine the location of the interfaces between the I, N, and

biphasic I+N regions, and thus to determine the temperatures of corresponding phase

transitions, as illustrated for a 0.9 mol/kg SSY solution doped with various amounts

of salt MgSO4 indicated on the left hand side in the mol/kg units (b).

64


X-ray diffraction measurements were performed at the Advanced Photon Source

of Argonne National Laboratory, as described in the previous chapter.


3.3.1 Effect of the simple salts on LCLCs

We discovered that the effect of salts strongly depends on the concentration of the

chromonic materials. In the N phase of SSY with a moderate concentration, typically the

salts stabilize the mesophases. In the case of a highly concentrated N phase, however,

opposite effect occurs. In the following we will discuss both cases.

Weakly concentrated N phase. Since the SSY aggregates are highly charged, the

phase behavior of SSY can be altered by a change in the ionic strength of the solution. To

demonstrate the simple salt effects on LCLCs, first we added several different

monovalent salts, such as sodium chloride (NaCl), lithium chloride (LiCl), ammonium

chloride (NH4Cl), and sodium acetate (CH3COONa), to 0.9 mol/kg SSY which is in the

N phase, but close to the I-N phase boundary at 23. These salts increased the

temperature !# ! of the SSY solution at which the first nuclei of the N phase appear

from the I melt upon cooling and also increase the width of the biphasic I+N region, Fig.

3.3. For further discussion, we defined the salt-induced temperature shifts of the phase

transitions with respect to the temperature !# ! of the I→I+N transition determined

for the salt-free SSY solutions. The shift in the I→I+N transition temperature,

65

Δ# !%+,b" # !%+,b" S # !%+,b 0" is positive, about (4-6) oC, for all

studied monovalent salts (%+,b 0.5 mol/kg) added to 0.9 mol/kg SSY, Fig. 3.3.

Figure 3.3. Effect of monovalent salts on the temperature of transition !"#! and

# !" in 0.9 mol/kg SSY solution.

Divalent cation salts, such as MgSO4 and MgCl2, also increase !# ! in

0.9 mol/kg SSY solution, Fig. 3.4. Quantitatively, the effect of divalent salts is

stronger than that of the monovalent salts such as NaCl, as the corresponding values of

Δ# !%+,b" are higher. Fig. 3.4 demonstrates the specificity effect of anions. MgSO4

and MgCl2 with the same cation Mg2+, but a different anion, impart different changes:

the sulphate anion −24SO produces a larger temperature shift Δ# !%+,b" as compared

66

to −Cl anion. Note that in the Hoffmeister series for anions, 2 24 4 3SO HPO CH COO Cl− − − −> > > ,

so that −24SO is more strongly hydrated as compared to −Cl .

Figure 3.4. I→(I+N) transition temperature shift ∆# !%" caused by divalent cation

salts added to a 0.9 mol/kg SSY solution, data with monovalent salt NaCl shown for

comparison.

The strong effect of divalent salts is also illustrated by the fact that MgSO4 added

with concentration §¨©) 0.54 mol/kg to the initially isotropic 0.7 mol/kg SSY

solution causes the appearance of the N phase coexisting with the I phase, Fig. 3.5(a),

which is also confirmed by x-ray measurement, Fig. 3.5(b). The x-ray diffraction patterns

of the LC region of this solution, Fig. 3.5, are the same as those obtained for the N phase

of pure SSY solutions in Chapter 2. From the full width as half maximum (FWHM) of

67

the large angle peak (2g 13.2°) in Fig. 3.5(c), we calculated the correlation length

associated with the KL 0.33 nm repeat distance, W: I 3.4 nm, which is practically the

same as for the N phase of the unsalted 0.9 mol/kg SSY solution, and larger than

W: I 2.4 nm measured in the unsalted SSY solution at 0.7 mol/kg SSY water

solution, Table 1. In the salt-condensed N phase of the biphasic region of 0.7 mol/kg

SSY with §¨©) 0.54 mol/kg, the aggregates are also packed more tightly as

compared to the original I phase, Table 1.

Table 1. X-ray diffraction date for 0.7 mol/kg and 0.9 mol/kg SSY solutions

with / without MgSO4 with §¨©) 0.54 mol/kg. All data have been taken at 28.4.

(mol/kg) §¨©)

(mol/kg) 2g at small angle (°) KL (nm) W: (nm) Phase

0.7 0 1.41 0.33 2.40 I

0.7 0.54 1.50 0.33 3.38 N

0.9 0 1.67 0.33 3.45 N

0.9 0.54 1.70 0.33 4.03 N

W: was calculated from the FWHM of a large angle peak at 2g 13.2°.

68

Figure 3.5. Polarizing micrograph (a) and x-ray diffraction pattern (b) of the N phase

induced by adding §¨©) 0.54 mol/kg of MgSO4 to the I phase of 0.7 mol/kg

SSY solution. The diffraction patterns show the N phase well aligned by the in-situ

magnetic field; (c) diffractographs for 0.7 mol/kg and 0.9 mol/kg water

solutions of SSY, both doped with §¨©) 0.54 mol/kg of MgSO4. All data have been

taken at 28.4.

69

X-ray data for a 0.9 mol/kg SSY solution doped with %+,b 0.54 mol/kg

of MgSO4, are shown in Fig.3.5.c and Table 1. In Fig. 3.5(c), it is clear that the large

angle peak for the 0.9 mol/kg solution is sharper than that for 0.7 mol/kg. The

correlation length W: I 4.0 nm corresponding to the stacking distance KL is somewhat

larger than W: I 3.5 nm for a 0.9 mol/kg SSY mixture with no salt, Table 1.

Although W: increases and the aggregates are closer to each other in the doped N phase as

compared to the un-salted N phase, we could not detect phase separation into the

coexisting regions of I and N phases in the doped N phase.

Highly concentrated N phase and C phase. We extended our studies of the

simple salt effect to include the higher concentrations of the N phase or even more the C

phase of the SSY solution. We added the different amount of NaCl, namely, !+, 0; 0.5; and 1 mol/kg, to the broad concentration range of SSY solution, 0.9 mol/kg 1.3 mol/kg. Fig. 3.6 shows that when the concentration of SSY is low, ~1 mol/kg, NaCl increases both # ! and !#!. However, when the concentration

of SSY is high, ª ~1.09 mol/kg, NaCl decreases both temperatures. NaCl added to

the solution with 1.3 mol/kg destabilizes the C phase by significantly decreasing

the # « ¬ transition temperature, Fig. 3.6. We thus conclude that the effect of

monovalent salts on SSY depends on the concentrations and that it can either enhance or

suppress the mesomorphic behavior.

70

Figure 3.6. Phase behavior of SSY solution in the presence of NaCl, !+, 0; 0.5; and 1 mol/kg. The transition temperatures were determined as the temperature

decreased.

3.3.2 Effects of spermine in a salt and free base form

Adding spermine in its salt form SpmCl4 and in the free base form lead to

different effects on SSY solution. The multivalent salt SpmCl4 added to the 1.14 mol/kg water solution of SSY in the N phase in the broad range of concentrations,

123% 0.1 S 0.7" mol/kg, decreases the temperatures of both I→I+N and I+N→N

phase transitions and widens up the range of the biphasic I+N region, Fig. 3.7, which is

somewhat similar to the simple salt effect on the high concentration of SSY solution, but

71

the shift in the transition temperature, Δ# ! is much larger than that caused by NaCl.

The correlation length W: corresponding to the stacking distance KL in x-ray

measurements reduces from 4.8 nm to 3.7 nm when 123% 0.5 mol/kg SpmCl4 added

to 1.14 mol/kg SSY solution.

Figure 3.7. Phase diagrams of 1.14 mol/kg SSY solution doped with the salt SpmCl4

The effect of Spm in its free base form also starts from the decrease of the

transition temperatures. For example, the addition of Spm with 1234 0.3 mol/kg to

0.9 mol/kg of SSY solution kept at 28.4, transforms the homogeneous N phase

into a homogeneous I phase; W: reduces from 3.5 nm to merely 2.4 nm. In more

concentrated SSY solutions, the effect is more dramatic: Spm free base separates the

homogeneous N phase into the coexisting C and I phase, Fig. 3.8. The C inclusions have

72

a hexagonal shape and develop a “petal” morphology [14] when the columns are

perpendicular to the bounding plates, Fig. 3.8. The hexagonal order of the C phase is

confirmed by the x-ray data, Fig. 3.9(a) and (b). In the three-component mixture with

1.14 mol/kg of SSY and 1234 0.3 mol/kg of Spm free base, one observes the

coexisting I and C phases, with two small angle diffractions at 2g 1.78° and 2g 2.18°, respectively. In the pure SSY solution (no Spm) at 1.14 mol/kg, 2g is

intermediate, 2g 1.87°, and the material forms a homogeneous N phase.

Figure 3.8. Phase diagrams of 1.14 mol/kg SSY solution doped with Spm free base;

the inset shows the polarizing micrograph of the I+C biphasic region corresponding to the

blue circle at the phase diagram.

73

As seen in Fig. 3.8, adding a moderate amount, 1234~0.1 mol/kg, of Spm free

base to the homogeneous N phase of 1.14 mol/kg SSY solution causes its separation

into the N and I phase. X-ray measurement of the “separated” N phase at small angle

shows that 2g increases with 1234, which suggests the aggregates are getting closer to

each other than in the original homogeneous N phase. At higher 1234, the separated

state is in the hexagonal C phase. For the hexagonal packing, the inter-aggregate axis-to-

axis distance M can be calculated from the diffraction lines (J) at small angles. For

example, the distance in 1.14 mol/kg SSY solution with 1234 0.2 mol/kg Spm

free base is M 2J √3 2.37 nm⁄ . According to [15, 16], there is just one molecule in

the cross-section of a SSY aggregate; its diameter J is in the range from 1 nm [16, 17]

to 1.4 nm [18]. Comparing M and J, one concludes that the lateral surfaces of

aggregates are separated by the distance Δ M S J I 1 S 1.4"nm in the C phase.

Therefore, the attractive forces triggered by Spm free base are balanced at some

equilibrium M by a short-range repulsion.

74

Figure 3.9. X-ray diffraction patterns, (a) wide angle and (b) small angle range, of the

coexisting I and C phases in the 1.14 mol/kg SSY water solution doped with

1234 0.3 mol/kg of Spm free base; note hexagonal symmetry in part (b). All data

have been taken at 28.4.

By adding Spm in its free base form, one inevitably changes the pH of the SSY

solution. For example, we determined experimentally using a pH meter (UB-10, Denver

Instrument) that the addition of 1234 0.2 mol/kg of Spm free base to a 1.14 mol/kg SSY water solution increases its pH from 6.49 to 11.33. For comparison,

pH=12.01 for 1234 0.2 mol/kg water solution of Spm free base with no SSY and

pH=6.08 for 1.14 mol/kg SSY doped with 123% 0.2 mol/kg of salt SpmCl4. At

elevated pH, as already indicated, Spm transforms into the mostly neutral form Spm0,

while SSY molecules at high pH increase their negative charge [8, 10, 11]. We observes

that pH can dramatically alter the changes introduced by Spm free base in the SSY phase

diagram. Namely, the addition of HCl to a SSY and Spm free base water solutions (in

75

which Spm has already caused a biphasic I+C state) decreases the pH of solutions, which

restores the homogeneous N phase and reverses the phase separation effect caused by

Spm free base, Fig. 3.10 and Fig. 3.11. This experiment demonstrates not only that pH

and the state in which Spm exists in the solution are important; it also underlines that

high concentrations of Spm free base that condense the SSY aggregates into the C phase

do not change irreversibly the structure of these aggregates.

Figure 3.10. Polarizing micrographs illustrating a transformation of a biphasic I+C state


into a N phase upon addition of HCl in concentrations 0.008 mol/kg (a), 0.04 mol/kg (b),

and 0.4 mol/kg (c). The polarizing micrographs show (a) C phase coexisting with I phase;

(b) N droplets surrounded by I phase and (c) Schlieren texture of a homogeneous N

phase. All pictures have been taken at o25 C.

76

Figure 3.11. X-ray diffraction patterns illustrating a transformation of a biphasic I+C state


into a N phase upon addition of acid HCl. The diffraction patterns for the mixture with no

HCl added, recorded at large (a) and small (b) angles. The diffuse (green) rings at large

and small angles and sharp reflections at small angle indicate a coexistence of I and C

phases. When 0.08 mol/kg (c) and 0.4 mol/kg (d) of HCl are added, the diffraction

patterns show the N phase well aligned by the in-situ magnetic field. The length scales

corresponding to the small angle (horizontal) and wide angle (vertical) reflections in (c)

and (d) are 22.5/24.4 Å and 3.33/3.33 Å. All data have been taken at 28.4.

77

3.3.3 Effects of monovalent pH changing agents

At neutral pH, SSY molecules in water are negatively charged with the molecular

charge ~2e, caused by the ionization of two sulfonate groups. However at a high pH, SSY

molecules increase their negative charge as a result of the additional ionization of either

OH or NH groups in the central part of the molecules [8, 10, 11]. To explore the pH

effect on SSY molecules, UV-visible spectra of the very dilute 2.3 5 1036 mol/kg SSY

solutions whose pH changed (6 pH 13) by adding the simple pH changing agents

NaOH or HCl, were measured using a spectrometer (Lambda 18, Perkin Elmer).

Spectroscopic data of 2.3 5 1036 mol/kg SSY solutions show the peaks centered at 315

nm and 478 nm gradually decreases with the increase of pH and concomitantly the new

peaks located at 289 nm and 337 nm emerge, Fig. 3.12. The spectra show a behavior with

a characteristic isosbestic point at 369 nm indicating that two absorbing species are

present and the relative concentration of these depends on pH. Using the values of the

acid dissociation constants pKa 10.4 from Ref. [11], we estimated their relative

population of two species, 3 S charged form and 2 S charged form, in water solutions as

a function of pH. At the neutral pH, most of SSY molecules exist in the 2 S charged

form. As pH increases, SSY becomes more charged; above pH 12.4, more than 99%

of SSY molecules have a charge 3 S.

78

Figure 3.12. Absorption spectra of 2.3 5 1036 mol/kg SSY solution at a different pH;

pH=6 (blue) caused by adding 1 5 1037 mol/kg HCl, pH=6.7 from pure SSY solution,

pH=10.5 caused by adding 1 5 1037 mol/kg NaOH, and pH=13 caused by adding

1 mol/kg NaOH.

¯°©± 0.04 mol/kg NaOH added to 0.9 mol/kg SSY solution transforms

the homogeneous N phase into the I+N biphase, Fig. 3.13. When ¯°©± 0.2 mol/kg

NaOH added, 0.9 mol/kg SSY solution melts into the I phase with pH=12.1 in

which more than 98% of SSY exist in 3 S charged form.

79

Figure 3.13. Polarizing micrographs for 0.9 mol/kg SSY solution doped with NaOH

at different concentrations: (a) ¯°©± 0 mol/kg, pH=6.5, homogeneous N phase; (b)

¯°©± 0.04 mol/kg, pH=11.3, coexisting N and I phases; (c) ¯°©± 0.2 mol/kg,

pH=12.1, coexisting C and I phases. All pictures have been taken at room temperature.

NaOH added to a 1.14 mol/kg SSY solution can induce a more condensed C

phase. At low concentrations (¯°©± 0.1 mol/kg), NaOH reduces the transition

temperatures of the I→I+N and N+I→N transitions. At higher concentrations, NaOH

produces first a biphasic region N+I (¯°©± 0.1 mol/kg) and C+I (¯°©± 0.2 mol/kg), then a completely homogeneous I phase, and finally a crystalline

precipitate, Fig. 3.14. The changes introduced by NaOH are qualitatively similar to the

changes caused by Spm free base, Fig. 3.8 and are in a marked contrast to the effect of

the monovalent salts such as NaCl on the N phase, Fig. 3.6. By adding the acid HCl to the

solutions of SSY with NaOH, and thus reducing pH, one effectively reverses the effect of

NaOH, restoring the N phase with somewhat different temperatures of phase transitions.

The later is easy to understand, as the simultaneous presence of both NaOH and HCl is

equivalent to adding the salt NaCl to the SSY water solution.

80

Figure 3.14. Polarizing micrographs for 1.14 mol/kg SSY solution doped with

NaOH at different concentrations: (a) ¯°©± 0 mol/kg, pH=6.5, homogeneous N

phase; (b) ¯°©± 0.1 mol/kg, pH=11.6, coexisting N and I phases; (c) ¯°©± 0.2 mol/kg, pH=11.9, coexisting C and I phases; (d) ¯°©± 0.5 mol/kg, pH=12.5,

homogeneous I phase; (e) ¯°©± 2 mol/kg, pH=13.2, precipitate. All pictures have

been taken at room temperature.

3.4 Discussion

Aggregation and the subsequent self-assembly of SSY molecules into the N and C

phases depends on a number of factors. (a) The intra-aggregate interactions responsible

for the monomer stacking and reversible aggregation are determined mainly by non-

covalent attractive forces such as π π− interaction and repulsive electrostatic forces

between the ionized groups, such as sulfonate. The extent of the ionization of these

81

groups depends on the pH of the solution. (b) The inter-aggregate interactions are mainly

controlled by excluded volume effects, as in the Onsager model of nematic order [19];

electrostatic forces, usually of a repulsive nature; and repulsive hydration forces (i.e.,

forces derived from the work needed to dehydrate the hydrophilic lateral surfaces of the

aggregates [20, 21]). The experimental data presented above indicate that how the

aggregate structure and phase diagrams of SSY solution depend on the presence of

various ionic additives such as salts of different valency and pH-controlling agents.

Mono- and divalent salts. The aggregated SSY molecules leave the charged

sulfonate groups at the aggregate-water interface and thus aggregates can be viewed as a

‘strong’ polyelectrolyte, fully charged in a solution. Adding salts decreases the Debye

screening length e _²³³´µs¶∑ ¡¸¹¸ and thus decreases the range of “soft” repulsion. Here º»

is the electric constant, º is the dielectric constant of water, is the ion’s valency. For

SSY, e is determined by (a) the “proper” counterions Na+, two per each SSY molecule;

(b) the co-ions and counterions that come from added salts. For example, for 300 mM

concentrations of 1:1 salt, such as LiCl or NaCl, and 1.1 M concentration of SSY, one

estimates ∑ %FF 2 « 2% I 2.8 MF and thus e 0.25 nm. For a salt 2:1, q=2,

such as MgCl2, at the same concentration, one finds ∑ %FF 2 « % « % IF4 M and e 0.21 nm. The sulfonate groups on SSY molecules contribute to intra-

aggregate electrostatic repulsion and to inter-aggregate repulsion. In terms of the

individual aggregate structure, the non-monotonous effect of a simple salt, such as NaCl,

on the temperatures of phase transitions suggests that the screening of electrostatic

82

repulsions by salt might lead to two opposite tendencies, one associated with the

increased physical length of aggregates and another with the decrease of their persistence

length. Following is a tentative explanation.

The screening of intra-aggregate electrostatic repulsion leads to an elongation of

the LCLC aggregates, as the average aggregation number,

m Xexp =3=_rst , (1)

is a strong function of the scission energy ( S C) as described in Chapeter 2.

Where C yJjxy 2X is the electrostatic correction, y C6krst is the Bjerrum

length, jO~ O y⁄ , and X is the volume fraction of SSY. As discussed by MacKintosh et

al. [22] for ideal cylindrical rods in isotropic solutions, the electrostatic correction

depends on the spatial distribution of the counterions and on X [22]. The addition of salt

is effectively equivalent to the increase of X # X « 2yJ %+,b, thus promoting

aggregation [22]. As %+,b increases, so does L. Salt-induced axial growth is known for

the wormlike micelles formed by ionic surfactants [23, 24]. X-ray measurements show

that the addition of a mono- or divalent salt into a moderately concentrated SSY solution

increases the correlation length W: and can even cause the appearance of the N phase

from an initial I phase, Table 1.

The possible mechanism of the salt-induced suppression of mesophases can be

related to the modification of persistence length. The persistence length, ¼, of the

aggregates thus decreases with %+,b. The persistence length of chromonic aggregates has

83

not been studied yet, but we can roughly estimate ¼ of SSY aggregates using a theoretical

model proposed by Manning [25]. According to the model, the addition of salts reduces

the persistence length as it reduces the stretching force caused by the electrostatic

repulsion of the electric charges at the surface of the polyelectrolyte/aggregate [25],

¼e" ½ ¾¿4À ,s+Á S 1Â ¿ +ÁlÃÂ CÄ]Á Ã⁄3CÄ]Á Ã⁄ " S 1 S ln1 S O3+Á lÃ⁄ "Å, (1)

where À 1 is the unsigned valence, KL 0.33nm is the stacking distance, and y 0.71nm is the Bjerrum length at room termperature. Using this equation, we estimate that

the persistence length of the SSY aggregate decreases by ~20 % if the concentration of

NaCl changes from 0 to 1 mol/kg. For flexible aggregates, the stability of LC is

determined by the persistence length rather than by the contour length of the aggregates.

According to the model proposed by Selinger et al. [26], as the linear rods become more

flexible (smaller ¼), the Æ # transition recedes progressively to higher concentrations,

which is in qualitative agreement with our experimental results. Therefore, one can

expect that the non-monotonous dependence of the temperatures !#! and ! # on

!+, is already contained in the effect of the salt on the intra-aggregate repulsions: at

small !+,, the aggregates elongate, which stabilizes the LC phase increasing !#! and ! #, while at large !+,, the aggregates become more flexible and the persistence

length decreases which destabilizes the N phase decreasing !#! and ! #.

Simple salt effect on the hexagonal C phase is more obvious. Salt destabilize the

C phase into the N phase, Fig.3.6, which can be attributed to the decrease of the

84

electrostatic repulsion between the aggregates by adding salts. In the I phase or N phase,

the change of the Debye screening length e is relatively small as compared to the space

between the aggregates and is not likely to influence the phase behavior. However, in the

C phase, the space between the aggregates is comparable to e. The screening of inter-

aggregate electrostatic repulsion produced by adding salts induces the fluctuation of

hexagonally packed aggregates with large undulation amplitude, which can cause the C

phase to melt to the N phase [26].

To conclude this section, we note that the MgSO4-induced condensation of the

isotropic solution of SSY with c=0.7 mol/kg into a N phase coexisting with the I phase

might reflect not only a higher degree of aggregation in the presence of salts, but also

correlation-mediated electrostatic attractions of aggregates. Typically, the electrostatic

attraction of similarly charged rods is observed in the presence of counterions of valency

higher than 2, but Qiu et al. [28] demonstrated recently that the attraction of the similarly

charged DNA strands can be caused by the divalent Mg+2 counterions.

Multivalent salt SpmCl4. SSY aggregates with fully dissociated ionic groups are

highly charged. The charge density per unit length, τ e ≃2e /0.33 nm=6 e /nm is the

same as that for the B-DNA. This similarity was one of the factors that stimulated our

studies of the ionic effects in LCLCs: It is well known that the electrostatics of highly

charged rods leads to a number of fascinating effects, such as the ‘condensation’ of DNA,

i.e. the attraction of the two similarly charged DNA molecules, in the presence of

multivalent ions [28-36]. The leading cause of this unusual effect appears to be

85

electrostatic correlations, but the concrete mechanisms remain elusive. Among the

possible models, one finds (1) a transient bridging, when a multivalent counterion such as

spermine connects two neighboring DNA rods [29], (2) causing the formation of patchy

regions of counterions condensed at the rods [30-32, 37, 38]. If either model of the

electrostatic attraction of similarly charged rods were applicable to the case of SSY, the

most likely candidate would be a mixture with an added spermine salt SpmCl4. In these

solutions, the pH remains low, about 6.1 (because of the presence of HCl); consequently

most of the Spm molecules are highly charged, q=4. The charged counterion Spm4+,

being relatively long, Fig. 3.1, can directly bridge the neighboring SSY aggregates.

Molecular dynamics simulations of Spm find that its mean end-to-end distance is about

1.2 nm [29]. Experimentally, the surface-to-surface separation Δ M S J I1 S 1.4" nm in the C phase, allows the Spm4+, in principle, to bridge the neighboring

SSY aggregates, serving therefore as “linkers”. This might lead to the interesting effects

described by Borukhov et al. [39] for linker-assisted filament aggregation, such as the

formation of macroscopic bundles or the transformation of the N phase into coexisting I

and C phases. This latter effect has indeed been observed in our system, but only when

Spm is added in the free base form, Fig. 3.8, which is likely acting through a different

mechanism of the excluded volume.

The condition for attraction through electrostatic correlations without bridging

can be written as Δ ΔL, where ΔL is the distance between the absorbed counterions at

the aggregate surface [38]. To estimate ΔL, let us assume that the adsorbed counterions

completely neutralize the charge at the aggregate surface. The condition of electro-

86

neutrality ΔL j⁄ then leads to ΔL 0.67 nm for Spm4+, which is smaller than the

typical ∆ in our experiments. This experiment, on the other hand, shows no significant

‘condensation’ of SSY, at least for 0.9 mol/kg when the system is already in the N

phase. The effect of added SpmCl4 is mostly in the suppression of the N phase, Fig. 3.7. It

appears that the strongest effect of SpmCl4 salt on concentrated SSY solutions is through

the structural reorganization of SSY aggregates. These aggregates are not bound by any

covalent bonds (unlike the DNA molecules) and can be easily disrupted by strongly

charged large counterions such as Spm4+. Apparently, these modifications and disruptions

of the aggregate structure suppress the ability of the system to preserve the N phase.

NaOH and Spermine in free base form. Both Spm free base and NaOH increase

the pH of the solutions. A high pH increases the negative charge of SSY and thus

weakens aggregation (a smaller ). Gooding et al.[8] studied the aggregation of SSY in

dilute solutions ( 0.1 mol/kg) and found that at pH≥13, SSY does not aggregate at

all, with which our experimental results were in aggrement. Taylor and Herzfeld [40]

demonstrated that when decreases, the N phase disappears, giving rise to the coexisting

I+C phases. At very low , the system might even crystallize [40]. All these predictions

are in qualitative agreement with our data, Fig. 3.14. Namely, at low concentrations

NaOHc of the added base NaOH, the increased pH narrows the temperature range of the N

phase. At higher ¯°©±, the base promotes biphasic regions I+N, I+C and a complete

isotropization of the solution with precipitation of a crystalline phase at a still higher

¯°©±, Fig. 3.14.

87

Weakened aggregation at a high pH is opposite to the effect of added monovalent

and divalent salts that enhance molecular attractions and thus elongate the aggregates.

The same consideration of pH-modified intra-aggregate attractions might apply to the

case of Spm free base, but here the situation is more complicated, as with increasing pH,

one also changes the state of Spm molecules themselves, most of which become neutral

at high pH. These neutral Spm0 molecules, being relatively large, act as “crowding”

agents that promote inter-aggregate attraction through the “excluded volume” effect, as

discussed below.

The free base form of Spm causes a similar peculiar change in the phase diagram,

Fig. 3.8, as NaOH does, Fig. 3.14, by first suppressing the N phase at low concentrations

12 and then replacing it with biphasic states, a densely packed N phase or even C

phase, coexisting with the I phase. The latter effect cannot be explained by the

multivalent salt-induced mechanism of electrostatic attraction, as Spm base raises pH to

the level at which Spm molecules become neutral. For example, in the mixture with

12 0.2 mol/kg of Spm and 1.14 mol/kg of SSY, one finds pH=11.3. Using the

Henderson-Hasselbach equation [41], one finds that at this pH, the majority (~70%) of

Spm molecules exist in the neutral form Spm0; less than 0.01% have the charge 3+ and

less than 10-5 % have the charge 4+. At high concentration of Spm free base, the system

contains mostly neutral Spm0. At a high concentrations of Spm0, the system would phase

separate into a Spm0-rich I phase and a SSY-rich orientationally ordered phase, if the loss

of the entropy of mixing is compensated by the gain in the translational entropy of the

components, as in the model proposed by Madden and Herzfeld [42, 43].

88

The model [42, 43] describes demixing in a ternary system comprised of a self-

assembling LCLC solute, a non-aggregating solute, and a solvent (water), thus extending

the Taylor-Herzfeld model [40]. The diameter of the non-aggregating spheres is close to

the diameter of the spherical monomers of LCLC (in our experiment, the diameter of the

neutral Spm0 is close to the diameter of the SSY “disk”, 1 nm). According to the

numerical simulations [43], the addition of the non-aggregating spheres to the N phase

produces a wider biphasic N+I region in which a dilute I phase enriched with additives

coexists with a concentrated N solution that is practically free of the additives. At a high

concentration of spheres, the N phase might be replaced by a wide I+C coexistence

region [43], in qualitative agreement with the data, Fig. 3.8.

Finally, we observed that acid HCl restores a homogeneous N phase from the I+C

phase separated state created by Spm free base, Fig. 3.10, or by NaOH. This effect falls

naturally into the mechanisms addressed above. Namely, HCl-induced reduction of pH

implies a decreased negative charge carried by the SSY molecules (promoting their

aggregation) and an increase of the positively charge of spermine molecules. In effect, a

simultaneous action of the appropriate amounts of HCl and Spm free base should be

similar to the effect of SpmCl4 salt, while simultaneous action of HCl and NaOH should

resemble the effect of added NaCl, as indeed observed in the experiments.

89

3.5 Conclusions

The experimental data presented on phase diagrams of LCLCs in the presence of

different ionic additives demonstrate a variety of effects.

First, simple salts such as NaCl enhance the stability of the N phase when : is

low, while they suppress the mesophases when : is high.

Second, a base, such as NaOH destabilizes the N phase at a small !+(8 and then

induces the biphasic regions I+N or I+C at higher concentration of NaOH. The

subsequent addition of the acid HCl reduces the pH and stabilizes the N phase, thus

reversing the effect of the bases.

Third, spermine in salt form, SpmCl4, suppresses the N phase. However, spermine

in free base form, Spm, causes a change in the phase diagram similar to that brought

about by NaOH, by first suppressing the N phase at low concentrations of 12 and then

replacing it with the biphasic states, a densely packed N phase or C phase coexisting with

the I phase. Tentatively we have related this transformation to the excluded volume effect

(crowding effect): in concentrated (“crowded”) solutions, the freedom gained by the

segregation of particles with different packing parameters (such as SSY aggregates and

neutral Spm molecules) can exceed the mixing entropy that is lost in demixing [44]. The

excluded volume effect of non aggregating additives on LCLCs will be explained in more

detail in the next chapter.

90

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97

Chapter 4 Phase separation and condensation of the self-

assembled lyotropic chromonic liquid crystals in

poly(ethylene glycol) solution

4.1 Introduction

The aggregated lyotropic chromonic liquid crystal (LCLC) molecules leave the

charged groups, such as sulfonate or carboxylate, at the aggregate-water interface and can

be thus viewed as macroions surrounded by counterions, such as Na+. Thus, electrostatic

interaction within and between the aggregates can be affected by the ionic additives, such

as salt, which produces the change in the phase diagram, as described in the previous

chapter. On the other hand, the neutral additives can also affect the self-assembled

structure of LCLCs and their phase behavior. Neutral additives such as poly(ethylene

glycol) (PEG) are known to condense and align macromolecular DNAs [1-4] and self-

assembled guanosine aggregates [5] through the excluded volume effects, in the presence

of salts. In the classic picture, the excluded volume effects are considered for colloidal

dispersions of particles with a fixed shape, say, solid rods of constant length and

diameter. In this picture, adding a depletion agent, such as neutral spheres, forces the rods

to pack more closely, as the spheres cannot penetrate the rods and the total volume

available for them is maximized when the two species are separated [6, 7]. In water, a

PEG molecule behaves as a random coil that can be approximated by a sphere of a certain

98

radius of gyration H'. The excluded volume effects of the depletion agents in LCLCs have

not been well studied. Simon et al. [8] demonstrated that some water-soluble polymers,

such as polyvinyl alcohol and polyacrylamide, added to isotropic (I) disodium

cromoglycate (DSCG) solutions cause the formation of birefringent droplets, with the

polymer creating a shell around the DSCG droplet. PEG of a molecular weight 600-1,500

was reported to have produced no effect on the I phase of DSCG solution [8].

We expect that the depletion effects in LCLCs are more complex than in

dispersions with particles of fixed shape and length. Since the LCLC assembly is non-

covalent, the neutral additives can influence the system at two different levels: at the

level of aggregate assembly from smaller aggregates and monomers, and at the level of

inter-aggregate interaction. Motivated by these considerations, in this chapter, we explore

the phase behavior of the reversible self-assembled chromonic aggregates of an anionic

dye, Sunset Yellow (SSY), in the presence of the electrically neutral polymer PEG. Three

component phase diagrams constructed for the entire composition range demonstrate that

the addition of PEG to a SSY solution leads to phase-separation into a liquid crystalline

(LC) region with a high concentration of SSY aggregates and a PEG-rich isotropic (I)

region. We found that in the condensed LC region, the distance between the SSY

aggregates decreases, while the average length of the aggregates increases as the

concentration of PEG increases. The addition of NaCl can either enhance the condensing

effect of the PEG, or suppress it, depending on the concentrations.

99


4.2.1 Materials

SSY of a purity of 95.7% was purchased from Sigma Aldrich, then purified and

dehydrated by the procedure described in the previous chapter. PEG (ACS reagent grade)

with a molecular weight of 3,350 was purchased from Sigma Aldrich and used without

further purification. Fluorescein isothiocyanate PEG (FITC-PEG) with a molecular

weight of 3,400 was purchased from Nanocs. Sodium chloride, NaCl (ACS reagent

grade) was purchased from Sigma Aldrich and used without further purification. Distilled

water further purified with a Millipore water purification system (resistivity ≥18.1

MΩ·cm) was used for preparing all the solutions. We use weight % unit for the

concentration of SSY, <= of PEG and > of water, so that in all the mixtures,

« <= « > 100 wt.%.

4.2.2 Phase diagram study

The phase identification was performed using polarizing optical microscopy

(POM) and X-ray diffraction measurements. For optical observations, the samples were

prepared by placing a drop of the mixture between two glass plates separated by mylar

spacers (thickness 12 µm) and sealing the edges with an epoxy glue (Davcon), as

described in the previous chapter.

4.2.3 Fluorescence microscopy

To visualize the spatial distribution of the macromolecule PEG in the ternary

mixture, we added a small amount (1~2wt.%) of FITC-PEG to its non-fluorescent

100

counterpart. For the purpose of observation, we used the Olympus Fluoview confocal

microscope BX50. Ar laser (λ=488nm) was used for the excitation of FITC-PEG and the

fluorescent light was detected in the spectral range 510-550 nm.

4.2.4 Cryogenic transmission electron microscopy (TEM)

To prepare the vitrified sample for cryo-TEM, 5 µl of mixture was dropped on a

holey carbon grid (Ted Pella, Redding, CA) in a controlled environment vitrification

chamber (CEVS, Vitrobot, FEI) in which the atmosphere surrounding the sample grid

was kept at room temperature and 100% relative humidity. The sample grid was

immediately vitrified in cryogen (50/50 ethane/propane) after blotting using Vitrobot

(FEI). The vitrified samples were examined on a FEI Technai G2 microscope operated at

200kV.

4.2.5 Density measurements

For the calibration plot, the densities (9) for the homogeneous I and N phase of

pure SSY water solutions were measured using a density meter (DE45, Mettler) at room

temperature. In phase separated samples, the LC region is at the bottom of a vial, while

the I region is at the top. The samples for density measurements were taken from the LC

region with a syringe.


X-ray diffraction measurements were performed at the Advanced Photon Source

of Argonne National Laboratory, as described in the previous chapter.

101


4.3.1 Phase diagram of ternary mixture

Fig. 4.1(a) shows the phase behavior of the ternary mixture of SSY, PEG, and

water at 296K in an equilateral triangle phase diagram. Each vertex represents a pure

component: water, SSY, or PEG. As one moves away from the vertex, the portion of the

corresponding component linearly decreases and goes to 0% at the opposite edge of the

triangle. To determine the composition at any point in the diagram, one uses grid lines

drawn through the point of interest parallel to the edges of the triangle. For example, for

the mixture labeled by a in Fig. 4.1(a) the composition is : <=: > 40: 20: 40 (in

weight %). The left edge of the diagram shows the phase behavior of a binary SSY:water

mixture, which agrees well with the previous studies from Horowitz et al.[9] and

Edwards et al. [10]. The homogeneous N and C phases are shown as bold lines drawn at

the left edge, their thickness corresponding roughly to the extension of the homogeneous

N and C phase upon the addition of a small amount of PEG. As the SSY concentration

increases, the I phase is replaced with a coexisting I+N region (26.5 28.2), then

an homogeneous N phase (28.2 34.9), a biphasic N+C region (34.9 36.1", and finally a homogeneous C region (36.1 49.5). At ª 49.5, one

observes crystals in solution. To trace the changes induced by the added PEG, we chose

the line A drawn from point b (: <=: > 28.9: 0: 71.1) on the left edge to the PEG

vertex, that corresponds to a homogeneous N phase, Fig. 4.1(b). All points on line A

represent the same weight ratio 28.9: 71.1 of SSY to water. The distance from point b

increases as one increases <=. The addition of <= 5wt. % to the homogeneous N

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phase SSY solution brings us to the point c, at which the system phase separates into the

coexisting N and I phases. The composition of these two phases is specified by ends of

the tie line passing through c, tentatively marked as points ÇÈ and ÇÉ, Fig. 4.1(a). At

<= 10wt. %, one observes three coexisting I+N+C phases (point d), Fig. 4.1(d). The

ternary mixture forms the I+C biphasic state when more than <= 11wt. % is added

(point e), Fig. 4.1(e).

103

Figure 4.1. The ternary phase diagram (a) and polarizing micrographs of SSY and PEG

water mixtures in the N phase (b), I+N phase (c), I+N+C phase (d), and I+C phase (e).

The density (9) of the homogeneous I and N phases of pure SSY aqueous

solution increases with , Fig. 4.2(a). Fig. 4.2(b) shows that the density (9:) of the

LC region condensed by the addition of PEG increases with <=. Using Fig. 4.2, one can

estimate the direction of the tie lines in the phase diagram, Fig. 4.1(a).

I+C

104

Figure 4.2. (a) Concentration dependency of density 9 for homogeneous I and N phase

of SSY solution at 296 K, (b) density 9: of the condensed LC region vs concentration of

PEG.

The equilibrium composition of the biphasic regions is given by the intersections

of a tie line with the phase boundaries. Even a relatively small amount of PEG (1 wt.% or

less) makes the homogeneous LC phases biphasic, through the separation of an I phase.

To draw the tie lines, we measured the density of the condensed LC regions, Fig. 4.2(b).

Assuming that all the PEG is expelled from the condensed regions, we can use these data

to estimate the SSY concentration at the left end point of the tie line, i.e., in the

condensed region. The concentration of SSY in the condensed regions is higher than the

SSY concentration in the SSY solution with <= 0, and increases with <=, Fig.

4.1(a). The right ends of the tie lines meet the phase boundaries of the isotropic regions,

which are composed of PEG, water, and some amount of SSY (in vials with phase

105

separated LC and I phases, the I phase is light absorbing and appears yellow/reddish

which indicates the presence of SSY).

The PEG induced condensation of SSY into orienationally and positionally

ordered phases occurs even if the initial concentration of SSY is low and corresponds to

the I phase. The effect is illustrated by line B in Fig. 4.1(a), where : > 24.1: 75.9.

When PEG is added at <= 6.5wt. %, the isotropic mixture transforms into the I+N

biphasic region. However, if is too low, 13wt. %, adding PEG does not cause

any LC condensation; see line C in Fig. 4.1(a).

4.3.2 Spatial distribution of components

To explore the spatial distribution of the components, we used samples with

4.9wt. % of PEG and a small amount, 0.1wt.%, of fluorescein isothiocyanate PEG (FITC-

PEG) added to the homogeneous nematic 29wt.% SSY solution. The fluorescently

labeled polymer FITC-PEG has been used previously to characterize the polymer phase

following separation [11, 12]. We expected that FITC-PEG and PEG, having a similar

molecular weight, would behave similarly as condensing agents. Fluorescence

micrographs clearly show that in the biphasic I and N region, FITC-PEG is expelled from

the ordered N phase, Fig. 4.3. The FITC-PEG depleted dark regions of the fluorescence

image, Fig. 4.3(a), perfectly match the SSY-rich birefringent regions of the N phase of

the polarizing micrograph, in Fig. 4.3(b). One can estimate the relative concentration of

FITC-PEG in both regions by comparing the fluorescent intensity, which linearly

106

increases with the concentration of FITC-PEG when its concentration is small [13]. The

average fluorescence intensity in the LC region is near zero and its value in the I region is

close to saturation, Fig. 4.3(a). Using the data in Fig. 4.3(a), we estimated that the

concentration of FITC-PEG in the LC region is ~106 times smaller than that in the I

region.

Figure 4.3. The phase separation in a 29wt.% SSY water solution caused by PEGs and

FITC-PEGs. (a) Fluorescence micrograph and the fluorescence intensity profile along the

dashed line (inset), and (b) polarizing micrograph of the same area of sample.

Cryogenic transmission electron microscopy (cryo-TEM) provides a high

resolution image of the phase-separated sample. Vitrification by rapid freezing (105Ks-1)

ensures the preservation of the assembled structure and phases while avoiding the risk of

the result being an artifact associated with crystallization or dehydration. The cryo-TEM

image shows the dark N region and the bright I region from the SSY-PEG-water mixture

107

(: <=: > 22.2: 7.8: 70.0, point f in Fig. 4.1(a)), in Fig. 4.4. Dark regions clearly

show dense collections of parallel self-assembled SSY aggregates. No long aggregates of

SSY can be resolved in the bright PEG-rich isotropic region. We note that at the LC-I

interface, the SSY aggregates can orient at different angles, Fig. 4.4. The angles α

between PQ, the average orientation of aggregates, and the normal ÊË to the boundary can

adopt all values in the range from 0° to 90°, Fig. 4.4. The result suggests that the surface

anchoring at the I-N interface of SSY is not as strong as in the case of DSCG.

In a separated experiment [17], we found that PEGs added to a 14.8 wt.% DSCG

solution induce the separation of the type N+I at low <= and C+I at high <=, showing

very different shape morphologies and patterns of the director PQ. The N+I regions feature

tactoids [14, 15], with PQ being tangential to the N-I interface and showing both splay and

bend distortions. The condensed C inclusions show a larger variety of shapes with a bent

PQ. Some of the shapes are close to those described theoretically by Starostin for

hexagonally ordered DNA [16], including elongated bundles and giant toroids of a

typically of a size ~10 µm. The toroids are frequently formed by merging the two ends

of elongated bundles [17]. These special morphologies of toroids and tactoids in the

condensed DSCG region closely resemble the morphologies of DNA condensation and

deserve further studies.

108

Figure 4.4. Cryo-TEM image of SSY-PEG-water mixture (: <=: > 22.2: 7.8: 70).

4.3.3 The correlation length of aggregates and the distance between the aggregates

Fig. 4.5(a) shows the x-ray diffraction pattern of the extracted LC region of a

29wt.% SSY solution with 7.5 wt.% PEG. The x-ray patterns are the same as the those

obtained for the N phase of pure SSY solutions presented in Chapter 2, and clearly

support the H-stacking model, with the planes of the SSY molecules being perpendicular

to the axis of aggregates and to PQ. Since the aromatic rings of the cores align along the

magnetic field, PQ aligns perpendicularly to the field. The walls of the vertical circular

capillary containing the sample, being perpendicular to the field, further assist in a

uniform alignment of PQ along the axis of the capillary. The diffraction pattern of the N

109

phase has two pairs of arcs in the orthogonal directions, Fig. 4.5(a). One pair of arcs in

the vertical direction at the large angle (2g 13.2°) is ascribed to the stacking distance

(KL 0.33 nm) between the SSY molecules in the aggregate. This broad diffraction

maximum at the large angle does not move with the addition of PEG, Fig. 4.5(c),

meaning that KL is not altered by PEG. The full width at half maximum (FWHM) of the

large angle peak, however, decreases in the N phase as <= increases, indicating that the

correlation length W: of stacking measured along the aggregate axis increases with the

addition of PEG, Table 1. Another pair of arcs in the horizontal direction, Fig. 4.5(a), is

from the small angle diffraction corresponding to a d-spacing of 2.63 nm, which provides

a measure of the average distance between the SSY aggregates. In the N phase, the

diffraction line d at the small angle shifts from 2.63 nm to 2.28 nm and the correlation

lengths (W) determined from the FWHM value of the diffraction peaks at 2g 1.64~1.92° increase as <= increases, Table 1.

110

Figure 4.5. X-ray diffraction patterns of 29wt.% SSY with <= 7.5wt. % (a) and

<= 20wt. % (b). Diffractographs of 29wt.% SSY in the presence of PEG with <= =

0, 7.5, and 20wt.% (c). The arrow in (a) represents the direction of the magnetic field.

20wt.% PEG added to the N phase of 29wt. % SSY induces the

coexistence of the C+I phases. The x-ray diffraction pattern of the condensed LC region

of this mixture at small angle (2g 2.32°) shows a strong sharp diffraction line (J) and

three faint, but sharp diffraction lines (J, J7, J6) whose diffraction spacings are in the

ratios 1: 1 √3 ⁄ : 1 √4⁄ : 1 √7⁄ , characteristic of the hexagonal packing of aggregates in

the plane perpendicular to their axes [18, 19], Fig. 4.5(b) and (c). The correlation length

W associated with the inter-aggregate distances, determined from the FWHM value of

this diffraction peak is much larger than W for the N phase, Table 1. For the hexagonal

packing, the inter-aggregate axis-to-axis distance M can be directly related to these

diffraction lines, M 2J √3 2J 4J7 √3⁄⁄ . The clear trend is that both d and M in

111

the condensed LC regions significantly decrease as cPEG increases, Table 1, thus

illustrating an increase of the osmotic pressure exerted by PEG onto the LC domains. The

x-ray diffraction pattern of the condensed C region also shows the same large-angle peak

at 2g 13.2° as the one in the N phase, indicating that the stacking distance along the

aggregate’s axis is not altered by PEG. The correlation length W: calculated from the

FWHM value of this peak is longer than W: for the initial PEG-free N phase, pointing to

the PEG-triggered enhancement of correlated molecular stacking along the axes of

aggregates. However, once the condensed region adopts a C phase, W: increases to some

saturated value that does not changed with the further addition of PEG, Table 1.

Table 1. X-ray diffraction date for the separated LC region of 29wt.% SSY mixtures

with PEG

<= (wt.%) J (nm) M(nm) W (nm) W: (nm) Phase

0 2.63 - 5.27 3.21 N

2.5 2.47 - 6.31 4.03 N

5 2.36 - 7.90 5.15 N

7.5 2.28 - 9.51 6.54 N

15 1.98 2.29 153.25 6.37 C

20 1.96 2.27 129.82 6.28 C

25 1.89 2.18 157.08 6.42 C


112

4.3.4 The effect of non-ionic additives on LCLCs in the presence of salts

Previous studies showed that the polymer induced DNA condensation is promoted

by the addition of monovalent salt [2-4]. Since the SSY aggregates are as highly charged

as the DNA, the phase behavior of SSY condensed by PEG can be altered by a change in

the ionic strength of the solution. However, the non-monotonic effect of salts on SSY

described in the previous chapter leads also to the new features of the PEG-induced

condensation of salted SSY. When is small, the addition of monovalent salt to the

(SSY + PEG) water solution enhances the phase-separation and condensation of LC

regions. For example, the mixture of 5wt.% PEG and 24 wt.% SSY (point g in Fig.

4.1(a)), a homogeneous isotropic phase, shows no phase separation, Fig. 4.6(a) and (b).

However, when NaCl is added to this mixture, it induces the phase separation of the N+I

type, Fig. 4.6(a) and (c). The volume of the separated N region increases as !+, increases, Fig. 4.6(a).

When is high, the addition of NaCl salt to (SSY + PEG) water solutions

causes an opposite effect, destabilizing the condensed LC regions. In the salt-free case

7.5wt.% PEG added to the N phase of 33wt. % SSY, Fig. 4.7(a), induces the

coexistence of the C+I type, Fig. 4.7(b) (point h in Fig. 4.1(a)). However, if one adds

NaCl, !+, 1 mol/kg, to this sample, the C+I coexistence is changed into an N+I

coexistence, Fig. 4.7(c).

113

Figure 4.6. (a) SSY-PEG-water mixture (: <=: > 23.1: 3.9: 73.0) in the presence

of NaCl !+, 0 mol/kg (first mixture from the left), !+, 0.2 mol/kg (second),

!+, 0.4 mol/kg (third), !+, 0.6 mol/kg (fourth). The polarizing micrograph of

(b) the I phase of the mixture (: <=: > 23.1: 3.9: 73.0) and (c) the N+I

coexistence induced by the addition of NaCl, !+, 0.2 mol/kg, into the mixture (b).

114

Figure 4.7. Polarizing micrographs of (a) the N phase in an additive-free 33 wt. %

SSY solution, (b) C+I coexistence for : <=: > 31.3: 5.2: 63.5 mixture, and (c)

the N+I coexistence induced by the addition of 1 mol/kg NaCl to the mixture (b).

4.4 Discussion

The experimental data presented above point to the following tendencies in the

phase behavior of aqueous solutions of SSY upon the addition of the neutral polymer

PEG with a molecular weight 3,350 and the monovalent salt NaCl.

(a) PEG added to the isotropic or nematic solutions of SSY causes a strong condensing

effect on SSY, triggering phase separation and condensation of the LC phases, either of

115

the N type or the C type. The PEG is excluded from the condensed LC regions into the I

phase.

(b) NaCl added to (SSY+PEG) solutions can either promote the phase separation and

condensation of the LC phases when and !+, are low, or destabilize the condensed

LC phases caused by PEG when the concentrations are high.

Below we describe how the excluded volume and electrostatic effects can contribute to

these experimentally observed features.

(1) Excluded volume effects: face-to-face vs side-by-side aggregation. The

excluded volume effects in LCLCs are more complex than in the systems studied

previously with colloidal particles of pre-fixed shape and length [6, 7, 20, 21], since the

main structural unit is a non-covalently assembled aggregate rather than an individual

molecule. The excluded volume effect can influence both the face-to-face stacking of

molecules (i.e., the length of aggregates or the aggregation number) and orientational and

positional order in the packing of aggregates. To illustrate this, let us approximate an

LCLC “unit”, an aggregate or a monomer, as a cylinder of diameter J and length

m 5 KL, where n is the aggregation number. Each unit creates an excluded volume

around itself that cannot be penetrated by spherical PEG molecules of radius H'. This

excluded volume is reduced when the two units are placed closely together. For

simplicity, we consider only arrangements in which the axes of all the units are parallel to

each other. For face-to-face (with respect to the molecular planes of SSY) placement of

116

the two units of aggregation number n, the overlapping excluded volume is n-

independent:

BCÍ°ÎÏ 2H' S KL"GÐÐÑ « H'". (1)

For the side-by-side placement of the same two units, the overlapping excluded volume

increases with n:

BCÒÓÔÏ Õ2H' « mKLÖ ×2GÐÐÑ « H'" cos3 Ø GÐÐÑ cÙÚ S M ¾GÐÐÑ « H'" S 6 ÅÛ, (2)

where M I 2 nm is the inter-aggregate axis-to-axis distance. Using Eq. (1) and (2) with

H' I 2 nm, J I 1nm, and KL 0.33 nm, one estimates that for m ~10, the face-to-

face stacking is more favorable, while for m ª ~10, the side-by-side placement is more

favorable, Fig. 4.8.

The simple geometrical argument above suggests that the excluded volume effect

of PEG on SSY occurs at two different levels: for dilute system with short aggregates, the

effect is mostly in the increase of the aggregate length, while for the longer aggregates in

more concentrated solutions, the effect is in more dense lateral packing with an ensuing

orientational and positional order, Fig. 4.9.

117

Figure 4.8. The schematic diagram shows the overlap of the excluded volumes of the

face-to-face and side-by-side configuration: (a) for individual molecules and (b) for

elongated aggregates. (c) The overlapping excluded volumes, BC, for face-to-face and

side-by-side placement as a function of the number of SSY molecules in an aggregate.

BCÍ°ÎÏ and BCÒÓÔÏ were calculated using H' I 2 nm, J I 1nm, KL 0.33 nm, and

M I 2 nm.

(b) BCÍ°ÎÏ BCÒÓÔÏ (a) BCÍ°ÎÏ ª BCÒÓÔÏ

(c)

118

Figure 4.9. Schematic illustration of the excluded volume effect of the increasing

concentration of PEG chromonic assembly: elongation of short aggregates (a), followed

by parallel arrangement in the N phase (b) and C phase (c).

119

(2) Excluded volume effects: spatial distribution of components. The experiments

(such as the one with fluorescently labeled PEG) suggest that PEG is excluded from the

condensed LC regions. This observation is in line with the geometrical consideration of

the typical length scale involved. The gyration diameter of PEG with a molecular weight

of 3,350 is J<= 2H' I 4 nm [22], which is larger than the diameter of the SSY

aggregate, J I 1nm. X-ray measurements show that the inter-aggregate axis-to-axis

distance, M, in the separated N or C region varies from 2.9nm to 2.2nm as <= increases.

Considering the actual space between two aggregates J% M S J I 1.2~1.9nm,

PEGs with J<= I 4nm are too big to be intercalated in the lateral gaps between parallel

rod-like aggregates of SSY in the N or C phase.

(3) Excluded volume effects: osmotic pressure vs electrostatic repulsion. PEG

molecules excluded from the SSY-rich regions apply osmotic pressure on the aggregates,

reducing the separation distance M between them, Table 1. At an equilibrium M, the

osmotic pressure is balanced by the screened electrostatic repulsions between the

similarly charged SSY aggregates: [17, 23, 24],

Ü<= √6πjCDD 6rstÔÞÞß ,sàÔÞÞß l"⁄ l"7 ⁄ exp S M e"⁄ , (3)

where Tá" is the first order modified Bessel function of the second type, y 0.71nm

is the Bjerrum length at room termperature, and e is the decay length, equal to the Debye

screening length e when the aggregates are considered as rigid rods. The Debye

screening length is e _²³³´µs¶∑ ¡¸¹¸ 0.32nm for 29wt.% SSY solution, where º» is the

electric constant, º is the relative dielectric constant of water, is the ion’s valency, and

120

O is the elementary charge. In the model [17, 23, 24] leading to Eq. (1), the aggregates

are charged cylinders with a diameter J I 1nm and “bare” dimensionless charge

density j 2y KL I 4.2⁄ . When the charge density is high, j ª 1, j is replaced with

jCDD since a certain portion of counterions bind to the aggregate surface [25]. Using Eq.

(3) with e e 0.32nm and jCDD I 2.8 [26], one estimates that for the condensed C

phase with M 2.29nm (which corresponds to <= 15wt. %", the needed osmotic

pressure is Πãä 7.3 5 10åN m⁄ , while for M 2.18nm (<= 25wt. %" , the

needed pressure is Πãä 1.1 5 10æN m⁄ . The actual concentrations of PEG in the I

phase are higher than the values of <=, and can be estimated from the relative volume

of the I and the condensed LC regions. We estimated the concentration of PEG in the I

phase to be in the range (22~36%) when <= is in the range (15~25%). For these

solutions of PEG, the osmotic pressure was measured directly [27] to be in the range

0.9 S3.1" 5 10æ N m⁄ which correlates well with the estimates that follow from Eq.

(3).

(4) Excluded volume effects in the presence of monovalent salts. PEGs added to

SSY solutions are excluded from the SSY-rich condensed regions in which SSY is

apparently the same as the pure SSY water solution. Thus, salts added to SSY+PEG

water solution can affect the intra-aggregate and inter-aggregate interactions of SSY by

reducing repulsive electrostatic forces in the same way we described in Chapter 3. When

and <= are low, the screening of intra-aggregate electrostatic repulsion caused by

adding salts produces an elongation of the SSY aggregates triggering side-by-side

assembly, leading to the phase-separation and condensation into N phase from the I phase

121

of the SSY+PEG solution. However, when and <= are high the screening of the

electrostatic repulsion can cause the fluctuation of hexagonally packed aggregates with

large undulation amplitude, which can cause the C phase to melt to the N phase. This is a

difference from the systems studied previously with colloidal particles of pre-fixed shape

and length, such as polymer salt induced DNA condensation [2-4].

4.5 Conclusions

We have demonstrated that the effect of a non-ionic additive on LCLCs is very

different from the effect of ionic additives.

1) The addition of PEG to a SSY solution leads to phase-separation into a

condensed LC region, either of the N type or the C type, and a PEG-rich isotropic region.

2) In the condensed LC region, the distance between the SSY aggregates

decreases, while the average length of the aggregates increases as the concentration of

PEG increases.

3) The addition of simple salts together with PEG to a SSY solution can either

promote the phase separation and condensation of the LC phase or suppress it by

screening the electrostatic repulsion forces within the aggregate and between the

aggregates.

122

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[20] Madden, T. L.; Herzfeld, J. Exclusion of spherical particles from the nematic

phase of reversibly assembled rod-like particles. Mat. Res. Soc. Symp. Proc.

1992, 248, 95-101.

125

[21] Madden, T. L.; Herzfeld, J. Crowding-induced organization of cytoskeletal

elements: 2. Dissolution of spontaneously formend filament bundles by capping

proteins, J. Cell Biol. 1994, 126, 169-174.

[22] Devanand, K.; Selser, J. C. Asymptotic behavior and long-range interactions in

aqueous solutions of poly(ethylene oxide). Macromolecules 1991, 24, 5943-

5947.

[23] Strey, H.H.; Parsegian, V. A.; Podgornik, R., Equation of state for polymer

liquid crystals: Theory and experiment. Phys. Rev. E 1999, 59, 999.

[24] Rau, D. C.; Lee, B.; Parsegian, V. A. Measurement of the repulsive force

between polyelectrolyte molecules in ionic solution: hydration forces between

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[25] Naji, A.; Jungblut, S.; Moreira, A. G.; Netz, R. R. Electrostatic interactions in

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[26] Aubouy, M.; Trizac, E.; Bocquet, L, Effective charge versus bare charge: an

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126

Chapter 5 Chromonic materials for nano-fabrication: Side-

by-side and end-to-end assembly of Au nanorods using self-

assembled chromonic stacks

5.1 Introduction

Recently, there has been an increasing interest in lyotropic chromonic liquid

crystals (LCLCs), not only because chromonic materials are a distinct class of soft matter

showing unique properties different from the conventional lyotropic liquid crystals, as

described in the previous chapters, but also because they have shown promise for

potential new applications, such as the preparation of optically anisotropic films [1-9],

micro-pattering [10, 11], and biosensing [12-14]. In this chapter we demonstrate that

chromonics can be used as functional materials for nano-fabrication. Assembled

structures of nanoparticles (NPs) can be tailored by placing the metallic NPs into the

solutions with self-assembled chromonic stacks. As we have demonstrated in the

previous chapters, the chromonic aggregate structure and phase behavior strongly depend

on many factors, such as chromonic concentration, temperature, ionic content and pH of

the solution, which thus can be utilized to control the assembly of metallic NPs caused by

chromonic materials.

Significant progress in the synthesis of diverse metal NPs of uniform size and

shape over the last decades has led to an increased understanding of the fundamental size-

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and shaped-dependent properties of NPs and to the development of numerous

applications [15-26] ranging from molecular [22] and biological sensors [23, 24] to solar

energy conversion [25] and the construction of optical cloaking devices [26]. One of the

most challenges of nanotechnology today is to develop simple and reliable technique to

produce the complex arrangements and assemblies of NPs which is the key to the

successful application of the nanoparticle-based devices. A good example is the

cylindrical cloak described by Cai et al. [26] in which metal nanorods (NRs) are aligned

along the radial directions in a cylindrical shell, their density being maximum at the inner

surface of a shell and minimum at the outer surface of the shell [27].

In general, the complex arrangements and assembly of NPs can be achieved using

the “top-down” approach, which is based on patterning on a large scale into the

nanostructures, such as lithography, or the “bottom-up” approach which arranges the

individual base elements, such as self-assembly. Since self-assembly is the simpler and

less expensive approach for generating the organized nanoscale structures, in recent years

this approach to engineering nanostructures has received increasing attention [17, 21].

The assemblies of metallic NPs show dramatic modification of their optical and electric

properties as compared to those of individual NPs [28, 29] and consequently can be used

as the “building units” for more complex architectures. This is particularly true for NRs

because their anisotropy implies additional functionality on the scale of individual NRs

and their assemblies.

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There are two basic approaches to NR assembly. The first one relies on the

anisotropy of repulsive forces and the excluded volume (Onsager) effect [30] to induce

parallel alignment of the NRs, achieved when the volume fraction X and shape

anisotropy of the NRs are large enough [31]. The second approach utilizes the anisotropy

of attractive forces, caused, for example, by linking agents that are chemically bound to

preselected sites on the surface of the NRs. For examples, Caswell et al. [32]

demonstrated end-to-end assembly of NRs through antibody-antigen interaction. The

ends of rods were first functionalized with covalently bound (through thiol groups) biotin

molecules. When the linking agent streptavidin was added to the solution, its specific

binding to biotin resulted in the end-to-end assembly. In a similar way, Thomas et al.

[33] used hydrogen bonding, while Sudeep et al. [34] functionalized NRs with

electrically charged linking agents that attracted to oppositely charged groups of the

neighboring NRs. Nie et al. [35] functionalized hydrophilic NRs with hydrophobic tails

and observed their spontaneous organization into bunches and circles driven by

differences in hydrophobic interactions. Khanal et al. [36] used the tendency of NRs

covalently modified with polystyrene chains to concentrate at the interface and to make

unique patterns taking advantage of water droplets as templates. One of the most

challenging problems in all these processes is to control the NRs assembly, its shape and

size.

Here we describe a technique both simple and universal for assembling metallic

NRs utilizing the anisotropic attractive forces between the NRs and aggregated stacks of

chromonic materials. We used disodium cromoglycate (DSCG), as the linking agent for

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NR assembly. As described in Chapter 2, DSCG molecules stack face-to-face, leaving the

charged groups, carboxylate, at the aggregate-water interface with counterions, Na+. The

repeat distance of stacking is 0.34nm. When the polar groups are fully ionized, the line

density of electric charge along the aggregate can be very high, e.g., ~6O/nm (O is the

electron’s charge) under an assumption of a single DSCG molecule in the cross-section

of the cylindrical stack (details about the stacking structure of DSCG are still the subject

of discussion). The stacks thus represent self-assembled macroions with the ability to

interact with other charged species, such as metallic NRs covered with ionic surfactants

or polymers. The anisotropic electrostatic interaction between the chromonic stacks and

NRs produces the different geometries of NR assembly, namely, side-by-side and end-to-

end, depending on the electric charge of the NRs. We also explore how the assembly

depends on the concentration of DSCG and pH of the solution. We extended the

technique by demonstrating that polyelectrolyte solutions could be used to arrest the

assembly of NRs and then transfer the aggregated NRs from the water solution into a film

of polyvinyl alcohol, while preserving the basic features of NR structural organization

and optical properties.


5.2.1 Materials

The DSCG was purchased from Spectrum Chemical Mfg. Corp. (Gardena. CA)

with a purity of more than 99 % and used without further purification. To avoid the

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effects of hydration, we placed the dried DSCG in a vacuum dessicator after dehydartion

as described in Chapter 2. Hexadecyltrimethylammonium bromide (CTAB), silver

nitrate, ascorbic acid, sodium borohydride, gold (III) chloride, L-cystine, thioglycolic

acid (TGA), poly(acrylic acid) (PAA) (mol. wt. 400,000), poly(sodium styrenesulfonate)

(PSS) (mol. wt. 100,000), and poly(vinyl alcohol) (PVA) (mol. wt. 100,000) were

purchased from Aldrich and used without further purification. Distilled water further

purified with a Millipore water purification system (resistivity ≥18.1 MΩ·cm) was used

for preparing all the solutions.

5.2.2 Synthesis of gold nanorods

Gold NRs with an average aspect ratio of 2.8 (length ~50nm and diameter

~18nm) were synthesized by using the well known seed mediated growth method [37,

38]. In this process, a 20ml solution of gold NRs is centrifuged twice and redispersed into

a final volume of 1 ml of deionized water to remove the excess stabilizer CTAB in the

solution. The purified gold NRs have a bilayer of surfactant on the surface with a net

positive charge (zeta potential +30mV). To make the gold NRs negatively charged,

100µl of 1% PAA solution in water is slowly added to 1ml of the purified gold nanorod

solution under vigorous stirring. The negatively charged PAA is adsorbed onto the

positively charged gold NRs by electrostatic attraction and the excess COOH groups

provide a net negative charge to the rods (zeta potential -25mV).

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5.2.3 Ionic polymer layer deposition on the assembled structure of NRs.

0.2 ml of 1% PSS water solution was added to 0.8 ml of pre-assembled gold NR

assembly mixture to quench the assembly reaction. After 30 min of adsorption time, the

excess ionic polymer and linking agent DSCG in supernatant fraction were removed by

centrifugation (9,000 rpm, 10 min), and NR superstructures were re-dispersed in water.

Gold NR-PVA composite films were prepared by drying the pre-assembled NR structure

in the presence of dissolved PVA (10 wt. %).

5.2.4 Transmission electron microscopy (TEM)

TEM samples were prepared by dropping the dilute mixture on a 300 mesh

carbon coated copper grid (Ted Pella, Redding, CA) and allowing the solvent to

evaporate. TEM images were obtained using a JEOL JEM-100S electron microscope at

the Northeastern Ohio Universities Colleges of Medicine and Pharmacy.

5.2.5 Dynamic Light Scattering

Dynamic light scattering (DLS) was performed using ALV/LSE-5004 (Germany)

with a He-Ne laser (maximal power 25mW, power stability better than ≤1% over 24h, ,

and wavelength e 632.8 nm). The intensity correlation functions were collected at

several different scattering angles, from θ=15° to θ=100°. The size of the aggregates was

estimated through measurement of the intensity correlation function of the scattered light

that probed the dynamics of the system [39]. The translation diffusion coefficient was

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determined as Mbc+i% 1 j⁄ , where 6kl m sin o, m 1.33 is the refractive index of

water, and j is the characteristic relaxation time obtained from the correlation function

[38]. The effective size of the aggregate was deduced as the radius of an aggregate that is

assumed to be spherical, by using the Stokes-Einstein equation [39].


We discovered that the chromonic stacks can serve as electrostatic linking agents

and assemble the metallic NRs in side-by-side fashion or in end-to-end fashion,

depending on the electric charge of the NR surface. Below, we discuss both NR

assemblies.

5.3.1 Side-by-side assembly

Gold NRs are coated with double layers of cationic surfactant CTAB. These

CTAB layers are positively charged and stabilize gold NRs dispersed in water against

aggregation [40]. In control samples with no DSCG, we observe no assembly of gold

NRs because the NRs covered by positively charged layers repel each other, Fig. 5.1(a).

The assembly of gold NRs starts immediately after the addition of 1mL of 0.5mM DSCG

solution to 1mL of ~2nM NR solution. TEM images show that side-by-side assembled

structures are induced by adding chromonic material DSCG, and the size of the

assembled structures increases with time, Fig. 5.1. The NRs in the superstructures are

parallel to and in registry with each other so that their ends are aligned fairly well along a

common line. The latter implies that the NRs are attracted to each other and that the force

133

of attraction is proportional to the length of the overlap of the neighboring rods [41]. We

observed one-layer “raft” formations yet didn’t “island” assembly in which the NR would

be surrounded by more than two neighbors like a membrane [41]. However, as time

progresses, separate side-by-side assembled structures might eventually overlap. In some

instances, the TEM image shows that in regions with a high concentration of NRs

individual assembled structures overlap and associate in an end-to-end fashion, Fig.

5.1(e).

In the TEM, the assembly might be an artifact of solvent evaporation. Further

evidence of the side-to-side assembly is obtained by UV-vis spectroscopy. The gold NR

solutions show two surface plasmon peaks associated with the oscillations of free

electrons. One is a transversal plasmon peak associated with the transversal scale,

namely, the diameter J" of NR, and the other is a longitudinal plasmon peak associated

with the length " [42]. In assemblies of NRs, the plasmon peaks depend not only on the

size and shape of the individual NRs, but also on the presence and orientation of

neighboring NRs [28, 42-44]. In fact, spectral changes in the solutions of NRs serve as a

good indicator of the kinetics and geometry of assembly. When the DSCG solution is

added to the NR solution, the longitudinal peak experiences a strong blue shift (towards

the shorter wavelengths) and decreases in amplitude, while the transverse peak becomes

red-shifted and increases in amplitude, Fig. 5.2. According to the numerical simulations

of spectral features [44, 45], these distinct changes correspond to the side-by-side

assembly of NRs.

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Figure 5.1. Side-by-side assembly of gold NRs induced by a 0.8mM DSCG solution

mixed with ~2nM gold NR solution at a 1:1 ratio. (a) TEM image of the control sample,

no DSCG; (b), (c), (d) and (e) the assembled structures of gold NRs formed after the

addition of the DSCG solution.

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Figure 5.2. The absorption spectra of ~2 nM Au NRs with CTAB coatings modified by

the addition of 0.5 mM DSCG, as a function of time; the longitudinal plasmon peak is

blue shifted and the transverse plasmon peak is red shifted. The inset shows the picture of

the NR solution immediately after (left) and 30 minutes after adding DSCG (right).

The weak concentration of DSCG solution does not produce any effect on the

gold NR solution. Fig. 5.3(a) shows the absorption spectrum of ~2 nM Au NR solution

with 0.04 mM DSCG solution at a 1:1 ratio, whose plasmon peaks do not change, even 1

day after the addition of the DSCG solution. When a high concentration of DSCG

solution is added, the color of the solution suddenly becomes bluish and then clear with a

dark precipitate at the bottom of a vial. UV-vis spectrum data of ~2 nM Au NR solution

with 40mM DSCG solution at a 1: 1 ratio shows that the longitudinal and the transverse

136

peaks become broad and red-shifted, then both peaks disappear ~6 minutes after the

addition, as shown in Fig. 5.3(b). Experiments at different concentrations of DSCG

revealed that the amount of DSCG is an important factor for gold NR assembly.

Assembly was especially efficient for DSCG concentrations in the range 0.4-0.8mM, and

the aggregation rate increased with the concentration.


addition of a 0.04 mM DSCG solution (a) and 40mM DSCG solution (b), as a function of

time.

At this point one might posit the intriguing question of what drives the assembly

of NRs upon the addition of DSCG. Below we argue that the reason might be associated

with the polyinioc character of DSCG stacks. An isolated DSCG molecule can be

considered as a di-valent salt. However, the ability to assemble the NRs into ordered

“rafts” is not a simple consequence of the “salt” character of isolated DSCG molecules.

137

When the typical salts such as NaCl and MgSO4 (concentrations up to 0.1M) are added to

gold NR solutions, they produce no changes in the plasmonic spectra. For example, the

addition of a 0.1M NaCl solution to the NR solution, Fig. 5.4(a), or the addition of a

0.1M MgSO4 solution to the NR solution, Fig. 5.4(b), at a 1: 1 ratio does not change the

absorption spectrum even after several days. Apparently, the mechanism of the DSCG-

induced assembly of NRs is in the ability of the DSCG molecules to assemble into

multivalent stacks that serve as polyionic condensing agents for charged NRs.


addition of 0.1 M NaCl solution (a) and 0.1 M MgSO4 solution (b).

To elucidate the role of DSCG stacking in NR assembly, we used the previously

observed effect that the chromonic stacks can be made shorter by increasing the pH of the

solution, apparently because of the increased effective negative charge on each DSCG

molecule, as described in Chapter 3. In the experiment, we first mixed a ~2 nM solution

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of NRs and a 0.5 mM solution of DSCG in the proportion of 1:1. 1 ml of the mixture was

left for 15 min to allow the NRs to assemble, after which time 8 µl of a 0.5 M solution of

NaOH in water was added. While the NR-DSCG mixture demonstrated the shift of the

longitudinal and transverse plasmon peaks towards each other, the addition of NaOH

reversed this trend, Fig. 5.5. Since the addition of NaOH increases the pH of the solution,

the effect can be associated with the shortening of DSCG at a high pH, rendering them of

lower charge and less capable of linking the NRs.


addition of 0.5 mM DSCG at a 1:1 ratio; the longitudinal plasmon peak is blue shifted

and the transverse plasmon peak is red shifted, as a function of time (dot lines). 15 min

after the addition of DSCG, 8µl of 0.5M NaOH is added; the longitudinal plasmon peak

reverses its shift back to the longer wavelength and the transversal peak shifts back to the

shorter wavelength (colored solid lines).

139

5.3.2 Quenching gold NR assembly by polymer coating.

Controlling their size presents one of the key challenges in producing the NRs

assemblies. In our system and in many other NR assemblies, the size of the assembled

NR structures increases with time, ultimately resulting in giant clusters that eventually

precipitate. As we demonstrate below, the growth of chromonic-mediated NR assemblies

can be arrested by using polyelectrolytes.

Figure 5.6. Absorption spectra (a) and picture (b) of the assembled NR structure solution

with the addition of 1% PSS solution at different reaction times; PSS added on NR

solution without DSCG and on NR solution 0, 5, 15, 30, 60, and 120 min after initiating

the assembly reaction by adding the DSCG solution. The plasmon peaks resulting from

the solutions quenched by PSS coating do not change even after a period of one month.

140

When the desired assembly reaction time in the solution of gold NRs and DSCG

has been reached, 1% of the PSS solution is added to the gold NR-DSCG solution at a 1:4

ratio. After the addition of PSS, the longitudinal and transverse plasmon peaks stop

shifting, Fig. 5.6(a). After the excess polymer and DSCG has been removed by

centrifugation, the resulting NR solutions demonstrate “arrested” spectral features that

are stable for a period of at least one month, Fig. 5.6(a). Fig. 5.6(b) shows the NR

assembled solution quenched by PSS at different reaction times: 0, 5, 15, 30, 60, and 120

minutes after adding DSCG. The control sample without DSCG (sample “w/o”) but with

added PSS shows the same color as the sample with no DSCG and no PSS (not shown).

Dynamic light scattering (DLS) experiments demonstrate that the addition of PSS

at different stages of the DSCG-triggered assembly of NRs produces a different average

size of the “arrested” aggregates. PSS was added to the NR solutions either

simultaneously with the DSCG (zero “assembly time” in Fig. 5.7) or at times 5, 15, 30,

60, and 120 minutes after the addition of the DSCG. The effective diffusion coefficients

Mbc+i%" for NR structures show a dramatic decrease as the assembly time (NRs exposed

to DSCG but not to PSS) increases. The effective hydrodynamic diameter

JqçGcèGçi+2FE" of assembled structures, roughly estimated using the Stokes-Einstein

equation for spherical particles [39], JqçGcèGçi+2FE xy 3zMbc+i% , increases

practically linearly from about 65 nm to about 110 nm as a function of the “assembly

time”, Fig. 5.7.

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Figure 5.7. DLS data for the NR assembled structure solution quenched at 0, 5, 15, 30,

60, and 120 min. The diffusion coefficient decrease with reaction time, while the

hydrodynamic diameters increase with reaction time.

5.3.3 Polymer composite of assembled NR.

Embedding assembled NR structures in a polymer matrix present a good method

to utilize and further modify their unique properties. Recently, several groups have

produced NR polymer composites with isolated gold NRs [46, 47]. In this present work,

we prepared a NR-PVA polymer composite film by transferring side-by-side assembled

NRs from the water solution. We prepared three different NR solutions, all quenched by

adding PSS. One solution represented isolated CTAB-functionalized NRs (no DSCG),

while our second and third represented NR solutions with DSCG at an assembly time of 5

min and 15 min, respectively. The pre-assembled gold NR solutions arrested with PSS

142

were added to the 10wt% of PVA solution in water. After water evaporation, the system

formed films with embedded NR structures. Their spectral properties are similar to those

of side-by-side assembled structures in water, Fig. 5.8. Because the dielectric permittivity

of PVA is much smaller than that of water, the plasmon peaks for the NR-PVA

composite film are red shifted as compared to the plasmon peaks for water solution [42].

The UV-vis spectra of the composite films with assembled NR structures, Fig. 5.8(a),

show that longitudinal plasmon peaks still have a blue shift (towards the shorter

wavelengths) and the transverse peaks have a red shift (towards the longer wavelength),

which means the assembled structures are preserved in the polymer medium.

Figure 5.8. Absorption spectra (a) and picture (b) of the assembled NR structure

embedded in PVA film. The composite films were prepared with three different NR

structure solutions, an isolated NR solution and assembled structure solutions with 5 and

15 min reaction times.

143

5.3.4 End-to-end assembly

The geometry of NR assembly can be altered by changing the surface charge of

NRs, and thus changing the nature of their electrostatic interactions with DSCG stacks.

We coated the isolated gold NR with PAA to make them negatively charged after

synthesizing the Au NR solution. A 0.5 ml of 0.2 M DSCG solution was added to a 0.5

ml gold NR solution and triggered an end-to-end assembly, Fig. 5.9. The TEM images

show that the NRs are connected end-to-end, with a small lateral shift apparently caused

by the prismatic geometry of the ends with tilted 111 triangular facets. UV-vis spectra

show that the amplitude of the longitudinal peak centered at 706 nm for isolated NRs

gradually decreases with time, and concomitantly, a new peak located at a longer

wavelength emerges and increases its amplitude after the addition of DSCG. The spectra

show a characteristic isosbestic point at 770 nm, indicating that two different species, an

isolated one and an assembled one, are present and the relative concentration changes

with time. For chains of NRs, one expects that the longitudinal peak is located at

wavelengths larger than 706 nm. The transverse peak centered at 514nm should remain

unaffected, Fig. 5.10. These distinct changes qualitatively agree with other experiments

and calculations for end-to-end assembly [33, 34, 44, 45].

144

Figure 5.9. End-to-end assembly of Au NRs with CTAB and PAA coating, induced by

0.1M DSCG added to ~2 nM NR solution at a 1:1 ratio. TEM images of the end-to-end

chains were taken at about 2 hours (a) and (b), 5 hours (c) and (d), and 24 hours (e) and

(f) after the preparation of the mixture.

145

Figure 5.10. Absorption spectra of Au NRs with PAA coatings modified by the addition

of 0.1 M DSCG, as a function of time. The inset shows the picture of the NR solution just

after (left) and 1day after adding DSCG (right).

The concentration of DSCG is also an important factor for the end-to-end NR

assembly. The end-to-end assembly requires a higher concentration of DSCG in the

solutions, as compared to the side-by-side NR assembly. For example, a DSCG solution

of concentration less than ~0.02M added to a PAA-coated gold NR in solution at the

proportion of 1:1 causes no superstructures. By simply diluting the sample at the desired

stage of DSCG-triggered assembly, this concentration effect can be used to control the

end-to-end NR assembly. Fig. 5.11(b) shows that the assembly is quenched when the

146

sample containing NRs and DSCG is diluted by adding water (the concentration of

DSCG changed from 0.05M to 0.01M). In the diluted samples, the plasmon peaks do not

change for at least several days, indicating the stability of the assembled structures.

Figure 5.11. The longitudinal plasmon peak from gold NRs with PAA coationgs modified

by the addition of 0.1M DSCG changes with time (a). The dilution of the concentration

of the DSCG quenches the assembly reaction (b).

147

5.4 Discussion

As described, the side-by-side and end-to-end geometries of gold NR assembly

clearly indicate the anisotropic character of their interactions in solutions containing gold

NRs and chromonic stacks. The observed assembly cannot be caused by the Onsager

mechanism, as the concentration of NRs is much lower than needed to satisfy the

Onsager criterion, é J 4⁄ [30, 48]. In our case, the length-to-diameter ratio is

J I 3 S 5⁄ for the used NRs, and X on the order of 10-4 – 10-5, too small to cause an

orientational order in the system of NRs. The anisotropic interaction responsible for

anisotropic assembly can be connected to the electrostatic attraction between negatively

charged DSCG stacks and gold NRs functionalized with coatings of different polarity.

First we discuss the case of positively charged NRs, functionalized with CTAB.

(1) Side-by-side assembly: CTAB is a cationic surfactant that forms a double layer

around each gold NR. The CTAB bilayer is packed more densely on the lateral surface of

the gold NR than on the end facets, as the affinity of CTAB to the “lateral” 100 and

110 crystal facets is higher than to the facets 111 forming the ends [32, 49, 50]. The

charge density at the lateral sides of NR is higher than at the ends, thus offering a

preferential sites for the attachment of the negatively charged DSCG stacks. As we

described in section 5.3.1, the simple mono- and di-valent salts do not induce the gold

NRs assembly. This fact indicates that it is the polyionic nature of DSCG stacks with a

large linear charge density [51-54] that makes the NR-DSCG stack binding effective. The

charged DSCG stacks act similarly to the multivalent counter-ions that are known to

induce aggregation (condensation) of stiff rod-like charged biopolymers, such as B-DNA

148

and F-actin [55-57]. In the absence of DSCG, the NRs experience Coulomb repulsion

which promotes their mutually perpendicular arrangements. As demonstrated

theoretically by Borukhov et al. [57], the addition of oppositely charged linkers that

absorb at the surface of the rods drive the formation of the bundles in which the rods are

parallel to each other, connected by linkers, Fig. 5.12. The effect of the linker-triggered

alignment of rods is predicted for a concentration of rods that is much lower than in the

Onsager criterium for the excluded volume induced alignment. Absence of side-by-side

assembly at low concentrations of DSCG linkers is also in qualitative agreement with the

model [57].

Figure 5.12. Schematic representation of the side-by-side assembly.

Note that in our experiments the rafts have a pronounced shape anisometry, being

only one NR diameter thick, at least at the early stages of association. Each NR is in

contact with only two neighbors (in a membrane-like structure with hexagonal packing,

each NR will be surrounded by 6 neighbors). This shape might be simply an experimental

artifact associated with the TEM sample preparation; however, we point out a physical

149

mechanism favoring rafts over membranes of a larger thickness. Since the DSCG stacks

bound to the same rod should strongly repel each other, they might prefer to bind on the

antipodal sides of the NR, thus triggering an assembly in which each NR has only two

neighbors.

The observation of end-to-end arrangements in some of the highly concentrated

samples observed in TEM, Fig. 5.1(d), is consistent with the idea that the affinity of

CTAB to the ends of NRs is low; the bare ends might experience attraction through van

der Waals forces when they find themselves in close proximity. Alternatively, the

arrangement might be promoted by the entrapment of DSCG stacks between the NR ends

covered with a small but finite amount of CTAB, or by the attraction of polyaromatic

cores of DSCG molecular to the bare Au end facets 111. Finally, capillary attractions

of the rafts in TEM samples prepared through solvent evaporation might also contribute

to the observed arrangements in Fig.5.1(d).

An important feature of the side-by-side NR assembly induced by chromonic

material DSCG is that it can be affected by a number of factors influencing the self-

assembly of DSCG itself, such as pH. Orendorff et al. [58] reported that by adding an

adipic acid whose charge depends on the pH and evaporating the solvent to facilitate the

Onsager type of alignment, one can assemble NRs into raft structures similar to those

shown in Fig. 5.1. For our system, an increased pH causes a different effect, a

disassembly of the NRs, Fig. 5.5. A higher pH increases the negative charge of the DSCG

molecules which increases the electrostatic repulsion of the DSCG molecules within the

150

stacks and decreases the sticking energy , as described in Chapter 3. In the isotropic

solution, the average length of a stack is expected to depend on the volume fraction X of

DSCG and , Xexp =rst, where xy is the Boltzmann constant. Clearly, smaller X

and smaller imply shorter DSCG stacks that are apparently less capable of electrostatic

binding to the NRs. Short stacks and individual DSCG molecules in solution are similar

to the low-valency salts such as NaCl or MgSO4 that are not capable of NR assembly as

demonstrated in the experiments above.

The side-by-side NR assembly can be easily controlled using a polyelectrolyte as

a quenching agent. When the positive charges of CTAB-functionalized NRs are screened

by a coat of polyelectrolyte such as PSS, the assembly through the negatively charged

DSCG stacks cannot take place. As demonstrated in Section 5.3.2, adding the PSS at

different time after the assembly started allows one to control the average size of the NR

assemblies. Moreover, PSS-covered assemblies can be transferred into a polymer film,

such as a film of PVA, in Section 5.3.3.

(2) End-to-end assembly: The plausible mechanism of the end-to-end NR

assembly is the difference in charge characteristics of the lateral and end facets of

individual gold NRs. The anionic polymer PAAs used to reverse the polarity of charge on

the gold NRs from positive to negative can easily bind to the densely packed CTAB

double layers at the lateral facets but not to the end facets deprived of CTAB. The

negatively charged DSCG stacks are then likely to repel the lateral facets and attract the

151

end facets of NRs either through van der Waals forces or electrostatic attraction, Fig.

5.13.

Comparing the time evolution of side-by-side assembly, Fig. 5.1 and end-to-end

assembly, Fig. 5.9, one notices that the latter is a much slower process. The feature is

natural, as the interaction cross section for NR particles in end-to-end assembly is much

lower than that in the side-by-side assembly. The end-to-end assembly also needs a much

higher concentration of DSCG in the solutions as compared to the side-by-side assembly.

The end-to-end assembly can be controlled by simply diluting the concentration of

DSCG, Fig. 5.11.

Figure 5.13. Schematic representation of the end-to-end assembly.

5.5 Conclusions

We described a simple yet universal technique of assembling a system of gold

NRs into orientationally ordered structures using the lyotropic chromonic material DSCG

that self-assembles into charged stacks. The proposed method shows advantages as

compared to the previously known NR assembly processes. First, the same agent, DSCG

152

stacks, can cause different geometries of aggregation. Depending on the type and

distribution of electric charges on the surface of NRs, the chromonic stacks assemble the

NRs either in side-by-side or end-to-end fashion. Second, the linkers responsible for NR

assembly are not covalently bound to the NRs, which allows for a high degree in

flexibility, for example, in the possibility of disassembling the NRs structures through the

increase of pH. Third, the average size of the assembled NR structures can be controlled

through a variety of approaches, such as the dilution of the solution and the addition of

quenching agents, such as PSS. Fourth, the assembled NR structures can be transferred

from the water solution into the polymer films preserving their basic structural and

optical properties. The self-assembled nature of the linking agents, chromonic stacks,

offers other possibilities in the control of NR assemblies, for example, through the

changes in chemical nature of chromonic materials. The new approach can expand the

opportunities for practical applications of assembled metallic NR structures.

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161

Chapter 6

6.1 Summary

This dissertation has explored how the aggregate structure and the phase diagrams

of lyotropic chromonic liquid crystals (LCLCs) in water depend on multiple factors, e.g.

concentration, temperature, the pH of the solution, as well as the presence of various

additives, such as salts and the neutral polymer. We also described a potential application

of LCLCs as a functional material for nanofabrication. The most important results are

summarized below:

First, we demonstrated that the chromonic molecules, such as Sunset Yellow

(SSY) and disodium cromoglycate (DSCG), stack on top of each other forming

aggregates which arrange themselves into ordered liquid crystal phases as a function of

both the concentration of LCLC and the temperature [1]. The very existence of the

nematic (N) phase in the typical LCLCs represents an apparent puzzle, since the

correlation length associated with the stacking measured in the x-ray measurement is too

short to explain the orientational order by the Onsager model. Consequently, we proposed

that the aggregate can be more complex than a simple rod and contain “stacking faults,”

such as junctions with a shift of neighboring molecules, 3-fold junctions, etc [1]; the

stacking faults explain why the correlation length measured in the x-ray experiments is

shorter than the actual length of the aggregates.

162

Second, we explained that simple salts, such as NaCl, enhance the stability of the

N phase of SSY solution when is low, while they suppress the N phase of SSY when

is high [1, 2]. This non-monotonous effect of salts on the temperatures of phase

transitions suggests that the screening of electrostatic repulsions within an aggregate

might lead to two opposite tendencies: the increased physical length of the aggregate

raises the transition temperatures !#! and ! #, while a decrease in their

persistence length lowers temperature !#! and ! #. The effect of a simple salt on

the hexagonal columnar (C) phase is more obvious. Salts can destabilize the C phase into

the N phase, which can be attributed to the decrease of the electrostatic repulsion between

the aggregates. The screening of inter-aggregate electrostatic repulsion by adding salts

produces the fluctuation of hexagonally packed aggregates with large undulation

amplitude, which can cause the C phase to melt to the N phase.

Third, we demonstrated that a base, such as NaOH, destabilizes the N phase of a

SSY solution at a low concentration of !+(8 and then replaces the N phase with

biphasic states, a densely packed N phase or C phase coexisting with the isotropic (I)

phase. These peculiar changes resulting from the addition of NaOH can be explained by

the pH of SSY solutions: higher pH increases the negative charge of SSY and thus

weakens the aggregation by decreasing the scission energy . Taylor and Herzfeld [3]

demonstrated that when decreases, the N phase disappears, giving rise to the

coexisting I+C phases, which is in qualitative agreement with our experimental results.

Interestingly, the subsequent addition of HCl reducing the pH stabilizes the N phase, thus

reverses the effect of the base.

163

Fourth, we showed that spermine in tetravalent salt form suppresses the N phase.

However, spermine in free base form causes a change in the phase diagram similar to that

brought about by NaOH, by first suppressing the N phase at low concentrations of 12

and then replacing it with biphasic states, a densely packed N phase or C phase coexisting

with the I phase [1]. The mechanism here is more complicated, as with increasing pH one

also changes the state of spermine (Spm) molecules themselves, most of which become

neutral at high pH. Spm adopts to different forms: a neutral Spm0, single-charged SpmH+,

and multiple-charged SpmH2+2, SpmH3

+3, and SpmH4+4. The relationship among the

concentrationss is a function of the pH. We proposed that these neutral Spm0 molecules,

being relatively large, act as “crowding” agents that promote the phase separation into a

Spm0-rich I phase and a SSY-rich LC phase through the excluded volume effect.

Fifth, we demonstrated that the addition of a non-ionic additive, poly(ethylene

glycol) (PEG), to a SSY solution leads to phase-separation into a condensed LC region,

either of the N type or the C type, and a PEG-rich isotropic region. This behavior can be

qualitatively explained by the depletion (excluded volume) effects [2, 4]. Since the main

structural unit of LCLC is a self-assembled aggregate rather than an individual molecule,

the excluded volume effect has two different levels of influence on the LCLC systems.

When chromonic aggregates are short, the face-to-face assembly caused by PEG

promotes elongation of the aggregate. When chromonic aggregates are long, the side-by-

side assembly is favored, inducing more densely packed arrays of aggregates and the

formation of the condensed phase with orientation and positional order, as <= increases.

The addition of salts together with PEG to a SSY solution can either promote the phase

164

separation and condensation of the LC phase or suppress it by screening the electrostatic

repulsion forces within the aggregate and between the aggregates.

Finally, we presented a simple and universal technique for assembling a system of

gold NRs into orientationally ordered structures using the lyotropic chromonic material

DSCG that self-assembles into charged stacks. This proposed method shows advantages

as compared to the previously known NR assembly processes. First, the same agent,

DSCG stacks, can cause different geometries of aggregation. Depending on the type and

distribution of electric charges on the surface of NRs, the chromonic stacks assemble the

NRs either in side-by-side or end-to-end fashion [5]. Second, the assembly of NRs can be

controlled by a number of factors influencing the self-assembly of chromonic materials,

such as the concentration and pH of the solution. Third, the assembled NR structures can

be transferred from the water solution into the polymer films preserving their basic

structural and optical properties [6]. The self-assembled nature of the linking agents,

chromonic stacks, offers other possibilities in the control of NR assemblies, for example,

through the changes in the chemical nature of chromonic materials.

To conclude, there is no unifying model that can describe the observed features of

the phase behavior of LCLCs. The experimental data presented on phase diagrams in the

presence of different additives demonstrate an extraordinarily rich variety of possible

effects and mechanisms and can serve as a basis for further studies. We hope that these

studies provide a basic understanding of phase behavior and the physical properties of the

165

reversible self-assembled chromonic materials and expand the opportunities for practical

applications of LCLCs.

6.2 References

[1] Park, H.-S.; Kang, S.-W.; Tortora, L.; Nastishin, Y.; Finotelle, D.; Kumar, S.;

Lavrentovich, O. D. Self-assembly of lyotropic chromonic liquid crystal Sunset

Yellow and effects of ionic additives. J. Phys. Chem. B 2008, 112, 16307-

16319.

[2] Park, H.-S.; Kang, S.-W.; Tortora, L.; Kumar, S.; Lavrentovich, O. D. Phase

separation and condensation of self-assembled lyotropic chromonic liquid

crystal Sunset Yellow aqueous solution crowded with poly(ethylene glycol).

Submitted to Langmuir.

[3] Taylor, M. P.: Herzfeld, J. Shape anisotropy and ordered phases in reversibly


[4] Tortora, L.; Park, H.-S.; Kang, S.-W.; Savaryn, V.; Hong, S.-H.; Kaznatcheev,

K.; Finotello, D.; Sprunt, S.; Kumar, S.; Lavrentovich, O.D. Self-assembly,

condensation, and order in aqueous lyotropic chromonic liquid crystals crowded

with additives. Soft Matter, 2010, 6, 4157-4167.

166

[5] Park, H.-S.; Agarwal, A.; Kotov, N. A.; Lavrentovich, O. D. Controllable side-

by-side and end-to-end assembly of Au nanorods by lyotropic chromonic

materials. Langmuir 2008, 24, 13833-13837.

[6] Park, H.-S.; Lavrentovich, O.D. Orientational order in systems of nanorods:

side-by-side and end-to-end controlled assembly using lyotropic chromonic

materials. Proceedings of SPIE - The International Society for Optical

Engineering 2009, 7392, 739210.

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