Structure of AOT reverse micelles under shear

Reverse micelles in the AOT/water/isooctane system, at various water contents (W(0)), were studied using rheometry and small angle X-ray scattering (SAXS) experiments under static conditions and under shear. The SAXS analysis confirmed the spherical shape of the micelles at low water content and revealed a transition into elongated micelles at higher water content. A population of spherical micelles was found to coexist with the cylindrical ones, even above the percolation threshold. The shape transformation was correlated with a viscosity leap observed in the rheometry measurements. Reverse micelles at low water content under shear act as a Newtonian fluid, without any detectable shape changes. In contrast, reverse micelles at high water content behave as a shear thinning fluid. SAXS measurements at high water content under shear force have shown that the shear forces induced alignment of the cylindrical micelles in the flow direction, without any other changes in the micelle dimensions. The anisotropy parameter, a measure of the degree of the spatial order, was found to increase with increasing water content and shear rate.     

Site-specific hydration status of an amphipathic peptide in AOT reverse micelles

Reverse micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in isooctane (IO) and water have long been used as a means to provide a confined aqueous environment for various applications. In particular, AOT reverse micelles have often been used as a template to mimic membrane-water interfaces. While earlier studies have shown that membrane-binding peptides can indeed be incorporated into the polar cavity of AOT reverse micelles where they mostly fold into an alpha-helical structure, the underlying interactions leading to the ordered conformation are however not well understood. Herein, we have used circular dichroism (CD) and infrared (IR) spectroscopies in conjunction with a local IR marker (i.e., the CN group of a non-natural amino acid, p-cyano-phenylalanine) and a global IR reporter (i.e., the amide I' band of the peptide backbone) to probe the conformation as well as the hydration status of an antimicrobial peptide, mastoparan x (MPx), in AOT reverse micelles of different water contents. Our results show that at, w0=6, MPx adopts an alpha-helical conformation with both the backbone and hydrophobic side chains mostly dehydrated, whereas its backbone becomes partially hydrated at w0=20. In addition, our results suggest that the amphipathic alpha-helix so formed orients itself in such a manner that its positively charged, lysine-rich, hydrophilic face points toward the negatively charged AOT head groups, while its hydrophobic face is directed toward the polar interior of the water pool. This picture is in marked contrast to that observed for the binding of MPx to phospholipid bilayers wherein the hydrophobic surface of the bound alpha-helix is buried deeper into the membrane interior.     

Oxidative refolding of reduced, denatured lysozyme in AOT reverse micelles

The refolding kinetics of the reduced, denatured hen egg white lysozyme in sodium bis(2-ethylhexyl)sulfosuccinate (AOT)-isooctane-water reverse micelles at different water-to-surfactant molar ratios has been investigated by fluorescence spectroscopy and UV spectroscopy. The oxidative refolding of the confined lysozyme is biphasic in AOT reverse micelles. When the water-to-surfactant molar ratio (omega 0) is 12.6, the relative activity of encapsulated lysozyme after refolding for 24 h in AOT reverse micelles increases 46% compared with that in bulk water. Furthermore, aggregation of lysozyme at a higher concentration (0.2 mM) in AOT reverse micelles at omega 0 of 6.3 or 12.6 is not observed; in contrast, the oxidative refolding of lysozyme in bulk water must be at a lower protein concentration (5 microM) in order to avoid a serious aggregation of the protein. For comparison, we have also investigated the effect of AOT on lysozyme activity and found that the residual activity of lysozyme decreases with increasing the concentration of AOT from 1 to 5 mM. When AOT concentration is larger than 2 mM, lysozyme is almost completely inactivated by AOT and most of lysozyme activity is lost. Together, our data demonstrate that AOT reverse micelles with suitable water-to-surfactant molar ratios are favorable to the oxidative refolding of reduced, denatured lysozyme at a higher concentration, compared with bulk water.     

Kinetics of photopolymerization of acrylamide in AOT reverse micelles

The kinetics of the photoinitiated polymerization of acrylamide in toluene/AOT/water inverse microemulsions have been examined for systems initiated by AIBN or a dye: triethanolamine redox reaction. The rates of polymerization were determined by dilatometry, the data being corrected for the effect of reaction exotermicity. For both the oil-soluble and the water-soluble initiating systems, the rate of polymerization was found to be proportional to the first power of the incident light intensity. For AIBN-initiated systems, the rate of polymerization was also found to be proportional to the first power of the initiator concentration. The molecular weights of the polymers produced were independent of the rates of polymerization and initiation. These results suggest that exclusively monoradical termination, involving a degradative chain transfer, is occurring in these systems     

Reactions of hydrated electron with hemoproteins in AOT reverse micelles

Reverse micelles formed from sodium bis (2-ethylhexyl)sulfosuccinate in n-heptane containing cytochrome c, horseradish peroxidase (HRP) and catalase in water pools preferentially scavenge electrons produced radiolytically in oil phase. Both initial absorbance at 720 nm and the half-lifetime of hydrated electron depend on the kind of hemoprotein and on the overall protein concentration. The possibility of electron capture by the protein molecule before its hydration is discussed. It is shown that the entrappment of cytochrome c and HRP into reverse micelles changes the reduction ability of the protein, whereas ferric iron in catalase remains unaccesible fore _aq ^– like in homogeneous aqueous solution.     

Higher order structure of proteins solubilized in AOT reverse micelles

The higher order structure of proteins solubilized in an bis(2-ethylhexyl) sulfosuccinate sodium (AOT) reverse micellar system was investigated. From circular dichroic (CD) measurement, CD spectra of cytochrome c, which is solubilized at the interface of reverse micelles, markedly changed on going from buffer solution to the reverse micellar solution, and the ellipticity values in the far- and near-UV regions decreased with decreasing the water content (W₀: molar ratio of water to AOT), indicating that the secondary and tertiary structures of cytochrome c changed with the water content. The ellipticity of ribonuclease A, which is solubilized in the center of micellar water pool, in the near-UV region was dependent on W₀ and became minimum when W₀ of ca. 8 while the ellipticity in the far-UV region was almost constant, indicating that the tertiary structure of ribonuclease A was affected by the water content, but the secondary structure was conserved. The degree of curvature of the micellar interface appears to influence the protein structure because the reverse micelle size is linearly proportional to the W₀ value. As evidence of this, when the micelle size was comparable to the protein's dimensions, the structures were more affected by the water content. Judging from the dependence of the factor influencing the protein structure on the protein species, the location of solubilized protein in reverse micelles is significantly related to whether the protein structure in the system is affected by the micellar interface. In the cases of cytochrome c and lysozyme, the ellipticity against W₀ was dependent on the AOT concentration. In contrast, ribonuclease A gave very similar ellipticity values whatever the AOT concentration. In the n-hexane micellar system, cytochrome c exhibited lower ellipticity values and ribonuclease A in the lower W₀ range (W₀ < ca. 8) higher ellipticity values. These results indicated that the interaction between the protein and the micellar interface is a dominant factor influencing the protein structure in reverse micelles, and that it is governed by the location of solubilized proteins and the state of the micellar interface.     

Spectral properties of carbocyanine dyes in solutions of reverse AOT micelles

Spectral and luminescence studies of an anionic carbocyanine dye in solutions of reverse AOT micelles have shown that the dye in micelle pools can occur only in the form of the trans-monomer; although aggregated forms of the dye including J aggregates can be present in micellar solution, they all are localized in the organic phase, not micellar pools.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 1, 2005, pp. 21–25.Original Russian Text Copyright © 2005 by Brichkin, Kurandina, Nikolaeva, Razumov.     

Spectroluminescent properties of cadmium selenide nanoparticles synthesized in AOT reverse micelles

Cadmium selenide nanoparticles have been synthesized in solutions of AOT/water/n-heptane reverse micelles with average micelle water pool diameters of 25, 30, and 47 Å using cadmium sulfate (CdSO₄) and sodium selenosulfate (Na₂SeSO₃) as precursors. Absorption and fluorescence spectra of the obtained nanoparticles were recorded. The picosecond dynamics of fluorescence decay over the entire range of their emission band have been investigated by time-resolved fluorescence spectroscopy. A procedure for the stabilization of nanoparticles by dodecanethiol was developed for electron microscopy analysis.     

The location of tryptophan, N-acetyltryptophan and alpha-chymotrypsin in reverse micelles of AOT: a fluorescence study

The spectroscopic properties of alpha-chymotrypsin (alpha-Chym), L-tryptophan (Trp) and N-acetyl-L-tryptophan (NAT) solubilized in hydrated reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate in iso-octane were followed by fluorescence as a function of the amount of intramicellar water and initial pH. The lack of pH dependence observed for Trp in these systems, as opposed to what occurs in bulk water, and the similarities found for the protein in both media foresee different locations of these probes. In reverse micelles, fluorescence quenching studies using acrylamide emphasize the existence of structural alterations within the protein when its global charge changes from positive (pH = 7) to negative (pH = 10). The ensemble of the data points to an interfacial location of the zwitterionic Trp, an intermediate region of less tightly bound water for the location of the anionic Trp and NAT and an almost bulk water environment for alpha-Chym.     

Lipase catalyzed esterification in AOT reverse micelles: A structural study

AOT reverse micelles are used to cosolubilize hydrophilic and hydrophobic reactants of lipase catalysed esterification. Depending on the nature of the alcohol, a drastic change of the initial rate of the esterification is observed. A structural study of the micellar system with and without reactant is undertaken to explain the change in the activity with the various alcohols.     

Photophysics and locations of IR125 and C152 in AOT reverse micelles

Many fluorescent chromophores have been employed to investigate the nature and dynamics of the water confined in reverse micelles (RMs). However, some questions remain as to the location of a probe in a RM and the diameter of the RM at which the physical characteristic of the water inside RMs becomes similar to that of bulk water. In this work, we systematically studied the photophysics of IR125 and C152 in AOT RMs at different w(0) by means of static absorption and fluorescence spectroscopy as well as time-resolved fluorescence spectroscopy. We obtained the absorption maxima, fluorescence emission maxima, fluorescence lifetime, and reorientation time of IR125 and C152 in AOT RMs at corresponding w(0). We found that all obtained photophysical parameters of IR125 and C152 in AOT RMs as a function of w(0) have a distinct changeover point around w(0) = 8, indicating that there is a dramatic change in the nature of the water confined in AOT RMs around w(0) = 8. The observed changeover point around w(0) = 8 is well in agreement with the Satpati's report (ChemPhysChem, 2009, 10, 2966). In addition, we observed that the measured reorientation time of IR125 in AOT RMs increases with the increase of w(0), which is opposite to the trend of change in the measured reorientation time of C152 in AOT RMs with the increase of w(0). We found that IR125 prefers to reside in the water pool of AOT RMs and that C152 prefers to reside in the outer side of the interfacial region or the nonpolar n-heptane phase of AOT RMs. Furthermore, we found that the time-resolved fluorescence anisotropy of IR125 in smaller w(0) AOT RMs primarily measures the reorientation of RMs and the time-resolved fluorescence anisotropy of IR125 in larger w(0) AOT RMs measures the reorientation of IR125 in the water pool confined in RMs. This work demonstrated that IR125 is an excellent probe to study the nature and dynamics of the water confined in AOT RMs.     

Thermochemicstry of the oxidative dissolution of silver nanoparticles in AOT reverse micelles

A procedure has been developed for thermochemical studies of two types of silver nanoparticles: coagulated nanoparticles in isolated nanopowders and unbound nanoparticles enclosed in micelles. The first type of nanoparticle was obtained and studied after destroying micelles. The heats and rates of dissolution of unbound nanoparticles (i.e., nanoparticles enclosed in micelles and involved in Brownian motion together with the latter) were determined by reacting them with nitric acid solubilized in micelle systems of the same composition. The heats of solution of Ag⁰ in HNO₃ found for isolated nanopowders were ?10 × (1 ± 0.3) kJ/mol; they were close to the results obtained for bulk samples under the same conditions. The heats of solution of nanoparticles in micelles were estimated at ?(17–25) kJ/mol. The dissolution rates of silver nanoparticles in micelles as functions of micelle composition were two to three times the dissolution rates of isolated nanopowders.     

Dynamics of low temperature induced water shedding from AOT reverse micelles

The effects of low temperature and ionic strength on water encapsulated within reverse micelles were investigated by solution NMR. Reverse micelles composed of AOT and pentane and solutions with varying concentrations of NaCl were studied at temperatures ranging from 20 degrees C to -30 degrees C. One-dimensional (1)H solution NMR spectroscopy was used to monitor the quantity and structure of encapsulated water. At low temperatures, e.g., -30 degrees C, reverse micelles lose water at rates that are dependent on the ionic strength of the aqueous nanopool. The final water loading (w0 = [water]/[surfactant]) of the reverse micelles is likewise dependent on the ionic strength of the aqueous phase. Remarkably, water resonance(s) at temperatures between -20 degrees C and -30 degrees C displayed fine structure indicating the presence of multiple transient water populations. Results of this study demonstrate that reverse micelles are an excellent vehicle for studies of confined water across a broad range of conditions, including the temperature range that provides access to the supercooled state.     

Excited-state proton transfer behavior of 7-hydroxy-4-methylcoumarin in AOT reverse micelles

The excited-state proton transfer and phototautomerization of 7-hydroxy-4-methylcoumarin (7H4MC) dye has been studied in the confined water pools of AOT reverse micelles using steady-state and time-resolved fluorescence measurements. In the "dry" reverse micelles ([water]/[AOT], w(0) = 0), only the neutral form of the dye is present both in the ground and the excited states. At higher w(0) values, three prototropic forms, namely, neutral, anionic, and tautomeric, can be identified in the excited state, although only the neutral form of the dye is present in the ground state. From steady-state fluorescence results and time-resolved area-normalized emission spectra (TRANES), it is indicated that the anionic and tautomeric forms of the dye are the excited-state reaction products and that they arise apparently independently from the excited neutral form of the dye. In bulk water, however, there is no evidence of the tautomeric species and only the anionic form is observed in the excited state. The fluorescence quenching results of the three forms of 7H4MC by the different quenchers, potassium iodide, aniline, and N, N-dimethylaniline, suggest that the distribution of 7H4MC molecules in the reverse micelles is not diverse but that the different prototropic forms arise from the same population of the excited dye in the interfacial region.     

Fourier transform infrared spectroscopy of azide and cyanate ion pairs in AOT reverse micelles

Evidence for ion pair formation in aqueous bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles (RMs) was obtained from infrared spectra of azide and cyanate with Li(+), Na(+), K(+), and NH(4)(+) counterions. The anions' antisymmetric stretching bands near 2000 cm(-1) are shifted to higher frequency (blueshifted) in LiAOT and to a lesser extent in NaAOT, but they are very similar to those in bulk water with K(+) and NH(4)(+) as the counterions. The shifts are largest for low values of w(o) = [water]/[AOT] and approach the bulk value with increasing w(o). The blueshifts are attributed to ion pairing between the anions and the counterions. This interpretation is reinforced by the similar trend (Li(+)>Na(+)>K(+)) for producing contact ion pairs with the metal cations in bulk dimethyl sulfoxide (DMSO) solutions. We find no evidence of ion pairs being formed in NH(4)AOT RMs, whereas ammonium does form ion pairs with azide and cyanate in bulk DMSO. Studies are also reported for the anions in formamide-containing AOT RMs, in which blueshifts and ion pair formation are observed more than in the aqueous RMs. Ion pairs are preferentially formed in confined RM systems, consistent with the well established ideas that RMs exhibit reduced polarity and a disrupted hydrogen bonding network compared to bulk water and that ion-specific effects are involved in mediating the structure of species at interfaces.     

Photophysics of 4-N,N-dimethylamino cinnamaldehyde in AOT reverse micelles and exploration of its position and orientation

An attempt has been made in this Letter to locate the position and orientation of 4-N,N-dimethylamino cinnamaldehyde (DMACA) inside sodium bis(2-ethylhexyl) sulfosuccinate (AOT)–n-heptane reverse micelle based on change in photophysical properties of DMACA compared to that in n-heptane. It has been proposed that the possibility of finding the donor moiety inside the small water pool of reverse micelle is maximum while the acceptor group straddles in the remaining part of the reverse micelle. The micropolarity in the vicinity of the donor moiety has been computed in terms of dielectric constant with varying water pool size.     

Physicochemical investigation of the solubilization of ytterbium nitrate in AOT reverse micelles and liquid crystals

A wide investigation of the solubilization of the water-soluble salt Yb(NO₃)₃ in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles and AOT liquid crystals has been carried out. After saturation of water/AOT/organic solvent w/o microemulsions with pure Yb(NO₃)₃, the Yb(NO₃)₃/AOT composites were prepared by complete evaporation under vacuum of the volatile components (water and organic solvent) of the salt-containing microemulsions. It was observed that these composites can be totally dissolved in pure n-heptane or CCl₄, allowing the solubilization of a noticeable amount of Yb(NO₃)₃ in quite dry apolar media. By UV–vis–NIR, FT-IR, and ¹H NMR spectroscopies, some information on the state of Yb(NO₃)₃ within AOT reverse micelles were acquired, whereas by small angle X-ray scattering (SAXS), it has been ascertained that Yb(NO₃)₃ is quite homogeneously distributed as very small clusters among the reverse micelles. An analysis of SAXS and wide-angle X-ray scattering spectra of Yb(NO₃)₃/AOT composites leads to the hypothesis that, also in these systems, Yb(NO₃)₃ is dispersed in the surfactant matrix as very small clusters.     

Proton transport and the water environment in nafion fuel cell membranes and AOT reverse micelles

The properties of confined water and diffusive proton-transfer kinetics in the nanoscopic water channels of Nafion fuel cell membranes at various hydration levels are compared to water in a series of well-characterized AOT reverse micelles with known water nanopool sizes using the photoacid pyranine as a molecular probe. The side chains of Nafion are terminated by sulfonate groups with sodium counterions that are arrayed along the water channels. AOT has sulfonate head groups with sodium counterions that form the interface with the reverse micelle's water nanopool. The extent of excited-state deprotonation is observed by steady-state fluorescence measurements. Proton-transfer kinetics and orientational relaxation are measured by time-dependent fluorescence using time-correlated single photon counting. The time dependence of deprotonation is related to diffusive proton transport away from the photoacid. The fluorescence reflecting the long time scale proton transport has an approximately t-0.8 power law decay in contrast to bulk water, which has a t-3/2 power law. For a given hydration level of Nafion, the excited-state proton transfer and the orientational relaxation are similar to those observed for a related size AOT water nanopool. The effective size of the Nafion water channels at various hydration levels are estimated by the known size of the AOT reverse micelles that display the corresponding proton-transfer kinetics and orientational relaxation.     

"Snap-shooting" the interface of AOT reverse micelles: use of chemical trapping

The first use of the phenyl cation trapping technique in "snap-shooting" the local molar concentrations of water and sulfosuccinate head-groups in the interfacial region of AOT-2,2,4-trimethylpentane-water reverse micelles has been accomplished. Our results demonstrate that the interfacial concentrations of the sulfosuccinate head-groups in AOT (0.1 M)-2,2,4-trimethylpentane-water reverse micelles are remarkably high (2.75-2.34 M) across the W0 (the molar ratio of water to surfactant) range 12 to 44. However, the interfacial concentrations of water in AOT- 2,2,4-trimethylpentane-water reverse micelles across the same range of solution compositions are significantly lower (27.9-32.0 M) than the molar concentration of bulk water (55.5 M). The present results provide new insight on the microenvironments of interfacially located enzymes such as lipases entrapped in AOT-2,2,4-trimethylpentane-water reverse micelles, the most extensively exploited reverse-micellar system in micellar biotechnology.