105

CHAPTER 3

PART A

SURFACE ACTIVE AND MORPHOLOGICAL BEHAVIOUR OF

AGGREGATES OF AILs BASED ON [Cnpy][Cl], n = 10, 12, 14, 16 AND 18 IN

AQUEOUS SOLUTIONS

3. A. 1 Introduction

Cationic surfactants or surface active agents with antibacterial properties

continue to play an important role as sensitizing and antiseptic agents, as components

in cosmetic formulations and as germicides and fungicides. Infact these surfactants

are used less for their wetting abilities than for their pronounced germicidal

properties. Their ability to adsorb on the surface of hair, skin and fabrics is well

known and they are effective in reducing static and interfibre fricition among

microfibrils. Because of these properties cationic surfactants are used world wide as

rinse added fabric softeners. The other applications of cationic surfactants are

antistatic agents, corrosion inhibitors, textile softeners, foam depressants, floatation

chemicals, asphalt and petroleum chemicals etc [1]. By shere bulk of the volume of

these surfactants, that are in use, cationic surfactants represent a broad family of

commercial compounds whose position has been raised from speciality chemicals to

bulk industrial chemicals.

As mentioned in the Chapter 1, cationic surfactants are a class of compounds

having at least one hydrophobic (R) moiety attached directly or indirectly to a

positively charged nitrogen atom. The hydrophobic groups are derived largely from

either long chain fatty acids, or from long chain alcohols derived from petrochemicals

106

[2]. Esterquats, are special novel class of surfactants, also known as biodegradable or

cleavable surfactants [3,4], have been gaining a great importance in recent years and

are characterized by the fact that hydrophobic parts of moieties ‘R’ linked to the

charge head groups via ester bonds. These are quaternary ammonium compounds

having two long (C16 or C18) fatty acid chains with too weak ester linkages and good

replacements for dialkyldimethyl ammonium based salts or surfactants. The ester

functional group allows improved kinetics of biodegradation of cationic surfactants.

The structure of esterquats is shown below:

Esterquats are modified am

widely used one are based on

alkyldimethyl ammonium halides (DMAC), alkylbenzyl dimethyl ammonium halides

(BDMAC), dialkyl dimethyl ammonium halides (DADMAC), dihydrogentaed tallow

dimethyl ammonium chloride

chlorides (DTTMAC) etc.

surface activity, and micelle formation have been extensively studied and reported [5

14]. Surfactant molecules, when dissolved in water, exist as monomer at low

concentration, and undergo self aggregation at an higher concentration (called as

critical micelle concentration, CMC), to form micelles with hydrophobic groups

located at the interior core part of the aggregates and the hydrophilic groups towards

the water. The CMC has been routinely determined by various methods as shown in

Fig.3A.1.

Fig. 3A.1. Changes in some physical properties for an aqueous solution of sodium dodecyl sulfate (SDS) in the neighborhood of the CMC

107

Esterquats are modified amonium based compounds (QAC) and among them, most

widely used one are based on alkyltrimethyl ammonium halides (TMAC),

alkyldimethyl ammonium halides (DMAC), alkylbenzyl dimethyl ammonium halides

(BDMAC), dialkyl dimethyl ammonium halides (DADMAC), dihydrogentaed tallow

dimethyl ammonium chlorides (DHTDMAC) and ditallow trimethyl am

(DTTMAC) etc. The fundamental properties of these surfactants namely

and micelle formation have been extensively studied and reported [5

Surfactant molecules, when dissolved in water, exist as monomer at low

and undergo self aggregation at an higher concentration (called as

critical micelle concentration, CMC), to form micelles with hydrophobic groups

located at the interior core part of the aggregates and the hydrophilic groups towards

MC has been routinely determined by various methods as shown in

Fig. 3A.1. Changes in some physical properties for an aqueous solution of sodium dodecyl sulfate (SDS) in the neighborhood of the CMC

onium based compounds (QAC) and among them, most

alkyltrimethyl ammonium halides (TMAC),

alkyldimethyl ammonium halides (DMAC), alkylbenzyl dimethyl ammonium halides

(BDMAC), dialkyl dimethyl ammonium halides (DADMAC), dihydrogentaed tallow

(DHTDMAC) and ditallow trimethyl ammonium

The fundamental properties of these surfactants namely

and micelle formation have been extensively studied and reported [5-

Surfactant molecules, when dissolved in water, exist as monomer at low

and undergo self aggregation at an higher concentration (called as

critical micelle concentration, CMC), to form micelles with hydrophobic groups

located at the interior core part of the aggregates and the hydrophilic groups towards

MC has been routinely determined by various methods as shown in

Fig. 3A.1. Changes in some physical properties for an aqueous solution of sodium dodecyl sulfate (SDS) in the neighborhood of the CMC

108

The geometrical features of the aggregates or micelles of surfactants in terms

of shape, size and dispersity are in general obtained from light scattering [8], small

angle neutron scattering [5,7-9], fluorescence quenching including time resolved

spectra [8,9] and speed of sound [11] etc. All these studies have concentrated on the

effect of alkyl chain length of hydrophobic part, on the nature and type of counter

anion, and temperature on CMC and aggregation properties. The effect of additives

based on both of electrolytic and nonelectrolytic nature also influences these

properties and details of such investigations would be discussed in next section of the

present chapter.

The micelle forming ability of cationic surfactants gives rise to their

detergency and solubilization properties. Surfactants in general and cationic

surfactants in particular solubilize more of hydrophobic organic compounds, than that

would be dissolved in water alone and hence affect the mobility and degradation of

hydrophobic organic compounds in soils and / or sediments [15]. Ying [15] reviewed

the fate, behaviour and effects of surfactants and their degradation products in the

environment. Quaternary ammonium based surfactants are biodegradable under

aerobic conditions and the biodegradability decreases with the increasing in number

of nonmethyl groups, and also with benzyl group. However, under anaerobic

conditions, QAC showed no or very poor primary degradation and no evidence for

ultimate biodegradation. Moreover cationic surfactants based on QAC, get sorbed

more on sludge, sediment or soil and hence persistent in the environment for longer

duration.

Quaternary pyridinium or alkylpyridinium based cationic surfactants have

been emerged as one of the promising class of cationic surfactants: they are the

unsaturated heterocyclic compounds and are prepared from the precursor pyridine,

109

which is derived from coaltar or synthetically prepared [16]. Pyridine is reported to be

toxic for several bacterial species at a concentration of 340 mgL-1 [17,18]. Hence

quaternary alkylpyridinium based cationic surfactants possess good geometrical

properties. Moreover, pyridine is reported to be biodegradable in aerobic conditions

within eight days [1 and references cited there in]. Pyridine has also been reported to

be degraded from mixed active sludge based reactor at faster rates [19]. Structurally,

pyridine based quaternary compounds offer wide possibilities for substitution of

different groups either on the pyridine ring or at nitrogen atom of the cycle [1].

Therefore various alkyl derivatives have been synthesized and the possible structural

variations are listed in Table 3A.1.

The advantages of quaternary alkylpyridinium based compounds enable them

to be used in various diverse areas ranging from biological and industrial applications,

such as cosmetics [20-22], pharmaceuticals [23], gene delivery [24,25],

polymerization [26,27] and many other industrial applications like dye removal from

waste water [28], drug delivery [29-31], electrolyte in lithium battery [32], textile

processing [33], floatation of mineral [34], modification of biological membranes [35-

37], dewatering of waste oil [38], cleaning of nonadsorbent articles [39] and

antimicrobial agents [23].

There are few studies in the literature describing the micelle formation,

aggregation behavior and rheology of micelle solutions of pyridinium based cationic

in presence of additives. It would be appropriate and pertinent to describe these

studies and main conclusions drawn.

110

Table 3A.1 Pyridinium derivatives

S. No. Derivative Substituent Position Structure 1

N-Alkylpyridinium compounds

Unsubstituted

2

L-Alkyl-N- alkylpyridinium compounds

Alkyl groups(R')

2,3,4

3

L-(Thio-ether)-N-alkylpyridinium compounds

Thio-ether(SR’)

2,4

4

Ring-acylated-N-Alkylpyridinium compounds

3

111

5

N-Aralkylpyridinium compounds

N

6

N-Alkoxymethylpyridi- nium compounds

N

7

N-(2-Alkoxyethylpyridi- nium compounds

ROCH2CH2

N

8

N-Alkoxybenzylpyridi- nium chlorides

N

112

9

Ester substituted pyridinium compounds

N

10

N-Amide substituted pyridinium compounds

N

11

N-Thioether substituted pyridinium compounds

CH2CH2SR

N

113

3A. 1. 1 Critical Micelle Concentration (CMC) and Aggregation Behaviour:

CMC is the useful property as it reveals property of surfactants to self

associate. Higher the number of surfactant molecules in a micelle, more highly

cooperative would be the association process. Longer the hydrocarbon chain of a

surfactant tail, the lower would be the CMC. Hydrophobic forces drive the

micellization process, while electrostatic repulsions among the head groups oppose

the association of molecules. The value of CMC and degree of ionization decrease

with the increase in the alkyl chain length [40], and the same increase with increase in

number of pyridinium head groups [41-43]. Brigma and Engberts [44], Brigma et al

[45] have described and discussed the influence of counter ions on the growth of the

micelles formed by alkylpyridinium surfactants in aqueous solutions. It has been

shown that the nature of the counter anion plays an important role on the geometrical

features or growth of the micelles. Three types of counter ions have been studied: (i)

halides (ii) alkylsulfonates and (iii) aromatic counterions. The CMC was found to

decrease with increasing counter ion size and hydrophobicity; whereas the degree of

counterion binding increases. The aggregation behavior of 1-methyl-4-n-

dodecylpyridinium surfactants with aromatic counterions was found to be markedly

dependant on the substituent (hydrophobicity) and the substitution pattern in the

aromatic ring of the counter ion. More hydrophobic counter ions induce the growth of

the spherical micelles to worm-like micelles. Next to the hydrophobicity, the type of

the substituent and the substitution pattern, the microenvironment of counterion in the

stern region, and the structure of the surfactant monomer also play a role on the nature

of the micellar structures. For example, the formation of a network of entangled,

elongated worm-like micelles by alkylpyridinium surfactants with o-

hydroxybenzoate, or p-chlorobenzoate counter ions was discussed by the authors in

114

terms of surfactant structure. The head group of counterions and surfactants are

responsible for the formation of worm-like micelles. Detailed small angle neutron

scattering (SANS) studies were reported on the aggregate structures of multi-headed

pyridinium surfactants in aqueous solutions [42,43]. The single headed pyridinium

surfactant formed lamellar structure, where as surfactants with double and triple head

groups formed simple micelles in water. The aggregates shrinked in size with the

increase in number of head groups. The aggregation number continually decreased

and the fractional charge increased with the more number of head groups. It was

suggested that the hydrocarbon chains in micelles of multi-headed pyridinium

surfactants adopt a bent conformation as compared to extended form in micelles of

single headed surfactants.

The aggregation number of micelles of alkylpyridinium halides was affected

by the nature of the counterions: the value increase in the order: IO3- < HCO2

- < BrO3-

< F- < Cl- < NO3- < Br- < ClO3

- < SCN- [41,47]. Similarly, the presence of alkyl

groups on the pyridine ring decreased the aggregation number [47], increased with

increase in partial charge in alkyl tails and decrease with increase in positive charge

on the pyridinium ring atom to which decyl chain was attached [48].

3A .1. 2 Thermodynamics of micellization:

There are few studies that deal with the thermodynamic properties of aqueous

solutions of 1-alkylpyridinium chlorides at temperatures from (283.15 to 343.15) K

[49,50] from the vapour pressure osmometric [49], molar integral enthalpic [51], and

also conductivity [51] measurements. The values of osmotic coefficients and molar

integral enthalpies of dilution were treated using a mass action model that includes

ion-interaction terms for the activity coefficient of both micellar and monomeric

115

species in solution [49]. The thermodynamic behavior of micelle formation was

represented in terms of an equilibrium constant, an activity coefficient equation and

their temperature derivatives. Chatterjee et al [50] assessed the thermodynamic energy

parameters (free energy, enthalpy, entropy and heat capacity) of micellization of

cetylpyridinium chloride in aqueous solutions of with or without sodium chloride. The

enthalpy and heat capacity of micellization values calculated by Van’t Hoff method

(which uses CMC of a surfactant at different temperatures and mass action and

pseudo-phase models) and directly determined by the microcalorimetric methods

were found to be significantly different. These results suggested that the enthalpy

values also arise from physicochemical processes other than amphiphilic association.

The change in the aggregation number and shape (as micellization is a dynamic

process) and the counter ion binding to micelles also influence the enthalpy of

micellization. Moreover, the calorimetry measures the integral heat of micellization,

while Van’t Hoff method gives differential heat of micellization. The overall enthalpy

change of micellization may also have contributions from heat of solvation–

desolvation of the species, ionization degree, mixing etc. Bhat et al [51] had

determined transport, solvation and micellization parameters for the micellization of

cetyl- and dodecylpyridinium chlorides and their analysis suggested that

dodecylpyridinium ion move more easily in aqueous solutions than cetylpyridinium

ions on account of enhanced hydrophobic interactions among the tails of former. It is

otherwise generally agreed that entropy change contributes to the over all negative

free energy of micellization of alkylpyridinium halides in water [52,53].

The interest in pyridinium cations or alkylsubstituted pyridinium as cations in

designing ionic liquids, with a pairing from different anions namely tetrafluoroborate,

triflate, dicyanamide and bromide etc is emerging, due to fact that ILs with special

116

properties that are attributable to pyridine ring can easily be incorporated into ILs.

Moreover, the ILs can be designed to have alkyl chain length varied from as low as

possible to long ones. By this way, one adds the amphiphilicity to the molecules. The

utility of ionic liquids is that hydrophobicity can be incorporated from cation or anion

or both the sides, and several structural possibilities exist as far as cation-anion

combinations are concerned.

Bendres et al [54] had reported a systematic study of the aggregation behavior

of a series of alkylpyridinium based ionic liquids (1-butylpyridinium

tetrafluoroborate, 1-butylpyridinium triflate, 1-butyl-2-methylpyridinium

tetrafluoroborate, 1-butyl-4-methylpyridinium tetrafluoroborate, 1-butyl-3-

methylpyidinium dicyanamide and 1-octyl-3-methylpyridinium tetrafluoroborate) in

aqueous solutions through density, speed of sound and conductivity measurements.

Critical aggregation concentration, degree of ionization of the aggregates, standard

Gibbs free energy of aggregation were obtained. The authors discussed variation of

partial molar properties or conductivity or isentropic compressibility with the

concentration of ILs in terms of effect of alkyl chain length on the interactions:

between the cationic head groups, between the cation and anion, which influence the

aggregation tendency. It was suggested that, even though the ILs with butyl or octyl

chain showed distinct CAC values, increase of carbon chain length decreased CAC,

due to increased hydrophobic character of the cation. The authors also compared their

CAC values with that of 1-butyl-3-methylpyridinium chloride [55,56] in water. It was

noted that the CAC values also depends on the size of the counter ion [57,58]. Greater

the anion size, the weaker the hydration and aggregation process is made easier since

they decrease the charge repulsion between the cations due to their adsorption on the

charged surface. Based on the relative values of over all conductivity of different ILs

117

in aqueous solutions, it was indicated that the degree of ionization decreases with the

increase in the alkyl chain length of the ILs, and counter ions bind with the aggregates

more easily [59].

Bendres et al [54,60-62] had also performed a comprehensive study of a series

of ILs based on pyridinium ring and it was observed that the aromatic protons of

pyridinium ring are less acidic than the nitrogen atoms of other ILs, such as those

derived from imidazolium ring, due the bigger size of the ring and the presence of

only one electronegative atom. Moreover, NMR measurements showed that

pyridinium based cations exhibit cation-cation Van der Waals attraction and favor a

configuration favorable for ring staking through π---π interactions at the aggregate

surface [56]. The increase of alkyl chain length would on the contrary increase the

cohesive interactions between the chains, also affect the attraction between the π-

electron clouds and coulombic interactions between anions and counterions due to

steric hindrance [63].

Cornellas et al [64] investigated the effects of alkyl chain length and the nature

of the cationic head group on micellization and antimicrobial activity of ionic liquids

based on long alkyl chain (C8 to C14) imidazolium or pyridinium bromides on the

micellization and antimicrobial activity using tensiometry, conductometry,

spectrofluorometry and PGSE-NMR. The authors did not notice any significant

difference between imidazolium and pyridinium ILs regarding surface activity and

aggregation behavior in aqueous solutions. Therefore, it was suggested that

methylimidazolium and pyridine rings have similar polarity. Otherwise micellization

was found to be driven by hydrophobic interactions between the alkyl chains. These

amphiphilic ILs showed higher surface activity and antimicrobial activity. It was also

118

suggested that log CMC can be used as good index of hydrophobicity to estimate the

efficiency as antimicrobial agents.

In order to investigate and understood the surfactant behavior of ionic liquids

derived from pyridinium cations and halide anions, with different alkyl chain length,

our laboratory [65] had reported the aggregation behavior of short alkyl chain ionic

liquids, namely 1-butyl, or 1-hexyl, or 1-octylpyridinium, and 1-octyl, or 3-, or 4-

methylpyridinium chlorides in water, using surface tension, electrical conductance, 1H

NMR, SANS and SAXS measurements. Even though the ILs were characterized by a

critical aggregation concentration (CAC), the nature of the aggregation process and

aggregates were found to complex and different for 1-butyl chain based ILs on one

hand, and 1-octyl chain based ILs on the other hand. The aggregation process was

found to be entropy driven. 1H NMR shift measurements revealed that the protons of

terminal methyl groups of 1-butylpyridinium chlorides are less shielded than their

counter parts of 1-octylpyridinium chlorides. The aggregates at best could be

considered as clusters of closely packed molecules or small open aggregates stabilized

by several types of interactions namely π……π stacking, hydrogen bond interactions

between the cations of the IL and Cl⊝, water…..Cl⊝ ion interactions and ring acidic

protons and water etc. There was no evidence for the formation of micelle like

aggregates structures, that are expected of conventional surfactant like amphiphiles.

In the present chapter, our studies on surfactant-like properties of

alkylpyridinium chlorides in water have been extended to ILs with longer carbon

chain length, namely 1-decylpyridinium-, or 1-dodecylpyridinium-, or 1-

tetradecylpyridinium, or 1-hexadecylpyridinium- and 1-octadecylpyridinium

chlorides. The critical aggregation concentrations, thermodynamic parameters of

aggregation, aggregation properties namely shape and size have been determined

119

using several techniques such as surface tension, electrical conductivity, steady state

fluorescence, static light scattering, and SANS. The effects of alkyl chain length and

interactions between cation-cation, cation-anion and hydrocarbon tails, cation/anion-

water, and concentration of ILs are monitored and discussed. It is hoped that though

this work, the conditions for micelle-like aggregate formations would be established.

3A. 2 Results and Discussion

3A. 2. 1 Critical Aggregation Concentrations (CAC):

Surface tension measurements on different concentrations of ionic liquid

solutions in water were made with an aim to understand the surface activity at the

air/water interface and also to determine the CAC. The data on surface tension of

aqueous solutions of five AILs namely [C10py][Cl], [C12py][Cl], [C14py][Cl],

[C16py][Cl] and [C18py][Cl] are listed in Table 3A.2. The surface tension, γ versus

logc isotherms are shown in Fig. 3A.2. With increase in the concentration of ILs, the

IL molecules adsorb on the air/water surface and reduce the surface tension values.

Once the surface gets saturated, the IL molecules undergo self association into

micelles and this phenomenon occurs at a certain concentration marked by constant

surface tension values. These concentrations are determined by the abrupt increase in

the first derivative of dγ/dc, as shown in the Fig. 3A.2. The surface tension isotherms

did not exhibit any minima around CMC values indicating that the samples are free of

impurities and micelle formation is not preceded by any other related phenomenon, as

observed in the surface tension isotherm of short alkyl chain ILs based on

imidazolium or pyridinium cations [64,66,67].

120

Table 3A.2 Surface tension values of aqueous solutions of alkylpyridinium chloride based amphiphilic ionic liquids at 298.15 K

Log C ST mNm-1

Log C ST mNm-1

Log C ST mNm-1

[C10py][Cl] [C12py][Cl] [C14py][Cl] -2.3010 54.76 -4.3010 69.55 -4.3010 69.12 -2.0000 48.54 -3.0000 65.78 -4.0000 68.56 -1.6990 41.66 -2.6990 62.71 -3.3010 63.11 -1.5229 37.41 -2.3979 55.81 -3.0000 57.89 -1.4559 35.74 -2.2218 50.37 -2.8539 53.55 -1.3979 34.57 -2.0969 46.13 -2.7447 50.13 -1.3565 34.16 -2.0000 43.37 -2.6576 48.02 -1.3188 34.17 -1.9208 41.12 -2.5850 45.45 -1.2840 34.18 -1.8539 39.21 -2.5229 43.49 -1.2518 34.18 -1.7959 37.56 -2.4685 41.58 -1.2218 34.19 -1.7447 36.16 -2.4202 40.05 -1.1871 34.20 -1.6990 36.18 -2.3979 39.26 -1.1549 34.21 -1.6198 36.21 -2.3768 39.34 -1.0969 34.22 -1.5229 36.24 -2.3565 39.45 -1.0458 34.22 -1.3979 36.26 -2.3372 39.57

-2.3010 39.62 -2.2596 39.75 [C16py][Cl] [C18py][Cl]

-4.6990 69.79 -5.0000 69.84 -4.3979 69.51 -4.3979 64.34 -4.2218 68.52 -4.0969 58.05 -4.0969 67.52 -3.9208 51.61 -4.0000 66.61 -3.7959 46.21 -3.6990 62.91 -3.7447 44.14 -3.3979 56.18 -3.6990 42.34 -3.2218 49.54 -3.6576 41.45 -3.0969 44.83 -3.6198 41.16 -3.0000 41.36 -3.5850 41.17 -2.9208 40.55 -3.5528 41.18 -2.8539 40.56 -3.5229 41.18 -2.7959 40.57 -3.4949 41.19 -2.7447 40.58 -3.4437 41.20 -2.6990 40.58 -3.3979 41.21 -2.5229 40.59 -3.3468 41.22 -2.3979 40.59 -3.3010 41.22 -2.3010 40.60

121

0.01 0.1 1 10 10030

40

50

60

70

80

γ, m

N.m

-1

Log C, mM.dm-3

Fig. 3A.2 Surface tension versus log C isotherms for alkylpyridinium chloride based ionic liquids aqueous solutions at 298.15 K : (∆∆∆∆) [C10py][Cl], () [C12py][Cl], () [C14py][Cl], (♦) [C16py][Cl], () [C18py][Cl]. Location of CAC is indicated by peaks (as calculated by Phillips method)

CAC of AILs are also determined by other two independent methods namely

electrical conductance and steady state fluorescence. The data on specific

conductance of IL aqueous solutions as a function of the concentration are listed in

Table 3A.3. The plots of specific conductivity versus concentration of AILs at T =

(298.15 to 318.15) K are displayed in Fig.3A.3.

122

Table 3A.3 Specific conductivity (κ in mScm-1) values as a function concentrations of aqueous solutions of alkylpyridinium chloride based ionic liquids at T = (298.15, 308.15 & 318.15) K

Conc.,

mM.dm-3

Specific Conductance (κ)

mScm-1

Conc.,

mM.dm-3

Specific Conductance (κ)

mScm-1

298.15 308.15 318.15 298.15 308.15 318.15 [C10py][Cl] [C12py][Cl]

5.00 0.743 0.899 1.136 0.05 0.102 0.135 0.149 10.00 1.133 1.377 1.688 1.00 0.146 0.185 0.208 20.00 1.912 2.333 2.790 2.00 0.234 0.285 0.326 30.00 2.692 3.288 3.893 4.00 0.410 0.485 0.562 35.00 3.081 3.766 4.444 6.00 0.586 0.685 0.797 40.00 3.471 4.244 4.995 8.00 0.762 0.885 1.033 44.00 3.783 4.626 5.436 10.00 0.938 1.085 1.269 48.00 3.944 4.796 5.836 12.00 1.114 1.285 1.505 52.00 4.095 4.994 6.074 14.00 1.290 1.485 1.740 56.00 4.246 5.192 6.312 16.00 1.466 1.685 1.976 60.00 4.397 5.390 6.550 18.00 1.633 1.853 2.212 65.00 4.586 5.637 6.848 20.00 1.713 1.947 2.332 70.00 4.774 5.884 7.146 24.00 1.874 2.135 2.572 80.00 5.152 6.379 7.741 30.00 2.115 2.417 2.931 90.00 5.529 6.874 8.337 40.00 2.516 2.888 3.530 100.00 5.906 7.368 8.932 50.00 2.917 3.359 4.129

[C14py][Cl] [C16py][Cl] 0.05 0.041 0.044 0.057 0.02 0.011 0.012 0.015 0.10 0.046 0.050 0.064 0.04 0.013 0.015 0.018 0.50 0.087 0.098 0.121 0.06 0.015 0.018 0.021 1.00 0.137 0.158 0.191 0.08 0.018 0.020 0.024 1.40 0.178 0.206 0.248 0.10 0.020 0.023 0.028 1.80 0.218 0.254 0.304 0.20 0.031 0.036 0.043 2.20 0.259 0.302 0.360 0.40 0.053 0.063 0.075 2.60 0.299 0.350 0.417 0.60 0.076 0.090 0.107 3.00 0.340 0.398 0.473 0.80 0.098 0.117 0.138 3.40 0.381 0.446 0.530 1.00 0.120 0.144 0.170 3.80 0.421 0.495 0.586 1.20 0.137 0.170 0.202 4.00 0.441 0.519 0.614 1.40 0.146 0.184 0.223 4.20 0.465 0.543 0.643 1.60 0.155 0.195 0.237 4.40 0.474 0.556 0.671 1.80 0.164 0.206 0.252 4.60 0.483 0.567 0.691 2.00 0.173 0.217 0.266 5.00 0.501 0.589 0.718 3.00 0.218 0.271 0.336 5.50 0.523 0.616 0.751 4.00 0.262 0.326 0.406 6.00 0.546 0.643 0.785 5.00 0.307 0.380 0.477

123

Conc.,

mM.dm-3

Specific Conductance (κ)

mScm-1

298.15 308.15 318.15 [C18py][Cl]

0.01 0.008 0.010 0.011 0.04 0.012 0.014 0.016 0.08 0.016 0.020 0.023 0.12 0.021 0.025 0.029 0.16 0.026 0.031 0.036 0.18 0.028 0.034 0.039 0.20 0.030 0.037 0.043 0.22 0.032 0.040 0.046 0.24 0.035 0.042 0.049 0.26 0.036 0.045 0.052 0.28 0.037 0.046 0.053 0.30 0.038 0.047 0.055 0.32 0.039 0.048 0.056 0.36 0.041 0.050 0.059 0.40 0.042 0.052 0.061 0.45 0.045 0.055 0.065 0.50 0.047 0.058 0.068

It can be seen from the plots that, specific conductivity gradually increases with the

concentration and the plots are characterized by two distinct linear dependencies,

marking the pre and post micellization region, and the transition between the two is

marked by CAC. The linear increase in specific conductivity upto CAC is attributable

to the increase in concentration of [Cnpy+] and [Cl-] ions. Above CAC, the

conductivity values tend to increase linearly with the concentration but with a low

value of slope. This increase of specific conductivity beyond CAC value can be

explained by (i) the condensation of counterions on the aggregate surface and (ii) the

charged aggregates along with the counter ions have lower mobility and hence

contribute to the charge transport to lesser extent as compared to unassociated free

ions [51,68]. The CAC values as determined by conductance measurements for AILs

are also listed in Table 3A.4.

124

0 20 40 60 80 100 1200

2

4

6

8

10

0 20 40 60 80 100 1200

2

4

6

8

10

0 20 40 60 80 100 1200

2

4

6

8

10

0 1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

a

Spe

cific

co

ndu

ctiv

ity, m

S.c

m-1

b

Conc., mM.dm-3

c

d

e

f

Fig. 3A.3 Specific conductance isotherms for alkylpyridinium chloride based amphiphilic ionic liquids in water : (a) 298.15 K; (b) 308.15 K; (c) 318.15 K : (∆∆∆∆) [C10py][Cl], () [C12py][Cl] ; (d)298.15 K; (e) 308.15 K; (f) 318.15 K : () [C14py][Cl], (♦) [C16py][Cl], () [C18py][Cl]

125

Table 3A.4 Critical aggregate concentrations, (CAC, in mMdm-3) of alkylpyridinium chloride based amphiphilic ionic liquids in water at different temperatures

IL T in K 298.15 308.15 318.15

Surface tension Electrical conductivity Steady state fluorescence

Electrical conductivity

Electrical conductivity

Exp. Lit. Exp. Lit. Exp. Exp. Lit. Exp. Lit. [C10py][Cl] 44.25 ± 2.00 44.05 ± 2.00 45.93 45.33 ± 2.00 47.32 ± 2.00 [C12py][Cl] 17.07 ± 1.00 17.25 [69] 17.17 ± 1.00 16.2 [51] 17.39 17.76 ± 1.00 17.80 [51] 18.01 ± 1.00 19.4 [51] [C14py][Cl] 4.19 ± 0.05 4.02 ± 0.05 4.25 4.28 ± 0.05 4.45 ± 0.05 [C16py][Cl] 0.98 ± 0.02 0.90 [69] 1.07 ± 0.02 1.05 [70]

0.96 [50] 0.916[51]

1.02 1.16 ± 0.02 1.04 [50] 0.974 [51]

1.23 ± 0.02 1.150 [50] 1.039 [51]

[C18py][Cl] 0.24 ± 0.02 0.24 [69] 0.25 ± 0.02 0.26 0.25 ± 0.02 0.26 ± 0.02

126

Steady state fluorescence is another useful method to determine CAC values

unambiguously. In this method, a fluorescent probe, pyrene was added to aqueous

AIL solutions and its polarity index as defined in terms of (I1/I3), where (I1/I3) stands

for the ratio of intensities of the first and third vibronic peaks for pyrene. The profiles

showing the dependence of (I1/I3) as a function of concentrations of different ILs are

shown in Fig. 3A.4. The profiles are sigmoidal in nature and the abrupt decrease of

intensities clearly indicates the formation of micellar like aggregates and the

hydrophobic pyrene tend to get solubilized and reside in the interior of micelles,

relative to bulk water. At high AIL concentrations, the values of intensity ratio

become near constant, as the entire volume of the solution now consist of micelles.

The interaction between the rapidly decreasing linear part of the sigmoidal curves and

of the near constant region at high concentrations represents the CAC value. The CAC

values as determined from the Fig.3A.4 for different AILs are also listed in Table

3A.4. The CAC values for some of the AILs of present study, were also reported in

the literature using same methods. The lit. values are also listed for comparison

purpose. It can be seen that the average values of CAC for a given AIL and at a

particular temperature, as obtained by three independent methods of ST, electrical

conductance and steady state fluorescence agree closely with each other. The CAC

values reported in the present study also are in the reasonable agreement with the

literature data.

The values of CAC of any single chain amphiphile depend mainly on the

nature and length of its alkyl chain and also equally on the size of the head group. A

balance between these factors influences the stability of micelles.

127

1E-3 0.01 0.1 1 10 1001.0

1.2

1.4

1.6

1.8

2.0

[I1/

I 3]

Conc. IL

, mM.dm-3

Fig. 3A.4 I1/I3 ratio of pyrene as a function of concentration for alkylpyridinium chloride based aqueous solutions of AILs at 298.15 K: (∆∆∆∆) [C10py][Cl], () [C12py][Cl], () [C14py][Cl], (♦) [C16py][Cl], () [C18py][Cl]

For a homologous series of single chain amphiphiles, the dependence of CMC values

on the length of carbon chain follows Stauff- Klevins rule [71,72]:

Log CMC = A – Bx (1)

where A and B constants for a particular homologous series and temperature,

respectively and x is the number of carbon atoms in the hydrocarbon tail. The

constant A varies with the nature and number of the hydrophilic groups and while B

remains constant which measures the effect of each additional alkyl group on CMC.

The data of CAC for homologous series of alkylpyridinium chloride of formula

[Cnpy+][Cl -], as determined from surface tension measurements at 298.15 K were

fitted through Stauff-Kleven rule and a value of A=1.686 and B=0.288 were obtained.

As compared to the value A=1.686 for [Cnpy][Cl] ILs, [Cnmim][Cl], [Cn4-mpy][Cl]

cationic alkylammonium bromides and anionic alkyl sulphates are characterized by

1.492, 1.377, 1.761 and 1.519 and these differences are expected as a consequence of

128

type and nature of the head group. The value of B represents the free energy of the

transfer of a methylene group from aqueous back ground to micellar environment. For

non ionic amphiphiles B is ~ 0.5, for a paraffin or alkyl chain based surfactants with a

single ionic head group B ranges 0.28 – 0.30 [72-74]. Therefore our calculated B

value of 0.288 for [Cnpy][Cl] ILs is in good agreement with the above. Therefore it

can be stated that the addition of a methylene unit approximately halves the CMC

values.

The comparison of CAC and γCAC for homologous series of AILs/surfactants

with different head groups but same alkyl chain length is made in Table 3A.5. It can

be seen that CAC values of [Cnpy][Cl], when n=10, 12 and 14 is higher than mim

based ILs and anionic surfactants and for n=16 and 18 homologues, it is less.

Correnellas et al [64] had reported that γCAC for [Cnmim][Br] and [Cnpy][Br], where

n= 8, 10, 12 and 14 is almost comparable within the experimental uncertainty,

contrary to our observation. Br⊝ ions are less hydrophilic than Cl⊝ ions. Therefore,

the condensation or neutralization of charge on the cations of [Cnpy][Cl] molecules on

the aggregate surface is minimum and head group---head group repulsions are

relatively more leading to high CAC values. When the carbon chain consists of 16 or

more methylene units, the hydrophobic interactions among long hydrocarbon tails

dominant and overcome the head group repulsions leading to low CAC values.

γCAC values for various ionic liquids and surfactants with similar alkyl chain

length are also compared in Table 3A.5. The overall efficiency of wetting i.e. surface

tension reduction is adjudged from γCAC values. As compared to alkyl mim based ILs,

[Cnpy][Cl] are less surface active at air/water interface on one hand, however they are

more surface active over the conventional cationic or anionic surfactants.

129

Table 3A.5 Comparison of critical aggregation concentration (CAC, in mMdm-3), surface tension at CAC, γCAC, of alkylpyridinium chlorides, alkylmethylimidazolium chlorides, alkyltrimethylammonium bromides and alkyl sulfates based amphiphiles at air / water interface at 298.15 K (determined from surface tension isotherms)

Alkylchain

(Cn)

[Cnpy][Cl] [Cnmim][Cl] Alkyl Quaternary

Bromides (AQB)

Alkyl sulfates

Exp. Lit. Lit. Lit. Lit.

CAC(mMdm-3)

C10 44.25 ± 2.00 40.8 ± 1.55 [75] 68.00 [69] 16.0 [69]

C12 17.07 ± 1.00 17.25 [69] 14.8 ± 1.00 [75] 16.00 [69] 8.2 [69]

C14 4.19 ± 0.05 - 3.60 [69] 2.1 [69]

C16 0.98 ± 0.02 0.90 [69] 1.14 ± 0.05 [75] 0.92 [69] 0.58 (400C) [69]

C18 0.24 ± 0.02 0.24 [69] 0.40 ± 0.02 [75] - 0.23 (400C) [69]

γCAC (mNm−1)

C10 34.07 27.3 [75] - 39.8 [69]

C12 36.01 38.8 [69]

35.8 [76]

33.1 [75] - 39.8 [69]

C14 39.16 - - 40.0 [69] 34.8 [69]

C16 40.47 42.0 [76] 34.8 [75] - -

C18 41.36 29.8 [75] - -

130

3A. 2. 2 Surface Active Parameters:

The adsorption behavior of surface active ionic liquids (or even surfactants) at

air/water interface is elegantly described in terms of various parameters namely

surface excess concentration, Γmax, adsorption efficiency, pC20 effectiveness of surface

tension reduction, πCAC and minimum area occupied per molecule, etc [77-80].

The data on various surface active parameters along with their comparison with mim

based ionic liquids or conventional ionic surfactants with similar hydrocarbon chain

length are listed in Table 3A.6. Γmax values systematically increased with the increase

in carbon chain length among [Cnpy][Cl] series of ILs and this behavior is similar to

other surface active substances. The increased Γmax values in general suggest the

enhanced hydrophobic interactions and increased number of IL molecules adsorbed at

the air/water interface. The Γmax values for AILs based on pyridinium and

methylimidazolium cations are comparable and close indicating that the interactions

of these head groups with water are almost equal in magnitude. As compared to

quaternary ammonium head group based cations, [Cnpy][Cl] based ILs are

characterized slightly by high Γmax, and by marginally lower values, than the alkyl

sulfate based anionic surfactants. Therefore, it is concluded that, besides the length of

the carbon chain in the hydrocarbon tail, the head group…….water interactions, i.e.

hydrophilicity also play an important role in the final adsorption efficiency of an

amphiphile.

pC20 values, where C is the molar concentration of surfactant and C20 stands

for the concentration required to reduce the surface tension of the water by 20 mN/m

and is also a measure of minimum concentration needed to saturate the air/water

interface. The increase in carbon chain length in [Cnpy][Cl] ILs, lead to the

systematic increase in pC20 and this is due to the enhanced adsorption efficiency of

131

longer alkyl chains at air/water interface. pC20 values for py or mim based ILs of

same carbon chain are almost comparable and while the same were less (carbon

chains of ≥ C14) than the linear alkyl sulphates.

The values decreased with increase in carbon chain length (please see

Fig.3A.5) and this trend is consistent with the enhanced adsorption efficiency under

the same conditions. The decreased values of suggests very close packing of

longer alkyl chains due to hydrophobic interactions.

8 10 12 14 16 18 200

1

2

3

4

5

pC2

0

nc

40

60

80

100

120

as 1 / Å

2

Fig. 3A.5 PC20 (open symbols) and surface area per molecule at air/water interface,

(close symbols) as a function of number of carbon atoms (nc) in the alkyl chains of pyridinium chloride based ionic liquids in water at 298.15 K

πCAC is the surface pressure at CMC and is defined as: πCAC = γ0 - γCAC,

where γ = surface tension of pure solvents i.e. water and γCAC is the surface tension at

CAC. This parameter is also a measure of the effectiveness of amphiphile as it

indicates how much the surface tension of water can be lowered. It can be seen that

πCAC decreases with the increase in the alkyl chain length and while pC20 increases,

indicating that longer the alkyl chain higher would be the lowering of surface tension

132

of water and more effective would be the adsorption of amphiphile molecules at the

air/water interface. A comparison of πCAC values of [Cnpy][Cl] ILs with mim based

ILs and also anionic surfactants reveals that, the former are equally or some instances

more effective in reducing the surface tension of water.

Table 3A.6 Comparison of maximum surface excess concentration Γmax, surface

tension reduction pC20, surface pressure at CAC πCAC, and surface area per molecule at air/water interface a

of alkylpyridinium chlorides, alkylmethylimidazolium chlorides, alkyltrimethylammonium bromides and alkyl sulfates based amphiphiles at air / water interface at 298.15 K

Alkylchain (Cn)

[Cnpy][Cl] [Cnmim][Cl] Alkyl ammonium Bromides

Alkyl sulfates

Exp. Lit. Lit. Lit. Lit. Γmax × 1010 ( molcm−2)

C10 2.1 ± 0.1 - 1.9 [75] - - C12 2.5 ± 0.1 2.7 [69] 2.3 [75] 2.7 (300C) [53] 2.9 [69] C14 2.9 ± 0.1 - - 2.7 (300C) [53] 3.1 [69] C16 3.3 ± 0.1 - 3.4 [75] - 3.7 [69] C18 3.6 ± 0.1 - 3.7 [75] 2.6 [69] -

pC20 C10 2.1 2.5 [75] - 1.8 (270C) [69] C12 2.3 2.1 [69] 2.4 [75] - 2.2 [69] C14 2.8 - - 3.1 [69] C16 3.3 3.2 [75] - 3.7 [69] C18 3.9 3.6 [75] - -

πCAC (mNm−1) C10 37.74 - 44.5 [75] - 32.01 [69] C12 35.80 - 33.6 [75] - 32.01 [69] C14 32.65 - - 31.81 [69] 37.01 [69] C16 31.34 - 34.8 [75] - - C18 30.35 - 29.8 [75] - -

a (Å2)

C10 81 ± 1 - 85 [75] - - C12 66 ± 1 62 [69]

52.5 (300C) [53] 72 [75] 61 (300C) [53] 57 [69]

C14 57 ± 1 - - 61 (300C) [53] 53 [69] C16 51 ± 1 42 (300C) [53]

49 [76] 49 [75] - 45 [69]

C18 48 ± 1 - 45 [75] 64 [69] -

133

3A. 2. 3 CAC and Thermodynamic Parameters of Aggregation of AILs:

The Gibb’s free energy of aggregation, ∆ infact represents the Gibb’s free

energy change for the transfer of one mole of amphiphile from the aqueous phase to

micellar pseudo phase and is calculated from the equation, [59,80]:

∆G = (1+β) RT ln XCAC (2)

where β = ionization degree of the aggregates, XCAC is the critical aggregation

concentration expressed in the mole fraction scale. The values of β are obtained by the

least square analysis of the two linear fragments of specific conductivity versus conc.

isotherms. The ratio of the slope of the linear fragments above and below the CAC

determines the degree of ionizations (β) of the aggregates. The values of various

thermodynamic parameters for a series of [Cnpy][Cl] ILs at T = (298.15 to 318.15) K

and a comparison of the same with the other mim based ILs and conventional ionic

surfactants are listed in the Table 3A. 7 and 8 respectively. The ∆ contains

contributions from the transfer of the IL segments from bulk water to the aggregates

[59] and can be divided in to contribution from terminal CH3 group of the alkyl chain

(∆ -CH3), the methylene group of the alkyl chain (nCH2∆ .2

), and the head group

(∆ head group), i.e.

∆ = ∆

head group + ∆ -CH3 + nCH2∆

-CH2 (3)

The individual contributions can be easily obtained by using the above linear

relationship and plotting ∆ values versus and nCH2. The intercept would be equal to

∆ head group + ∆

-CH3 = -7.77 kJ.mol-1 and slope = ∆ -CH2 = -2.01 kJ.mol-1 at T =

298.15 K. These values are comparable to those of alkyl-3-methylimidazolium

134

chlorides, or alkylmethylmorpholinium chlorides, or alkylmethylpiperidinium

chlorides or alkyl 4-methylpyridinium chlorides (-9.295 and -1.895; -8.142 and -

2.070; -6.828 and -2.114; -5.539 and -2.132) respectively [81]. The CAC values and

hence ∆ are very sensitive to the nature of anion Wang et al [59] had reported the

values of ∆ head group + ∆

-CH3 = -3.7 kJ.mol-1 and ∆ -CH2 = -3.0 kJ.mol-1 at

298.15 K for a series of ILs based on [Cnmim][Br], n = 4-12. Similar data for n-

alkylpyridinium bromides in water at 298.15 K were -2.957 and 25.317 and -2.805

and 24.255 for alkyltetraammonium bromides [53]. Close scrutiny of data presented

in Table 3A.8 reveals that, ∆ -CH2 is close in value for homologous amphiphiles

with same anion but different cations and hence it can be stated that the transfer of a

methylene group from bulk solution to the aggregates is independent of the structure

and nature cation. The negative values of ∆ at three different temperatures

increased with the increase in the alkyl chain length, due to enhanced hydrophobic

interactions among the hydrocarbon tails that facilitate easy formation of aggregates.

∆ for [Cnpy][Cl] and [Cnmim][Cl] ILs is comparable but more negative than for

alkylammonium bromides and also for alkylpyridinium bromides. Therefore it can be

stated that ∆ is highly dependent not only on the nature of the anion but also

cationic head group. The thermodynamic properties of especially surface active

agents are often derived from the measurements of the variation of the CMC with

temperatures. The enthalpies obtained from these indirect methods have an intrinsic

large uncertainty [82-84]. Therefore researchers have been using isothermal

calorimetry as one of the most sensitive techniques for measuring directly and

precisely, the thermodynamic properties (especially entropy of aggregation) [83-88].

The ∆ for series of ILs based on [Cnpy][Cl] along with the literature data for other

similar systems is listed in Table 3A.9. The ∆ values are in general negative and

135

become more negative with increase in the carbon chain length of the alkyl tail. This

trend is similar to other class of ILs, based on similar alkyl chain and counter anion

but different cationic head group. Among the ILs, the ∆ values with py or mim, or

mmor head groups, are close in magnitude. The negative enthalpy indicates that the

energy is released from the system (exothermic process) in the form of heat. Such

exothermic effects can be due to not only enhanced but also dominant role of

hydrophobic interaction among long hydrocarbon chain based tails. Moreover, since

overall ∆ for the systems under study are negative and therefore negative enthalpic

contributions to ∆ are substantial. Interestingly, the ∆

values for alkyl

methylpyrrolidinium or 4-methylpyridinium cation based ILs are less negative as

compared to [Cnpy][Cl] ILs. This indicates besides the hydrophobic interactions

among hydrocarbon chains, the hydrophilicity of head group also contribute to the

overall magnitude of ∆. Introduction of an anion of relatively less hydrophobicity,

such as Br-, or I-, would increase their condensation on the charged surface and

neutralize the cationic charge partially. This causes less repulsions among cationic

head groups and promote more hydrophobic interactions and hence more negative

∆ (please see and compare the data from columns 1 & 2 with the last two column

of Table 3A.9).

136

Table 3A.7 Degree of counter ion binding parameter, β, Gibbs free energy, ∆G, standard enthalpy, ∆H

and standard entropy, ∆S of

aggregation for alkylpyridinium chloride based amphiphilic ionic liquids at different temperatures

T / K β ∆G

kJmol-1 ∆H

kJmol-1

∆S

Jmol-1 K-1

Exp. Lit. Exp. Lit. Exp. Lit. Exp. [C10py][Cl]

298.15 0.484 -26.3 ± 0.2 -3.95 ± 0.1 74.8 ± 1.0 308.15 0.517 -27.6 ± 0.2 -4.31 ± 0.1 75.6 ± 1.0 318.15 0.541 -28.8 ± 0.2 -4.67 ± 0.1 75.8 ± 1.0

[C12py][Cl] 298.15 0.456 0.40 [51] -29.1 ± 0.2 37.0 [69] -2.58 ± 0.1 -2.0 [69] 89.1 ± 1.0 308.15 0.471 0.45 [51] -29.9 ± 0.2 -2.70 ± 0.1 89.7 ± 1.0 318.15 0.508 0.50 [51] -31.6 ± 0.2 -2.95 ± 0.1 91.3 ± 1.0

[C14py][Cl] 298.15 0.443 -34.1 ± 0.1 -5.44 ± 0.1 96.1 ± 1.0 308.15 0.452 -34.7 ± 0.1 -5.66 ± 0.1 96.0 ± 1.0 318.15 0.475 -36.2 ± 0.1 -6.13 ± 0.1 91.2 ± 1.0

[C16py][Cl] 298.15 0.399 0.44 [70]

044 [51] -37.6 ± 0.2 -42.0 [70]

-41.8 [50] -7.24 ± 0.1 -4.5 [50] 101.9 ± 0.5

308.15 0.406 0.46 [51] -38.3 ± 0.2 -42.1 [50] -7.52 ± 0.1 -7.8 [50] 101.6 ± 0.5 318.15 0.446 -40.4 ± 0.2 -41.4 [50] -8.20 ± 0.1 102.8 ± 0.5

[C18py][Cl] 298.15 0.380 -42.1 ± 0.3 -2.04 ± 0.1 134.4 ± 0.5 308.15 0.384 -42.9 ± 0.3 -2.11 ± 0.1 134.5 ± 0.5 318.15 0.401 -44.8 ± 0.3 -2.28 ± 0.1 135.8 ± 0.5

137

Table 3A.8 Comparison of Gibbs free energy, ∆G, (in kJ.mol-1) of alkyl (pyridinium, imidazolium, morpholinium, piperidinium, pyrrolidinium

and γ- picolinium) chlorides, alkyl (trimethylammonium and pyridinium ) bromides based amphiphiles at air / water interface at

298.15 K

Alkylchain

(Cn)

[Cnpy][Cl] [Cnmim][Cl] [Cnmmor][Cl] [Cnmpip][Cl] [Cnmpyrr][Cl] [Cn-4mpy][Cl] CnTAB [Cnpy][Br]

Exp. Lit. Lit. Lit. Lit. Lit. Lit. Lit.

C10 -26.3 - 25.6 [75] - - - - - -

C12 -29.1 - 30.7 [75] -30.7 [81] -30.0 [81] -30.8 [81] -28.8 [81] -6.58 (300C) [53] -7.33 (300C) [53]

C14 -34.1 - - - - - -12.25 (300C) [53] -12.90 (300C) [53]

C16 -37.6 - 38.4 [75] -39.1 [81] -38.8 [81] -39.3 [81] -38.1 [81] -17.80 (300C) [53] -19.16 (300C) [53]

C18 -42.1 - 40.7 [75] -42.9 [81] -42.6 [81] -45.8 [81] -41.4 [81] - -

138

Table 3A.9 Comparison of standard enthalpy, ∆H, (in kJ.mol-1) of alkyl (pyridinium, imidazolium, morpholinium, piperidinium or

pyrrolidinium and γ- picolinium) chlorides, alkyl (trimethylammonium and pyridinium) bromides based amphiphiles at air / water

interface at 298.15 K

Alkylchain

(Cn)

[Cnpy][Cl] [Cnmim][Cl] [Cnmmor][Cl] [Cnmpip][Cl] [Cnmpyrr][Cl] [Cn-4mpy][Cl] CnTAB [Cnpy][Br]

Exp. Lit. Lit. Lit. Lit. Lit. Lit. Lit.

C10 -3.95 -1.70 [75] - - - - - -

C12 -2.58 -2.62 [75] -0.58 [81] -1.32 [81] -0.40 [81] -1.06 [81] -2.73 (300C) [53] -2.11 (300C) [53]

C14 -5.44 - - - - - -9.45 (300C) [53] -2.74 (300C) [53]

C16 -7.24 -7.15 [75] -5.09 [81] -1.56 [81] -1.58 [81] -1.59 [81] -9.26 (300C) [53] -8.04 (300C) [53]

C18 -8.53 -7.50 [75] -7.76 [81] -1.99 [81] -2.51 [81] -2.27 [81] - -

139

The hydrophobic concentrations to the overall negative ∆ , is also often

adjudged from the entropy of aggregation, ∆, defined as ∆

= 1/T (∆ − ∆

).

A perusal of data from the last column of Table 3A.7 and Table 3A.10, clearly

indicates that ∆ is largely positive and increases with increase in the carbon chain

length of alkyl chain, irrespective of the nature of the cationic ring or anion. The high

positive ∆ can be related to gain of kinetic energy by the water molecules, as a

result of breaking of water clusters surrounding the hydrophobic alkyl chains in the

aggregate environment. The large and positive ∆, contributions predominantly

contributes to the large and negative ∆ . Therefore it is concluded that the process of

aggregation of amphiphiles in general is driven by entropic contributions, an effect

common among ILs with different head groups or anions. However, for the AILs of

type [Cnpy][Cl], both the enthalpic and entropic contributions are important for the

aggregation process.

The fraction of counter anions condensed on the charged aggregate surface is

estimated in terms of β values, calculated as β = (1-α), with α obtained from

conductivity measurements, as described earlier. The values of β for [Cnpy][Cl] ILs

and also their comparison among of AILs /amphiphiles with different head groups are

listed in Table 3A.7 and 3A.11. A perusal of data shows that β values decreases with

the increase in the alkyl chain length at three different temperatures and while for a

given AIL, β values increase with increase in temperature. This phenomenon is found

to similar to that of other ILs and surfactants. The results reflect that the binding of

counter Cl⊝ or B⊝ ions is easier and facilitated by the longer alkyl chains, probably

due to the close proximity of head groups in the aggregates. The increase in

temperature increases the thermal motion of anions and hence their binding to cations

is decreased as indicated by increased β values (i.e. increased ionization degree).

140

Table 3A.10 Comparison of standard entropy, ∆S, (in J.mol-1K-1) of alkyl (pyridinium, imidazolium, Morpholinium, Piperidinium,

Pyrrolidinium and γ- picolinium) chlorides, alkyl (trimethylammonium and pyridinium) bromides based amphiphiles at air /

water interface at 298.15 K

Alkylchain

(Cn)

[Cnpy][Cl] [Cnmim][Cl] [Cnmmor][Cl] [Cnmpip][Cl] [Cnmpyrr][Cl] [Cn-4mpy][Cl] CnTAB [Cnpy][Br]

Exp. Lit. Lit. Lit. Lit. Lit. Lit.(kJmol-1) Lit.( kJmol-1)

C10 74.8 80.2 [75] - - - - - -

C12 89.1 94.0 [75] 101.0 [81] 93.1 [81] 102.2 [81] 93.1 [81] 0.013 (300C) [53] 0.017 (300C) [53]

C14 96.1 - - - - - 0.009 (300C) [53] 0.034 (300C) [53]

C16 101.6 105.0 [75] 114.3 [81] 124.9 [81] 126.5 [81] 122.4 [81] 0.028 (300C) [53] 0.038 (300C) [53]

C18 134.4 111.4 [75] 118.0 [81] 139.8 [81] 145.3 [81] 131.4 [81] - -

141

Table 3A.11 Comparison of counter ion binding parameter, β, of alkyl (pyridinium, imidazolium, Morpholinium, Piperidinium, pyrrolidinium

and γ- picolinium) chlorides, alkyl (trimethylammonium and pyridinium) bromides based amphiphiles at air / water interface at

298.15 K

Alkylchain

(Cn)

[Cnpy][Cl] [Cnmim][Cl] [Cnmmor][Cl] [Cnmpip][Cl] [Cnmpyrr][Cl] [Cn-4mpy][Cl] CnTAB [Cnpy][Br]

Exp. Lit. Lit. Lit. Lit. Lit. Lit. Lit.

C10 0.484 0.490 [75] - - - - - -

C12 0.456 0.560 [75] 0.58 [81] 0.53 [81] 0.55 [81] 0.44 [81] 0.278 (300C) [53] 0.315 (300C) [53]

C14 0.443 - - - - - 0.239 (300C) [53] 0.272 (300C) [53]

C16 0.399 0.450 [75] 0.53 [81] 0.51 [81] 0.53 [81] 0.48 [81] 0.290 (300C) [53] 0.232 (300C) [53]

C18 0.380 0.399 [75] 0.50 [81] 0.49 [81] 0.58 [81] 0.43 [81] - -

142

3A. 2. 4 Geometrical features of aggregates: Small angle neutron scattering (SANS)

SANS is now a very standard technique used to determine the size, shape and

polydispersity of aggregates in solution [89,90]. In a SANS experiment, the intensity

of a scattered neutron beam with wave length λ is measured as a function of the wave

vector transfer or a scattering vector Q, where Q = 4π sin (θ/2)λ, where θ is the angle

between the straight-through direction and the scattered direction. The details of the

technique and instrument parameters are given in Chapter 2. AILs being structured

liquids, the interest about the organization of ILs in aqueous media has been

emerging. As far as initial SANS measurements on ILs in aqueous solutions are

concerned, [C4mim][BF4] aqueous solutions were first investigated [66,91]. It was

observed that significant SANS curves were detected, especially around XIL = 0.075

and this scattering was attributed to the heterogeneous mixing in the system. Since,

the solution structures in IL/water systems depend on the several factors such as the

length of the alkyl chain within the cation, combination of cation/anion and

hydrophobic/hydrophilic balance etc, SANS experiments were also extended to higher

alkyl chain based ILs in water [92-94]. It was reported that IL cations with relatively

long alkyl chain, i.e. n ≥ 6 showed self organization into micelles analogous to

cationic surfactants in water. Vaghela et al [67] have also concluded that nano scale

aggregates or micelles of ellipsoidal shape are formed by [Cnmim][X -], where n = 4,

6, or 8 and X- = Cl-, Br-, or I-, when the Cn ≥ 6, while no aggregates were detectable

with ILs of C4 chain. Goodchild et al [95] investigated the aqueous solutions of

[Cnmim+][Br -] (n = 2, 4, 6, 8 and 10) using SANS measurements and the authors

proposed the formation of spherical micelles when n = 8 and 10. Bowers et al [66]

have probed the aggregates structure of 1-alkyl-3-methylimidazolium based ionic

liquids in aqueous solutions by SANS from the view point of alkyl chain length, n,

143

anions (Cl-, Br- and trifluoromethane sulfonate, CF3SO3-). The authors also did not

notice any significant SANS profile for ILs with Cn= 4, in contrary to significant

SANS profiles shown by [C4mim+][BF4-] in aqueous solutions [90]. These

observations suggest that [C4mim][Br] or [Cl] or [CF3SO3] mixes homogeneously

with water, unlike [C4mim][BF4] system, due to preferential hydration. The authors

further reported that, the aqueous [C12mim][Br] solutions showed typical SANS

curves, that can be model fitted to a nano scale micellar structures of ellipsoidal

shape. Sastry et al [75] have also investigated the effect of chain length and head

group on the aggregation behavior of [Cnmim][Cl], n= 10, 12, 14, 16 or 18,

[C16mpip][Cl], and [C16mpyrr][Cl] in aqueous solutions using SANS method. The ILs

formed close nano aggregates of dimensions typical of surfactant micelles. The

increase in alkyl chain length facilitate the parallel alignment of chains and increase

the aggregation number of ellipsoidal micelles. The head groups consisting of cations

of rings with π electrons pack closely as compared to the rings with point charge.

In the present section, we describe below the SANS curves obtained for a

series of IL based on [Cnpy][Cl] structure, where n = 10, 12, 14, 16 and 18 in D2O

solutions at T = 303.15 K. The qualitative analysis of SANS data takes in to account

the calculation single particle form factor, F(Q) and inter particle structure factor,

S(Q), after giving allowance to the probable shape of the micelle-like aggregates. In

the analytical program, the scattering length and volume of hydrophobic part are

required and the calculated values of the same for the ILs are summarized in Table

3A.12. Besides this data, the trial values for the size related parameters and fractional

charge on the aggregate surface are feeded as input parameters.

144

Table 3A.12 Scattering length densities (SLD) and volumes of tail and head group of ionic liquids (used in SANS data analysis)

IL SLD of molecule Volume of single molecule

Alkyl chain

(core) × 10-10

cm-2

Head group

(shell) × 10-10

cm-2

Alkyl chain

(core)

Å3

Head group

(shell)

Å3

[C10py][Cl] -0.407 1.78 296.4 134.9

[C12py][Cl] -0.392 1.78 350.2 134.9

[C14py][Cl] -0.382 1.78 404.0 134.9

[C16py][Cl] -0.373 1.78 457.8 134.9

[C18py][Cl] -0.366 1.78 511.6 134.9

The SANS curves typically consists of plots of coherent differential cross

section, dΣ/dQ versus scattering wave vector, Q. The SANS curves for amphiphilic

ionic liquids with different lengths of carbon chain and at different concentrations of a

given AIL in water are shown in Fig. 3A.6. The theoretical scattering functions (solid

lines) for AIL aqueous solutions were obtained by the analysis based on modified

Hayter-Penfold type for the colloidal solutions of an ellipsoidal shape [96,97]. The

detailes of analysis and analytical relations employed have already been discussed in

experimental section. It is found that the oblate core-shell model F(Q) in combination

with the Hayter-Penfold charge sphere, S(Q) fitted the experimental data (shown as

points). The results of the analysis directly gave the semi major, or semi minor axis,

thickness of the shell and fractional charge of the aggregates. By our procedure, we

have kept the fitting parameters to a minimum of four for better statistics. Thus

extracted micellar parameters for series of ILs at different concentration at T = 303.15

K are summarized in Table 3A.13.

145

0.0 0.1 0.2 0.3 0.40

1

2

3

4

0.0 0.1 0.2 0.3 0.40

1

2

3

4

5

6

7

0.0 0.1 0.2 0.3 0.40

2

4

6

8

0.0 0.1 0.2 0.3 0.40

2

4

6

8

0.0 0.1 0.2 0.3 0.40

2

4

6

8

10

12

a

b

dΣ/d

Ω, c

m-1

C

Q , Å-1

d

Q , Å-1

e

Fig. 3A.6 Concentration dependence of SANS distribution curves and model fits (Ellipsoidal core-shell model) for alkylpyridinium chloride based AILs (in mMdm-3) in D2O at 303.15 K: (a) [C10py][Cl] : (∆∆∆∆) 100, (οοοο) 400, () 800; (b) [C12py][Cl] : (∆∆∆∆) 100, (οοοο) 300, () 600; (c) [C14py][Cl] : (∆∆∆∆) 50, (οοοο) 100, () 300; (d) [C16py][Cl] : (∆∆∆∆) 25, (οοοο) 50, () 100; (e) [C18py][Cl] : (∆∆∆∆) 25, (οοοο) 50, () 100. The lines are model fits and the symbols are data points

146

Table 3A.13 Semi major axis, b, semi minor axis, a, shell thickness, t, axial ratio, b/a, fractional charge, α and aggregation number, N, water

fraction of the micellar associates (ellipsoidal core-shell type) of alkylpyridinium chlorides based amphiphilic ionic liquids in D2O

at 303.15 K

Conc. Semi major axis, b

Semi minor axis, a

Shell thickness, t

Axial ratio

Fractional Charge, α

Aggregation number

Water fraction

mMdm-3 Å Å Å b/a N [C10py][Cl]

100 24.9 ± 1.0 8.1 ± 0.2 3.3 3.1 0.21 ± 0.02 23 0.63 400 27.9 ± 1.0 9.7 ± 0.3 3.4 2.9 0.28 ± 0.02 37 0.56 800 31.8 ± 1.0 10.6 ± 0.2 3.5 3.0 0.33 ± 0.02 50 0.53

[C12py][Cl] 100 33.4 ± 1.0 10.2 ± 0.3 3.6 3.2 0.31 ± 0.02 41 0.62 300 37.1 ± 1.1 11.8 ± 0.3 3.6 3.1 0.32 ± 0.02 62 0.55 600 39.8 ± 1.5 13.2 ± 0.4 3.8 3.0 0.36 ± 0.02 82 0.53

[C14py][Cl] 50 32.6 ± 2.1 14.6 ± 0.5 3.7 2.2 0.25 ± 0.02 72 0.55 100 34.8 ± 2.2 15.1 ± 0.5 3.8 2.3 0.23 ± 0.02 82 0.54 300 36.3 ± 2.2 16.8 ± 0.5 3.7 2.2 0.22 ±0.02 105 0.48

[C16py][Cl] 25 34.7 ± 2.7 16.2 ± 0.4 3.7 2.1 0.19 ± 0.01 83 0.56 50 36.1 ± 2.8 17.1 ± 0.4 3.8 2.1 0.21 ± 0.01 96 0.55 100 37.7 ± 2.9 17.9 ± 0.5 3.8 2.2 0.22 ± 0.01 110 0.49

[C18py][Cl] 25 40.3 ± 2.9 19.2 ± 0.5 3.7 2.1 0.14 ± 0.01 121 0.52 50 40.6 ± 3.1 19.8 ± 0.5 3.8 2.1 0.16 ± 0.01 130 0.52 100 41.9 ± 3.2 20.4 ± 0.7 3.9 2.5 0.17 ± 0.01 142 0.51

147

The semi major and minor axes increase with increase in concentration of AILs. The

increase is pronounced in aggregates of AILs of C10, C12 and C14 chains and is

minimal for aggregates of ILs with C16 or C18 chains. The maximum length of semi

minor axis of the aggregates can be equated to the length of the fully extended (or all

transform) form of respective alkyl chain, lextend , as estimated by using Tanford

formula [98], 1.54 + 1.265 nc, where nc is the number of carbon atoms in the alkyl

chain. We estimate the lextend values for the decyl, dodecyl, tetradecyl, hexadecyl and

octadecyl chains within [Cnpy][Cl] to be 14.2, 16.7, 19.3, 21.8 and 24.31 Å

respectively. The concentration averaged values of a for aggregates of AILs are: 9.5,

11.7, 15.5, 17.06 and 19.8 Å respectively for [Cnpy][Cl] ILs, with n=10, 12, 14, 16

and 18. These values are too small than the actual ‘a’ values of aggregates extracted

from SANS data.

The fully extended form of alkyl chains within the core of the aggregates is

possible only, when it is completely dry and consists entirely of hydrophobic alkyl

chains only. However, our data analysis procedure enabled us to calculate the water

fraction of the aggregates and same are listed in last column of the Table 3A.13. The

water of hydration is held mostly by the hydropylic cationic head group, the

percolation of water into the interior of aggregates cannot be ruled out. Therefore, the

alkyl chains of ILs would prefer a zig-zag or not fully extended conformation and

hence the calculated values for semi-minor axis are justified considering the hydration

effect and also the size of the head group. The shape of the aggregates is ellipsoidal

but with core-shell structure. The shell consists of hydrophilic head group and

associated water. The value of shell thickness as expected to close to each other,

irrespective of the alkyl chain length. The axial ratio values, measure the asymmetry

of aggregates and the same decreases by factor of almost one unit in AILs with C16 or

148

C18 chains. Similarly, the water fraction gets decreased with the chain length of alkyl

tails. Therefore one expects that the individual molecules of AILs (with C16 or C18

chains) pack parallel to each other in the aggregates and the same is reflected in the

increased aggregation numbers, with the increase in the alkyl chain length. A perusal

of variation of aggregation number (i.e. number of IL molecules in an aggregate) for a

given IL at different concentrations, a low increase with increase in concentration.

This indicates that more and more IL molecules get into the aggregate structures with

the increase in the concentration. Similarly, the volume of the aggregates i.e. ba2 show

a continuous increase with the concentration of respective IL (Fig. 3A.7).

0 200 400 600 800 10000

4000

8000

12000

16000

20000

240008 10 12 14 16 18 20

ba2 ,

Å3

Conc., mM.dm-3

nc

Fig. 3A.7 Effect of concentration and number of carbon atoms (nc) of the alkyl chain on the micellar volume for 100 mMdm-3 of alkylpyridinium chloride based ionic liquids in D2O at 303.15 K : (∆∆∆∆) [C10Py][Cl], () [C12Py][Cl], () [C14Py][Cl], (♦) [C16Py][Cl], () [C18Py][Cl] ; () 100 mMdm-3 [CnPy][Cl] where n= 10, 12, 14, 16, 18

149

3A. 2. 5 Static Light Scattering Measurements (SLS):

Light scattering techniques have been traditionally used to characterize the

equilibrium properties of colloidal and micellar solutions. Since, the change in

intensity of scattered light and its angular distribution can easily be monitored from

very dilute to dilute solutions, light scattering methods, both in static and dynamic

modes have been developed to study the micellar characteristics of surfactants in

aqueous solutions [99-102]. SLS measurements are particularly important and

interesting because one can obtain critical micelle concentration, micellar molar mass,

radius of gyration and mass averaged aggregation numbers of micelles or aggregates

[99, 103-109]. The basis of Debye and Zimm plots for treating static light scattering

data has been presented in Chapter 2.

Debye method, also known as disymmetry method, requires measurements of

the scattered intensity at different angles for single concentrations (five times more

than the CAC of respective IL). In this method, Kc/Rθ vs sin2(θ/2) plots are generated

and they adhere to a linear relation as per the theoretical formalism. Such linear plots

for AILs of type, [Cnpy][Cl], where n = 12, 14, 16 and 18, in aqueous solutions are

shown in Fig. 3A.8. From the extrapolated Kc/Rθ value to zero angle and slope of the

straight line, mass and rms radius of the aggregates are obtained. The value of rms

radius depends on the P(θ), the particle scattering factor, which in turn is a

characteristic of shape of the aggregates. Therefore, the radius value obtained includes

a presumption about the shape of the aggregates. This is considered to be one of the

disadvantages of Debye plots. Zimm method offers a solution to this problem and it is

based on double extrapolation procedure. Kc/Rθ vs sin2(θ/2) plots are constructed to

their concentrations and angular dependence followed by simultaneous extrapolation

to zero angle and concentration. The Zimm plots for IL aqueous solutions are shown

150

in Fig. 3A.9. The data of properties of aggregates of AILs, as extracted from both the

methods is given Table 3A.14. The radius of gyration, weight averaged aggregate

mass and aggregation numbers increase with the number of carbon atoms in the alkyl

chain. Zimm method, based on double extrapolation procedure gives high aggregation

numbers than Debye method. In the Debye method, the scattering data is treated in

terms of zero value for the second virial coefficient, A2 and while the value of A2 can

be obtained from Zimm plots. The A2 values are positive and small. The positive

values of A2 suggest favorable solute or IL---water interactions and the value of A2

becomes less positive with the carbon chain length and this trend is not unexpected

and can be attributed to the increased hydrophobicity of longer hydrocarbon chains.

151

0.0 0.2 0.4 0.6 0.8 1.0 1.29.62

9.64

9.66

9.68

9.70

0.0 0.2 0.4 0.6 0.8 1.0 1.26.71

6.72

6.73

6.74

0.0 0.2 0.4 0.6 0.8 1.0 1.22.72

2.73

2.74

2.75

2.76

2.77

0.0 0.2 0.4 0.6 0.8 1.0 1.23.09

3.10

3.11

3.12

K*c

/R (

θ) ×

105

a

b

sin2(θ/2)

c

d

Fig. 3A.8 Angular dependence of light scattering from alkylpyridinium chloride based ionic liquids aqueous solutions at 298.15 K.: (a) [C12py][Cl], (b) [C14py][Cl], (c) [C16py][Cl], (d) [C18py][Cl]

152

4

6

8

10

12

1

2

3

4

5

0.0 0.4 0.8 1.2 1.6 2.01

2

3

4

sin2(θ / 2) + 75.5*c

a

K*c

/R (

θ) ×

105

sin2(θ/2) + 107*C

b

sin2(θ / 2) + 186*C

c

Fig. 3A.9 Concentration and angular dependence of light scattering (Zimm style plots) from alkylpyridinium chloride based ionic liquids aqueous solutions at 298.15 K: (a) [C14py][Cl], (b) [C16py][Cl] and (c) [C18py][Cl]

153

Table 3A.14 Specific refractive index (dn/dc), weight average aggregate weight (a.w.), root mean square radius (rrms),

aggregation number (N) and second virial coefficient, (A2) from static light scattering measurements on

aggregates of alkylpyridinium chloride based ionic liquids in water at 298.15 K

Single concentration

(Extrapolation to zero angle)

Multiple concentration

(Double extrapolation to zero angle and zero conc.)

AIL dn/dc

cm3/g

M m

g/mol

rrms

nm

Nagg M m

g/mol

A2

mol.cm3/g2

Nagg

[C12py][Cl] 0.167 (1.128 ± 0.001) ×104 3.1 ± 0.1 40 - - -

[C14py][Cl] 0.156 (1.625 ± 0.001) ×104 3.6 ± 0.1 52 (1.769 ± 0.074) ×104 (1.861 ± 0.026) ×10-3 57

[C16py][Cl] 0.144 (3.559 ± 0.002) ×104 3.7 ± 0.1 105 (4.345 ± 0.046) ×104 (1.040 ± 0.033) ×10-3 128

[C18py][Cl] 0.145 (4.121 ± 0.002) ×104 4.3 ± 0.1 102 (4.912 ± 0.095) ×104 (0.940 ± 0.050) ×10-3 133

154

3A. 2. 6 Steady State Fluorescence Quenching:

Fluorescence quenching techniques has been successfully applied to determine

the aggregation number of aggregates of IL/surfactants in aqueous solutions [78, 110-

113]. It is preassumed that the equilibrium distribution of the fluorescence probe

between the aqueous and aggregate phases follows the poisson distribution and the

equation to be applied is [110,114].

Ln I = ln Io – Cq/Ca = ln I0 – N Cq / (Ct - CAC) (4)

where, Cq, Ca and Ct are the concentrations of the quencher, aggregate and total ILs,

respectively, while I and I0 are the fluorescence intensities with and without the

quencher respectively. Pyrene is a strongly hydrophobic probe and its fluorescence

emission spectrum exhibits characteristic five bands in the wave length region of 370

to 425 nm, as shown in parts a-e of Fig. 3A.10. It is known that pyrene preferentially

dissolves in to hydrophobic region of IL aggregates. The first band is known as O-O

band, peak 1 may be enhanced in polar micro environment, due to vibronic coupling.

The third band is not sensitive to the surrounding microenvironment. The ratio of the

intensities of the first to third band (I1/I3) is not only useful to probe the micropolarity

but also yield characteristic break points, corresponding to CAC, as already discussed

in previous sections of the present chapter. The emission spectra of pyrene in aqueous

solutions of AILs in the presence of quencher, benzophenone, are shown in Fig.

3A.11. It can be seen that the characteristic vibronic peaks of pyrene are gradually

depressed by benzophenone. The plots of ln (I1/I0) versus quencher concentration are

displayed in Fig. 3A.12. The aggregation numbers calculated from the slopes of the

linear plots are listed in Table 3A.18.

155

340 360 380 400 420 440 4600

100

200

300

400

500

600

700

800

340 360 380 400 420 440 4600

100

200

300

400

500

600

700

800

340 360 380 400 420 440 4600

200

400

600

800

1000

1200

340 360 380 400 420 440 4600

100

200

300

400

500

600

340 360 380 400 420 440 4600

100

200

300

400

500

600

700

800........ Above cmc 18 mM.dm-3

Below cmc 0.5 mM.dm-3

........ Above cmc 50 mM.dm-3

Below cmc 5 mM.dm-3

I, a

.u.

a

........ Above cmc 0.26 mM.dm-3

Below cmc 0.01 mM.dm-3

........ Above cmc 3 mM.dm-3

Below cmc 0.01 mM.dm-3........ Above cmc 5 mM.dm-3

Below cmc 0.5mM.dm-3c

λ ,nm

d

e

λ, nm

b

Fig. 3A.10 Representative pyrene fluorescence emission spectra in aqueous solutions of ILs: (a) [C10py][Cl], (b) [C12py][Cl], (c) [C14py][Cl], (d) [C16py][Cl], (e) [C18py][Cl] corresponding to the concentrations at the below and above CAC. Excitation wavelength: 335 nm pyrene concentration 2 ×××× 10-6 mol.dm-3

156

340 360 380 400 420 440 4600

100

200

300

400

500

600

340 360 380 400 420 440 4600

100

200

300

400

500

600

340 360 380 400 420 440 4600

100

200

300

400

500

340 360 380 400 420 440 4600

100

200

300

400

500

600

340 360 380 400 420 440 4600

100

200

300

400

I, a.

u.

a

0.0 mM.dm-3

0.2 mM.dm-3

[Q]

b

0.0 mM.dm-3

0.7 mM.dm-3

[Q]

c

0.0 mM.dm-3

0.7 mM.dm-3

[Q]

λ, nm

d

0.0 mM.dm-3

0.2 mM.dm-3

[Q]

e

λ, nm

0.0 mM.dm-3

0.1 mM.dm-3

[Q]

Fig. 3A.11 Changes in the pyrene fluorescence emission spectra as a function of the concentration of quencher, benzophenone IL aqueous solutions: (a) [C10py][Cl], (b) [C12py][Cl], (c) [C14py][Cl], (d) [C16py][Cl], (e) [C18py][Cl]

157

0.0000 0.0002 0.0004 0.0006 0.0008-4

-3

-2

-1

0

ln [I

1/I 0]

[Quencher], mol⋅ dm-3

Fig. 3A.12 Linear plots of ln(I/I0) for 1 ×××× 10-6 mol.dm-3 pyrene in aqueous solution of alkylpyridinium chloride based ionic liquids aqueous solutions as a function of the benzophenone concentration as quencher at 298.15 K : () [C10py][Cl], () [C12py][Cl], ()[C14py][Cl], () [C16py][Cl], () [C18py][Cl]

3A. 2. 7 Aggregation Numbers from Densities and Molar Volumes:

The basis of utilizing densities of apparent molar volumes of aqueous

solutions of ILs has already been discussed in chapter 2. The data of experimental

densities, ρ, speed of sound, υ, isentropic compressibility, кs for aqueous solutions of

[Cnpy][Cl] ILs at T = 298.15 K are listed in Table 3A.15.

158

Table 3A.15 Density (ρ), speed of sound (υ) and isentropic compressibility (κs) of aqueous solutions of alkylpyridinium chloride based

amphiphilic ionic liquids at 298.15 K

Molality, m

mol⋅kg-1

ρ, gm⋅cm-3

υ m.s-1

κs TPa-1

Molality, mol⋅kg-1

ρ, gm⋅cm-3

υ, m.s-1

κs, TPa-1

[C10py][Cl] [C12py][Cl] 4.9852 0.995675 1510.00 440.482 0.9970 0.995652 1508.99 441.082 9.9704 0.995692 1511.20 439.776 1.9941 0.995656 1509.22 440.949 19.9409 0.995724 1513.20 438.600 3.9882 0.995666 1509.65 440.691 29.9113 0.995752 1515.20 437.430 5.9823 0.995675 1510.06 440.447 34.8965 0.995766 1516.00 436.963 7.9763 0.995684 1510.45 440.216 39.8817 0.995778 1516.80 436.497 9.9704 0.995693 1510.82 439.998 43.8699 0.995788 1517.60 436.032 11.9645 0.995701 1511.16 439.793 47.8581 0.995797 1518.40 435.569 13.9586 0.995710 1511.49 439.601 51.8462 0.995806 1518.80 435.336 15.9527 0.995718 1511.79 439.422 55.8344 0.995814 1519.60 434.874 17.9468 0.995727 1512.07 439.257 59.8226 0.995822 1520.00 434.641 19.9409 0.995735 1512.32 439.104 64.8078 0.995831 1520.80 434.180 23.9290 0.995750 1512.77 438.836 69.7930 0.995839 1521.60 433.720 29.9113 0.995773 1513.28 438.532 79.7634 0.995854 1522.80 433.031 39.8817 0.995807 1513.68 438.283 89.7339 0.995866 1524.00 432.344

[C14py][Cl] [C16py][Cl] 0.0997 0.995651 1508.52 441.358 0.0399 0.995649 1508.48 441.384 0.4985 0.995679 1508.64 441.275 0.0598 0.995651 1508.48 441.380 0.9970 0.995713 1508.76 441.190 0.0798 0.995653 1508.49 441.377

159

1.3959 0.995739 1508.88 441.108 0.0997 0.995655 1508.49 441.373 1.7947 0.995764 1508.96 441.050 0.1994 0.995666 1508.51 441.355 2.1935 0.995789 1509.04 440.992 0.3988 0.995685 1508.56 441.321 2.5923 0.995813 1509.16 440.912 0.5982 0.995704 1508.60 441.289 2.9911 0.995836 1509.24 440.855 0.7976 0.995721 1508.64 441.259 3.3899 0.995858 1509.32 440.798 0.9970 0.995737 1508.67 441.230 3.7888 0.995879 1509.40 440.742 1.1965 0.995753 1508.71 441.203 3.9882 0.995889 1509.44 440.715 1.3959 0.995767 1508.74 441.178 4.1876 0.995900 1509.44 440.710 1.5953 0.995780 1508.77 441.155 4.3870 0.995910 1509.48 440.682 1.7947 0.995792 1508.80 441.133 4.5864 0.995919 1509.52 440.655 1.9941 0.995803 1508.82 441.113 4.9852 0.995938 1509.60 440.599

[C18py][Cl] 0.0399 0.995650 1509.16 440.984 0.0798 0.995655 1509.22 440.947 0.1196 0.995660 1509.28 440.909 0.1595 0.995665 1509.33 440.878 0.1795 0.995667 1509.36 440.860 0.1994 0.995669 1509.38 440.847 0.2193 0.995672 1509.4 440.834 0.2393 0.995674 1509.42 440.821 0.2592 0.995676 1509.44 440.809 0.2792 0.995678 1509.46 440.796 0.2991 0.995680 1509.48 440.784 0.3191 0.995682 1509.49 440.777 0.3589 0.995686 1509.52 440.758 0.3988 0.995690 1509.54 440.744

160

The apparent molar volumes (Vϕ ) for [Cnpy][Cl], n = 10, 12, 14, 16, 18 were

calculated from density data by using the equation,

VØ = M / ρ + 103 (ρo - ρ) / m⋅· ρo · ρ (5)

Where M and m are the molar mass and molality of IL and ρ and ρo are densities of

solutions and pure water respectively. Following pseudo phase model of aggregation, the

apparent molar volume may be equated to:

VØ = CAC/m · Vϕ + (m – CAC) / m · Vϕ

!"# (6)

Where Vϕ is the infinite dilution apparent molar volume of ILs in monomer form,

Vϕ!"# is the apparent molar volume of ILs in aggregate phase. In fact above equation

can be rewritten as,

VØ = Vϕ!"# + CAC / m · ( Vϕ

- Vϕ!"#) (7)

And from the thermodynamic point of view, Vϕ can be considered as the standard

partial molar volume of monomer of an IL.

The plots of apparent molar volumes against the reciprocal of molality (m-1)

for [Cnpy][Cl], n = 10, 12, 14, 16 and 18 are shown in Fig. 3A.13. It can clearly be

seen that, these plots consists of two linear regions and the intercept of the line at

concentrations above the CAC gives the value of Vϕ!"# and that of concentrations

below CAC gives the value for Vϕ. The concentration and the apparent molar volumes

at which the two lines intersect corresponds to CAC value.

161

0.00 0.05 0.10 0.15 0.20 0.25256.88

256.90

256.92

256.94

256.96

0.0 0.2 0.4 0.6 0.8 1.0 1.2285.06

285.07

285.08

285.09

285.10

285.11

285.12

0 2 4 6 8 10 12313.08

313.12

313.16

313.20

313.24

313.28

0 5 10 15 20 25341.34

341.35

341.36

341.37

341.38

0 5 10 15 20 25 30369.517

369.518

369.519

369.520

369.521

a

b

Vφ,

cm

3 mo

l-1

c

1/m, kg mol-1

d

1/m, kg mol-1

e

Fig. 3A.13 Apparent molar volume versus (1/m) plots of alkylpyridinium

chcloride based ionic liquids at 298.15 K : (a) [C10py][Cl], (b)

[C12py][Cl], (c) [C14py][Cl], (d) [C16py][Cl] and (e) [C18py][Cl]

162

From the values of the apparent molar volume in the aggregate phase and the

apparent molar volume at CAC, the change in the apparent molar volume upon

aggregation, Vϕ∙! can be calculated using the relation,

Vϕ∙! = Vϕ!"# - Vϕ∙% (8)

The values of Vϕ , Vϕ∙%, Vϕ

!"# and Vϕ∙! for [Cnpy][Cl] ILs in

aqueous solutions are summarized in Table 3A.16. The values of Vϕ∙! for

[Cnpy][Cl] (n = 10, 12, 14, 16 & 18) are small and positive. The positive values of the

transfer volumes are attributed mainly due to the release of structured water in the

hydration shell of the monomeric ILs, when the aggregates are formed [59, 115-117].

Table 3A.16 Apparent molar volume at infinite dilution (Vϕo ), , apparent molar

volume at the critical aggregation concentration (V*CAC), apparent molar

volume in the aggregation phase (Vφ,mic),the change of apparent molar

volume upon aggregation (∆V∅,!), the coefficients Bv, volume of the

hydrophobic portion of the monomer in a micelle (Vh) for the aqueous solutions of alkylpyridinium based ionic liquids at T = 298.15 K

IL 012

cm3mol-1

VφCAC

cm3mol-1

0134

cm3mol-1

∆0∅,

cm3mol-1

Bv

cm3mol-1 Vh

[C10py][Cl] 256.21 256.90 256.93 0.03 -0.21 152.47

[C12py][Cl] 285.69 285.09 285.11 0.02 -0.52 181.50

[C14py][Cl] 313.15 313.14 313.21 0.07 -1.53 210.00

[C16py][Cl] 341.38 341.35 341.37 0.02 -1.89 239.22

[C18py][Cl] 369.51 369.50 369.52 0.02 -2.53 267.86

A linear relation is also observed between Vϕ and the number of carbon atoms

in the alkyl chain of the alkylpyridinium cation: Vϕ = (115.585 ± 1.185) + (14.11 ±

163

0.081) nc with a correlation coefficient of 0.999. In the above equation, the slope

values of 14.11 ± 0.081 cm3·mol-1 represents the concentration per methylene group in the

alkyl chain of the pyridinium cation to the standard partial molar volumes of the ILs.

However, the over all nature of the aggregation process depends on the nature and size of the

head group on one hand and the type and nature of the counter ion on the other. This is

reflected by the fact that our calculated value of 14.11 cm3 · mol-1 for V5 of

alkylpyridinium cations is smaller than 15.9 cm3·mol-1 derived from homologous

[Cnmim][Br] (n = 6, 8, 10, 12) [79], 15.5 cm3·mol-1 derived from a series of homologous

alkyldimethylbenzylammonoim chlorides [118], 15.4 cm3·mol-1 for alkylpyridinium

bromides [119], 15.9 cm3·mol-1 derived from RCOONa [120] and 15.8 cm3

·mol-1 derived

from amino acids [121]. Otherwise, the value of V5 , reported in present work is within

the group addivity rules and within the limits of above considerations.

3A. 2. 8 Aggregation Numbers from Isentropic Compressibilities:

Singh and Kumar [56] have determined the aggregation number of

[C8mim][Cl] and [C8mim][BF4] in aqueous solutions from isentropic compressibility

data using the mass action law for free IL-aggregate. The detailed treatment of the

data has been discussed in chapter 2. Plots of isentropic compressibility against the

reciprocal concentration of [Cnpy][Cl] ILs in aqueous solutions at T = 298.15 K are

given in Fig. 3A.14. It can be seen that, the кs values increase linearly with the

concentration and get levels off after a certain concentration. The sudden change in

the δкs/dc or inflection points corresponding to the plots in the figure indicates

aggregate formation and the concentration corresponding to interaction relates to

CAC of given IL.

164

0.00 0.05 0.10 0.15 0.20 0.25430

432

434

436

438

440

442

0.0 0.2 0.4 0.6 0.8 1.0 1.2438

439

440

441

442

0 2 4 6 8 10 12440.4

440.6

440.8

441.0

441.2

441.4

441.6

0 5 10 15 20 25 30441.0

441.1

441.2

441.3

441.4

441.5

0 5 10 15 20 25 30440.7

440.8

440.9

441.0

441.1

a

b

κ s ,

TP

a-1

c

1/C, mM-1dm3

d

1/C, mM-1dm3

e

Fig. 3A.14 Variation of кs against the reciprocal concentration for various

alkylpyridinium chloride based ionic liquids at 298.15 K: (a) [C10py][Cl], (b) [C12py][Cl], (c) [C14py][Cl], (d) [C16py][Cl] and (e) [C18py][Cl]

165

From the values of кs in the aggregation phase, кs,agg and кs at CAC i. e. кs,CAC, the

change in кs upon aggregation can be calculated by,

∆кs,agg = кs,agg - кs,CAC (9)

The values of кs,CAC, кs,agg and ∆кs,agg are listed in Table 3A.17. The aggregation

numbers, determined from the plots of log (C1κs) versus n log [C1(κs,agg - κs)] (see

Fig. 3A.15) are given in Table 3A.18. The κs,agg values are small and positive and

decrease with increase in the number of carbon atoms, Cn of cation of [Cnpy][Cl]s,

These results indicate that the water structures around the aggregates are

compressible, due to the break down of water…..water structures (hydrophobic effect)

around the hydrocarbon chains of ILs.

In conclusion, the aggregation numbers of [Cnpy][Cl] IL aggregates in water,

as determined by scattering, or spectroscopic, or thermodynamic properties are close

to each other and the values in general increase with the increase in the carbon chain

length of the alkyl tails of cations. Therefore, it is stated that, the longer alkyl chains

pack parallel to each other within the aggregate structures.

166

Table 3A.17 Isentropic compressibility at the critical aggregation concentration (кs,CAC), isentropic compressibility in the aggregation phase (кs,agg), and the change of isentropic compressibility upon aggregation (∆кs,agg) for different ILs in aqueous solution at 298.15 K

IL кs,CAC

TPa-1

кs,agg

TPa-1

∆кs,agg

TPa-1

[C10py][Cl] 436.1 439.2 3.1

[C12py][Cl] 439.4 440.8 1.4

[C14y][Cl] 440.7 441.2 0.5

[C16py][Cl] 441.2 441.3 0.1

[C18py][Cl] 440.8 440.9 0.1

-2 0 2 4 6-5

0

5

10

15

20

log

(Ctκ

s)

log [Ct(κsagg - κs)]

Fig. 3A.15 Plots of isentropic compressibility against the log of concentration for aggregated ILs () [C10py][Cl], () [C12py][Cl], () [C14py][Cl], () [C16py][Cl] and (∆∆∆∆) [C18py][Cl]

167

Table 3A.18 Comparison of aggregation numbers (Nagg) of ILs as calculated by

different methods at room temperature

IL SANS SLS (Zimm)

double

extrapolation

Fluorescence Density

and molar

volume

Speed of sound

and isentropic

compressibilities

[C10py][Cl] 23 (100 mMdm-3) -- 50 37 34

37 (400 mMdm-3)

50 (800 mMdm-3)

[C12py][Cl] 41 (100 mMdm-3)

62 (300 mMdm-3) 40 57 50 41

82 (600 mMdm-3)

[C14py][Cl] 72 (50 mMdm-3) 57 59 65 68

82 (100 mMdm-3)

105 (300 mMdm-3)

[C16py][Cl] 83 (25 mMdm-3) 128 65 82 82

96 (50 mMdm-3)

110 (100 mMdm-3)

[C18py][Cl] 121 (25 mMdm-3) 133 95 102 103

130 (50 mMdm-3)

142 (100 mMdm-3)

168

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