Nonaqueous Polar Solvents in Reverse Micelle ... - ACS Publications

Review pubs.acs.org/CR

Nonaqueous Polar Solvents in Reverse Micelle Systems N. Mariano Correa,† Juana J. Silber,† Ruth E. Riter,‡ and Nancy E. Levinger*,§ †

Departamento de Química, Universidad Nacional de Río Cuarto, Agencia Postal #3, C.P. X5804BYA Río Cuarto, Argentina Department of Chemistry, Agnes Scott College, Decatur, Georgia 30030-3770, United States § Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States 1. INTRODUCTION Water’s ability to interact with itself and other molecules through a number of intermolecular interactions including hydrogen bonding leads to interesting and useful assemblies of molecules in a wide range of naturally occurring systems from the precise folding of proteins to the winding of DNA on a chromatin core to the organization of millions of lipids into sheets that delineate the inside and outside of a cell. In nature, water plays a key role for self-assembly; in the laboratory researchers seek motifs that arise in nonaqueous solutions to provide new media for chemistry. The self-assembly of amphiphiles in the absence of water forms the focus of this review. In particular, we discuss the assembly of amphiphiles CONTENTS into microemulsions and reverse micelles in nonpolar solvents while sequestering a polar nonaqueous core. We include a 1. Introduction A glossary of terms used in this review in Table 1. ‡

2. Background 2.1. Aqueous Reverse Micelles 2.2. Definition of Aqueous and Nonaqueous Reverse Micelles 3. Characterization of Reverse Micelles 3.1. Methods Demonstrating the Presence of Reverse Micelles 3.2. Surfactant Packing at the Reverse Micelle Interface 4. Reverse Micelles Encapsulating Nonaqueous Polar Organic Solvents 4.1. Reverse Micelles Containing Glycerol 4.2. Reverse Micelles Containing Other Polyols 4.3. Reverse Micelles Containing Formamide 4.4. Reverse Micelles Containing Other Amides: DMF and DMA 4.5. Other Polar Solvents 5. Requirements for Effective Reverse Micelle Formation 5.1. Polar Solvent Properties 5.2. Problem Systems: Methanol and Acetonitrile 6. Room Temperature Ionic Liquids in Reverse Micelles 7. Nonaqueous Reverse Micelles as Nanoreactors 8. Future Directions and Cautions Author Information Corresponding Author Notes Biographies Acknowledgments References

A A

2. BACKGROUND 2.1. Aqueous Reverse Micelles

D E

The unique properties of water including its polarity and highly structured hydrogen bond network have led to the widespread use of amphiphilic molecules to form self-assembled structures. Certain amphiphilic molecules can form reverse micelles in nonpolar solvents in the presence of water and some polar solvents.1−5 Reverse micelles form when an amphiphile delineates a nanoscale droplet of the aqueous phase from a nonpolar medium, coating the surface of an isolated water droplet in solution, as depicted in Figure 1. These nanoscopic water pools have shown utility for a range of applications from nanoparticle synthesis,3,6−11 to enhancement of chemical reaction rates,12−15 and to models for water in biological confinement.16−20 Reverse micelles have been explored for many years and several excellent reviews about their properties have appeared in the literature.1−5,21−27 Varied techniques have been applied to learn about the special properties of these systems, especially focusing on the changes incurred to the nanoscopic pool of water on the reverse micellar interior. Reverse micelle shape and size has been characterized through a range of methods, including scattering techniques such as small angle neutron scattering (SANS), 4,28−32 small-angle X-ray scattering (SAXS),4,33,34 and both dynamic and static light scattering.21,35−38 The nature of the water inside the reverse micelles has been explored through various steady-state and timeresolved spectroscopies. These studies have found that properties of the water in smaller reverse micelles depart

E F J J L N P Q R R S U Y AB AC AC AC AC AD AD

Received: July 7, 2011

© XXXX American Chemical Society

A

dx.doi.org/10.1021/cr200254q | Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review a

Table 1. Glossary of Terms and Abbreviations Used in This Review abbreviations

name

property

ACN bmim+ bm2im+ bm2im+N(CN)2− bm2im+NCS− bmim+BF4− bmim+PF6− CiEj (Cm)2DABr N(CN)2− DEG DMA EAN emim+ emim+EtOSO3− emim+Tf2N− emim+HexOSO3− ϕc P13+Tf2N− pmim+BF4− 9-AM ACN ANS AOB AOT AuO BC 1 BHDC Brij 30 Brij-52 Brij-56 Brij-72 Brij-92 C10E5 C12E5 C152 C152A C153 C343 C480 C490 cmc CTAB DLS DMABA DMABN DMF DMSO DOSY DQ EG FA FFEM FTIR GY HLB HSq ICT IL/O microemulsions MB MeOH

acetonitrile 1-butyl-3-methylimidazolium 1-butyl-2,3-dimethylimidazolium 1-butyl-2,3-dimethylimidazolium dicyanamide 1-butyl-2,3-dimethylimidazolium thiocyanate 1-butyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium hexafluorophosphate alkyl oligoethyleneoxide dialkylmethylammonium bromide dicyanamide diethylene glycol dimethylacetamide ethyl ammonium nitrate 1-ethyl-3-methylimidazolium 1-ethyl-3-methylimidazolium-ethylsulfate 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-ethyl-3-methylimidazolium-hexylsulfate critical volume fraction N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide 1-pentyl-3-methyl-imidazolium tetra-fluoroborate 9-anthracene methanol (Figure 11) acetonitrile 8-anilino-1-naphthalenesulfonic acid (Figure 13) acridine orange base (Figure 11) sodium bis (2-ethylhexyl) sulfosuccinate (Figure 2) Auramine O (Figure 13) cyano derivative of 7-diehylamino coumarinyl benzopyrano pyridine (Figure 15) benzylhexadecyldimethylammonium chloride polyoxyethylene(4) lauryl ether polyoxyethylene(2) cetyl ether polyoxyethylene(10) cetyl ether polyoxyethylene(2) stearyl ether polyoxyethylene(2) oleyl ether pentaethylene glycol mono-n-decyl ether pentaethylene glycol mono-n-dodecyl ether coumarin 152 (also known as coumarin 485) (Figure 18) coumarin 152A (also known as coumarin 481) (Figure 18) coumarin 153 (also known as coumarin 540A) (Figure 18) coumarin 343 (also known as coumarin 519) (Figure 11) coumarin 480 (also known as coumarin 102) (Figure 8) coumarin 490 (also known as coumarin 151) (Figure 8) critical micelle concentration cetyltrimethyl ammoniun bromide dynamic light scattering p-N,N-dimethylaminobenzoic acid (Figure 19) p-N,N-dimethylaminobenzonitrile (Figure 19) N,N′-dimethylformamide dimethyl sulfoxide diffusion ordered spectroscopy NMR duroquinone (Figure 26) ethylene glycol formamide freeze-fracture electron microscopy Fourier transform infrared spectroscopy glycerol hydrophile−lipophile balance bis[4-(dimethylamino)phenyl] squaraine (Figure 7) intramolecular charge transfer ionic-liquid-in-oil microemulsions methylene blue (Figure 20) methanol

polar solvent ionic liquid component ionic liquid component ionic liquid ionic liquid ionic liquid ionic liquid surfactant surfactant ionic liquid component polar solvent polar solvent ionic liquid component ionic liquid component ionic liquid ionic liquid ionic liquid characteristic ionic liquid ionic liquid molecular probe polar solvent molecular probe molecular probe surfactant molecular probe molecular probe surfactant surfactant surfactant surfactant surfactant surfactant surfactant surfactant molecular probe molecular probe molecular probe molecular probe molecular probe molecular probe characteristic surfactant method molecular probe molecular probe polar solvent polar solvent method reactant polar solvent polar solvent method method polar solvent characteristic molecular probe method characteristic molecular probe polar solvent

B


Chemical Reviews

Review

Table 1. continued abbreviations MO MV2+ N3111+ NaDEHP NCS− N-EtFOSA NMA NMF NMR omim+ o-NA P P-13+ PFG-NMR PFGSE-NMR PG Pluronic surfactants PRODAN QB rH ROESY RTIL SANS SAXS SDS SNAr Surfynol-2502 TEG TEM Tf2N− TGE TMGA TMGL TMGT TTEG Tween80 TX-100 Vm w0 w/o WS ZnTPP a

name

property

molecules methyl orange (Figure 20) methyl viologen (Figure 26) N,N,N-trimethyl-N-propyl ammonium sodium diethylhexylphosphate (Figure 30) thiocyanate N-ethyl perfluorooctylsulfonamide (C2H5NHSO2C8F17) N-methylacetamide N-methylformamide nuclear magnetic resonance octylmethylimidazolium ortho-nitroaniline (Figure 17) effective packing parameter N-methyl-N-propylpyrrolidinium pulsed-field gradient NMR pulsed-field gradient spin-echo NMR propylene glycol ethylene oxide/propylene oxide block copolymers 6-propionyl-2-(N,N-dimethyl) aminonaphthalene, (Figure 27) 1−8-methylquinolinium (Figure 6) hydrodynamic radius rotating-frame nuclear Overhauser effect correlation spectroscopy room temperature ionic liquids small angle neutron scattering small angle X-ray scattering sodium dodecyl sulfate nucleophilic aromatic substitution proprietary alkoxylated acetylenic diol-based nonionic surfactant triethylene glycol transmission electron microscopy (trifluoromethylsulfonyl)imide tri(ethylene glycol) monomethyl ether 1,1,3,3- tetramethylguanidinium acetate (Figure 21) 1,1,3,3-tetramethylguanidinium lactate (Figure 21) 1,1,3,3-tetramethylguanidinium trifluoroacetate (Figure 21) tetraethylene glycol ethoxylated sorbitan monooleate polyoxyethylene tert-octylphenyl ether solvent molar volume [water]/[surfactant] water-in-oil [polar solvent]/[surfactant] zinc tetraphenylporphyrin (Figure 26)

molecular probe reactant ionic liquid component surfactant ionic liquid component surfactant polar solvent polar solvent method ionic liquid component molecular probe characteristic ionic liquid component method method polar solvent surfactant reactant molecular probe characteristic method polar solvent method method surfactant reaction surfactant polar solvent method ionic liquid component surfactant model ionic liquid ionic liquid ionic liquid polar solvent surfactant surfactant characteristic characteristic characteristic characteristic reactant

For molecular structures provided, we list the figure showing the structure.

from the properties of bulk water. For example, experiments utilizing fluorescent probe molecules have shown increased microviscosity and slower solvation dynamics.18,21,39−44 Likewise, molecular probes have allowed researchers to map the polarity within reverse micelles.45 Although other surfactants have been used, most of the studies about reverse micelles have focused on systems utilizing the surfactant Aerosol OT (sodium diethylhexyl sulfosuccinate, sodium docusate, AOT) as the amphiphile (see Figure 2).1 This surfactant forms reverse micelles in a wide range of nonpolar environments including alkanes, aromatic solvents, haloalkanes, and supercritical alkane solutions. The sizes of particles formed depend on the water to surfactant ratio, often described by the equation,

Figure 1. Cartoon representation of a reverse micelle.

w0 = [H 2O]/[surfactant] C

(1)


Chemical Reviews

Review

Using a unique IR pump-anti-Stokes Raman probe technique, Dlott and co-workers have followed energy transfer from high energy OH and CH stretching vibrations to lower frequency vibrational modes in the intramicellar water, the surfactant layer and surrounding nonpolar solvent.95,96 Owrutsky and coworkers have measured vibrational relaxation of small ionic probes such as azide or cyanoferrate to explore the micelle interior using time-resolved IR spectroscopy.103−107 As new tools become available, researchers will undoubtedly apply them to explore structure and dynamics of these confined systems. For example, very recently, Penwell and Wright have investigated AOT reverse micelles using ultrafast coherent multidimensional spectroscopies.110 Many other techniques have been used to study reverse micelles encapsulating water; we focus here on the formation and properties of reverse micelles that encapsulate a nonaqueous polar solvent rather than water as a solvent pool formed in the nonpolar solvent. We aim to document the formation and properties of reverse micelles and microemulsions formed with polar solvents other than water. Although many reports of nonaqueous reverse micelles have appeared, pitfalls exist to trap researchers exploring these nonaqueous systems.

Figure 2. Chemical structure for the common surfactant AOT (sodium diethylhexylsulfosuccinate or sodium dioctylsulfosuccinate).

(w0 is sometimes also referred to as R or ω). Properties of the reverse micelles including particle size also depend on other factors such as the nonpolar phase, temperature, presence of a cosurfactant, etc.1 Although AOT is without doubt the most thoroughly studied surfactant for reverse micelles, other surfactants effectively form reverse micelles in a range of nonpolar solvents with and without cosurfactant.46 In addition to traditional methods used to explore reverse micelles, in the past decade, several new methods have been applied to investigate reverse micelles. For example, although only four research groups published papers reporting computer simulations of reverse micelles before 2000,47−50 since then several groups have actively simulated these systems.37,38,49,51−78 With a model that combined atomistic calculations of water and sodium ions but approximated the impact of the AOT headgroup with a single charged site with nonpolar surfactant tails and nonpolar solvent modeled using a dielectric continuum, Faeder and Ladanyi stimulated new interest in modeling reverse micelles.52 More recently, several groups, including Ladanyi, have modeled various reverse micelles including all atoms explicitly or through coarse grain models.56,57,59,62,63,74−76,79 Recently Laria and co-workers have modeled reverse micelles sequestering solvents other than water.71 Another new method recently applied to the study of reverse micelles utilizes time-resolved vibrational spectroscopies.80−109 For example, Fayer and co-workers82−94 and Bakker and coworkers97−102 followed the vibrational relaxation of isotopically labeled water, HOD through transient absorption experiments probing the OD stretch in H2O (Fayer) or the OH stretch in D2O (Bakker). Cringus et al. have explored the relaxation and energy transfer of H2O stretching vibrations for pure water in reverse micelles.108,109 The Fayer group has also used vibrational echo measurements to explore water inside the reverse micelles.83,90,91 Various groups have measured deposition of energy in the reverse micelles as heat.80,81,90,94,99,101,109

2.2. Definition of Aqueous and Nonaqueous Reverse Micelles

Reverse micelles form a subset of structures that can exist in water-in-oil (w/o) microemulsions. Before discussing reverse micelle formation in systems lacking water, it is useful to define terms we use in this review. Microemulsions include a wide range of macroscopically homogeneous, thermodynamically stable liquid systems of water, oil, and amphiphile. Danielsson and Lindman have suggested the definition of a microemulsion as a system of water, oil, and amphiphile which forms a single, optically isotropic phase in a thermodynamically stable liquid solution.111 Microemulsion systems have no definitive stoichiometry but include a certain degree of intermolecular arrangement. Within their definition of a microemulsion, Danielsson and Lindman suggest a range of examples of microemulsions given in Table 2. Their definitions include lipophilic micelle surfactant solutions containing solubilized water, “water-in-oil microemulsions”, in which reverse micelles form. Friberg agrees with the Danielsson and Lindman’s definitions but points out that that the condition of thermodynamic stability appears cumbersome and unnecessarily limits the possible systems.112 He suggests that a change of the definition requiring spontaneous formation would be more suitable.

Table 2. Examples of Systems Considered To Be Microemulsions and Contrasting Systems That Are Not Microemulsions, According to the Definition of Danielsson and Lindemann (ref 111)a systems considered to be microemulsions

systems that are NOT microemulsions

• Aqueous micellar surfactant solution containing solubilized surfactant; “oil-in-water microemulsion” • Lipophilic micellar surfactant solution containing solubilized water, “water-in-oil microemulsions” • Systems displaying a continuous transition from an aqueous to a lipophilic solution

• Aqueous solutions of surfactants, micellar or nonmicellar, without additives

• “Surfactant phase” in nonionic surfactant systems

a

• Aqueous solutions of surfactants containing only inorganic electrolytes as additives • Aqueous solutions of surfactants containing only water-soluble nonelectrolytes as additives • Liquid crystalline systems • Real emulsions that are thermodynamically unstable • Surfactant-free systems of any kind

Excerpt of text reprinted with permission from ref 111: Danielsson, I.; Lindman, B. Colloids Surf. 1981, 3, 391. Copyright 1981 Elsevier. D


Chemical Reviews

Review

The examples in Table 2 present a generalized definition for a microemulsion; as a subset of microemulsions, the term reverse micelle requires further discussion. It is clear that reverse micelles are w/o microemulsions but not all w/o microemulsions can be considered reverse micelles. In the literature, reverse micelles have been defined as noncontinuous aggregates of surfactant molecules that delineate a polar nanophase from a nonpolar phase.1,21,25,46 For the purposes of this review, we define reverse micelles with the following properties: (1) The overall viscosity of the solution is the same as or very nearly the same as the viscosity of the neat nonpolar solvent in which the reverse micelles form. (2) The solution contains individual particles that diffuse in the nonpolar solvent. A diffusion coefficient can be defined and measured for individual particles. (3) The solution includes an interface that delineates polar and nonpolar parts. (4) The solution includes three identifiable phases, that is a continuous nonpolar phase, a polar phase and the interface between them. (5) The size of the particles in the solution depends upon the amount of polar solvent. Ideally, reverse micelles form as discrete, spherical droplets that interact only through interparticle collisions.113 However, sometimes droplets may be noncontinuous but adopt a form other than spherical.114−119 Water is usually considered a necessary component of microemulsions. However, if water in usual microemulsion recipes is replaced with a polar organic compound that is also immiscible with the nonpolar phase, then microemulsions can form. Several key characteristics of polar organic solvents like propylene glycol (PG), ethylene glycol (EG), and formamide (FA) are similar to water, for example, the hydrogen-bond network, relatively high dielectric constants, high surface tension between the polar solvents and a nonpolar phase leading to strong immiscibility. For the purposes of this review, in analogy with w0 for aqueous systems, we define the parameter WS as the molar ratio of the nonaqueous polar component to the surfactant, WS = [polar solvent]/[surfactant]

Figure 3. Droplet diameter values (nm) of polar solvents/AOT/nheptane RMs varying WS determined from dynamic light scattering measurements. [AOT] = 0.1 M. (Reproduced by permission from ref 144. Copyright 2009 PCCP Owner Societies.)

3. CHARACTERIZATION OF REVERSE MICELLES 3.1. Methods Demonstrating the Presence of Reverse Micelles

Researchers have used a wide range of techniques to study reverse micelles. However, to demonstrate the presence of reverse micelles, as we have defined them in section 2.2, requires certain methods. As noted above, unlike microemulsions, defined only by the presence of a clear single macroscopic phase in solution, reverse micelles exist in only a subset of these phases. Ideally, to determine if reverse micelles are present in a solution, one must be able to observe the individual particles existing in solution. The best ways to do this is to employ scattering methods, namely, dynamic light scattering (DLS),1,35,36,38,113,120 SANS,28,119,121 or SAXS.34,122,123 Historically, small angle scattering techniques have been considered the premiere methods to uncover details about the structures in microemulsions.28,124 These techniques demonstrate the size of nanoscopic features in the microemulsions, and, with fitting to appropriate form and structure factors, can also demonstrate particle shapes.28 However, the generation of neutron and /or high intensity and energy X-ray beams needed for these experiments require large accelerator facilities. Furthermore, analysis of SANS and SAXS data requires some knowledge of the system and state-of-the-art fitting to the data. This places these techniques out of the reach of many researchers who do not have access to the specialized and sophisticated equipment. Signals from DLS, also known as photon correlation spectroscopy, arise from the correlation of light scattered by the same particle.125 DLS data coupled with knowledge of the solution viscosity makes it possible to determine the diffusion coefficient for the particles. To determine a size parameter for the particles, most DLS analyses of reverse micelles assume that the particles adopt a spherical form. With the benchtop DLS instruments available at large from several different vendors, this technique has become a staple of reverse micelle characterization. Each of these methods, SANS, SAXS, and DLS, provides direct evidence for several properties associated with reverse micelles. SANS, SAXS, and DLS signals all arise from scattering at the particle surfaces. They directly demonstrate the presence of an interface in the solution, and delineation between the

(2)

For example, for a system in which FA is emulsified using AOT, WS = [FA]/[AOT] = nFA/nAOT where nFA and nAOT are the number of moles or molecules of formamide and AOT, respectively. The polar solvent and surfactant for a specific use of WS pertains to the system under discussion. In true reverse micelles formed without added water, the system should display a size dependence on WS, as shown in Figure 3 for several nonaqueous polar solvents and for water. This review describes the studies of nonaqueous reverse micelles and microemulsion systems utilizing polar solvents other than water. In reality, unless working in a completely water-free, controlled environment, trace amounts of water will be present in any nonaqueous reverse micelle formed. Here, we review results as reported by the original authors without discussing the role of trace water in these systems. We discuss in more detail the first reports and some of the controversies that are still not resolved about these systems. E


Chemical Reviews

Review

3.2. Surfactant Packing at the Reverse Micelle Interface

polar and nonpolar phases by the amphiphiles. Additionally, all three methods measure particle size, so they can follow the swelling of particles with added polar solvent. The correlation of photons scattered that comprises DLS data directly reflects the diffusion of particles in solution. DLS data in Figure 3 show the dependence of the reverse micelle hydrodynamic radius, rH, as a function of w0 (water) or WS (nonaqueous polar solvent) for a few different systems, where rH is the apparent radius calculated with the assumption that the reverse micelle is a hard sphere diffusing through a solvent. Reverse micelle presence and size can also be determined through fluorescence correlation spectroscopy measurements.126−129 Correlation between fluorescent photons collected from molecules in the same reverse micelles provides a measure of diffusion coefficient similar to DLS. This technique could be particularly useful for understanding systems where the presence of a dye molecule impacts the character of the reverse micelles.130 In addition to scattering techniques, researchers have utilized other methods to infer the presence of isolated particles in clear solutions showing a single macroscopic phase. If reverse micelles diffuse freely in solution without significant interparticle interactions, then the solutions should have essentially the same viscosity as the pure nonpolar solvent. Thus, measures of viscosity have been used to infer the presence of isolated particles diffusing in nonpolar solvent, that is, reverse micelles.36,115,131,132 Although experiments measuring viscosity do not directly show the presence of particles, they provide strong inference for their presence in the microemulsion solutions. Similarly, researchers have used conductivity to infer the presence of isolated particles in solutions supposedly containing reverse micelles.133,134 To gauge the presence of isolated particles in solutions of normal or direct micelles that form in water, conductivity can be highly effective. However, for reverse micelles, the conductivity of the supporting nonpolar solvent is usually very low, often below the sensitivity for most standard conductivity meters. This makes it difficult to measure the solution conductance accurately enough to determine solution structure. NMR spectroscopy is another method that has demonstrated the presence of reverse micelles in microemulsions. In particular, NMR techniques like pulsed-field gradient (PFGNMR) or pulsed-field gradient spin echo (PFGSE-NMR) that measure diffusion of particles can, in principle, yield information about solutions similar to DLS.22,46,135−137 Molecules diffusing as a part of a reverse micelle aggregate display substantially slower diffusion than the isolated molecule; determination of the diffusion coefficient can provide a measure of particle volume similar to DLS. Finally, it is very common for researchers to assume the presence of isolated reverse micelle particles in microemulsion solution simply in analogy to experiments that have been reported in the literature. For solution parameters, that is, components, concentrations, temperatures, pressures, etc., that have been amply documented, it is likely that samples prepared have similar characteristics to those reported in the literature. However, given the sensitivity of solutions including interfaces to the presence of impurities, characterization of solutions by techniques that provide unequivocal evidence for reverse micelle particles is highly recommended.

In the self-assembly of any amphiphiles, the surface area of each amphiphilic molecule plays a significant role in the supramolecular structures.124,138 The nature of the phases created in these mixtures depends intimately on the geometry of molecules at the interface.1,133,139−141 Among other variables, the propensity for a particular solution to lead to reverse micelle formation and the sizes of the particles formed depend on the effective packing parameter of the surfactants, P = v /alc

(3)

Here, v and lc are the volume and the length of the surfactant hydrocarbon chain(s), respectively, while a is the effective surfactant headgroup area.1,133,139−141 When alc equals v, then the surfactant has a cylindrical geometry and does not work well to form reverse micelles. Researchers have shown that effective reverse micelle formation occurs when P > 1, that is, when the excess volume of the hydrocarbon chains leads to a truncated cone shaped form. The interaction of polar solvents with the surfactant at the reverse micelle interface can lead to an increase in a. This is well established for AOT in water/AOT/isooctane RMs.1,141 Maitra demonstrated that the AOT a value increases from 36 to 51 Å2 as w0 increases from 4 to 20; they attribute this change in headgroup area to water binding to the AOT polar headgroup at the reverse micelle interface.141 That is, the reverse micelle size is larger when P is smaller.133,139,140 Consequently, any factor that causes an increase in the surfactant effective a value will cause a decrease of the effective packing parameter and the consequent increases in the reverse micelle droplet size. Understanding the relationship between the reverse micelle size and WS can provide information about the surfactant packing parameter and surface area. Very recently, Durantini et al. presented a detailed analysis of the differing interactions between the polar solvents and the AOT surfactant in nonaqueous polar solvent encapsulating reverse micelles. Through FTIR measurements marking changes in the AOT carbonyl and sulfonate vibrational modes, they were able to demonstrate which region of the AOT molecular structure interacts with the nonaqueous polar solvents.142 Importantly, they discuss how these interactions could affect the surfactant a value and consequently the nonaqueous AOT reverse micelle sizes. Various papers have noted that reverse micelles containing hydrogen-bond donating solvents, like water, glycerol (GY), and EG have larger sizes than those encapsulating solvents that do not donate hydrogen bonds, like dimethylformamide (DMF) or dimethylacetamide (DMA).113,120,143,144 Durantini et al. correlated changes in the AOT vibrational spectra with the reverse micelle size to find how a, the effective surfactant area, and P, the packing parameter, depend on the nonaqueous polar solvent properties.142 For example, they showed that GY and water bind to the AOT sulfonate group which increases the AOT a value beyond that for reverse micelles encapsulating only water. EG interacts with the AOT carbonyl, leading to greater oil penetration of the interface and a larger effective a value. The spectroscopic studies show that intramicellar DMF and DMA interact primarily with the Na+ counterions without dissociating them from the AOT sulfonate. Because neither DMF nor DMA solvates the AOT carbonyl nor sulfonate, the AOT a value should remain constant with WS. Indeed, DMF and DMA/AOT/n-heptane RMs droplet sizes increase with increasing WS as expected for a system with constant a.2 F


Chemical Reviews

Review

Table 3. Compositions and Methods Used to Study Reverse Micelles Containing Nonaqueous Organic Polar Solvent polar solvent glycerol (GY)

ethylene glycol (EG)

propylene glycol (PG)

surfactant

nonpolar solvent

AOT CTAB AOT AOT

n-heptane n-heptane/chloroform n-heptane n-heptane

AOT AOT AOT AOT AOT AOT AOT AOT C12E5 C12E5 C12E5 AOT AOT AOT AOT AOT AOT AOT AOT

n-heptane n-heptane n-heptane n-hexane isooctane isooctane isooctane isooctane n-heptane n-dodecane n-hexadecane decane ethylbenzene n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane

AOT AOT AOT

n-heptane n-heptane n-heptane

AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT

n-heptane n-heptane n-heptane heptane/ chloroform n-hexane n-hexane isooctane isooctane isooctane isooctane isooctane

AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT

isooctane isooctane isooctane octane octane toluene xylene decane n-heptane n-heptane n-heptane n-heptane

AOT AOT AOT AOT AOT AOT AOT AOT

n-heptane n-heptane n-hexane isooctane isooctane octane toluene xylene

experiment

refs

DLS, viscosimetry DLS, viscosimetry, phase-separation studies QB absorption spectroscopy and AOB fluorescence quenching of 9-AM, absorption spectroscopy of AOB time-resolved emission spectroscopy of AOB conductivity, viscosimetry, dielectric relaxation DLS, FTIR absorption spectroscopy of QB and AOB conductivity, viscosimetry, dielectric relaxation DLS FTIR, 1H NMR fluorescence depolarization of HSq NMR self-diffusion, SANS NMR self-diffusion, SANS NMR self-diffusion, SANS DLS, absorption spectroscopy of C343 conductivity, viscosimetry conductivity, viscosimetry, calorimetry DLS conductivity, viscosimetry, dielectric relaxation DLS, FTIR absorption spectroscopy of QB and AOB fluorescence quenching of 9-AM, absorption spectroscopy of AOB time-resolved emission spectroscopy of AOB photophysical properties of PRODAN photophysical properties of ZnTPP FTIR,1H NMR fluorescence spectroscopy of AuO and ANS kinetics study absorption spectroscopy of QB and AOB fluorescence spectroscopy of AuO and ANS conductivity, viscosimetry, calorimetry DLS, absorption spectroscopy of C343 DLS fluorescence depolarization of HSq absorbance, emission, and time-resolved spectroscopies of C343 photophysical properties of ZnTPP FTIR, 1H NMR fluorescence spectroscopy of AuO and ANS conductivity, viscosimetry, calorimetry fluorescence spectroscopy of AuO and ANS conductivity, viscosimetry, calorimetry conductivity, viscosimetry, calorimetry DLS, absorption spectroscopy of C343 conductivity, viscosimetry, calorimetry DLS, FTIR absorption spectroscopy of QB and AOB fluorescence quenching of 9-AM, absorption spectroscopy of AOB time-resolved emission spectroscopy of AOB photophysical properties of PRODAN absorption spectroscopy of QB and AOB conductivity, viscosimetry, calorimetry DLS, absorption spectroscopy of C343 conductivity, viscosimetry, calorimetry conductivity, viscosimetry, calorimetry conductivity, viscosimetry, calorimetry G

Fletcher et al.145 Fletcher et al.146 Falcone et al.150 Silber et al.152 Falcone et al.153 Mehta et al.185 Durantini et al.142 Falcone et al.150 Mathew et al.131 Laia et al.120,147 El Seoud et al.148 Laia and Costa151 Martino and Kaler167 Martino and Kaler167 Martino and Kaler167 Riter et al.113 Mehta et al.156 Ray and Moulik132 Laia et al.120 Mehta et al.185 Durantini et al.142 Falcone et al.150 Silber et al.152 Falcone et al.153 Novaira et al.282 López-Cornejo and Costa;271 Costa et al.272 Novaki et al.157 Moore and Palepu163 Atay and Robinson266 Falcone et al.150 Moore and Palepu163 Ray and Moulik132 Riter et al.113 Laia et al.120 Laia and Costa151 Correa and Levinger158 Lopez-Cornejo and Costa271 Novaki et al.157 Moore and Palepu163 Ray and Moulik132 Moore and Palepu163 Ray and Moulik132 Ray and Moulik132 Riter et al.113 Ray and Moulik132 Durantini et al.142 Falcone et al.150 Silber et al.152 Falcone et al.153 Novaira et al.282 Falcone et al.150 Ray and Moulik132 Riter et al.113 Ray and Moulik132 Ray and Moulik132 Ray and Moulik132


Chemical Reviews

Review

Table 3. continued polar solvent

diethylene glycol (DEG)

triethylene glycol (TEG)

tetraethylene glycol (TTEG)

formamide (FA)

N-methylformamide (NMF) dimethyl formamide (DMF)

surfactant

nonpolar solvent

experiment

refs

C12E5 C12E5 C12E5 AOT

n-heptane n-dodecane n-hexadecane n-heptane

NMR self-diffusion, SANS NMR self-diffusion, SANS NMR self-diffusion, SANS fluorescence spectroscopy of AuO and ANS

Martino and Kaler167 Martino and Kaler167 Martino and Kaler167 Moore and Palepu163

AOT AOT AOT AOT

n-hexane isooctane octane n-heptane

fluorescence fluorescence fluorescence fluorescence

spectroscopy spectroscopy spectroscopy spectroscopy

of of of of

AuO AuO AuO AuO

and and and and

ANS ANS ANS ANS

Moore Moore Moore Moore

and and and and

Palepu163 Palepu163 Palepu163 Palepu163

AOT AOT AOT AOT

n-hexane isooctane octane n-heptane

fluorescence fluorescence fluorescence fluorescence

spectroscopy spectroscopy spectroscopy spectroscopy

of of of of

AuO AuO AuO AuO

and and and and

ANS ANS ANS ANS

Moore Moore Moore Moore

and and and and

Palepu163 Palepu163 Palepu163 Palepu163


n-hexane isooctane octane cyclohexane decane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane

Moore and Palepu163 Moore and Palepu163 Moore and Palepu163 Pomata et al.71 Riter et al.113 Ray and Moulik132 Laia et al.120,147 Arcoleo et al.173 Correa et al.135 Falcone et al.150 Hazra et al.177 Silber et al.152


n-heptane n-heptane n-heptane n-heptane n-hexane isooctane isooctane isooctane isooctane isooctane isooctane isooctane

AOT AOT AOT Ci E j (Cm)2DABr CTAB(1butanol) AOT

isooctane divinylbenzene styrene n-octane n-octane isooctane

fluorescence spectroscopy of AuO and ANS fluorescence spectroscopy of AuO and ANS fluorescence spectroscopy of AuO and ANS molecular dynamics simulation DLS, absorption spectroscopy of C343 conductivity, viscosimetry, calorimetry DLS conductivity, viscosimetry, permittivity, FTIR FTIR, 1H NMR absorption spectroscopy of QB and AOB time-resolved emission spectroscopy of C152 fluorescence quenching of 9-AM, absorption spectroscopy of AOB time-resolved emission spectroscopy of C480 transient vibration spectroscopy of azide time-resolved emission spectroscopy of AOB photophysical properties of PRODAN absorption spectroscopy of QB and AOB conductivity, viscosimetry, calorimetry DLS, absorption spectroscopy of C343 DLS FTIR, 1H NMR time-resolved fluorescence spectroscopy of C343 photophysical properties of BC I absorbance, emission, and time-resolved spectroscopies of C343 FTIR of azide and cyanate probes DLS DLS conductivity, cmc measurements, SANS conductivity, cmc measurements SAXS

n-heptane

Arcoleo et al.173

AOT

decane

conductivity, viscosimetry, permittivity, density, FTIR DLS, absorption spectroscopy of C343

AOT AOT AOT AOT

n-heptane n-heptane n-heptane n-heptane

Ray and Moulik132 Durantini et al.142 Falcone et al.150 Silber et al.152

AOT AOT AOT AOT AOT AOT

n-heptane n-heptane n-heptane n-heptane n-heptane n-hexane

conductivity, viscosimetry, calorimetry DLS, FTIR absorption spectroscopy of QB and AOB fluorescence quenching of 9-AM, absorption spectroscopy of AOB time-resolved emission spectroscopy of C480 time-resolved emission spectroscopy of AOB photophysical properties of PRODAN kinetics study kinetics of SNAr reaction absorption spectroscopy of QB and AOB H

Shirota and Segawa176 Sando et al.175 Falcone et al.153 Novaira et al.282 Falcone et al.150 Ray and Moulik132 Riter et al.113 Laia et al.120,147 Correa et al.135 Riter et al.159 Raju and Costa174 Correa and Levinger158 Owrutsky et al.317 Sapp and Elliot286 Sapp and Elliot286 Schubert et al.179,180 Schubert et al.179 Auvray et al.170

Riter et al.113

Shirota and Segawa176 Falcone et al.153 Novaira et al.282 Atay and Robinson266 Correa et al.269 Falcone et al.150


Chemical Reviews

Review

Table 3. continued polar solvent

dimethyl acetylamide (DMA)

dimethylsulfoxide (DMSO) methanol (MeOH)

acetonitrile (ACN)

2-pyrrolidinone acetamide acrylamide N-methylurea urea

surfactant

nonpolar solvent

experiment

refs Ray and Moulik132 Riter et al.113 Correa and Levinger158

octane toluene xylene cyclohexane

conductivity, viscosimetry, calorimetry DLS, absorption spectroscopy of C343 absorbance, emission, and time-resolved spectroscopies of C343 conductivity, viscosimetry, calorimetry conductivity, viscosimetry, calorimetry conductivity, viscosimetry, calorimetry conductivity, viscosimetry

decane

phase behavior, critical behavior, refractive index

Peng et al.318

AOT AOT AOT AOT AOT

n-heptane n-heptane n-heptane n-heptane n-heptane

Ray and Moulik132 Durantini et al.142 Novaira et al.282 Falcone et al.150 Silber et al.152

AOT AOT AOT AOT

n-heptane n-hexane isooctane isooctane

AOT AOT AOT

octane xylene toluene cyclohexane

conductivity, viscosimetry, calorimetry DLS, FTIR photophysical properties of PRODAN absorption spectroscopy of QB and AOB fluorescence quenching of 9-AM, absorption spectroscopy of AOB time-resolved emission spectroscopy of AOB absorption spectroscopy of QB and AOB conductivity, viscosimetry, calorimetry absorption, emission, and time-resolved spectroscopies of C343 conductivity, viscosimetry, calorimetry conductivity, viscosimetry, calorimetry conductivity, viscosimetry, calorimetry DLS, FTIR, absorption spectroscopy of QB

AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT AOT lecithin AOT AOT AOT AOT AOT

decane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane isooctane decane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane isooctane n-hexane carbon tetrachloride carbon tetrachloride carbon tetrachloride carbon tetrachloride carbon tetrachloride carbon tetrachloride n-heptane n-heptane

DLS, absorption spectroscopy of C343 conductivity, viscosimetry, dielectric relaxation time-resolved emission spectroscopy of C343 time-resolved emission spectroscopy of C152A time-resolved emission spectroscopy of C152 time-resolved emission spectroscopy of C490 time-resolved emission spectroscopy of C153 photophysical properties of DMABA and DMABN photophysical properties of DDPT DLS, absorption spectroscopy of C343 DLS, absorption spectroscopy of C343 time-resolved emission spectroscopy of C343 time-resolved emission spectroscopy of C152A time-resolved emission spectroscopy of C152 time-resolved emission spectroscopy of C490 time-resolved emission spectroscopy of C153 photophysical properties of DMABA and DMABN DLS, absorption spectroscopy of C343 conductivity, viscosimetry, dielectric relaxation FTIR FTIR, 1H NMR, SAXS FTIR, 1H NMR, SAXS FTIR viscosimetry, permittivity, density, X-ray diffraction FTIR FTIR, SANS time-resolved spectroscopy of C343

Riter et al.113 Mehta et al.185 Shirota and Horie208 Hazra et al.212 Hazra et al.177 Hazra and Sarkar214 Hazra et al.213 Hazra et al.211 Ozcelik and Atay319 Riter et al.113 Riter et al.113 Shirota and Horie208 Hazra et al.212 Hazra et al.177 Hazra and Sarkar214 Hazra et al.213 Hazra et al.211 Riter et al.113 Mehta et al.185 Ruggirello and Liveri189 Calandra et al.122 Calandra et al.122 Calvaruso et al.188 Ruggirello and Liveri186 Caponetti et al.121 Caponetti et al.121 Sengupta et al.

AOT AOT AOT

isooctane isooctane isooctane

AOT AOT AOT TX-100 (1propanol) AOT

Ray and Moulik132 Ray and Moulik132 Ray and Moulik132 Tessy and Rakshit183

Falcone et al.153 Falcone et al.150 Ray and Moulik132 Correa and Levinger158 Ray and Moulik132 Ray and Moulik132 Ray and Moulik132 Elles and Levinger184

headgroup.135 At the same time, FA maintains hydrogen bonds between FA molecules. Given its interactions with AOT, one might ponder why this solvent leads to the largest reverse micelles. One logical explanation is that through its electrostatic interaction, FA forms part of the AOT molecular structure, possibly as counterion, leading to dramatic increases in the a value of the surfactant.

The case of formamide (FA)/AOT/n-heptane is perhaps the most interesting because AOT reverse micelles with FA are the largest reverse micelles for a given WS.142,144 Although FA is an excellent hydrogen-bond donor and acceptor, it does not donate hydrogen bonds to AOT.135 Instead, it has electrostatic interactions with AOT’s sulfonate and the Na+ counterions, effectively removing the counterions from the sulfonate I


Chemical Reviews

Review

thermodynamically stable, discrete GY droplets stabilized by the surfactant. Similar to the AOT system,145 the droplet size depended primarily on WS = [GY]/[CTAB], with sizes predicted by

Interestingly, the reverse micelle sizes do not depend on the solvent molar volumes, Vm.144 For example, sizes are not so different for EG, GY, and water despite the very large differences in their Vm. At the same time, DMF or DMA/ AOT/n-heptane lead to smaller reverse micelles even though these polar solvents have the highest Vm values. These results most likely indicate that the solvents are distributed differently in these reverse micelles.

rH/nm = 1.8 + 1.2WS

with a maximum WS = 9. The clear growth of particle size with increasing WS dispelled concerns that GY might partition between the particle central cores and a state in the continuous−oil solvent (50%v/v heptane/chloroform) solution. Fletcher et al. observed increasing viscosity of the GY/ CTAB/(heptane/chloroform) solutions with decreasing temperature. They suggested this could reflect increasing solvation of the polar head by GY with decreasing the temperature or deformation to slightly nonspherical reverse micelle particles at low temperature. While AOT and CTAB reverse micelles formed with GY exhibit different characteristics, Fletcher et al.146 hypothesize that the differences are due to the surfactant structures. The double-tail structure of AOT leads it to be “wedge-shaped” with a packing parameter, P > 1; hence it is expected to pack well in a spherical annular shell. In contrast, the single tailed CTAB surfactant, which has a P < 1,139−141 might be expected to have voids in the interfacial region allowing it to entrap larger amounts of solvent than AOT does. This could also lead to the departure from a spherical form. Others followed the first studies of GY-encapsulating reverse micelles. Mathew et al. explored viscosity, conductivity and dielectric relaxation of GY/AOT/isooctane reverse micelles specifically focusing on percolation phenomena.131 In w/o systems, percolation implies that reverse micelles become sufficiently close to cause effective interaction and transfer between droplets. The increase of the system’s conductivity accompanied by changes in viscosity and dielectric relaxation seen in the work of Mathew et al. clearly indicate this phenomenon. Using DLS, Laia et al.120,147 also investigated interactions between GY/AOT reverse micelles with isooctane or n-heptane as the continuous phase. They reported more attractive interactions between spherical reverse micelle droplets of GY than between similar water/AOT reverse micelles. Both these results show that percolation in GY/AOT/ isooctane reverse micelles occurs in a fashion similar to or enhanced compared to water/AOT reverse micelles. The works mentioned thus far focused on the preparation and macroscopic properties of reverse micelles containing GY. Others have attempted to characterize the nature of GY, and other polar solvents in reverse micelles. We continue by reviewing studies that have aimed to determine the structure of the different polar solvents encapsulated in the reverse micelle polar core. We assess the polar solvent−surfactant interactions upon encapsulation and on how encapsulation affects both the bulk polar solvent structure and the reverse micelle interfacial properties. To explore intramicellar structure of GY, El Seoud et al. used FTIR and 1 H NMR spectroscopy to investigate the solubilization of GY and the GY−water mixture in AOT reverse micelles.148 Their spectra showed that GY interactions with AOT are similar to water−AOT interactions; namely, the GY solvates the surfactant headgroup and ions through hydrogen-bond interactions, destroying the GY bulk hydrogen-bond network structure. They also showed that like water, GY resides in a well-defined pool in the polar interior of the AOT reverse micelles. Unlike water, there is no evidence that

4. REVERSE MICELLES ENCAPSULATING NONAQUEOUS POLAR ORGANIC SOLVENTS Many organic reactants are not easily solubilized in water. Moreover, dielectric constants at the interface of aqueous organized media are lower than in water and are closer to those found in salt solutions or in methanol. These considerations led to the use of nonaqueous solvents that enhance solubility and reactivity in nonhomogeneous media including microemulsions, direct and reverse micelles. Here we review several classes of solvents that have been used to form reverse micelles in the absence of water. Table 3 lists nonaqueous polar organic solvents that have been demonstrated in a range of microemulsions and the techniques used to explore these systems. 4.1. Reverse Micelles Containing Glycerol

One of the first reports of nonaqueous reverse micelle formation was by Fletcher et al.145 who used DLS and viscosimetry to study thermodynamically stable AOT stabilized dispersions of GY in n-heptane. Their experiments showed the presence of spherical GY droplets in solution whose droplet size was independent of temperature and primarily depended on the parameter like that used to describe aqueous systems, that is, the molar ratio of GY to AOT, WS = [GY]/[AOT]. From their data, they derived an equation describing the hydrodynamic radius (rH), rH/nm = 1.7(± 0.2) + 0.88(± 0.15)WS

(5)

(4)

Although AOT reverse micelles have been shown to solubilize large amounts of water, up to w0 = 60 or higher, Fletcher et al. found that the highest WS = 5 using GY as the polar solvent. At the same time, the apparent area occupied per AOT molecule drops ∼20% when using GY rather than water to form reverse micelles.21 This suggests that AOT desorbs from the GY polar solvent−surfactant interface more than it does from the water interface or that GY causes AOT to pack more closely at this interface. However, the approximate nature of the AOT molecular area calculations, ignoring possible contributions from effects such as particle size polydispersity, suggests that one should focus on the significant similarities between GYand water-containing reverse micelles rather than their relatively minor differences. Fletcher et al. interpret their viscosity data to demonstrate that although the shape of the particles is constant with increasing temperature, interdroplet interactions clearly increase with temperature. Thus attractive interactions between droplets increase as the microemulsion phase stability approaches. Subsequently, Fletcher et al.146 demonstrated the formation of reverse micelles encapsulating GY with the cationic surfactant, cetyltrimethylammonium bromide (CTAB), with and without the long-chain alcohol cosurfactants that are necessary for CTAB reverse micelles encapsulating water. They performed DLS, viscosity, and phase-separation studies for GY/ CTAB/50/50 v/v heptane/chloroform mixtures at different temperatures and showed that the solutions consist of J


Chemical Reviews

Review

GY exists in “layers” of different structures, as suggested by the multistate water solubilization model.90,149 As noted in section 3.2, Durantini et al. reported on their FTIR spectroscopy study of nonaqueous polar solvents in AOT reverse micelles.142 Rather than following the polar solvent vibrational modes, they used the changes in various vibrational modes of AOT, specifically the CO stretch and asymmetric SO3− vibration modes to understand the polar solvent interactions with the surfactant. Figure 4 contrasts the Figure 5. Schematic representation of the proposed interactions of the solvent sequestered in GY/AOT/n-heptane reverse micelles.

Figure 4. FTIR spectra of AOT vibrational modes. (A) Vibrational signatures of carbonyl (CO) and sulfonyl (SO) antisymmetric and symmetric stretching modes in AOT/n-heptane solution; (B) vibrational signatures of carbonyl (CO), and sulfonyl (SO) antisymmetric and symmetric stretching modes in GY/AOT/nheptane solutions with varying amounts of glycerol (GY). Spectral shifting in the sulfonyl peaks reveal GY interactions with AOT.

Figure 6. (A) Absorption spectra of 1−8-methylquinolinium betaine (QB) in DMF, benzene, and water showing how the B1 and B2 bands vary with solvent polarity and hydrogen bond donating ability; (B) absorption spectrum for QB in bulk glycerol and in glycerol/AOT/nheptane reverse micelles ([AOT] = 0.15 M. WS = 1.5).

with the absorption bands shifts, provides an effective method to determine the properties of the microenvironment surrounding the probe.143 The spectra shown in Figure 6b indicate that in dry AOT/hexane reverse micelles, QB resides in a relatively nonpolar and non-hydrogen-bond donating environment. With increasing water, the ratio between AB2/ AB1 reaches a plateau (data not shown) indicating that QB remains anchored in the reverse micelle interface.143 Addition of GY to AOT reverse micelles leads to a hypsochromic shift in both the B1 and B2 bands but the AB2/AB1 ratio indicates that the QB remains anchored to the interface. This shows that GY causes the interface to become more polar as it strongly solvates the AOT SO3− polar headgroup.150 Laia and Costa have explored details of GY/AOT/hexane RMs through spectroscopy of the fluorescent bis[4(dimethylamino)phenyl] squaraine (HSq) molecular probe (structure shown in Figure 7).151 Through steady-state fluorescence and fluorescence depolarization as well as timeresolved fluorescence, they estimated average rotational

vibrational features from AOT reverse micelles with and without GY. Lack of spectral shifting in the CO mode and spectral shifting for the SO3− peaks indicate that GY forms a hydrogen bond only with the SO3− group effectively separating the Na+ counterions from the interface, as depicted in a cartoon shown in Figure 5. Another effective method to explore the interior of reverse micelles employs molecular probes whose spectroscopy is sensitive to the environment. The absorption spectroscopy of 1-methyl-8-oxyquinolinium betaine, QB, has been used to investigate the GY structure in AOT reverse micelles.150 The UV−vis absorption spectrum of QB has two major features, as shown in Figure 6; the peak at longer wavelength, band B1, is primarily sensitive to polarity, while the band peaking at shorter wavelength in the UV, B2, reflects the hydrogen-bond donor capability of the solvent.143 The absorbance of the B2 band is highly sensitive to the molecule’s environment, and hence the ratio of the B2 to B1 absorbance, AB2/AB1, used in combination K


Chemical Reviews

Review

Figure 7. Structure of bis[4-(dimethylamino)phenyl] squaraine (HSq).

relaxation times for HSq providing estimates for microviscosity in these confined systems. Although dielectric friction dominates the microviscosity in water/AOT/hexane reverse micelles, the GY containing reverse micelles have comparable contributions from dielectric and hydrodynamic friction indicating that the two processes may be coupled. Using steady-state absorption and emission spectroscopies as well as measurements of fluorescence lifetimes, Silber and coworkers determined the partitioning of anthracene methanol (9-AM) and acridine orange base (AOB) in GY/AOT/nheptane reverse micelle solutions.152,153 In bulk solution, the probes reside preferentially in the nonpolar phase, but they found strong propensity for these probes to partition into the reverse micelles. This indicated that the solvent sensed by the probes has different properties from the bulk solvent. Solvent− AOT headgroup interactions as well as probe-AOT headgroup and counterion interactions provide the driving force to incorporate 9-AM and AOB into the GY/AOT/n-heptane reverse micelles. Sarkar and co-workers154 have probed GY/AOT/isooctane reverse micelles through steady-state and time-resolved fluorescence spectroscopy of two solvatochromic dyes, coumarin 480 (C480, also known as coumarin 102155) and coumarin 490 (C490, also known as coumarin 151155), whose structures are shown in Figure 8. They observed slower solvent

Figure 9. Absorption spectroscopy of C480 in GY/AOT/n-heptane solutions. Spectral structure suggest that the dye resides in a highly nonpolar environment (Reprinted with permission from ref 154. Copyright 2006 American Chemical Society).

In a comprehensive study, Ray and Moulik demonstrated formation of AOT reverse micelles in a number of nonpolar solvents, including heptane, octane, isooctane, xylene, and toluene, and for a range of polar nonaqueous solvents, including EG and PG.132 Results from experiments measuring phase behavior, conductance, and viscosity showed that reverse micelles formed in all these ternary systems. Furthermore, the internal structure is more or less spherical in shape, and the reverse micelles obey well established viscosity equations with a good degree of correlation. This work demonstrated most of the known systems that truly form nonaqueous reverse micelles. Metha and Kawaljit156 performed detailed volumetric and transport studies of the ternary system EG/AOT/ethylbenzene. The conductivity and viscosity of these systems change exponentially with increasing volume fraction of dispersed phase but lack a sharp increase of conductivity with respect to temperature. This indicates percolation occurs with increasing concentration but not with temperature. In addition, viscosity data suggest an increase in the particle interaction as the WS value increases. These results are similar to observations noted in section 4.1 for GY/AOT reverse micelles. Mehta et al. claim that WS as large as 10 can be reached in this system, which is very surprising because EG in AOT/isooctane, benzene, or nheptane reverse micelles display phase separation near WS = 4.150 Riter et al. explored a range of nonaqueous polar solvents, including EG and PG in AOT/isooctane and decane.113 Their DLS measurement showed that when EG and PG are dissolved in the AOT systems, the droplet sizes increase as the WS values increase demonstrating the formation of true reverse micelles. As reported above, several groups have demonstrated the propensity for EG and PG to form reverse micelles in nonpolar solvents with AOT. Others have utilized spectroscopic methods to explore the nature of the sequestered polar solvents. The FTIR and 1H NMR studies of EG/AOT reverse micelles performed by Novaki et al.157 show that EG solvates the AOT polar headgroup through hydrogen bonding similar to the water/AOT interaction. The spectra also do not indicate the existence of bulk-like solvent inside the reverse micelles at the maximum Ws values between 2 and 4. This suggests that, like results for GY, the EG does not form a well-defined pool separated from the interface.157 Marking changes in the AOT carbonyl and sulfonyl modes through FTIR, Durantini et al. demonstrated that PG and EG interact through hydrogen bonding with the AOT CO group

Figure 8. Molecular structures of coumarin 490 (C490, also known as coumarin 151) and coumarin 480 (C480, also known as coumarin 102).

relaxation compared to the relaxation of these probes in pure GY. However, the structures of these probes should lead to their solubility in the nonpolar phase.21 The significant structure appearing in steady-state absorption spectra shown in Figure 9 suggests that rather than residing in the reverse micelles where they could interact with GY, these probes reside in the nonpolar environment or at the interface between nonpolar solvent and the surfactant tails. Thus, the dynamics measured likely do not reflect dynamics of the encapsulated GY. Rather, this could indicate changes to the reverse micelles occurring as they swell with GY. 4.2. Reverse Micelles Containing Other Polyols

In addition to GY, researchers have explored the propensity for other poly alcohols to form reverse micelles. These include ethylene glycol (EG), propylene glycol (1,2-propandiol, PG), diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (TTEG). Many of these studies explore reverse micelles formed from a range of nonaqueous polar solvents rather than a single system. L


Chemical Reviews

Review

that penetrates into the oil side of the interface.142 Thus, they interact weakly with the Na+ counterion, which remains close to the AOT sulfonate group. Figure 10 presents a schematic representation of one possible configuration for EG in the reverse micelles demonstrating the interaction between the EG and AOT.

structure at the micelle interface; the main driving force to incorporate the probes into the nonaqueous reverse micelles is related to solvent−AOT headgroup, probe−AOT headgroup, and counterion interactions. Because hydrogen-bond donor solvents mainly solvate the polar head of the AOT through hydrogen-bond interactions, counterion solvation for either 9AM or AOB, which are both good electron donors, could be an important factor assisting the partitioning. Similar spectral responses from AOB absorption and emission spectra indicate that partitioning of AOB does not change for the molecule in its excited state. C343 has been used to study various nonaqueous reverse micelle solutions, including EG and PG.113,158 Often used as a probe for solvation dynamics,160,161 the C343 absorption spectrum is very sensitive to the environment and, with a pKa ≈ 4.5, to pH.113,158 In aqueous solution above pH = 5, C343 generally exists in its anionic form; the absorption spectra for anion and neutral C343 differ significantly allowing the ionic state to be determined. The insolubility of both anion and neutral dye lead to J-aggregation at very low concentrations in nonpolar media.158 Dissolved in the AOT reverse micelle systems at low concentration, C343 exists as a monomer, and when introduced to the reverse micelles samples in its protonated form it remains protonated driving it to reside in the interface.158 Correa and Levinger used C343 to probe water and EG/AOT/isooctane reverse micelles, using absorption, emission, and time-resolved spectroscopies.158 The solvatochromic behavior of the dye showed that EG and water hydrogen-bond with the AOT sulfonate group destroying their bulk hydrogen-bond network structures. While water remains well segregated from the nonpolar regions, EG appears to penetrate into the oil side of the interface. Despite the similarities between GY and EG, some interesting differences exist between the solvents encapsulated in reverse micelles. FTIR and 1H NMR spectroscopy142,157 both show strong interactions of GY and EG with the AOT polar headgroup; the interactions appear even stronger than water interactions with the headgroup, with the strongest interaction by GY. The virial coefficient calculated for EG/ AOT reverse micelles is more negative than that for GY/AOT reverse micelles162 and has been interpreted as evidence that the diol penetrates into the interface; that is, EG acts as cosurfactant as suggested in Figure 9. In 1H NMR studies, chemical shifts from AOT also indicate that EG penetrates to the oil side of the interface.157 Usually, the H1 proton on AOT senses the nonpolar side of the interface, as indicated in Figure 12. In water and GY containing reverse micelles, 1H NMR spectroscopy confirms this orientation by the constant chemical shift of the H1 signal with increasing polar solvent content.148 However in EG containing reverse micelles, the H1 signal shifts with increasing WS demonstrating that EG penetrates the interface and interacts with H1 on the AOT. This indicates EG containing AOT reverse micelles display a more fluid interface and a lower threshold for phase separation relative to those encapsulating water or GY. Moore and Palepu explored the propensity for a series of glycols, that is, EG, DEG, TEG, and TTEG, to form reverse micelles with AOT in hexane, heptane, octane, and isooctane.163 They interpreted changes in the fluorescence spectroscopy of auramine-O (AuO) and 8-anilino-l-napthalene sulfonic acid, ammonium salt (ANS), a cationic and anionic probe, respectively (Figure 13), indicating formation of a bulklike solvent pool in the reverse micelles. The WS value where

Figure 10. Schematic representation of the proposed interactions of EG sequestered in EG/AOT/n-heptane reverse micelles.

As noted in section 4.1, a wide range of molecular probes has been used to characterize these reverse micelles. Each probe possesses slightly different properties; taken together, the results provide a glimpse into the nature of these nonaqueous self-assembled systems. Absorption spectroscopy of QB molecular probe in EG and PG/AOT reverse micelles shows significant blue shifting and diminution of the B2 peak. This shows that the probe interacts with the EG and PG through hydrogen bonding, possibly by QB displacement into the reverse micelle polar core or by penetration of the EG and PG into the reverse micelle interface. The significant hydrogenbond interactions observed indicate a stronger EG−AOT and PG−AOT interaction than water−AOT interaction. Spectroscopic investigations using several different solvatochromic dyes have been enlisted to investigate reverse micelles;21 these include absorption and emission spectroscopy of 9-anthracenemethanol (9-AM),152 acridine orange base (AOB),152,153 and coumarin 343 (C343).113,158,159 Structures for these molecules are shown in Figure 11. The solvatochromic nature of these dyes’ spectra make them particularly good

Figure 11. Molecular structures of 9-anthracene methanol (9-AM) acridine orange base (AOB) and coumarin 343 (C343).

for exploring partitioning of the dye between various environments in the reverse micelle samples. For example, Silber and co-workers showed that the strong interactions of EG, PG, or GY in AOT reverse micelles/n-heptane strongly impacted the partitioning of both 9-AM and AOB. The solvent−AOT headgroup interaction modifies the solvent M


Chemical Reviews

Review

Some of the first reports of the propensity for FA to drive the self-assembly of surfactants to direct micelles and vesicles in bulk FA, and also in reverse micelles and microemulsions with nonpolar solvents were presented by Rico, Lattes, and coworkers,168−170 and Friberg and co-workers.171,172 In one of the first studies, Auvray et al. used X-ray scattering to explore the structure of an oil-rich solution containing CTAB, 1-butanol, and isooctane and found small filament-like structures.170 Since these first reports, several other groups have explored FA as a water replacement in reverse micelles. There exist quite a few studies of FA/AOT reverse micelles using a range of techniques. Unlike the results from studies of polyol-containing reverse micelles, various interpretations have been presented describing the structure of the aggregates as well as the FA structure upon encapsulation. The presence of reverse micelles in FA/AOT/isooctane or nheptane solutions has been demonstrated through DLS measurements113,147 as well as viscosity and conductivity.173 Arcoleo et al. noted that the maximum amount of FA that could be sequestered depended on the volume fraction but always WS < 4. Increasing viscosity, conductance, and dielectric permittivity indicated interactions between the reverse micelles with increasing WS suggestive of clustering or percolation. Arcoleo et al. interpreted their data to indicate that AOT effectively encapsulates both FA for all WS studied and that these systems consist of discrete nanodroplets of FA coated by AOT molecules and dispersed in the oil. Laia et al. investigated FA/AOT reverse micelles formed in isooctane and in n-heptane using DLS.120,147 They found that above a critical volume fraction, ϕc , reverse micelles demonstrate strong attractive interactions which prevent the formation of monodisperse reverse micelles, instead leading to larger aggregates, but below ϕc solutions did not show any evidence of cluster formation. The aggregates appear to be globular, having an interface with low bending constants, which causes large variations of the aggregates size with temperature and AOT concentration. They hypothesize that interactions between FA molecules are stronger than those in water leading to a loss of stabilization of the AOT polar surfactant headgroup compared to similar systems created with water. Thus, they claim that the FA/AOT reverse micelles are more complex, showing characteristics observed in systems with neutral surfactants because the strong association at the interface between AOT and Na+. For constant WS, Riter et al. observed increasing particle size with increasing volume fraction suggesting similar percolation phenomena.113 Several groups have used IR spectroscopy to explore FA in AOT reverse micelles.135,159,173 Correa et al. demonstrated that FA interacts strongly with the Na+ counterions of AOT through electrostatic interactions, maintaining the hydrogen-bond network present in the FA bulk.135 Moreover, their work shows that the Na+ counterions are separated from the sulfonate group leaving FA “frozen” inside the reverse micelles. Similarly, vibrational spectra of deuterated FA in AOT/ isooctane reverse micelles measured by Riter et al. indicate that the intramicellar FA retains a large degree of hydrogenbonding character, and its structure appears significantly less perturbed by the restricted environment than water in comparable systems.159 In contrast, Arcoleo et al. studied spectral shifting of the NH stretch in FA/AOT/n-heptane for small WS values.173 They interpreted these results as an indication that the reverse micelle environment causes significant destruction of the hydrogen-bond structure FA

Figure 12. (Top) Structure of AOT with standard numbering used to identify specific hydrogen atoms; (bottom) rotational isomers of AOT.

Figure 13. Molecular structures for Auramine O (AuO) and 8-anilino1-naphthalenesulfonic acid (ANS).

the pool formed depended on the nonpolar solvent. However, their studies never demonstrated the presence of individual reverse micelle particles so it is unclear what the results really mean with respect to reverse micelle formation. Departing from studies utilizing AOT, one of the first observations of a nonaqueous microemulsions with polyols was found in a single-phase mixture of EG/decane/lecithin structure determined by turbidity measurements.164 Further studies of lecithin probed the role of polar solvents and nature of intermolecular interactions on the ability to induce the liquid to gel transition in polar solvent/heptane solutions.165 These studies showed that GY and EG added in small quantity could induce the formation of lecithin organogels and hypothesized that the GY and EG polarity as well as their ability to form hydrogen bonds with the lecithin were responsible for gel formation. Martino and Kaler166 reported the effect on microemulsion phase behavior and microstructure occurring when replacing water with PG, GY, and their mixtures in systems made with the nonionic surfactants pentaethylene glycol mono-n-decyl ether (C10E5), pentaethylene glycol mono-n-dodecyl ether, (C12E5) and several hydrocarbons such as n-heptane, ndodecane, and n-hexadecane. Using SAXS they found microstructure in polar solvent/dodecane/C12E5 systems indicating that they are reverse micelles, as we define above. The surfaceto-volume ratio in the nonaqueous systems is approximately twice as large as in the comparable water reverse micelles suggesting that the characteristic size of the microstructure in the polar organics is smaller than that in equivalent water mixtures. Related studies showing similar results also were performed using SANS and NMR self-diffusion measurements.167 4.3. Reverse Micelles Containing Formamide

The strong hydrogen-bonding character, very large dielectric constant, and other similarities to water have led researchers to use formamide (FA) in the place of water in microemulsions. N


Chemical Reviews

Review

emission due to the solubilization of the dye in two different environments, most likely solvated by the nonpolar solvent and in the reverse micelle interface. The comparative effect of the sequestered polar solvent in AOT/heptane reverse micelles upon the partitioning of probes was also investigated for FA/AOT/n-heptane systems using 9AM and AOB molecular probes, as described for GY in section 4.1 and EG in section 4.2.152,153 Their good electron donor qualities and differences in hydrogen-bond donating ability make these probes effective for probing the reverse micelle environment. Spectroscopic studies show that both probes partition effectively into the FA/AOT/n-heptane reverse micelles. The unique FA structure discussed above may explain the results.120,157 Thus solvent−probe interactions do not provide the driving force for 9-AM and AOB partitioning. Rather, probe interactions with the AOT headgroup and counterion could be an important factor in the probe behavior in these reverse micelles. Sando et al. have utilized both steady-state and time-resolved vibrational spectroscopy of azide antisymmetric stretching mode as a small molecular probe for FA/AOT/n-heptane and reverse micelles.175 In the reverse micelles, the azide vibrational frequency displays a blue-shift compared to the frequency in bulk solution that is most pronounced for the smallest WS value. Time-resolved studies measuring vibrational energy relaxation of the azide ion in FA/AOT/n-heptane showed much slower relaxation for FA containing reverse micelles than in comparable water containing reverse micelles. Both spectral shifts and vibrational energy relaxation approaches the bulk value for the largest WS values measured. They interpreted their results as indicating a different structure for the FA in the reverse micelles. Several research groups have applied ultrafast time-resolved spectroscopy to study FA in reverse micelles. In the first such study, Riter et al. followed the dynamic fluorescence Stokes shift of C343 in FA/AOT/isooctane reverse micelles.159 Given the insolubility of C343 in isooctane, the dye molecule response should reflect only the dynamics of the encapsulated FA. The solvation dynamics of FA inside the reverse micelles differs to a large extent from bulk FA solvent motion. While 20% of the solvent relaxation of the intramicellar FA does occur on an ultrafast time scale similar to bulk FA, this component corresponds to a mere 10 cm−1 relaxation. In comparison with the bulk dynamics, this component is not only slower, 0.5 ps vs 0.11 ps, but also of substantially reduced amplitude. Thus, their results showed that FA in a reverse micelle is nearly completely immobilized in the sub-picosecond to hundreds of picoseconds time scale. These results agree well with the authors’ results for water motion in AOT reverse micelles.42 Using a different coumarin probe, C480 (identified as C102 in their paper, Figure 8), Shirota and Segawa followed solvation dynamics in the FA/AOT/n-heptane system to understand how the amide hydrogen-bond affected the dynamical features of these liquids in the aggregate nanocavities.176 Additionally, they explored WS values above the region where reverse micelles have been shown to form. In related experiments, Hazra et al. have measured solvation dynamics for C152 in FA/AOT/n-heptane reverse micelles.177 Both studies report that the solvation dynamics of the systems are much slower compared to the response of the pure solvents. Hazra et al. also report no dependence on WS, which they interpret as the small size of the core of the FA reverse micelle restricting the motion of the FA molecules. However, unlike C343, both C480 and C152 are

normally found in the bulk solvent conflicting with the interpretations by Correa et al.135 and Riter et al.159 They also conclude that the structure of FA trapped in the reverse micelles differs from bulk solvents and suggest FA solvates the AOT ionic headgroup more strongly than water does. The NH stretching band for protonated FA is much broader than its deuterated counterpart. Furthermore, the presence of residual water in the reverse micelles could lead to additional broadening and the appearance of shifting in the NH band. In complementary 1H NMR spectroscopy experiments, Correa et al. found FA and water chemical shifts indicated strong preferential solvation; FA interacts strongly with the Na+ counterion through electrostatic interaction while water interacts with the AOT sulfonate group through hydrogenbonding.135 Interestingly, FA may be the only polar solvent other than water that really forms a “polar-solvent pool” with macroscopic properties similar to the bulk, even though only low Ws values can be reached. Studies employing molecular probes have also been applied to FA/AOT reverse micelles. Correa and Levinger explored reverse micelles composed of FA/AOT/isooctane with C343.158 In agreement with results from Laia et al.120,147 and Correa et al.,135 these measurements demonstrated that FA interacts with the Na+ counterions but retains its bulk hydrogen-bond network (see Figure 14). Similar results were

Figure 14. Schematic representation of the proposed interactions of FA sequestered in AOT/n-heptane reverse micelles.

obtained using QB as a molecular probe. Here FA displays substantial association with QB, which may indicate that FA retains much of its bulk hydrogen-bond network when it is dissolved in the AOT/hexane.135 Raju and Costa explored the ground and excited state properties of the pretwisted 7diethylaminocoumarin dye (BC I, Figure 15) in isooctane/ AOT/FA reverse micelles.174 Although BC I is soluble in bulk FA, the intramicellar FA does not appear to solvate it. Nevertheless the dye’s spectroscopy shows sensitivity to the changes in the polarity of the interfacial region with increasing WS. The BC I spectroscopic properties and dynamics as well as steady-state fluorescence anisotropy experiments indicate dual

Figure 15. Structure of the cyano derivative of 7-diehylamino coumarinyl benzopyrano pyridine, BC I. O


Chemical Reviews

Review

DMA complex the Na+ ions through their carbonyl and nitrogen groups, as depicted for DMF in Figure 16.

hydrophobic and hence have significant solubility in the nonpolar region of the reverse micelles system.178 So, the solvation dynamics measurements show a very slow response by the environment, but exactly what that environment the molecule probes remains unknown. In addition to their steady-state spectroscopic studies, Sando et al. have used ultrafast time-resolved vibrational spectroscopy to measure energy relaxation times for the antisymmetric stretching vibrational band of azide ion in FA-containing reverse micelles of AOT/n-heptane.103 This work points out the differences between the solvent structures of water and FA within the micelle and in bulk solution and points to the need for further investigation of these structures. Although the great majority of FA encapsulating reverse micelles use AOT as the surfactant, Strey and co-workers have explored the propensity for FA microemulsions to form with nonionic and cationic surfactants.179,180 They note that because hydrocarbons are slightly more soluble in FA than they are in water, the repulsive hydrophobic interaction between the hydrocarbon tails of the amphiphiles and FA is weaker leading to higher operational critical micelles concentration (cmc) values for the FA reverse micelles. In addition, one group has reported results for the related solvent N-methylformamide (NMF).173 Through studies of density, viscosity, conductivity, and dielectric permittivity, they show that the maximum WS reached is around 2. FTIR spectra show significant perturbation to the NH stretching mode for the smallest WS and the perturbation decreases with increasing WS.

Figure 16. Schematic representation of the proposed interactions of DMF sequestered in AOT/n-heptane reverse micelles.

Results from FTIR studies have led to some interesting observations regarding a range of polar nonaqueous solvents used to form reverse micelles.144 Via DLS measurements of GY, water, EG, DMF, DMA, or FA/AOT/n-heptane, Falcone et al. demonstrated that the size of the reverse micelles formed depends intimately on the polar solvent used.144 Specifically, the reverse micelle size depends on the interactions between different polar solvents and AOT, which determine the packing parameter defined in section 3.2, not on their molar volume, Vm. The interactions significantly affect the packing parameter with concomitant changes in the reverse micelle sizes. Thus, for solvents that donate hydrogen bonds, for example, water, GY, EG, the micelle sizes are similar for the same value of WS. When molecules cannot donate hydrogen bonds, for example, DMF and DMA, the resulting reverse micelles are smaller. FA/AOT/ n-heptane reverse micelles are the largest because FA interacts electrostatically with AOT removing the Na+ counterion from the interface.144 As with investigations of other nonaqueous (and aqueous) reverse micelles, researchers have used spectroscopy of molecular probes to explore reverse micelles formed with DMF or DMA. Using QB as molecular probe, Falcone et al.144,150,153 found that DMF and DMA encapsulated inside AOT reverse micelles led QB to detect an increase in polarity with increasing WS. Moreover, the micropolarity of the reverse micelles at the maximum WS value is higher than that of the neat solvent. This has interesting implications for these systems as nanoreactors as we will show in section 7. Falcone et al. also employed the solvatochromic behavior of ortho-nitroaniline (o-NA, Figure 17) evident from its absorption

4.4. Reverse Micelles Containing Other Amides: DMF and DMA

All the work reviewed to this point has included nonaqueous polar solvents that share similarity with water in their ability to both donate and accept hydrogen bonds. Although this may seem like a prerequisite for replacing water in reverse micelles, two solvents that do not donate hydrogen bonds stand out for their efficacy at forming reverse micelles, dimethyl formamide (DMF) and dimethyl acetamide (DMA). Several studies have demonstrated the utility for these solvents to form true reverse micelles with DMF 113,142,144,150,153,158,176,177,181 and DMA.142,144,150,153,182 Despite the slight solubility of DMF in nonpolar solvents, Riter et al. demonstrated the propensity for DMF to facilitate reverse micelle formation using DLS.113 They noted a critical dependence on the AOT concentration; that is, for reverse micelles to form requires a high AOT concentration. Otherwise, the polar solvent simply dissolves in the nonpolar phase of the system forming a microemulsion and displays a decrease in size as the WS value increases. Recently, Durantini et al. have shown that reverse micelles form in DMF/AOT/nheptane solutions.181 With 0.1 M AOT in n-heptane, the maximum DMF in the solution was WS around 4. Durantini et al.142 have also characterized DMF/AOT/nheptane and DMA/AOT/n-heptane reverse micelles using FTIR spectroscopy.142 Following the changes in the AOT C O, symmetric, and asymmetric SO3− vibrational modes with the increase in the polar solvent content in the AOT reverse micelles, they demonstrated that DMF and DMA encapsulated inside the reverse micelles interact neither with the CO nor with the SO3− groups. Instead, their weakly bulk associated structure is broken through interactions with the AOT surfactant counterion, Na+. They suggested that DMF and

Figure 17. Molecular structure of ortho-nitroaniline (o-NA).

UV−visible spectroscopy to explore AOT nonaqueous reverse micelles.182 They determined the binding of o-NA to the micelle interface, calculated through the changes in the UV spectra, for n-heptane/AOT/water, EG, FA, GY, DMA, and DMF reverse micelles.182 They found the lowest binding constant, Kb, for o-NA in reverse micelles encapsulating GY, and that Kb increases for water-containing reverse micelles. The binding constant is even larger for o-NA in EG, FA, and dry P


Chemical Reviews

Review

example, Elles and Levinger explored reverse micelle formation using DMSO in AOT/cyclohexane solutions.184 The authors used DLS, FTIR spectroscopy, and the absorption spectra of QB to characterize the novel media. The results show that reverse micelles solubilizing only DMSO form relatively small droplets and their size grows with WS until the maximum WS = 2.5. These reverse micelles are larger than the corresponding micelles containing an equal amount of water, which they attributed to the larger molar volume of the DMSO molecule compared to water. However, results from Falcone et al. suggest that the reason for larger particle formation may arise from the specific interaction of the DMSO with the AOT headgroup.144 Similar to DMF and DMA, although DMSO can accept hydrogen bonds, it lacks the ability to donate hydrogen bonds so a similar interaction could dominate the structure. It is also possible that trace amounts of water in the systems probed by Elles and Levinger account for the sizes measured.184 Elles and Levinger also used QB to measure polarity in the system. They found that the polarity sensed by QB is similar to that that of an empty micelle, WS = 0, intermediate between bulk water and bulk DMSO, and does not change with increasing WS. In addition, they explored the impact of adding water to these systems, that is, DMSO/water/AOT/cyclohexane, and found that when water is added to the reverse micelles, the polarity sensed by QB shifts toward the polarity in bulk water. FTIR studies following the water OH stretching mode showed a red-shift of the absorption when DMSO was added to the reverse micelles and suggest increasing hydrogen bonding with increasing DMSO content. Acrylamide is another polar molecule that has been used to form microemulsions.122 Calandra et al. investigated acrylamide entrapped in the core of AOT and lecithin reverse micelles using FTIR, 1H NMR, and SAXS. SAXS measurements show that acrylamide addition leads to one-dimensional growth of AOT and lecithin reverse micelles, involving a sphere to cylinder transition. Thus, although the particles do not swell as spheres, they do show volume changes with increasing WS suggesting that these systems form reverse micelles. FTIR and NMR experiments reveal that the solubilized acrylamide is preferentially located within the surfactant headgroup domain and oriented so that the ethylene group extends beyond the hydrophobic surfactant alkyl-chain region. This suggests that acrylamide−surfactant headgroup interactions through hydrogen bonding between the -NH2 and the surfactant polar heads drive reverse micelle formation. Mehta et al. have explored phase diagrams for several nonaqueous polar solvents in AOT/hexane.185 Most of the solvents they have explored have already been mentioned (EG, GY). One unusual solvent they studied is 2-pyrrolidinone. Through conductivity, dynamic viscosity, density, and ultrasonic velocity measurements, they calculated the aggregation number, core radius, and surface number density of the surfactant molecules at the interface. They note that replacing water with a nonaqueous polar solvent changes the hydrogenbonding equilibria and electrostatic interactions in the microemulsion. In particular, the nonaqueous polar solvents have enhanced molecular interactions with the hydrophobic part of the microemulsion leading to their higher interfacial penetration and tighter packing for AOT at the interface. They predict that this effect will be more pronounced with 2pyrrolidinone due to its cyclic structure and better hydrogenbonding capability compared to other nonaqueous solvents they probed.

micelles (containing no polar solvent) all of which show similar values, and the highest Kb is observed for reverse micelles encapsulating DMF or DMA. The results reflect the different parts of the AOT polar headgroup involved in the polar solvent−AOT interaction and demonstrated that o-NA interacts with AOT through the sulfonate moiety. Thus, the more accessible the sulfonate group, the higher the o-NA Kb value. Coumarin probes have also been used to characterize reverse micelles with DMF and DMA.113,158,176 In their early studies, Riter et al. measured the C343 spectrum in DMF/AOT/ isooctane solutions.113 More recently, Correa and Levinger interpreted the C343 spectroscopy as indicating that strong interactions with the AOT Na+ counterions disrupt the normal dipole−dipole interactions that dominate intermolecular interactions in DMF and DMA and destroy the solvents’ bulk structures.158 Shirota and Segawa enlisted C480 to measure solvation dynamics of DMF encapsulated in AOT/n-heptane reverse micelles.176 The solvation dynamics they measured were extremely slow in comparison with that of pure DMF, and they observed very little WS dependence. However, as we noted in section 4.3, C480 is soluble in the nonpolar phase; lack of sensitivity to WS in the steady-state absorption spectra reported in the paper suggests that the C480 likely probes a relatively nonpolar environment. Hence, results from the solvation dynamics experiments are ambiguous. In addition to their solvation dynamics measurements, Shirota and Segawa also estimated interaction energies of FA and DMF with AOT through Møller−Plesset geometryoptimized solvent clusters with a simple model of the AOT polar headgroup (CH3SO3−).176 Interestingly, the calculations indicate strong interactions between DMF and the SO3− group, and surprisingly the energies predicted for FA and DMF interacting with the AOT polar headgroup are similar. For FA, they found that the number of hydrogen bonds formed strongly impacted the hydrogen-bond strengths calculated. Calculations such as these can be intriguing but caution should be exercised when considering their validity for interactions occurring in the reverse micelles. Although virtually all studies reported utilize AOT as the surfactant, one report of a nonaqueous microemulsion explores the phase diagram of the system cyclohexane/TX-100/DMF at 40 °C.183 In this paper, they report physical properties like viscosity, adiabatic compressibility, specific conductance, and a contact angle to characterize the nonaqueous microstructure. Changes in the physical properties measured tracked the DMF concentration. Importantly, in the concentration range used, no percolation behavior was observed which suggests a single structural form at the different compositions studied. Tessy and Rakshit also explored how the addition of 1-propanol affected the system.183 They have found that the one-phase microemulsion area increased dramatically with the presence of 1propanol and suggested that the short chain alcohol increases the efficiency of the amphiphile by reducing the interfacial tension between cyclohexane-DMF. However, the presence of 1-propanol in the microemulsion causes some percolation. Although it was not an aim of this study, the results reported seem consistent with the presence of reverse micelles in the solution. 4.5. Other Polar Solvents

Other than polyols and amides, few other solvents have been used as effective replacements for water in reverse micelles. For Q


Chemical Reviews

Review

the intramicellar mixture each solvent behaves as if the other solvent were not present inside reverse micelle media especially at low WS values. Moreover, the molecular probe QB interacts almost exclusively with the GY at the AOT reverse micelle interface. In homogeneous media QB detects a strong preferential solvation by GY that almost disappears inside the reverse micelles. This unexpected and interesting result demonstrates the way that the unique intramicellar environment can modulate molecular properties and shows that results found in homogeneous environment not always can be extrapolated to confined media such as reverse micelles.181

Although many investigations of reverse micelles have been performed to study confinement effects of water and other polar solvents in AOT reverse micelles, there are practically no studies on solubilization of discrete amounts of strongly polar, solid substances in reverse micelles. These studies could be of interest in the preparation of nanoparticles of such solid substances or in novel solid−solid reactions in liquid phases. Moreover, solubilization of solid substances within the micelle core poses the interesting question of whether they are dispersed as single molecules or as large nanoparticles coated and stabilized by a surfactant monolayer. Addressing this issue, Turco-Liveri and co-workers investigated the solubilization of urea in several different systems including AOT/CCl4,186 lecithin/CCl4,187 sodium diethylhexylphosphate (NaDEHP)/ CCl4,187 and AOT/n-heptane121 reverse micelles, as well as Nmethylurea/AOT/CCl4188 and acetamide dispersed in AOT/ CCl4.188,189 For all these systems, FTIR spectra indicate that at low WS values, urea, N-methylurea, or acetamide disperses as monomers in the AOT aggregates. For higher WS values, vibrational spectra show significant interaction between the solids and AOT. Specific interactions occur between NH and CO groups on urea, N-methylurea, and acetamide with the AOT SO3− headgroups but not with the AOT CO moieties. They have made additional measurements on some of the systems. SANS experiments of urea/AOT/n-heptane121 show that the particles group asymmetrically with increasing WS. Density, viscosity, and dielectric permittivity data are consistent with reverse micelles that encapsulate urea as molecular clusters.186 They demonstrate that the reverse micelle environments influence homogeneous nucleation processes that normally occur.189 Recently, Mitra and Paul190 showed that nonaqueous microemulsions can be created using anionic−nonionic mixed surfactants in an AOT/Brij-52/FA, EG, PG, and triethylene glycol system. The phase diagram for this mixture is characterized by the presence of a small monophasic region and a large biphasic region along with mesophases like viscous, gel, and turbid phases. Replacing Brij-32 with Brij-72 or Brij-92 produces a negligible effect on the extent of the monophasic domain. The results have been explained in the light of monomeric solubility of surfactants in the polar solvents and hydrophobicity of the nonionic surfactants. Although these mixtures display a monophasic microemulsion region, it is unclear if reverse micelles comprise any part of the possible phases. A plethora of experiments have explored the effects of adding nonaqueous polar solvents to aqueous reverse micelles; the significant number of these studies makes them beyond the scope of this review. However a recent study reported the first results for a mixture of nonaqueous polar solvents in reverse micelles.181 Durantini et al. characterized the mixture of GY and DMF in AOT/n-heptane reverse micelles using UV−vis absorption of QB and 1H NMR spectroscopies and DLS. In bulk homogeneous media, GY and DMF interact strongly through hydrogen-bond interactions;191 unexpectedly, no evidence exists for hydrogen bonds between GY and DMF in AOT/n-heptane reverse micelles. Interactions of both GY and DMF with AOT components preclude a strong GY−DMF interaction. Specifically, when the GY−DMF mixture is encapsulated in the polar core of the AOT reverse micelles, GY binds through hydrogen-bonding to the AOT SO3− group at the interface and DMF makes complexes with the Na+ counterions in the polar core of the aggregates. Therefore, in

5. REQUIREMENTS FOR EFFECTIVE REVERSE MICELLE FORMATION 5.1. Polar Solvent Properties

In the preceding section, several nonaqueous polar solvents were demonstrated to form reverse micelles as defined in section 2.2. These solvents possess several common characteristics that lead to effective reverse micelle formation. As noted by Lattes et al., water has several key features that encourage self-assembly. Water has high polarity and is highly structured because it can both donate and accept hydrogen bonds. Additional hydrophobic interactions and internal pressure effects are important in stabilizing self-assembly.192 Considering whether nonaqueous polar solvents can serve as alternatives to water in these systems, Lattes et al. give the following specifications192 for a water substitute: (1) High polarity: the solvent’s dielectric constant must be sufficiently high to dissociate ions associated with their headgroups of ionic amphiphilic molecules. Solvent polarity can be gauged with several parameters including dielectric constant, molecular dipole moment, and various polarity measures, e.g., ET(30),193−195 the Kamlet−Taft π*,196 Kosower Z probe.197 (2) High solvating power: the solvent should have strong attractive interactions with polar species including charged particles with the high dielectric constant favoring dipole interactions over associations between oppositely charged ions. (3) High structure: generally, structure is based on the strength of intermolecular bonds and the geometry of the molecules. Both specific (hydrogen bonds) and nonspecific (electrostatic and dispersive) interactions can both contribute to this structure. Various parameters can indicate the structure of a solvent. Several parameters include the internal pressure (Pi), the density of cohesion energy (DCE), and the surface Gibbs energy (γ). Replacing water in reverse micelles with a polar organic solvent that also forms hydrogen bonds helps to meet the criteria set by Lattes et al.192 As far as we are aware, the firstreported nonaqueous reverse micelles replaced water with GY145,198 and FA,168 two highly structured, polar solvents that both donate and accept hydrogen bonds. In addition as discussed in section 4.4, hydrogen bonding within the solvent is not an absolute requirement, as demonstrated by reverse micelle formation using highly polar solvents like DMF,132,199 DMA,132 or DMSO,184 that can accept but not donate hydrogen bonds.200,201 Another important property correlated with solvents that leads to effective reverse micelle formation is sufficiently low solubility of the polar solvent in the hydrocarbon phase. A high interfacial tension between the polar and nonpolar phases leads R


Chemical Reviews

Review

Table 4. Properties of Polar Organic Solvents Used for Forming Reverse Micelles Described in This Reviewa solvent

εb

μc 1030/ Cm

π*d

αd

βd

ETNe

water glycerol (GY) ethylene glycol (EG) propylene glycol (PG) methanol (MeOH) formamide (FA) dimethyl formamide (DMF) dimethylacetamide (DMA) n-methyl formamide (NMF) acetonitrile (ACN) dimethylsulfoxide (DMSO) diethylene glycol triethylene glycol tetraethylene glycol

78.30 46.5i 37.7 32.0i 32.66 109.0 36.71

6.2 8.9i 7.7 7.6i 9.6 11.2 12.7

1.09 0.62 0.92 0.92 0.6 0.97 0.88j

1.17 1.21 0.9 0.9 0.98 0.71 0.00

0.47 0.51 0.52 0.52 0.66 0.48 0.69

1 0.903 0.790

37.78

12.5

0.85j

0.00

0.76

0.377

33.09

12.8i

0.93k

0.722

38.52

182.4 35.94 46.45

13.0 13.5

31.82i 23.69i 20.44i

7.8i 18.6i 19.5i

j

0.66 1.00l

0.19 0.00

0.762 0.775 0.386

j

0.30 1.66m

0.723 0.444

solubility in isooctane (mole fraction)f

solubility in decane (mole fraction)f

0.00059 0.0004 0.004 0.21

Nonaqueous Polar Solvents in Reverse Micelle ... - ACS Publications

Recommend Documents