i
A STUDY OF ORGANOGELS AND THEIR SOLUTE INTERACTIONS
BY
MELISSA ANN STOUFFER
A Thesis Submitted to the Graduate Faculty of
WAKE FOREST UNIVERSITY GRADUATE SCHOOL OF ARTS & SCIENCES
In Partial Fulfillment of the Requirements
For the Degree of
MASTER OF SCIENCE
In the Department of Chemistry
May 2009
Winston-Salem, North Carolina Approved By: Willie Hinze, Ph.D., Advisor ____________________________ Examining Committee: Brad Jones, Ph.D., Chair ____________________________ Christa Colyer, Ph.D. ____________________________
ii
ACKNOWLEDGMENTS
First of all, I would like to acknowledge Dr. Willie Hinze for his time and dedication
to this project. I would like to thank him for his persistence and patience throughout these
years. Without his tenacity I would have not been able to finish this thesis. I would also like
to thank Dr. Brad Jones and Dr. Christa Colyer for serving on my committee and being ready
for the examination on such short notice. Furthermore, I would like to thank the Graduate
Office for ensuring my continued enrollment when I was unable to enroll in person and Dean
Moore for granting my extension. I would also like to thank fellow students, Jen Rust and
Amanda Davis for being good friends during my time at Wake Forest University. I enjoyed
their company during my tenure in Salem Hall. I would also like to thank the TAA program
for supporting me financially during the time I attended school. Finally, I would like to thank
Performance Fibers, my current employer for allowing me to work on my thesis and to take
time off work for my defense.
iii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ……………………………………………………………….. ii TABLE OF CONTENTS ………………………………………………………………. iii LIST OF TABLES ……………………………………………………………...… v LIST OF FIGURES …………………………………………………………….… vi ABSTRACT …………………………………………………………….... vii Chapter 1 INTRODUCTION TO REVERSE MICELLES AND ORGANOGELS ........ 1
1.1 Surfactants ………………………………………………………………... 1
1.2 Normal Micelles ……………………………………………………………...… 1
1.3 Reverse Micelles ……………………………………………………………...… 3
1.4 Organogels ……………………………………………………………..... 13
1.5 Statement of Thesis Research Objectives ……………...……………………..... 21
Chapter 2 EXPERIMENTAL …………...………………………………………….…. 23
2.1 Materials ………………………………………………………………………. 23
2.2 Equipment & Instrumentation …………...…………………………………...… 24
2.3 Organogel Preparation ………………………...……………………………….. 24
2.4 Organogel Stability in Different Solvent Systems …………...………………… 26
2.5 Density Determination ………………………..………………...…………...… 27
2.6 Analyte Solution Preparation …………………………...……………………… 27 2.7 Determination of Solute – Organogel Partition Coefficients and Organogel Extraction Efficiency …………...……………………………………… 27 2.8 Determination of the Time Course for the Absorption Process …..……………. 29 2.9 Data Analysis ………………………………………………………….…… 30
iv
Chapter 3 RESULTS AND DISCUSSION……………………………………………. 31
3.1 Brief Characterization Studies ………………..……………………………...… 31
3.2 Interaction of Organic Solutes with AOT Organogels ……………………..….. 34
3.3 Extraction of Polar Analytes from Nonpolar Solvents using AOT Organogels ………………………………………………………………. 41 3.4 Conclusions ………………………………………………………………. 49 Chapter 4 FUTURE STUDIES …………………...……………………………………..…. 51 4.1 Recommendations ……...………………………………………………………. 51 REFERENCES ………………………………………………………………………. 53 VITA ………………………………………………………………………. 58
v
LIST OF TABLES
1 CMC and Aggregation Numbers of AOT in Various Solvents……………………… 5 2 Solubilities of some Compounds in Bulk Solvent and Reverse Micellar Systems…... 6
3 Results of Some Enantioselective Enzymatic Reactions in Organogels.................... 19 4 Solute in Solution Spectral Parameters…………………………………………...… 28
5 Appearance of Organogels after Exposure to Different Solvent Systems……...…… 33 6 Swelling of Organogels when in Contact with Different Media……………………. 34
7 Partition Coefficients of Solutes in AOT Reverse Micelles and AOT Organogels…. 36 8 Summary of Solutes, their AOT Organogel-Hexane Partition Coefficients and their
Abraham Solvation Descriptor Values…………………………………………….... 38
9 Interaction Coefficients for Solute Partitioning into AOT Organogel and AOT Reversed Micellar Solutions……………………………………................................ 40
10 Pseudo-Second Order Parameters for the Sorption of Solutes on AOT Organogels .. 45
11 Extraction Efficiency and Concentration Factors Achieved for the Extraction of
Analytes from Hexane with the AOT Organogel Materials….................................... 47
vi
LIST OF FIGURES
1 General Diagram of a Surfactant Molecule………………………………………….. 1
2 Simple Representation of an Aqueous Normal Micelle……………………...……… 2
3 Schematic Representation of a Reverse Micelle……………………………………... 4
4 Structure of Bis(ethylhexyl) Sodium Sulfosuccinate (AOT)…………………...……. 5
5 Schematic Representation of the Production of Long Chain Lactones from HHA Catalyzed by the Enzyme Lipase, E, in a Reverse Micellar System……………….... 8
6 The Reaction of of 1-Fluoro-2, 4-Dinitrobenzene with an Amine to Form the
Corresponding N-(2,4-Dinitrophenyl)-Piperidine……………………………........… 9
7 Schematic of Experimental DNA Amplification Procedure……………………...… 11 8 Proposed Structure of Organogel where 1 represents the W/O
Microemulsion-Type Droplets and 2 the Gelatin/Water Channels………………..... 14
9 SEM and TEM Micrographs of Organogel……………………………………….... 15 10 Photo of an AOT Organogel that had been in Contact with a 0.50 M NaCl Solution
for 7 days ………………………………………………………………………….... 32
11 Plot of the Solute – AOT Reverse Micelle Binding Constant, Kb (M-1) versus the Solute – AOT Organogel Partition Coefficient …………......…………………...… 37
12 Comparison of the Predicted and Experimentally Observed Values of the
Partition Coefficients for Solute Incorporation into AOT Organogels…………...… 41
13 Plot of the Absorbance of 2-Nitroaniline at 376 nm in Hexane versus Time (minutes) after exposure to 3.945 grams of AOT Organogel at 25.0o C………….… 42
14 Plot of the Amount of 2-NA (mmol/gram gel) Sorbed onto the AOT
Organogel as a Function of Time (minutes)…………………………………...…… 43
15 Plot of the Second-Order Sorption Model Equation (Eq.7), t/Qt as a function of Time for the Solute 2-NA………………………………………….....… 44
16 Plot of Absorbance (at 318 nm) of 4-Nitroaniline that Sorbed onto the
Organogel versus Time at 25.0o C……………………………………………….…. 46
vii
ABSTACT
A brief background on bis(2-ethylhexyl) sodium sulfosuccinate (AOT) reversed
micelles and organogels and survey of their general properties and some possible
applications is presented. The AOT organogels are characterized in terms of their relative
stability when contacted with different solvent systems (water, salt water, alcohols, alkanes).
The gels were found to retain their structure in salt (NaCl) water and nonpolar solvents
(hexane, heptanes). The density of the AOT organogel materials was found to be 1.05 g/ml.
The partition coefficients for the interaction of solute molecules (substituted phenols,
anilines, naphthols) with the organogels were found to be in the range of 1.3 to 500. These
partition coefficients are roughly 40 times less relative to the partitioning of the same solutes
to AOT reverse micelles in solution. The AOT organogels were found to function as a
sorbent for the extraction of nitroanilines, phenols, and 2-naphthol from a bulk hexane
solution. The extraction efficiencies ranged from 72 – 97% with enrichment factors from 3 –
13 depending upon the amount of AOT organogel employed. A pseudo-second order kinetic
model was found to best fit the sorption the data for the solutes examined (2- and 4-
nitroaniline, 2,4-dinitroaniline, and 2-naphthol). Finally, the Abraham solvation model was
applied and binding to the AOT organogels was found to be dominated by hydrogen bond
basicity and acidity with a secondary contribution from solute polarizability.
1
Chapter 1
INTRODUCTION
Reverse Micelles and Organogels: Brief Background Description and Applications 1.1 Surfactants
Surfactants, or surface-active agents, are molecules that have distinct polar and
nonpolar regions (Figure 1). The apolar tail of the surfactant molecule is typically a
hydrocarbon chain consisting of eight or more carbon atoms. The surfactant charge-type,
i.e., anionic, cationic, zwitterionic and nonionic, is dictated by the nature of the polar head
group.1 Surfactants are considered amphipathic molecules because of their dual nature of
polarity. Soaps and detergents are examples of surfactants. Surfactant molecules have been
employed in a variety of analytical chemistry and separation science applications.2
Figure 1: General Diagram of a Surfactant Molecule
1.2 Normal Micelles
In an aqueous environment, surfactants can form aggregates. In the interior of these
aggregates the nonpolar regions are associated together where they are shielded from the
aqueous solvent and the polar regions are oriented into the solvent.1 These aggregates,
shown in Figure 2, are termed aqueous or normal micelles.
Polar head regionNonpolar, typicallyhydrocarbon tail
2
Figure 2: Simple Representation of an Aqueous Normal Micelle
Such micellar systems are characterized by two parameters, aggregation number (N)
and critical micelle concentration (CMC). N is defined as the average number of surfactant
molecules that comprise the aggregate. This number generally ranges from 40 to 140
surfactant molecules.1 The aggregation number depends upon both the nature of the micelle
forming surfactant (hydrocarbon chain length, head group, counter ion for ionic surfactants)
and the experimental conditions (ionic strength, temperature, presence and concentration of
other additives, etc.).2
The CMC is defined as the minimum concentration of surfactant needed for
micellization. The CMC for most surfactants in pure water are in the range of 0.01 – 10 mM
depending upon the charge type and nature of the specific surfactant molecule involved. For
instance, the CMC values for nonionic surfactants are lower relative to charged surfactants
and as the alkyl chain length of the surfactant molecule increases, its CMC value decreases.1
The presence of additives in water can lead to an increase or decrease in a surfactant’s CMC
H H
O
H H
O
H H
O
H H
O
H H
O
3
value relative to that observed in pure water; i.e., the CMC value of a surfactant typically
decreases with added salt while it increases in the presence of added organic solvents (such
as an alcohol).1,2
Therefore, a surfactant’s N and CMC values are dependent upon both nature and
charge type of the surfactant and the experimental conditions. CMC values are typically
experimentally determined using surface tension, conductivity, or various spectroscopic
methods.1
1.3 Reverse Micelles
1.3.1 Description
In an organic nonpolar solvent, surfactants can also form aggregates in which the
nonpolar tail region of the surfactants are oriented toward, and in contact with bulk nonpolar
solvent while the polar moieties are located in the interior of the aggregate where they are
shielded from the bulk nonpolar solvent. These aggregates are termed reverse micelles or
inverted micelles (Figure 3) since they are inverted (or reversed) compared to the normal
aqueous micelle.3 If prepared in the presence of trace amounts of water, the reverse micelles
can contain a water pool in the interior of the micelle. These inner water molecules are
shielded from the bulk organic solvent. Depending on the amount of water present, such
systems are also referred to as water-in-oil microemulsions (w/o μE). When the system
contains just enough or less water to hydrate the hydrophilic groups of the surfactant, it is
considered a true reverse micelle.3 W/O μE’s contain more water than necessary for
surfactant head group hydration and have an increased micelle diameter and larger
aggregation numbers.3 This distinction between reverse micelles and w/o μE’s is
characterized by the water to surfactant ratio, which is typically designated as wo or Ro and is
4
equal to the concentration of water present divided by the concentration of the reverse
micelle forming surfactant . For convenience, the term reverse micelle will be typically used
although the system may actually be considered a w/o μE.
Figure 3: Schematic Representation of a Reverse Micelle
Compared to normal micelles the aggregation number and CMC are more difficult to
determine for surfactants in nonpolar solvents.1 The problem with determining the
aggregation number arises from the small number of surfactant molecules that form such
reverse micelle aggregates, about 4-20.1 Thus, aggregation numbers are typically smaller for
reverse micelles than normal micelles. The determination of CMC using surface tension is
not feasible in an organic solvent due to the small surface activity of these solutions.1 The
CMC value for a commonly used surfactant., bis(2-ethlyhexyl) sodium sulfosuccinate
(Aerosol OT or AOT), shown in Figure 4, has been determined using a positron annihilation
technique.4 The CMC value was found to be between 2.0 and 2.2 mM in benzene. CMC
H H
O
H H
O
H H
O
H H
O
5
values for AOT previously reported in the literature range from 0.4 to 2.8 mM (Table 1)
depending upon the specific solution conditions (bulk nonpolar solvent employed) and the
detection technique.3, 4 On the other hand, some have even argued against the existence of a
CMC for surfactants in non-aqueous solvents.1 Although this is an ongoing debate, newer
methods have been used to study reverse micellar formation. These include fluorescence,
dynamic light scattering, and NMR spectroscopy, among others.3
Figure 4: Structure of Bis(ethylhexyl) Sodium Sulfosuccinate (AOT)
Table 1 CMC and Aggregation Numbers of AOT in Various Solvents
Solvent CMC (mM)a Na Heptane None detected 13-16 Hexane None detected 15 Benzene 0.4 – 2 11-24
Cyclohexane 0.5 17 a Values taken from the literature1.
Reverse micelles were thought to be spherically shaped but some studies have shown
that they may actually be discs or elongated discs.3 Using dynamic light scattering
OO
S
O
OO
Na
O
O
6
measurements, it was found that the shape of AOT aggregates are dependent upon the
amount of water present, i.e., the wo value.5 When the wo value was greater than 20, the
aggregate resembles deformed discs, whereas at wo values less than 20, they are spherical in
shape.5 Thus, the amount of water present dictates the shape (and size) of reverse micelles.5
1.3.2 Selected Applications
1.3.2.1 Solubilization
One of the most basic and important applications of reverse micelles is their ability to
increase the solubility of compounds that would otherwise be insoluble or sparingly soluble
in a nonpolar solvent. Reverse micelles can solubilize alcohols, amines, alkynes and aqueous
solutions of acids, bases, and buffers.3 Table 2 compares the solubilities of some substances
in organic solvents to those in reverse micelle systems. From the data in Table 2, it can be
seen that the solubilities of these compounds are greatly improved in the reverse micellar
system. AOT is one of the most commonly employed reversed micelle forming surfactants
because it can solubilize a large amount of water relative to other surfactants.
Table 2
Solubilities of Some Compounds in Bulk Solvent and Reverse Micellar Systems
Substrate Solubility in Bulk Solvent Solubility in Reverse Micelle
Pyridine-2-azo-p-dimethylaniline
1.5 X 10-4mol/L (in H2O) 1.9 X 10-3mol/L (in heptane) 3.3 X 10-3 mol/L a
1-(Pyridyl-2-azo)-2-naphthol (PAN) 3.3 X 10-3mol/L (in heptane) 7.0 X 10-3 mol/L a
Lysozyme < 5.0 X 10-6 mol/Lb 2.2 X 10-4 mol/La a In AOT/heptane/H2O reverse micelle; molar ratio of water to surfactant concentration is equal to ten.3 bEstimated from the literature.6
7
Reverse micelles can also solubilize, with retention of activity, a number of
hydrophilic biomolecules, which is of particular importance in biochemistry, biotechnology
and medicine.5,7 Proteins and enzymes can be selectively solubilized in reverse micelles for
extractive separation purposes with subsequent recovery achieved by various techniques.8,9
In most cases, binding is thought to occur due to electrostatic interactions between the
charged surface of the biomolecule and the reverse micelle surfactant charged head group.10
Therefore, a particular protein or enzyme can be selectively extracted from an aqueous
solution mixture to the water pool of the reverse micelle if the biomolecules have different
isoelectric points.3 For example, Vinogradov et al. have selectively extracted two different
enzymes from inclusion bodies grown in E.coli and shown that they both refolded to their
native active conformations in a reverse micelle system.11 These inclusion bodies contain
mainly misfolded and inactive products that are otherwise difficult to extract and activate by
conventional techniques.11 Therefore the use of reverse micelles is advantageous in this
regard. The primary disadvantage of this technique is in the recovery process where it is
often difficult to recover pure protein void of any water or surfactant. However, a new
recovery technique has been investigated using pressurized CO2 with increased purity of
precipitated protein.10 Numerous other examples of the utilization of reverse micelles for
solubilizaton and extractive separations have been reported.9, 10, 11
1.3.2.2 Synthesis
In addition to solubilization, reverse micelles can be used as novel reaction media. In
many situations, the utilization of reverse micelles for biosynthesis is more desirable than the
use of aqueous solutions because biomolecules often exhibit enhanced activity in their
8
presence and many chemical reagents have poor solubility or are insoluble in water.7 In
reverse micelles the size of the “nanoreactor” water pool can be controlled, and once the
reactants and reagents are contained reaction conditions can be manipulated for the desired
products. An example of a reaction that is more desirable in reverse micelles than in an
aqueous solution is the production of long-chain lactones that are used as perfume
ingredients.12, 13 Figure 5 shows a representation of the lactonization of 16-
hydroxyhexadecanoic acid (HHA) catalyzed by the enzyme lipase in a reverse micellar
system.13
Figure 5: Schematic Representation of the Production of Long Chain Lactones from HHA Catalyzed by the Enzyme Lipase, E, in a Reverse Micellar System. Reproduced with permission from article by Rees, Robinson and Stephenson13
In aqueous solutions intermolecular esterification between long-chain hydroxy acid
molecules is favored over the desired intramolecular reaction. In reverse micelles these
lactones have been successfully synthesized without this problem.12
9
Another example is the reaction of 1-fluoro-2, 4-dinitrobenzene with n-butylamine or
piperidine to form n-butyl-2, 4-dinitroaniline or n-(2, 4-dinitrophenyl) piperidine,
respectively (Figure 6). The kinetics of the reaction were examined in reverse micelles and
compared to the reaction in bulk hexane. The reaction rates for this reaction were increased
by at least 2 orders of magnitude in the reversed micelle medium relative to that in bulk
hexane.14
Figure 6: The Reaction of of 1-Fluoro-2, 4-Dinitrobenzene with an Amine to Form the Corresponding N-(2,4-Dinitrophenyl)-Piperidine. Reprinted with permission from an article by Correa, Durantini and Silber14
Reverse micelles have also been utilized in the amplification of DNA. PCR is an
important technique used for amplifying DNA from a small amount of initial template DNA.
Traditional PCR theoretically can occur with only a single template molecule. However, this
can be complicated because reactions have to take place with single or extremely low
concentration of reactants and can produce undesired products.15
Many PCR modifications have been explored to overcome these complications.
These modifications include such techniques as Nested PCR, Booster PCR, and Homo-
primer PCR.15 All three techniques have drawbacks because they include the use of carefully
designed primers. In the case of Nested PCR and Booster PCR there is also a chance of
10
contamination because they require a two step processes where the reaction vessel must be
opened during amplification.15
Micro-devices have been utilized for single molecule PCR. This technique requires
special equipment, which will increase the cost of amplification. Another limitation of using
micro-devices is the possibility of the template DNA being absorbed by the reaction vessel
wall. Along this same line of thought, reverse micelles have been explored as the micro-
device for single molecule PCR where their water pools are used as a micro-reactor without
the added cost of specialized equipment.15
In a study by Nakano et al., single molecule PCR was explored using reverse
micelles.15 The two step method proposed is simple compared to other single molecule PCR
methods. First, the template DNA is emulsified into the bulk oil phase and amplified in the
water droplets after the addition of PCR substrates. After amplification the mixture is
centrifuged so the water pools are merged and the unused PCR substrates can be employed
for further amplification.15 A schematic representation of this procedure is shown in Figure
7. This technique allows the use of only one DNA template and amplification only occurs in
a single water pool so that excess PCR substrates cannot form any undesired products.
Another advantage of this technique is that opening of the reaction vessel is not necessary.
Therefore, the possibility of contamination is eliminated.15
11
Figure 7: Schematic of Experimental DNA Amplification Procedure.
Reproduced with permission from an article by Nakano, et. al.15
Reverse micelles have also been examined as a medium for enantioselective
synthesis. Chiral products have been produced using reverse micelles containing chiral
surfactants. Wu and Zhang reported a synthetic approach for preparation of optically active
amino acids in which an enantiomeric excess of 59.5% was attained.16 In another example
of a stereospecific conversion, Zhang and Sun reported on the reduction of prochiral ketones
to form optically active alcohols (enantiomeric excesses in the range of 3.8 – 26.6% were
achieved depending upon the exact reaction conditions).17
Enzyme mediated enantioselective synthesis in the presence of reverse micelles can
also produce chiral molecules.7,18 Optically active epoxides are used in the production of β-
adrenergic receptor blocking agents, or β-blockers.18 Large scale production of these types
of epoxides is difficult because many substrates have limited solubility, can further
metabolize into undesired products, and the biocatalysts can be sensitive to the reaction
environment.18 However the whole cell mycobacterium sp. strain M156 exhibited enhanced
catalytic activity when incorporated in a reversed micellar water pool resulting in chiral
12
epoxide production (enantiomeric excess of 86%). However, there are limitations to using
reverse micelles as reaction media for epoxide production including the difficulty of
recovering the desired pure chiral product, ability to recycle the enzyme (or biomaterial), and
the inactivation of the enzyme after about 150 minutes.18
1.3.2.3 Size Tailoring / Nanoparticle Synthesis
Compared to their regular or micro-sized counterparts, nanoparticles often exhibit
unique magnetic, optical and mechanical properties. Nanoparticles are employed for a
variety of uses, including ceramics, UV stabilizers, biomedical applications, and in the
manufacturing of electronics, among others.19 Due to the ideal properties of nanoparticles
much attention has recently been given to their synthesis. The hydrophilic microcore (water
pool) of reverse micelles can be controlled which in turn facilitates the production of
nanoparticles which are more homogeneous in size relative to their production in a bulk
solvent medium. The reversed micellar approach has advantages over the traditional
methods of nanoparticle production in terms of size control, size distribution and purity.19
Two representative examples of the many different nanoparticles that have been
successfully synthesized in reverse micelles are production of ZnS particles by Zhang et al.20
and AgI particles by Tamura et al.21 ZnS nanoparticles have superior properties compared
to that of the bulk zinc sulfide. Zhang et al. showed that the reverse micelle water pool size
can be varied (increased or decreased) by merely varying the wo ratio (larger ratio results in
larger water pool and vice versa).20 Thus, the water pool size can be tailored to dictate the
size of a desired particular nanoparticle. This was demonstrated for the synthesis and size
control of ZnS nanoparticles in reverse micelles.20 Likewise, silver iodide nanoparticles
13
with properties closer to that of high conductivity α-AgI (which are characterized by a phase
transition at 147 0C from the β-AgI of the bulk material) were successfully synthesized in a
reverse micelle medium.21
1.4 Organogels
1.4.1 Description
Organogels are formed by the addition of gelatin at an elevated temperature to a
reverse micellar system that contains the surfactant AOT. Upon addition of gelatin the entire
system forms a stable organogel. This phenomenon was a surprising result discovered by
Luisi and Haering.22 They were expecting the gelatin that was present in the reverse micelle
to form a gel within the water pool but instead gelation of the entire AOT/isooctane/water
reverse micelle system was observed. Since this initial discovery, AOT organogels have
been the subject of many physicochemical and application studies.21-28
Organogels have a number of advantageous characteristics and properties. For
instance, they can exist intact for several months if stored in a closed container without the
loss of physical properties and they exhibit high electrical conductivity and viscosity.22 They
are optically transparent, which facilitates their utilization in spectroscopic applications.2
Organogels can also be prepared from the zwitterionic surfactant lecithin without the use of
gelatin.22 Lecithin organogels exhibit similar properties to AOT - gelatin based gels.25
The formation of organogels depends on the wo, the nature of the surfactant, and the
amount of gelatin present. It was determined by Luisi and Haering that the minimum wo
ratio required was 20 using AOT, independent of the amount of surfactant present.22 This is
probably the consequence of insufficient water to dissolve or hydrate the gelatin.22 Other
14
surfactants (cationic CTAB, CTAC) have been investigated for formation of gelatin
organogels but have been unsuccessful in the absence of AOT or other cosurfactant.25
The exact structure of organogels is still an open question although several structures
have been proposed. The basic structure is a network of tubules, as shown in Figure 8.
These tubules consist of a network of gelatin and water coated with the surfactant. The
tubules are surrounded by the solvent and microemulsion water droplets.22 Scanning
electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs of
AOT organogels are shown in Figure 9.22 These micrographs clearly indicate a network
system in which the major components of the organogel have surprisingly relatively high
mobilities given the viscosity of the gel.22
Figure 8: Proposed Structure of Organogel where 1 represents the W/O Microemulsion-Type Droplets and 2 the Gelatin/Water Channels. Reproduced with permission from an article by Kantaria, Rees and Lawrence and Luisi26
15
Figure 9: SEM and TEM Micrographs of Organogel [TEM: A & B; SEM: C & D; panels A & C are of gels that contained 9 g gelatin per 100 mL and panels B&D are of gels that contained 4.5 g gelatin per 100 mL]. Reproduced with permission from an article by Haering and Luisi22
1.4.2 General Properties and Applications
Although based on limited data, it appears that organogels exhibit many of the same
general basic properties as reverse micelle solutions and thus can be utilized for similar
purposes and applications. The difference between organogels and reverse micelles is that
organogels are in a “solid” form because of gelation and can be used in situations that would
otherwise be difficult with a liquid, i.e. the reversed micelle solution. This “solid” form
allows for easier handling and reuse. Also, they are stable in many different types of solvents
which is useful in analyte (or product) recovery schemes where a product can be extracted
16
without the loss of physical properties or formation of emulsion systems that results when
reverse micelles in solution are utilized. These features have led to their application in
transdermal drug delivery, as fluorescent sensor platforms and as a catalytic medium for
synthetic reactions, particularly for enzyme mediated reactions and production of pure chiral
molecules.26-28
1.4.2.1 Transdermal Drug Delivery
In order to employ organogels for the purpose of transdermal drug delivery
pharmaceutically acceptable alternatives must replace the common use of AOT surfactant
and hydrocarbon solvents. This has been achieved using a variety of formulations including
the use of Tween 21, 81, and 85 surfactants with isopropyl myristate (IPM) as the solvent.26
In this example the use of AOT as a co-surfactant was greatly reduced but not eliminated.26
Such organogels can solubilize pharmaceuticals and have been successfully used in pig skin
models and the different factors impacting their drug delivery ability have been
investigated.26. The advantages of using organogels over other methods for transdermal drug
delivery include simple preparation and handling, resistance to microbial contamination, and
ease of use.
1.4.2.2 Fluorescent Sensors Fluorescent indicators incorporated into organogels have been successfully used as
humidity and oxygen sensors. 27,28 By adding the fluorescent dye sulforhodamin 101 (S101)
into a reverse micelle solution and then preparing an organogel, an organogel film humidity
sensor or optode membrane can be fabricated.27 S101 fluoresces at 608 and 588 nm in
17
solution and the luminescent intensity decreases with an increase in the water vapor
concentration. This same behavior was demonstrated when this dye molecule was
incorporated into an AOT organogel film optode.27 This AOT sensor system was
successfully employed for the determination of the relative humidity and the method was
validated by comparison of the results to that obtained using a hygrometer. 27
Another fluorescent indicator, tris-(2,2’-bipyridyl)ruthenium(II) complex
[Ru(bpy)3]2+, is highly fluorescent at 580 nm and its fluorescence intensity decreases in the
presence of increasing oxygen concentration in organic solvents.28 This indicator was also
incorporated into an AOT organogel membrane and evaluated for the quantification of
dissolved oxygen in hexane, toluene, and isooctane. 28
In both cases the organogel indicators performed as desired and the results were
comparable to those obtained with traditional methods. Also, the membranes were
completely reversible and reproducible allowing them to be reused repeatedly without the
loss of sensitivity.27, 28
1.4.2.3 Synthesis
The most frequent application of surfactant based organogels is as a medium for the
immobilization of enzymes for use in enzyme catalyzed reactions, especially in
enantioselective reactions for the production of optically pure chiral molecules. Enzymes
can be easily incorporated, i.e. solubilized, into the organogel system by either being
introduced in the solvent/surfactant or water/gelatin component before they are mixed
together.25 Once the enzyme is incorporated into the organogels, enzyme catalyzed reactions
can be performed. In the literature, most of the enzymatic reactions reported involve
18
hydrolytic or esterification reactions. Although a variety of different enzymes have been
utilized for an array of substrates, lipase seems to be the most frequently employed enzyme.
In addition, enzyme catalyzed reactions in AOT organogel systems have also been
employed to synthesize desired stereospecific products. Table 3 lists some examples of these
syntheses and the percent enantiomeric excess (%EE) found. As can be observed, very high
enantiomeric excesses have been reported.
Zhou et al. examined the lipase-catalyzed esterification of octanoic acid with 1-
octanol, which formed octyl octoate in an AOT/isooctane organogel system.29 The organogel
immobilized lipase was effective with recycling (after 10 rounds of esterification, only an
8.6% reduction in the percent conversion was observed) and stable upon storage (after 10
months of storage, only a 1.7% decrease in conversion was noted).29 This indicates that the
enzyme is contained in the stable organogel microenvironment with little loss of activity and
can be reused without modification. Similar results were also reported by Soni and
Madamwar who investigated the lipase-catalyzed esterification of n-caprylic acid with
ethanol to form ethylcaprylate in AOT organogels.30
19
Table 3
Results of Some Enantioselective Enzymatic Reactions in Organogels
Substrate Reactant Reaction Product Enzyme Reverse Micellar
System %EE Ref
Hexanol Hexanoic acid/ Dodecanoic acid (-)2-Hexanol
Chromobacterium viscosum (C.V.)
Lipase AOT/Hexane 96 31
Octanol Hexanoic acid / Dodecanoic acid (-)2-Octanol C.V Lipase AOT/Hexane 99 31
Phenylethanol Hexanoic acid/ Dodecanoic acid
(+)1-Phenyl-1-ethanol C.V. Lipase AOT/Hexane 98 31
Decanoic acid (-) 2-Octanol (-)1-Methyl-heptyl decanoate C.V. Lipase AOT/Heptane 92 32
(RS)-cyano(3-phenoxyphenyl)methyl butyrate
1-butanol (S)2-hydroxy-(3-
phenoxy) phenylacetonitrile
Lipase AOT/Isooctane/ Glutaraldehyde >99 33
20
Synthesis using enzymes immobilized in organogels offers advantages over the same
reaction in bulk organic solvent or aqueous solutions.23 For instance, enzymes can suffer a
loss of activity or may not be soluble in organic solvents. However, when solubilized in
reverse micelles or immobilized in organogels, they retain their native activity or even
exhibit enhanced activity relative to that seen in bulk solvent systems. Enzymes
encapsulated in organogels can be easily recycled and reused without a significant loss of
activity, which is not possible with reversed micelle solutions. Also, product recovery is
much easier when using enzymes immobilized in organogels since it is a “solid” entity and
thus the product can be extracted from the bulk external organic solvent. Attempted
extraction of a desired product(s) from a similar reverse micelle solution results in
troublesome microemulsion formation. 34
1.4.3 Advantages and Limitations of Organogels Relative to Reverse Micelle
Solutions
The unique properties of reverse micelles and organogels allow for a variety of uses.
These systems have advantages over classical methods of separation and synthesis. The
reaction conditions can be easily manipulated for size tailoring and nanoscale synthesis.
Also, in these systems, enzymes and proteins are easily solubilized along with other
compounds that may not be soluble in one particular bulk solvent or mixture of solvents.
In the case of reverse micelles, there is a major disadvantage compared to the
traditional methods of synthesis or separation. Product recovery is more difficult with
reverse micelles. First, there is a problem with the purity of the recovered sample. It is often
difficult to recover pure sample without surfactant contamination. The further purification of
21
the sample may be more time consuming or costly than the conventional methods. Second,
the destruction of the system may be necessary to recover a sample and in the case of enzyme
catalyzed synthesis, the enzyme may be deactivated. This leads to the use of more starting
materials, which also may be more costly. Finally, conventional methods may have a greater
percent yield of recovered product. Despite this disadvantage reverse micellar systems are
being studied extensively and product recovery may improve in the near future.
On the other hand, the use of organogels may solve many of these problems
encountered with reverse micelles. Organogels are in a “solid” form and are easier to
manipulate/handle but still retain the ability to incorporate enzymes/reactants. Enzymes
retain their activity or even show enhanced activity when immobilized in organogels. Since
the organogel is a “solid” gel, it can be extracted using bulk nonpolar solvents without the
formation of microemulsions as in the case of reverse micelle solutions. This facilitates
product recovery and the ability to recycle and reuse the organogel containing enzyme, etc..
1.5 Statement of Thesis Research Objectives
This research was undertaken with the aim to: (1) briefly characterize AOT
organogels with respect to their stability in aqueous solution (literature reports indicate that
they swell and leak their components when in contact with water); (2) determine their
density; (3) determine the partition coefficients for the interaction of a number of model
solute molecules with the AOT organogel; (4) determine whether or not any correlations
exist between the partition coefficient of the same solute for the AOT organogel relative to
its reported partition coefficient to an AOT reversed micelle solution; (5) evaluate the
feasibility of using AOT organogels to serve as an “extractant” medium for the extraction of
22
some environmentally relevant polar organic molecules from a bulk nonpolar solvent; and (6)
determination of the pertinent extraction parameters.
23
Chapter 2
EXPERIMENTAL 2.1 Materials
Sodium bis(2-ethylhexyl) sulfosuccinate (AOT) (f.w. 444.56 g/mol) and
gelatin (type A, 300 Bloom) were obtained from Sigma-Aldrich (St. Louis, MO) and used as
supplied. Heptane (99% laboratory grade) was also obtained from Sigma-Aldrich.
Isooctane and n-hexane (>98% purity) were obtained from Fisher Scientific (Raleigh, NC)
and used as received. Distilled, deionized water was obtained from a Milli-Q System
(Millipore Corp., Billerica, MA.). Solvents used for analyte preparation and organogel
stability studies included n-propanol, heptane, heptanol, carbon tetrachloride, and
chloroform; all reagent grade materials obtained from Fisher Scientific or Sigma-Aldrich.
Sodium chloride (analytical grade) was also obtained from Fisher Scientific Company. A
potassium phosphate / sodium hydroxide pH 6.0 buffer solution was prepared using
potassium phosphate monobasic from Sigma-Aldrich and sodium hydroxide from Fisher
Scientific. Both were used as supplied.
Model analytes employed in the partitioning and extraction studies included para-
nitrophenol (4-NP; f.w. 139.1 g/mol), para-bromoaniline (4-BrA; f.w. 172.0), ortho-
nitroaniline (2-NA; f.w. 138.1 g/mol), 2,4 dinitroaniline (2, 4-dNA; f.w. 183.1), phenol (f.w.
94.1), meta-nitroaniline (3-NA; f.w. 138.1) which were received from Fisher Scientific; para-
nitroaniline (4-NA; f.w. 138.1), tryptophan butyl ester (f.w. 296.8), tryptophan ethyl ester
(f.w. 268.7), and Nile Red (NR; f.w. 318.38) obtained from Sigma-Aldrich; 1-napthol and 2-
24
napthol (1-Nap or 2-Nap; f.w. 144.17), 2,4-dichlorophenol (f.w. 163.0), 2,4,5-trichlorophenol
(f.w. 197.45), 3-nitroaniline (m-NA; f.w. 138.1), phenol blue (f.w. 226.28) obtained from
Aldrich (Milwaukee, WI), and para-iodoaniline (f.w. 219.0) received from Eastman
Chemical Corp. (Kingsport, TN). All of these analytes were used as supplied from the
manufacturer. For the enzyme experiments, laccase, derived from Agaricus bisporus was
obtained from Sigma-Aldrich and used as supplied.
2.2 Equipment & Instrumentation
Standard laboratory equipment and glassware were used for all procedures.
Organogels were sectioned and cut on a glass plate or weighing paper with a single-edged
razor blade. A Mettler balance was employed for mass measurements.
Absorbance measurements were made using a Hewlett-Packard UV/Vis model 8452
diode array spectrophotometer. Standard one-centimeter Fisher quartz cuvettes were used for
all absorbance measurements.
2.3 Organogel Preparation
2.3.1 Standard Organogel Preparation Organogels were prepared by adding 2.4 ml water to 1.41 g gelatin type A in a 30 ml
beaker. In a separate 30 ml beaker, 6.0 ml of heptane were added to 0.89 g AOT. Both
beakers were heated in a water bath to 550C and allowed to remain at that temperature for a
few minutes until the gelatin was completely dissolved. The solutions were then removed
from the water bath and the AOT solution was poured into the beaker containing the gelatin
solution. The mixture was stirred vigorously until a homogeneous solution formed. The
solution was then allowed to cool to room temperature. After cooling, the gel was covered
25
with SealView laboratory film and placed in a refrigerator (4 ± 20C). All gels were allowed
to age for at least 3 days to allow for complete gelation prior to their utilization.
2.3.2 Alternative Organogel Preparation Organogels employed for some of the analyte absorption studies were prepared
according to the standard method except the amounts of the components were scaled up (i.e.,
3.75 ml water, 2.29 g gelatin (type A), 9.3 ml of heptane and 1.39 g AOT were utilized). In
addition, 50 ml sized beakers, instead of 30 ml, were employed. This organogel preparation
was altered from the standard method due to the need for more organogel materials for some
of the analyte absorption studies. This alteration allowed the gel thickness to remain
consistent with the standard method.
2.3.3 Organogels with Enzyme Preparation
Organogels were prepared in which the enzyme laccase was incorporated into the gel.
A solution of 3.75 ml 5.0 X 10-2 M potassium phosphate monobasic- sodium phosphate, pH
6.0, buffer solution, containing 1.9 X 10-2 g laccase was substituted for the water in the
alternative organogel preparation method. Alternatively, organogels with laccase were
formed by adding 1.50 ml of the buffer solution to 1.4 g of the gelatin in a 50 ml beaker. In a
separate 50 ml beaker 5.3 ml of isooctane were added to 0.89 g AOT. Both beakers were
heated to 550C simultaneously and allowed to remain heated until the gelatin dissolved. The
solutions were removed from the water baths and the AOT solution was poured into the
gelatin solution. Immediately afterwards, a solution of 0.90 mL of the buffer and 2.9 X 10-3 g
laccase was poured into the hot solution. The mixture was stirred vigorously until a
homogeneous solution formed. The solution was then allowed to cool to room temperature.
26
After cooling the laccase containing gel was covered with Seal View laboratory film and
placed in a refrigerator (4± 20C). The laccase gel was stored for a minimum of 3 days in
order to ensure complete gel formation before use.
2.4 Organogel Stability in Different Solvent Systems
The prepared organogels were sectioned to approximately one-centimeter cubes using
a razor blade. A cube was then placed in a flask containing the test solvent and then
stoppered. The gels were visually inspected as to their size, shape, color, etc. in order to
ascertain their stability in the test solvent or solution. The solvent systems examined
included n-propanol, heptane, aqueous 0.10 M NaCl, aqueous 0.25 M NaCl, aqueous 0.5 M
NaCl and bulk water. The organogel cubes were initially visually observed and inspected
and again inspected after three days of storage in the different solvent systems. The
procedure was repeated with an additional set of organogel cubes prepared in the same
manner.
A third set of gels was also thus inspected for stability but their uptake of solvent
was also quantified. This was performed by recording the initial mass of each organogel
cube prior to its immersion in the solvent system under study. Each organogel cube was then
submerged in a flask containing 10 ml of the particular solvent under study. The flasks were
then stoppered. Changes in size and appearance were visually noted over a 2 week period.
After the 2 weeks, the organogels were removed from the test solutions and their masses
again determined.
27
2.5 Density Determination
Organogel density measurements were made using approximately one centimeter
cubes of the organogel. The mass of the organogel cube was determined and then the gel
was carefully dropped into a 10.00 ml volumetric flask containing heptane. The displaced
solvent was removed from the flask and the volume of the displaced solvent was measured in
a graduated cylinder. Density measurements were repeated for a second set of gels that were
stirred for approximately 18 hours in 100 ml hexane. The density of the gel was also
measured after absorption of 2, 4-dinitroaniline in hexane.
2.6 Analyte Solution Preparation Solute solutions were prepared by accurately massing out the specific analyte,
transferring to a volumetric flask, and diluting to final volume using the bulk solvent.
Typically, the concentration of the stock solutions was on the order of 10-5 to 10-4 M. In the
case of sparingly soluble solutes, starting concentrations were between 3.3 x 10-7 and 2.6 x
10-5 M. In these latter cases, a saturated solution was prepared, left overnight, filtered and the
supernatant was used for analysis. Appropriate dilutions were made to obtain solutions at a
desired analyte concentration. The flasks containing these solutions were wrapped in Al foil
and stored in the dark when not in use.
2.7 Determination of Solute – Organogel Partition Coefficients and Organogel Extraction
Efficiency
In the majority of experiments performed, the organogels were cut into 8 to 12
sections of appropriate size for each analysis, massed on an analytical balance and placed
into a 25 or 50 ml beaker. The total gel mass ranged from ca. 1 to 4 grams. Appropriate
28
blanks were also prepared; i.e., a solution containing a section of organogel without analyte,
and another flask containing the analyte but without an added organogel, i.e. a control.
Typically, 10 to 50 ml of the analyte containing solution was then added to the gel containing
beakers and aliquots periodically taken, analyzed spectroscopically and immediately returned
to the beaker. The solutions were monitored until a constant absorbance was achieved
which indicated equilibration of the system. Table 4 summarizes the spectral data
(wavelength maximum and molar absorptivity values) for the solutes in bulk solvent and
reverse micellar systems.
Table 4
Solute in Solution Spectral Parameters
Solute Solubility, M
Reverse Micelle Bulk Organic Solvent λ,
nmε, M-
1cm-1 System λ, nm
ε, M-
1cm-1 Solvent
2-Nitroaniline 3.2 X 10-3 Hexanea 398 5500 AOT/Hexanea 376
3725370 5250
Hexanea
Isooctaneb 3-Nitroaniline ------- --- --- ---- 340 1300 Isooctaneb
4-Nitroaniline 2.3 X 10-4 Hexanea 358 18200 AOT/Hexanea
320318316
15500 10500 14100
Heptane Hexanea
Isooctaneb 2,4-
Dinitroaniline 3.1 X 10-5
Hexanea 334 17000 AOT/Hexanea 310 15500 Hexanea
Phenol ----- -- ---- ------ 269271
1410 2150
Isooctaneb Hexane/CCl4
d
4-Nitrophenol 4.6 X 10-4 Isooctanec 307 AOT/Isooctanec 288
27810500 10000
Heptane Isooctaneb
2,4-Dichlorophenol ---- ---- ---- ------ 285 2050 Carbon
tetrachloride 1-Naphthol ---- ---- ---- ------ 230 4890 Heptane 2-Naphthol ---- ---- ---- ------ 330 2210 Heptane
aData taken from literature.35 bData taken from literature.36
cValues taken from literature.37
dValues taken from literature.38
29
The partition coefficient (P) was defined as the solute concentration in the organogel
divided by the solute concentration in the bulk external solvent.32 To simplify the
calculations, Equation 1 was used for determination of the partition coefficient:
( )VV
AAAP
g
s
f
fi ×−
= (1)
where Ai is absorbance of the initial analyte containing solution prior to the addition of the
organogel, Af is the corrected absorbance of analyte solution with gel at equilibrium, Vs is
volume of analyte solution used, Vg is volume of the gel. All experiments were conducted at
room temperature (25 ± 2oC).
The extraction efficiency (percent extraction) was determined using an identical
experimental procedure as detailed for the solute partition coefficient determinations.
The extraction efficiency (%E) was calculated using Equation 2:
VVPPE
og/100(%)+
= (2)
where P is the partition coefficient from Equation 1, V is the volume of the analyte solution
used, and Vog is the volume of the organogel used.
2.8 Determination of the Time Course for the Absorption Process
The experimental protocol for determination of the time course for solute sorption to the
organogel was the same as noted for the partition coefficient determination except that
absorbance measurements of the solute containing solutions in contact with the gel were
taken at regular time intervals. From the data, the absorbance versus time profile could be
plotted.
30
In a few instances, absorption was monitored by measuring the solute absorbance
within the organogel. This was done by first preparing the organogel using the alternative
gel preparation method. After the gel formed it was reheated and poured into a quartz
cuvette allowing a thin layer, about 0.5 mm thick, to form on the inside face of the cuvette.
The solute containing solution was then contacted with the gel in the cuvette for a specified
time period, then the solute solution was removed and the absorbance due to the solute that
had absorbed into the gel was measured. This process was repeated at regular time intervals.
The blank measurement was made employing a similar gel lined cuvette that had not been
contacted with the solute containing solution. The data allowed for the determination of the
solute absorbance in the gel versus time profile.
2.9 Data Analysis
Required linear regression, multiple linear regression analyses, and statistical
calculations were performed using Excel (Microsoft, USA).
31
Chapter 3
RESULTS AND DISCUSSION
3.1 Brief Characterization Studies 3.1.1 Organogel Stability Applications involving the utilization of AOT organogels depend in part on their
robustness and ability to maintain their integrity under a variety of conditions. Previously,
AOT organogels have been shown to be quite stable when stored in closed containers and
when contacted with nonpolar organic solvents (hydrocarbons such as isooctane,
cyclohexane, n-heptane, etc.)22,32,39 However, the literature is sparse in regards to organogel
stability in polar organic solvents although it has been noted that organogels may be less
stable in more polar media, particularly in bulk water. 22,39
In this work, sections of organogels were contacted with selected solvents or aqueous
solutions and monitored over a one week time period with changes in size, appearance and
optical clarity noted. The swelling ratio was also determined. The swelling ratio, Esw, is
defined as Esw = [(We – Wo) / Wo], where We is the mass of the organogel after equilibration
in the external solvent system and Wo is the initial mass of the organogel before immersion.
The results obtained are summarized in Tables 5 and 6.
As can be observed from the Tables, in the presence of distilled water, the gel
significantly swelled (swelling ratio ca. 22, Table 6), became opaque and lost its integrity.
Such behavior has been previously noted in the literature.25 However, in the presence of salt
(NaCl), the gels appeared to be much more stable and did not significantly swell (swelling
ratio, 1.2 to 1.7 depending upon the salt concentration). It appears that AOT organogels
32
when contacted with aqueous salt solutions are stable and thus may also be utilized in such
aqueous solutions in addition to nonpolar organic solvents. This should serve to extend the
scope of applications possible with such materials. Figure 10 shows a photograph of the
appearance of a 1cm square cube AOT organogel after it had been immersed in a 0.50M salt
water (NaCl) solution for one week.
Figure 10: Photo of an AOT Organogel that had been in Contact with a 0.50 M NaCl Solution for 7 days
In the presence of propanol, the AOT organogel appeared unchanged after 1 day and
did not swell. However, after one week, it appeared to have shrunk in size and it became
more opaque. Likewise after one week’s exposure to isopropanol, the AOT organogel
shrank and became slightly opaque. These findings are in agreement with reports that AOT
organogels are not stable in polar organic solvents.39,25
When in contact with the nonpolar solvents cyclohexane and heptane, the AOT
organogels remained intact and maintained their integrity. Although there appeared to be
some swelling after 24 hours, the AOT swelling ratio was only 0.09 when in the presence of
33
cyclohexane and actually slightly negative if in bulk heptane. These observations are in
agreement with prior literature reports. 22,32,39
Table 5
Appearance of Organogels after Exposure to Different Solvent Systems
Solvent Observations
After 24 hours After one week
Propanol No observable change Slight whitening of gel
Isopropanol Slight whitening Shrinking of gel, slight whitening
Heptane Slight swelling Gel and solution cloudy / more swelling
Cyclohexane Some swelling / very clear almost transparent Some swelling, clearing
Water Whitening, swelling Much more swelling,
almost translucent, lost shape
Water (+ 0.50 M NaCl) Some gel swelling Greater swelling
34
Table 6
Swelling of Organogels When in Contact with Different Media
Solvent Initial Massa (Wo)
End Mass (We)
Swelling Ratio (Esw)
Water 0.0942 2.1435 21.75
Isopropanol 0.5082 0.4096 -0.19
NaCl 0.1M 0.0978 0.2255 1.31
NaCl 0.25M 0.1021 0.2646 1.59
NaCl 0.5M 0.0864 0.2307 1.67 NaCl (Saturated, ca.
5.3 M) 0.0941 0.2078 1.21
Cyclohexane 0.9591 1.0481 0.09
Heptane 0.9548 0.7800 -0.18 aInitial mass of gel prior to contacting it with the indicated solvent system. 3.1.2 AOT Organogel Density
Density represents another important property of organogel materials. The density of
AOT organogels prepared with hexane as the nonpolar solvent was found to be 1.05 g/mL (n
= 7; standard deviation 0.08 g/mL). To the best of my knowledge, there has been no prior
density value for AOT organogels reported in the literature.
3.2 Interaction of Organic Solutes with AOT Organogels
3.2.1 Determination of Partition Coefficients
Similar to AOT reverse micelles in solution, AOT organogels possess both ionic and polar
binding sites. Although a vast amount of literature is available regarding the binding or
partitioning of inorganic, biochemical and organic solutes to AOT reverse micelles in
solution40, there has been only one prior report concerning the binding of solutes to AOT
35
organogels.32 Consequently, in this work, the partition coefficients for the interaction of
twelve solutes with AOT organogels were determined. The results are summarized in Table
VII. Also included for comparison purposes are available literature values for binding of the
same solutes to AOT reverse micelles in solution. As can be observed, the values for the
partitioning of all solutes to the AOT reverse micelles in solution are greater (by factors of
roughly about 6 to 40) relative to that observed for their partitioning to the AOT organogels.
In addition, for the series of isomeric nitroanilines, the trends are the same; i.e., the 2-
nitroaniline exhibits the smallest partition coefficient while the 4-nitroaniline exhibits the
greatest (see Table 7) with respect to their interaction with AOT reverse micelles or AOT
organogels. For the nitroaniline derivatives, the order of their solubility in the nonpolar
solvent hexane follows the trend: 2,4-dinitroaniline (3.1 x 10-5 M) < 4-nitroaniline (2.3 x 10-
4 M) < 2-nitroaniline (3.2 x 10-3 M).6 Their binding/partitioning to either AOT reverse
micelles in solution or the AOT organogels appears to be inversely related to their solubility
in a nonpolar organic solvent like hexane; i.e., the greater the solubility in hexane, the
smaller is its binding or partitioning to the AOT reverse micelle/organogel. Thus, the more
nonpolar the solute, the lesser is its partitioning interaction with the AOT organogel. In
agreement with this hypothesis, two other nonpolar solutes, p-xylene and nitrobenzene, did
not exhibit any apparent partitioning to the AOT organogels (Table 7).
36
Table 7
Partition Coefficients of Solutes in AOT Reverse Micelles and AOT Organogels
Solute Kb M-1 (AOT RM) P (AOT RM)a P(AOT gel) Ref
2-Nitroaniline 45-70 35 119 – 185 17.9 ± 7 3-Nitroaniline ------ -- ------- 32.9 4-Nitroaniline 350-400 35 922 – 1054 122.4 ± 57
2,4-Dinitroaniline 3100-3350 35 8159 – 8817 276.5 ± 224 Phenolc 327-350 37 862 - 922 24.9
4-Nitrophenolb 3670-7510 37 9659 – 19737 512.3 ± 347 2,4-dichlorophenolc ------- -- --------- 1.31 ± 0.6
2,4,5-trichlorophenolc ------- -- -------- 3.9 ± 1.7 1-Naphthol ------- -- -------- 17.7 ± 0.9 2-Naphtholb 140-335 44 369 - 883 25.4 ± 5.3 Nitrobenzene -------- -- -------- 0
p-Xylene -------- -- ------- 0 aP represents the partition coefficient for solute distribution between AOT organogel and bulk solvent phase (hexane). P was calculated from Kb using the Berezin equation7; i.e., Kb = [P-1] v, where v is the molar volume of the AOT surfactant [v was taken to be 0.380 M-1 which is the average of literature values].42,43 bBulk solvent was heptane. cBulk solvent was carbon tetrachloride.
Figure 11 shows the plot of the solute – AOT reverse micelle binding constants, Kb
(median of reported values), versus their partition coefficient, P for interaction with the AOT
organogels obtained in this work. As can be seen, a good correlation (n = 6; R2 = 0.97) was
observed. Thus, if the solute – AOT reverse micelle binding constant is known, one can get a
rough estimate of that solute’s anticipated partition coefficient with AOT organogels using
such a plot.
37
0
1000
2000
3000
4000
5000
6000
0 100 200 300 400 500 600P value (AOT organogel)
K b (M
-1) (
AO
T R
ever
se M
icel
les)
Figure 11: Plot of the Solute – AOT Reverse Micelle Binding Constant, Kb (M-1) versus the Solute – AOT Organogel Partition Coefficient
3.2.2. Characterization of AOT Organogel Phase
In order to gain greater insight into the interactions that are involved in solute
partitioning to the organogel, the Abraham solvation parameter model was employed to
characterize the AOT organogel material.45-49 The Abraham model utilizes a linear free
energy equation that relates a dependent variable to a series of interaction parameters of the
solute molecule (general equation (3)):
log SE = c + r R2 + s π2H + a α2
H + b β2H + v V (3)
The dependent variable (SE) may be a chromatographic retention factor (log k’), partition
coefficient (log P), binding constant (log Kb), or Ostwald solubility coefficient (log L),
etc.45,46 Each solute molecule possesses a unique set of descriptors that are dependent upon
38
its specific structural features, which in turn governs the types of solute – host organogel
interactions that it may undergo. These descriptors include the solute’s excess molar
refraction (R2), polarizability (or dipolarity) (π2H), hydrogen bond acidity (α2
H), hydrogen
bond basicity (β2H ), and McGowan molar volume (V).45-47 These descriptors for all probe
molecules employed in this study are summarized in Table 8. It should be noted that in
addition to the eleven solute partition coefficients determined in this study, literature values
are included for octanol and decanoic acid32 in order to expand the data set.
Table 8
Summary of Solutes, their AOT Organogel-Hexane Partition Coefficients and their Abraham Solvation Descriptor Values
Solute Pa log Pa R2 π2
H α2H β2
H V Phenolb 24.90 1.40 0.81 0.89 0.60 0.30 0.775
4-nitrophenolb 512.32 2.71 1.07 1.72 0.82 0.26 0.949
2,4-dichlorophenolb 1.31 0.12 0.96 0.84 0.53 0.19 1.02
2,4,5-trichlorophenolb 3.87 0.59 1.07 0.92 0.73 0.10 1.14
2-nitroanilineb 17.93 1.25 1.18 1.37 0.30 0.36 0.99
3-nitroanilineb 32.90 1.52 1.20 1.71 0.40 0.35 0.99
4-nitroanilinec 128.87 2.11 1.22 1.93 0.46 0.35 0.99
1-naphtholc 17.74 1.25 1.52 1.05 0.60 0.37 1.144
2-naphtholc 25.45 1.41 1.52 1.08 0.61 0.40 1.144
Octanolc 0.94e -.03 0.20 0.42 0.37 0.48 1.30
Decanoic Acidc 1.02e .01 0.12 0.64 0.62 0.45 1.59
4-Xylenec,d 0 --- 0.61 0.52 0 0.16 1.00
Nitrobenzeneb,d 0 --- 0.87 1.11 0 0.28 0.89
aValues calculated from experimental data from Table 7. bSolvation parameters taken from the literature.47 cSolvation parameters taken from the litetarure.48
dValue was not used in the regression analysis ePartition coefficient taken from the literature.32
39
In this approach, the AOT organogel phase is characterized by independently
determining, from spectroscopic measurements, the partition coefficients for the interaction
of a number of solute molecules with the organogel. Using the measured partition
coefficients and multiple linear regression analysis (Eq. 3), the interaction parameters of the
AOT organogel (r, s, a, b v) can be determined. The coefficient r represents the ability of
the AOT organogel phase to interact with pi- and nonbonding electrons of the solute, s
indicates the polarizability/dipolarity of the organogel phase, a and b represent the AOT
organogel phase hydrogen bond acidity and basicity, respectively, and v represents its
hydrophobicity.
The calculated interaction parameters of the AOT organogel phase are summarized in
Table 8. In addition, the interaction parameters for AOT reverse micelles in solution as
reported by Zingaretti et al.49 are included for comparison purposes. In the case of AOT
reverse micelles in solution, solute incorporation is strongly favored by increasing solute
hydrogen bond acidity and slightly disfavored by increasing solute molar
volume/hydrophobicity (see Table 9). In contrast, for the AOT organogels, solute
incorporation is dominated by hydrogen bond basicity and acidity with a secondary
contribution from solute polarizability. It is also negatively impacted by increasing the solute
molar volume. For the reverse micelles in solution, there is no contribution to partitioning
due to solute hydrogen bond basicity whereas that is the dominant term in AOT organogels.
An explanation for this behavior might lie with the gelatin that is present in the organogel
system (and not solution reverse micelle). Each gelatin present contains a number secondary
amine and carbonyl oxygen moieties that contain lone pairs of electrons that are capable of
accepting a hydrogen bond, thus enhancing hydrogen bond basicity.
40
It should be cautioned that the interaction coefficients determined for both the reverse
micelles in solution and organogel systems were based on rather limited solute data sets (nine
amines and nine alcohols for the AOT reverse micelle systems and eleven solutes for the
AOT organogels) and thus probably reflect only crude approximations of the true interaction
coefficients.
Table 9
Interaction Coefficients for Solute Partitioning into AOT Organogel and AOT Reversed Micellar Solutions
aAOT Organogel values as determined in this work. bAOT Reverse Micelle values based on amine solutes reported in the literature.49
cAOT Reverse Micelle values based on alcohols as reported in the literature.49
A plot of the predicted partition coefficients (as calculated using the AOT organogel
interaction coefficients and equation 3) versus the experimentally observed partition
coefficients for solute incorporation into the AOT organogels is shown in Figure 12. The R2
value for this plot is 0.97, indicating a fair correlation. Thus, the partition coefficient for a
desired solute could be predicted with some confidence using equation 3 and the determined
AOT organogel interaction parameters provided that the solute’s descriptors are known. As
an example, the predicted partition coefficients for the solutes p-xylene and nitrobenzene are
calculated to be 0.025 and 0.60, respectively. The experimentally determined values were
zero within experimental error.
AOT System c r s a b v R2
Organogela -0.94 0.10 1.20 2.75 3.63 -1.93 0.975
Reverse Micelleb -1.4 0 0 7.0 0 -1.9 0.985
Reverse Micellec -2.8 0 0 12.7 0 -0.76 0.98
41
-0.30
0.30.60.91.21.51.82.12.42.7
-0.3 0.1 0.5 0.9 1.3 1.7 2.1 2.5log P observed
log
P pr
edic
ted
Figure 12: Comparison of the Predicted and Experimentally Observed Values of the Partition Coefficients for Solute Incorporation into AOT Organogels.
3.3 Extraction of Polar Analytes from Nonpolar Solvents using AOT Organogels
3.3.1 Extraction Time Profiles and Sorption Kinetics
Figure 13 shows the time course profile for the removal of the polar aromatic
solute 2-nitroaniline from hexane due to the presence of an AOT organogel extractant phase.
As can be observed, equilibrium is achieved fairly rapidly; i.e., within three hours and the
AOT organogel served to extract a significant amount of the 2-NA from the hexane solution.
42
0
0.2
0.4
0.6
0.8
1
1.2
0 25 50 75 100 125 150 175Time (min)
Abs
orba
nce
Figure 13: Plot of the Absorbance of 2-Nitroaniline at 376 nm in Hexane versus Time (minutes) after exposure to 3.945 grams of AOT Organogel at 25.0o C.
The amount of any solute absorbed by the organogel at any time, t, Qt (mmol
solute/gram organogel), was determined via use of equation (4):17
Qt = (Co – Ct) V / W (4)
where Co and Ct are the organic liquid phase solute concentrations (molar or millimolar) at
time equals zero (prior to addition of the gel to the hexane solution) and time t, respectively,
V is the volume (in mL) of the hexane solution, and W is the mass (in grams) of the AOT
organogel. Knowledge of the solute molar absorptivity in the organic solvent allows for
conversion from the absorbance reading to Qt value at each measurement time, t. Figure 14
shows the corresponding plot of the amount of 2-NA sorbed onto the AOT organogel as a
function of time.
43
0.0E+00
1.0E-04
2.0E-04
3.0E-04
4.0E-04
5.0E-04
6.0E-04
7.0E-04
8.0E-04
0 25 50 75 100 125 150 175Time (min)
Qt
Figure 14: Plot of the Amount of 2-NA (mmol/gram gel) Sorbed onto the AOT Organogel as a Function of Time (minutes) [other conditions are as noted in Figure 13 legend].
In order to identify the rate-controlling step for sorption of solutes onto the organogel,
three potential rate-limiting steps are typically considered:51
(i) Mass transfer of the polar solute from the bulk organic solution to the AOT
organogel surface
(ii) Sorption of the organic solute onto organogel sites, and
(iii) Internal diffusion of the organic solutes in the organogel
If the rate limiting step is due to solute mass transfer (i), then the first order equation first
proposed by Lagergren, equation (5) should best fit the data (from Fig. 14): 51,52
log (Qe – Qt) = log Qe - k1 (t) / 2.303 (5)
where k1 is the first-order sorption rate constant and Qe is the equilibrium sorption capacity
which is calculated from equation 6 as:
Qe = (Co – Ce) V / W (6)
where Ce is the liquid phase solute concentration (millimolar or molar) at equilibrium and the
other terms as defined in Equation 4.
44
0.0E+00
5.0E+04
1.0E+05
1.5E+05
2.0E+05
2.5E+05
0 50 100 150 200Time (min)
t/Qt
If the rate determining step is due to solute adsorption onto organogel sites (ii), then the
sorption data should best fit the following pseudo-second-order sorption model equation:51
t / Qt = 1 / (k2 Qe2) + t / Qe (7)
where k2 is the pseudo-second-order sorption rate constant.
Alternatively, if internal intraparticle diffusion (iii) is the rate limiting step, the
experimental data should best fit the Weber-Morris equation:
Qt = kp t0.5 (8)
where kp is the pore diffusion rate constant.20 Treatment of the 2-NA data shown in Figures
13 and 14 utilizing these three treatments revealed that 2-NA sorption onto the AOT
organogel sites (Eq. 7) best fit the data (see Fig. 15, correlation coefficient 0.9980).
Figure 15: Plot of the Second-Order Sorption Model Equation (Eq.7), t/Qt as a function of Time for the Solute 2-NA
45
The pseudo-second order sorption rate constant for 2-NA was found to be 139.5 g/mmol/min.
The correlation coefficients for treatment of the same data by the first order (Eq. 5) and
particle diffusion (eq. 8) models were 0.8663 and 0.8811, respectively.
The same treatment was applied to several other solutes (4-nitrophenol, 2-naphthol,
and 2,4-dinitroaniline) and in each instance, the pseudo-second order model (Eq. 7) was
found to best fit (gave the greater R2 value for) the experimental data. Table 10 summarizes
these data (i.e., the best fit model, rate constants, equilibrium sorption capacity, and the
correlation coefficient for each solute). There has not been any prior report in the literature
concerning an analysis of the sorption kinetics for solute partitioning to AOT organogels.
Table 10
Pseudo-Second Order Parameters for the Sorption of Solutes on AOT Organogels
Solute Best Fit Model
k2 (g/mmol/min) R2 Qe
a
Observed Qe
a
Calculatedb
2-Nitroaniline 2nd order (Eq.3.5) 139.5 0.998 7.96E-04 7.17E-04
4-Nitroaniline 2nd order (Eq.3.5) 3709.9 0.983 5.54E-04 5.27E-04
2,4-Dinitroaniline 2nd order (Eq.3.5) 306.1 0.999 2.68E-04 3.06E-04
2-Naphthol 2nd order (Eq.3.5) 92.2 0.999 2.19E-03 2.27E-03
aUnits for Qe are mmol*mL/g bValues calculated from the slope of the best fit model, equation 7. It should be noted that AOT organogels are optically transparent and thus in addition
monitoring the solute absorbance decrease in the bulk external organic solvent (as shown in
Figure 13), one can also monitor the absorbance due to solute incorporation into the
organogel directly. Figure 16 illustrates the time profile for the incorporation of the solute
46
0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200Time (min)
Abs
orba
nce
4-nitroaniline into an AOT organogel that was obtained by measuring the 4-NA absorbance
in the gel directly.
Figure 16: Plot of Absorbance (at 318 nm) of 4-Nitroaniline that Sorbed onto the Organogel versus Time at 25.0o C. 3.3.2 Extraction Parameters and Considerations Although a wide variety of sorbent materials have been employed for extraction of
organic solutes from aqueous solutions, a more limited number have been reported for
extraction of polar organic solutes from nonpolar organic solvents. Thus, the utilization of
AOT organogels for the extraction of polar aromatic solutes from nonpolar solvents was
evaluated. The extraction efficiencies achieved for different substituted anilines and phenols
are summarized in Table 11. As can be observed, the extraction efficiencies ranged from 72
to 98 % for the analytes examined for a single step extraction. The percent extraction
achieved roughly parallels the magnitude of the respective partition coefficients for the
47
analytes examined. Table 11 also gives the concentration factor achieved by the extraction,
which ranged from ca. 3 to 13 depending upon the amount of extractant AOT organogel that
was employed. A greater concentration factor can be achieved if one uses a smaller amount
of the gel (in this work, ca. 1.0 gram AOT organogel was the smallest amount of gel that was
employed for an extraction).
Table 11
Extraction Efficiency and Concentration Factors Achieved for the Extraction of Analytes from Hexane with the AOT Organogel Materials
Solute %Ea Concentration Factorb
2-Nitroanaline 71.7 5.3 3-Nitroanaline 75.8 --- 4-Nitroanaline 94.6 2.9 – 10c
2, 4 - Dinitroanaline 96.8 12.9 4-Nitrophenol 77.8 ---
2-Naphthol 74.8 8.3 Phenol 82.6 7.3
aCalculated using equation 2. bConcentration (or enrichment) factor was calculated as the ratio of the solute concentration in the AOT organogel phase to the original solute concentration in the bulk organic phase. cDepends upon the mass of gel (2.9 if 10 grams and 10.0 if 1 gram organogel employed).
These preliminary results indicate that the AOT organogel can serve for the extractive
preconcentration of polar organic analytes initially present in nonpolar organic solvent.
Previously, such polar analytes as anilines, nitroanilines, and phenols had been extracted
from nonpolar solvents using salt water as the extractant phase or different ion exchange
resins.54-56 None of these literature reports indicated the extraction efficiencies or enrichment
factors that were attained using these approaches.
48
It is anticipated that such AOT organogels will most likely be employed for the
extractive purification of nonpolar organic solvents rather than for analyte preconcentration
prior to its quantification. Previously, one report noted the utilization of AOT organogel
films for the extractive removal of wastewater pollutants and their detoxification.57 The
organogel films contained appropriate entrapped (immobilized) enzymes that allowed for the
in situ conversion of the original toxic pollutant material into a less toxic (or nontoxic)
product(s) within the film. A problem with this approach is that with extended use, the
organogel materials swelled and lost their integrity. This problem might be overcome if the
ionic strength of the contaminated water were increased by addition of salt prior to addition
of the enzyme-containing organogel material (refer to section 3.1.1, p. 31).
In this work, the incorporation of the enzyme laccase into the organogel with a
potassium phosphate buffer solution was performed. The uptake of some phenols into the
enzyme containing gel was measured with the intent of the enzyme detoxifying the phenols.
Little or no absorbance into the organogel was observed.
One consideration in the utilization of AOT organogels for extractions and/or
purifications is that of the subsequent desorption and recovery of the target analyte or
contaminant if required. In this context, the ability to use the organogels in salt water might
prove very beneficial. Some preliminary experiments revealed that when an AOT organogel
was contacted with an aqueous solution (containing NaCl) of 4-nitroaniline, the gel only
bound a small amount of the 4-nitroaniline. In other words, the partition coefficient for the
sorption of 4-NA from salt water as external solvent was only ca. 2.2. This means that if an
AOT organogel that had been previously employed for the removal of this analyte from a
nonpolar organic solvent were then subsequently immersed in a salt water solution, most of
49
the 4-NA should then be desorbed. Of course, experiments to conclusively demonstrate this
need to be conducted but this preliminary work bodes well for the likely success of this
desorption approach.
3.4 Conclusions For organogels to be a viable medium for solute extraction they must be stable and
maintain their integrity in the relevant solvent. In this work it was found that organogels are
stable in a variety of nonpolar solvents and varying strengths of aqueous sodium chloride
solutions. The stability in salt solutions is significant because this allows the possibility of
developing novel uses for organogels in such solutions. Conversely, the organogels were not
stable in water alone as was demonstrated by the large swelling ratio. This indicates that the
organogels absorb water and may be useful in extracting water from organic solvents.
The determined density of the AOT organogels is important in that it allows for the
interconversion from mass of the gel used to the corresponding volume. The volume is
required for some calculations that might involve such gels, as in the determination of the
sorption kinetics and/or calculation of extraction parameters, such as the enrichment factor.
The determined partition coefficient for the interaction of a number of solutes to AOT
organogels extends the data pool available in the literature. More important, the relative
binding/partitioning trends observed for the binding of solutes to AOT reverse micelles is
mirrored for the AOT organogels. This allows for rough estimates to be made if
corresponding reverse micelle binding data is available in the literature. Whereas, the more
nonpolar the solute the less it may be absorbed by the organogel.
Building upon the partition coefficient, further insight into the characterization of
organogel interactions was gained. The Abraham solvation model was employed to
50
determine the interaction parameters for the AOT organogel. The dominant terms for
organogels were determined and compared to those of AOT reverse micelles. It was also
noted that these interactions for both AOT containing systems were based on limited data and
represent only an estimate of the true systems.
Furthermore, this work determined the rate constants of uptake into the organogels
for a few solutes. The pseudo-second order sorption model was found to best fit the solute
sorption data for the four solutes examined in this study. Lastly, some extraction
parameters were determined for the extraction of polar aromatic analytes from hexane (or
heptane) as the nonpolar solvent. The extraction efficiencies were in the range of >70% with
enrichment factors of ca. 10 achieved via use of 1.0 gram of the AOT organogel as the
extractant phase.
51
Chapter 4
FUTURE STUDIES
4.1 Recommendations Given the findings in this work, there are a few recommendations for future studies
involving organogels. These recommendations may promote potential new applications and
further extend the characterization of organogels.
Although it was demonstrated that the organogels were unstable in water it would be
interesting to investigate the possibility of employing organogels for extraction of water from
organic or other non aqueous solutions (serving as a dehydration agent). Along with this,
one may want to quantify the maximum amount of water that can be extracted prior to the
organogel losing its integrity, i.e. the organogel cannot be removed from the solvent.
Furthermore, if the organogels are stable in salt water (such as NaCl), the minimum
salt concentration required to maintain organogel stability should be determined. Once this is
established, an investigation as to whether or not organogels can extract a target solute
initially present in an aqueous salt solution should be undertaken. In addition, what are the
factors (salt concentration, solute polarity) that impact such extraction? It might be expected
that relative to water as the initial phase, the AOT organogels might prove useful for the
extraction of “more” nonpolar analytes. Even if organogels prove to be ineffective for the
removal of analytes initially present in water, such data would be useful in the context of
using salt water for the desorption of analytes that had been removed from a nonpolar
52
organic solvent (as detailed in this work). It would be beneficial to determine the extent of
desorption possible and effectiveness of the subsequent reuse (recycling) of the spent
organogel after contact with an appropriate aqueous salt solution.
Due to the limited number of solutes examined in the Abraham treatment, the
determination of partitioning data for a wider range of solutes (particularly various
nonaromatic organic molecules such as aliphatic aldehydes, ketones, amines, alcohols,
carboxylic acids, etc.) would allow for further refinement of the AOT organogel interaction
parameters. Lastly, some extractive purification applications should be attempted. For
instance, ethers are subject to slow build up of peroxides that may become explosive. The
removal and detoxification of such peroxides initially present in bulk solvents have been
reported.58,59 Organogels have been shown to be stable in ethers,60 and the possibility of
organogels absorbing and detoxifying organic peroxides should be studied.
53
REFERENCES 1. Attwood, D. F., A.T., Surfactant System: Their chemistry, pharmacy and biology. ed.; Chapman and Hall: New York, 1983; p 794.
2. Pramauro, E.; Pelizzetti, E. Surfactants in Analytical Chemistry: Applications of Organized Amphiphilic Media, Vol. XXXI of Wilson & Wilsons Comprehensive Analytical Chemistry (edited by SG Weber), Elsevier: New York, 1996, 521 pp. 3. Hinze, W. L., Organized Assemblies in Chemical Analysis. ed.; JAI Press Inc.: Greenwich, CT., 1994; 'Vol.' 1, p 186. 4. Jean, Y.; Ache, H.J. Study of the Micelle Formation and the Effect of Additives on This Process in Reverse Micellar Systems by Positron Annihilation Techniques. J. Am. Chem. Soc. 1978, 100, 6320-6327. 5. Luisi, P. L.; Giomini, M.; Pileni, M.P.; Robinson, B.H. Reverse Micelles as Hosts for Proteins and Small Molecules. Biochim. Biophy. Acta, 1988, 947, 209-246 6. Grandi, C.; Smith, R.E.; Luisi, P.L. Micellar Solubilization of Biopolymers in Organic Solvents, J. Bio. Chem., 1981, 256, 837-843. 7. Orlich, B.; Scchomacker, R. Enzyme Catalysis in Reverse Micelles, Advances in Biochemical Engineering & Biotechnology, 2002, 75, 185-208 8. Bohidar, H.B.; Behboudnia, M. Characterization of reverse micelles by dynamic light scattering. Colloids and Surfaces A 2001, 178, 313-323. 9. Krishna, S. H.; Srinivas, N.D.; Raghavarao, M.S.; Karanth, N.G. Reverse Micellar Extraction for Downstream Processing of Proteins/Enzymes, book title: History and Trends in Bioprocessing and Biotransformation which is part of the series Advances in Biochemical Engineering/Biotechnology, Vol. 75, 2002, Springer, Berlin, pp. 119-183 10. Zhang, H.; Lu, J.; Han, B. Precipitation of Lysozyme Solubilized in Reverse Micelles by Dissolved CO2. J. Supercrit. Fluids 2001, 20, 65-71. 11. Vinogradov, A. A.; Kudryashova, E. V.; Levashov, A. V.; Van Dogen, W. Solubilization and Refolding of Inclusion Body Proteins in Reverse Micelles. Anal. Biochem. 2003, 320, 234-238. 12. Klyachko, N. L.; Levashov, A.V. Bioorganic Synthesis in Reverse Micelles and Related Systems. Curr. Opin. Colloid Interface Sci. 2003, 8, 179-186.
54
13. Rees, G.D.; Robinson, B.H.; Stephenson, G.R.; “Macrocyclic Lactone Synthesis by Lipases in Water-in-Oil Microemulsions” Biochem. Biophy. Acta, 1995, 1257, 239-248. 14. Correa, N. M.; Durantini, E. N.; Silber, J. J. Catalysis in Micellar Media. Kinetics and Mechanish for the Reaction of 1-Fluoro-2, 4-dinitrobenzene with n-Butylamine and Piperidine in n-Hexane and AOT/n-Hexane/Water Reverse Micelles. J. Org. Chem. 1999, 64, 5757-5763. 15. Nakano, M. K.; Matsuura, J.; Takashima, S.; Katsura, K.; Mizuno, S. Single-Molecule PCR using Water-in-Oil Emulsion. J. Biotechnol. 2003, 102, 117-124. 16. Wu, W.; Zhang, Y. Enantioselective synthesis of α-amino acids in chiral reverse micelles. Tetrahedron: Asymmetry 1998, 9, 1441-1444. 17. Zhang, Y.; Sun, P. The Asymmetric Induction and Catalysis of Chiral Reverse Micelle: Asymmetric Reduction of Prochiral Ketones. Tetrahedron: Asymmetry 1996, 7, 3055-3058. 18. Prichanont, S.; Leak, D.J.; Stuckey, D.C. Chiral Epoxide Production using Mycobacterium Solubilized in a Water-in-Oil Microemulsion. Enzyme and Microbial Technology. 2000, 27, 134-142.
19. Klabunde, K. J., Nanoscale Materials in Chemistry. ed.; Wiley-Interscience: New York, 2001; 'Vol.' p 292.
20. Zhang, J.; Han, B.; Liu, J.; Zhang, X.; Yang, G.; Zhao, H. Size Tailoring of ZnS
Nanoparticles Synthesized in Reverse Micelles and Recovered by Compressed CO2. J. Supercrit. Fluids, 2004, 30, 89-95. 21. Tamura, S.; Takeuchi, K.; Mao, G.; Csencsits, R.; Fan, L.; Otoma, T.; Saboungi, M. Colloidal Silver Iodide: Synthesis by a Reverse Micelle Method and Investigation by a Small-Angle Neutron Scattering Study. J. Electroanal. Chem., 2003, 559, 103-109. 22. Haering, G.; Luisi, P.L. Hydrocarbon Gels from Water-in-Oil Microemulsions. J. Phys. Chem. 1986, 90, 5892-5895. 23. Rees, G.D.; Robinson, B.H. Microemulsions and Organogels: Properties and Novel Applications, Advanced Materials, 1993, 5, 608-619. 24. Murdan, S. Organogels in Drug Delivery, Expert Opinion on Drug Delivery, 2005, 2, 489-505. 25. Hinze. W. L.; Uemasu, I.; Dai, F.; Braun. J. M. Analytical and Related Applications of Organogels. Curr. Opin. Colloid Interface Sci. 1996, 1, 502-513. 26. Kantaria, S. R., Rees, G.D.; Lawrence, M. J., Gelatin-Stabilised Microemulsion-Based Organogels: Rheology and Application in Iontophoretic Transdermal Drug Delivery. J.
55
Controlled Release 1999, 60, 355-365. 27. Choi, M.M.F.; Shuang, S. Fluorescent Optode Membrane Based on Organogel for Humidity Sensing, Analyst, 2000, 125, 301-305. 28. Velasco-Garcia, N.; Valencia-Gonzalez, M.J.; Diaz-Garcia, M.E. Fluorescent Organofilms for Oxygen Sensing in Organic Solvents Using a Fiber Optic System. Analyst, 1997, 122, 1405-1409. 29. Zhou, G.; Li, G.; Xu, J.; Sheng, Q. Kinetic Studies of Lipase-Catalyzed Esterificaiton in Water-in-Oil Microemulsions and the Catalytic Behavior of Immobilized Lipase in MBGs. Colloids Surf. A, 2001, 194, 41-47. 30. Soni, K.; Madamwar, D. Ester Synthesis by Lipase Immobilized on Silica and Microemulsion Based Organogels (MBGs). Process Biochem. 2001, 36, 607-611. 31. Jesus, P.C.; Rezende, M.C.; Nascimento, M.G. Enzymatic Resolution of Alcohols Via Lipases Immobilized in Microemulsion-Based Gels. Tetrahedron: Asymmetry 1995, 6, 63-66. 32. Rees, G.D.; Nascimento, M.G.; Jenta, T.R.J.; Robinson, B.H. Reverse Enzyme Synthesis in Microemulsion-Based Organo-Gels. Biochim Biophy Acta 1991, 1073, 493-501. 33. Fadnavis, N.W.; Babu, R.L.; Sheelu,G.; Deshpande, A. ‘Gelozymes’ in Organic Synthesis; Synthesis of Enantiomerically Pure (S)-2-Hydroxy-(3-Phenoxy)Phenylacetonitrile with Lipase Immobilized in a Gelatin Matrix. Tetrahedron; Asymmetry. 2000, 11, 3303-3309. 34. Fadnavis, N.W.; Deshpande, A.; Synthetic Applications of Enzymes Entrapped in Reverse Micelles & Organo-Gels. Current Organic Chemistry 2002, 6, 393-410. 35. Correa, N.M.; Silber, J.J. Binding of Nitroanilines to Reverse Micelles of AOT n-Hexane, J. Mol. Liquids, 1997, 72, 163-176. 36. Prabhumirashi, L.S.; Kunte, S.S. Solvent Effects on Electronic Absorption Spectra of Nitrochlorobenzenes, nitrophenols and nitroaniline-II. Studies in polar solvents, Specrochimica Acta, 1998, 44, 213-220. 37. Magid, L.J.; Kon-no, K.; Martin, C.A. Binding of Phenols to Inverted Micelles and Microemulsion Aggregates, J. Phys. Chem., 1981, 85, 1434-1439. 38. Mitsuo, I. Ultraviolet Absorption Study of the Molecular Association of Phenols, J. Mol. Spect., 1960, 4, 125-143. 39. Aguiar, L.M.Z.; Nascimento, M.G.; Prudencio, G.E.; Rezende, M.C.; Vecchia, R.D. The Preparation of Microemulsion-Based Gels of n-Hexane or Cyclohexane for Enzyme Immobilization. Quimica Nova, 1993, 16, 414-415.
56
40. Silber, J.J.; Biasutti, A.; Abuin, E. Interactions of Small Molecules with Reverse Micelles. Adv. Colloid Interface Sci., 1999, 82 (1-3), 189-252. 41. Martinek, K.; Yatsimirski, A. K.; Levashov, A. V. The Kinetic Theory and the Mechanisms of Micellar Effects on Chemical Reactions, in (Mittal, K.L., Ed.) Micellization, Solubilization, and Microemulsions. ed.; Plenum Press, New York, 1977, 2, 489-507. 42. Garcia-Rio, L.; Ramon Leis, J.; Elena Pena, M.; Iglesias, E. Transfer of the Nitroso Group in Water/AOT/Isooctane Microemulsions: Intrinsic and apparent Reactivity. J. Phy. Chem. 1993, 97, 3437-3442. 43. Montenegro, M.A.; Nazareno, M.A.; Durantini, E.N.; Borsarelli, C.D. Singlet Molecular Oxygen Quenching Ability of Carotenoids in a Reverse Micelle Membrane Mimetic System. Photochem. Photobiol., 2002, 75, 353-361. 44. Bardez, E.; Monnier, E.;Valeur, B. Absorption and Fluorescence Probing of the Interface of Aerosol OT Reversed Micelles and Microemulsions. J. Colloid Interface Sci., 1986, 112, 200-207. 45. Anderson, J.L.; Ding, J.; Welton, T.; Armstrong, D.W. Characterizing Ionic Liquids On the Basis of Multiple Solvation Interactions. J. Am. Chem. Soc., 2002, 124, 14247-14254. 46. Vitha, M.; Carr, P.W. The Chemical Interpretation and Practice of Linear Solvation Energy Relationships in Chromatography. J. Chrom. A, 2006, 1126, 143-194. 47. Torres-Lapasio, J.R.; Coque, M.C.G.; Bosch, M.R.E.; Zissimos, A.M.; Abraham, M.H. Analysis of a Solute Polarity Parameter in Reverse-Phase Liquid Chromatography on a Linear Solvation Relationship Basis. Anal. Chim. Acta, 2004, 515, 209-227.
48. Sprunger, L.; Blake-Taylor, B.H.; Wairegi, A.; Acree Jr., W.E.; Abraham, M.H.; Characterization of the Retention Behavior of Organic and Pharmaceutical Drug Molecules on an Immobilized Artificial Membrane Column with the Abraham Model. J. Chrom. A, 2007, 1160, 235-245.
49. Zingaretti, L.; Correa, N.M.; Boscatto, L.; Chiacchiera, S.M.; Durantini, E.N.; Bertolotti, S.G.; Rivarola, C.R.; Silber, J.J. Distrubution of amines in water/AOT/n-Hexane Reverse Micelles; Influence of the Amine Chemical Structure. J. Colloid Int. Sci., 2005, 286, 245-252.
50. El-Rahman, K.M.A.; El-Sourougy, M.R.; Abdel-Monem, N.M.; Ismail, I.M. Modeling the Sorption Kinetics of Cesium and Strontium Ions on Zeolite A, J. Nuclear & Radiochemical Sciences, 2006, 7, 21-27.
57
51. Gerentr, C.; Lee, V.K.C.; Le Cloirerc, P.; McKay, G. Application of Chitosan for the Removal of Metals from Wastewaters by Adsorption – Mechanisms and Models Review. Critical Reviews in Environmental Science and Technology, 2007, 37, 41-127.
52. Ho, Y.S. Citation Review of Lagergren Kinetic Rate Equation on Adsorption Reactions, Scientometrics, 2004, 59, 171-177.
53. Weber, W.J.; Morris, J. C. Kinetics of Adsorption on Carbon from Solutions. J. Sanit. Eng. Div. Am. Soc. Civ. Eng., 1963, 89, 31-59.
54. Seoud, A.A.; Doheim, M. A Solvent Extraction Study of the Thermodynamics of Aniline and its Nitrate, Canadian J. Chem., 1966, 44, 521-525.
55. Akre, K.P.; Morankar, R.K.; Gaikar, V.G. Selective Solubilization of Nitrophenols and Absorption on Ion Exchange Resins in Nonaqueous Conditions. Separation Sci. Technol., 2006, 41, 3409-3430.
56. Joshi, U.V.; Gaikar, V.G. Adsorption of Nitroanilines on Ion Exchange Resins in Nonaqueous Conditions, Separation Sci. Tehnol., 2004, 39, 1125-1147.
57. Crecchio, C.; Ruggiero, P.; Pizzigallo, N.D.R. Polyphenoloxidases Immobilized in Organic Gels: Properties and Applications in the Detoxification of Aromatic Compounds, Biotechnol. Bioeng., 1995, 48, 585-591. 58. Hamstead, A.C. Destroying Peroxides of Isopropyl Ether. Ind. Eng. Chem., 1964, 56, 37-42. 59. Feinstein, R. Notes: Simple Method for Removal of Peroxides from Diethyl Ether.J. Org. Chem., 1959, 24 (8), 1172-1173. 60. Braun, J.M. PhD Disseration, Wake Forest University, 1999.
58
VITA
Melissa Ann Stouffer was born on April 24th 1972 in Hagerstown, Maryland. She
completed her undergraduate work at Lenoir-Rhyne College in Hickory, North Carolina, and
received a Bachelor of Science in Chemistry in 2000 while working at Alcatel N.A. as an
associate chemist. Melissa entered Wake Forest University in 2001 as a part time student.
After a reduction in workforce at Alcatel she was able to attend full time thanks to the
support of the TAA program. She is now employed with Performance Fibers in New Hill,
North Carolina as a quality control chemist.