Micelle Interface Research Articles

On the Behavior of Indole-Containing Species Sequestered within Reverse Micelles at Sub-Zero Temperatures

We report on the effects of temperature (+30 to -100 degrees C) on the fluorescence from N-acetyl tryptophanamide (NATA) and human serum albumin (HSA) sequestered within Aerosol-OT (AOT) reversed micelles. NATA reports simultaneously from the polar and non-polar side of the reverse micelle interface. As the sample temperature decreases, the relative fraction of NATA molecules associated with the polar side increases. This redistribution process is characterized by DeltaH = -14.8 +/- 0.6 kJ/mol and DeltaS = -54 +/- 2 J/(K mol). The activation energy for thermal quenching (E(a,TQ)) associated with the polar side NATA molecules is 6.7 kJ/mol before the micelles have shed water and 1.0 kJ/mol after water shedding (below approximately -20 degrees C). The time-resolved fluorescence intensity decay for tryptophan-214 in HSA is triple exponential. We suggest that these lifetimes arise from three indole residue conformations in equilibrium. Cooling the sample causes a freezing-in of the least quenched conformer; the other conformers are frozen out. The E(a,TQ) value for the shortest lifetime component is 6 kJ/mol. The E(a,TQ) for the long and intermediate lifetime components are equivalent (approximately 1.5 kJ/mol).

In‐Situ Polymerization at the Interface of Micelles: A Novel Method to Control Functionality and Morphology

AbstractA new method to control the morphology and functionality of micelles is reported. Triblock copolymer micelles with atom transfer radical polymerization initiators at the interface are prepared in aqueous solution. After in‐situ polymerization at the interface, the structures of the interface and corona change, and micelles with PDMAEMA‐PEG comb–coil coronal chains are obtained. In aqueous solution, the pH exerts an influence on the morphology of the micelles. The coronal chains adopt different conformations at different pH values. Upon drying, the two coronal chains phase separate and form nanometer‐sized domains.magnified image

EFFECT OF THE ORGANIC SOLVENT ON THE INTERFACIAL MICROPOLARITY OF AOT -WATER REVERSE MICELLES

A study has been made on the effect of the organic solvent upon the properties of the interface of reverse micelles formed by sodium bis (2-ethylhexyl) sulfosuccinate (AOT) - water. The study has been performed using fluorescence and absorption spectroscopies. The probes considered were 8-anilinonaphtha lene-1-sulfonate (ANS-) and ruthenium tris [(bipyridine)]2+, [Ru (bpy)3]2+, in fluorescence spectroscopy, and ET(30) in absorption spectroscopy. These probes were selected since its spectroscopic behaviour depends on the micro polarity of the medium. Furthermore, they have equal charge (ANS-), opposite charge [Ru(bpy)3]2+ than that of the micelle, or its nature is zwitterionic (ET (30)), which allows for the sensing of the interfacial properties at different depths. The study has been performed at different W = [water]/[AOT] values ranging from ca. 1-2 up to saturation in the external solvent considered. Organic solvents employed were: n-heptane, n-hexane, isooctane, dodecane cyclohexane, benzene, toluene, cumene, chlorobenzene, dichlormethane (DCM), 1,2-dicholoroethane (1,2-DCE) and tetrachloroethylene (TCE), and the results are discussed taking n-heptane as reference solvent. For the three probes considered, n-hexane, isooctane, dodecane and cyclohexane behaves similarly to n-heptane over all the range of W values considered, irrespective of the probe employed. ANS- behaviour in benzene, toluene and TCE is similar to that observed in aliphatic hydrocarbons. Cumene, chlorobenzene, DCM and 1,2 - DCE show a microenvironment more polar than that of aliphatic hydrocarbons. The behaviour of dichloromethane and 1,2 - DCE is similar for the three probes and very different than that observed in aliphatic hydrocarbons. The microenvironment that they offer in front of [Ru(bpy)3]2+ is considerably more polar than that corresponding to n-heptane, while in front of ET(30) is less polar. The results obtained using [Ru(bpy)3]2+ are the more difficult to explain, since displacement towards more polar (in 1,2-DCE and DCM) or less polar (in benzene, cumene, TCE) media were observed

Control of wall thickness in the formation of ordered mesoporous silica films

Ordered mesoporous silica thin films using block copolymer have drawn an attention for low-k application due to its ordered pore structure. From the respect of dielectric and mechanical properties of the film, there is trade-off between pore size and wall thickness. In this work, factors for increase of wall thickness were investigated. It was found that body-centered cubic structure was maintained irrespective of the concentration of catalytic acid. The catalytic acid thickens the framework wall because counterion reduces the repulsion force between silicic acids. The highly ordered mesoporous silica films were obtained although high concentration of acid was added to the silica sol. However, wormlike micelle exists more with high HCl concentration due to fast gellation rate. And excess water, which has the role similar to the humid atmosphere, also increases the thickness of silica wall. However, large amount of excess water at the micelle interface disrupts organic-inorganic electrostatic interaction. As a conclusion, optimization of HCl concentration in the silica sol and control of humidity during spin coating can simultaneously increase the framework thickness while maintaining the pore periodicity.

Stochastic modelling of roto-translational motion of dyes in micellar environment

Time decay of the fluorescence anisotropy ratio due to diffusion of dyes in micelles is usually interpreted by decoupling the reorientational dynamics of the molecule with respect to the local director from the translational diffusion of the dye. However, while such a kind of assumption is justified in other contexts (like for reorientations of a small mobile fragment in a macromolecule, decoupled from the motion of the macromolecule as a whole, as invoked in the well-known model by Lipari and Szabo), here it is not based on physical grounds. In this work we develop the stochastic model for the full description of the roto-translational dynamics of a dye in the micellar environment, by employing the Fokker–Planck–Smoluchowski equation for the positional and orientational variables. Then we simplify the model to the situation of strong confinement of the molecule at the micelle interface. Finally, by employing a time-scale separation between fast reorientational dynamics and slow lateral diffusion of the dye (which holds if the micelle radius is much larger than the size of the dye), and by resorting to a model like the “wobbling in a cone”, we show that a bi-exponential form can be obtained for the fluorescence anisotropy ratio, but with the remarkable difference that the fast-relaxing component is not affected by the slow motion.

Solute−Solvent Contact by Intermolecular Cross-Relaxation. 2. The Water−Micelle Interface and the Micellar Interior

The intermolecular dipole-dipole cross-relaxation is measured between 19F nuclei of sodium perfluorooctanoate in micelles and 1H nuclei of the water solvent. The cross-relaxation rates for fluorines in the different moieties along the surfactant vary strongly by the resonance frequency in the investigated range of 188-470 MHz. This frequency dependence indicates that the cross-relaxation between water and amphiphilic aggregates is not controlled solely by the fast local water dynamics but significantly contributed to by the long-range translational diffusion of water. The cross-relaxation rates, analyzed in the framework of a model (Nordstierna, L.; Yushmanov, P. V.; Furó, I. J. Chem. Phys. 2006, 125, 074704), provide information about the dynamic retardation of water molecules by the micellar headgroup region and the location of the various moieties along the hydrophobic tail with respect to the water-micelle interface. Both intermolecular cross-relaxation and aggregation-induced 19F chemical shift changes indicate no direct water contact to fluorines except for those closest to the head group.

Formation of Disperse Nanoparticles at the Oil/Water Interface in Normal Microemulsions

An artificial oil/water interface was created in normal microemulsions. Various well-dispersed inorganic nanoparticles were successfully fabricated at this micelle interface, and a "hot liquid annealing" process was used to crystallize the products. Owing to the large solubility of the source materials in the water phase, the colloidal nanoparticles can easily be prepared on a large scale. Compared with traditional reverse-microemulsion methods, the method reported here yields larger amounts of colloidal particles but with the same quality.

The Influence of Urea on the Structure of Proteins in Reversed Micelles

Static fluorescence measurements from the protein Tryptophan (Trp) residues and circular dichroism (CD) spectroscopy were used to investigate changes on the tertiary and secondary structures of the protein bovine serum albumin (BSA) and the dimeric enzyme hexokinase (HK) type PII from yeast, entrapped in reversed micelles (RMs). The latter were obtained from the amphiphilic AOT (sodium bis-2-ethylhexyl sulfosuccinate) in n-hexane, at several water to surfactant ratios, W. BSA and HK were found to be anchored at the RM interface in close contact with the surfactant layer, regardless of the size of the waterpool. For BSA, such interaction promotes partial protein unfolding, according to CD data that showed a decrease in the content of helical structure from 66% in a buffer solution to 48% in the micellar moiety. When urea was present in the micelle, further loss in helical structure occurred, thus indicating that the combined effect (micellar environment and urea) altered the BSA conformation to a greater extent than did RM or urea alone. Interestingly, Trps probed the same environment in the micelle, regardless of the presence of urea, but the fluorescence was quenched to a higher extent with urea. Thus, the fluorofore emission must have been affected either by the direct interaction of urea or by indirect exchange of water structure caused by urea interacting with water and the micellar interface. Both mechanisms might be of relevance in the solvation properties nearby the Trps. For HK, an association between the enzyme and the micelle interface was indicated in the CD spectra, which exhibited a randomized structure upon interaction, whatever the RM droplet size. The urea addition to the micelle water pool did not cause further impact on the HK conformation. In addition, the influence of urea at the RM interface was not sensed by the exposed tryptophans of the enzyme, unlike the results for BSA.

Are the mixtures of homologous surfactants ideal?

The interaction between homologous surfactants in mixed micelles was studied by the Regular Solution Theory of mixed micelles. The interaction is independent of the nature of the polar head groups and attractive and the interaction parameter βM depends linearly on the difference in chain length ΔnC. The interaction becomes ideal at ΔnC=0.75±0.06. Above ΔnC≈5, the dependence remains linear but the slope increased 2.7 times. The phenomenon is explained as the effect of the reduction of the hydrocarbon/water micelle interface and a better packing of the chains in the micelle core, caused by the inclusion of a shorter homologous surfactants. This reduction can be more effective when ΔnC⩾5.

Molecular-Thermodynamic Theory of Micellization of pH-Sensitive Surfactants

A predictive, molecular-thermodynamic theory is developed to model the micellization of pH-sensitive surfactants. The theory combines a molecular-thermodynamic description of micellization in binary surfactant mixtures with the protonation equilibrium of the surfactant monomers. The thermodynamic component of the theory models the pH-mediated equilibrium between micelles, surfactant monomers, and counterions. These counterions may originate from the surfactant or from added salt, acid, or base. The molecular component of the theory models the various contributions to the free energy of micellization, which corresponds to the free-energy change associated with forming a mixed micelle from the protonated and deprotonated forms of the surfactant and from the bound counterions. The free energy of micellization includes hydrophobic, interfacial, packing, steric, electrostatic, and entropic contributions, which are all calculated molecularly. The theory also requires knowledge of the surfactant molecular structure and the solution conditions, including the temperature and the amount of any added salt, acid, or base. To account for the pH sensitivity of the surfactant, the theory requires knowledge of the surfactant monomer equilibrium deprotonation constant (pK1), which may be obtained from experimental titration data obtained below the critical micelle concentration (cmc). The theory can be utilized to predict the equilibrium micelle and solution properties, including the cmc, the micelle composition, the micelle shape and aggregation number, the solution pH, and the micelle deprotonation equilibrium constant (pKm). Theoretical predictions of the cmc, the micelle aggregation number, and the pKm compare favorably with the available experimental data for alkyldimethylamine oxide surfactants. This class of pH-sensitive surfactants exhibits a form of self-synergy, which has previously been attributed to hydrogen-bond formation at the micelle interface. Instead, we show that much of the observed synergy is related to the electrostatic contribution to the free energy of micellization. Although we do not explicitly include hydrogen bonding in the molecular model of micellization, we briefly discuss how it may be incorporated and its anticipated effect on the predicted micellization behavior.

Unusual Rate Enhancement of Bimolecular Dehydrocondensation To Form Amides at the Interface of Micelles of Fatty Acid Salts

Die Grenzfläche Micelle/Wasser ist der geeignete Ort für Dehydrokondensationen. Entsprechend verlief die Kupplung von Fettsäuresalzen mit einem amphiphilen Triazin-Dehydrokondensationsagens im micellaren System (siehe Bild) bis zu 2000-mal schneller. Die Reaktionsbeschleunigung wird mit der höheren lokalen Konzentration und der Ausrichtung der Reaktanten in den Micellen erklärt.

Unusual Rate Enhancement of Bimolecular Dehydrocondensation To Form Amides at the Interface of Micelles of Fatty Acid Salts

Unusual Rate Enhancement of Bimolecular Dehydrocondensation To Form Amides at the Interface of Micelles of Fatty Acid Salts

Nanosecond Dynamics of a Mimicked Membrane-Water Interface Observed by Time-Resolved Stokes Shift of LAURDAN

We studied the dipolar relaxation of the surfactant-water interface in reverse micelles of AOT-water in isooctane in the nanosecond and subnanosecond time ranges by incorporating the amphipathic solvatochromic fluorescent probes LAURDAN and TOE. A negative component was observed in the fluorescence decays in the red edge of the emission spectrum—the signature of an excited state reaction—with LAURDAN but not for TOE. The deconvolution of the transient reconstructed spectra of LAURDAN based on a model constructed by adding together three log-normal Gaussian equations made it possible to separate the specific dynamic solvent response from the intramolecular excited state reactions of the probe. The deconvoluted spectrum of lowest energy displayed the largest Stokes shift. This spectral shift was described by unimodal kinetics on the nanosecond timescale, whereas the relaxation kinetics of water-soluble probes have been reported to be biphasic (on the subnanosecond and nanosecond timescales) due to the heterogeneous distribution of these probes in the water pool. Most of this spectral shift probably resulted from water relaxation as it was highly sensitive to the water to surfactant molar ratio ( w 0) (60–65 nm at w 0 = 20–30). A small part of this spectral shift (9 nm at w 0 = 0) probably resulted from dipolar interaction with the AOT polar headgroup. The measured relaxation time values were in the range of the rotational motion of the AOT polar headgroup region as assessed by LAURDAN and TOE fluorescence anisotropy decays.

Uranine Chemiluminescence in Aggregates Formed from Aqueous Mixtures of Dodecyltrimethylammonium Chloride and Sodium Dodecyl Sulfate

Aggregates (mixed micelles, bilayer membranes, vesicles) formed from aqueous mixtures of dodecyltrimethylammonium chloride (DTAC) and sodium dodecyl sulfate (SDS) were evaluated for chemiluminescence (CL) reaction media by means of a batch method. The uranine CL based on oxidation by hypochlorous acid was chosen as a model CL system. The uranine CL intensity was enhanced by a factor of 91 in mixed micelles ([DTAC]/[SDS] = 1.5) compared with that in other aggregates (bilayer membranes and vesicles). This enhancement is thought to be caused by the increased CL quantum yield, which results from stabilization of the emitter (uranine) on the interface of the rigid mixed micelles.

Solvent effects versus concentration effects in determining rates of base-catalyzed keto-enol tautomerization

Solvent effects of homogeneous media (such as solvent–water mixtures) on chemical reactivity may be interpreted as due to solvent polarity and/or molecular structure of solvent molecules. In microheterogeneous media (such as aqueous micellar solutions), solvent effects on reaction rates must include concentration effects, in addition to changes in the solvent polarity of the micelle interface where the reaction is assumed to occur. In this work, we measured the rates of keto-enol tautomerization of the 2-acetylcyclohexanone (ACHE) and 2-acetyl-1-tetralone (ATLO) systems in dimethylsulfoxide (DMSO)–water mixtures and in aqueous micellar solutions with both anionic and cationic surfactants and in the presence of buffers. The results appear as an ideal framework to understand the paramount importance of the specific molecular structure of solvent molecules in determining chemical reactivity versus solvent polarity or even concentration effects.

Reverse Micellar Aggregates: Effect on Ketone Reduction. 2. Surfactant Role

Kinetics of the reduction of 3-chloroacetophenone (CAF) with sodium borohydride (NaBH(4)) were followed by UV-vis spectroscopy at 27.0 degrees C in different reverse micellar media, toluene/BHDC/water and toluene/AOT/water, and compared with results in an isooctane/AOT/water reverse micellar system. AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate, and BHDC is benzyl-n-hexadecyl dimethylammonium chloride. The kinetic profiles were investigated as a function of variables such as surfactant and NaBH(4) concentration and the amount of water dispersed in the reverse micelles, W(0) = [H(2)O]/[surfactant]. In all cases, the first-order rate constant, k(obs), increases with the concentration of surfactant as a consequence of incorporating the substrate into the interface of the reverse micelles where the reaction takes place. The reaction is faster at the cationic interface than at the anionic one probably because the negative ion BH(4)(-) is part of the cationic interface. The effect of the external solvent on the reaction shows that reduction is favored in the isooctane/AOT/water reverse micellar system than that with an aromatic solvent. This is probably due to BH(4)(-) being more in the water pool of the toluene/AOT/water reverse micellar system. The kinetic profile upon water addition depends largely on the type of interface. In the BHDC system, k(obs) increases with W(0) in the whole range studied while in AOT the kinetic profile has a maximum at W(0) approximately 5, probably reflecting the fact that BH(4)(-) is part of the cationic interface while, in the anionic one, there is a strong interaction between water and the polar headgroup of AOT below W(0) = 5 and, above that, BH(4)(-) is repelled from the interface once the water pool has formed. Application of a kinetic model based on the pseudophase formalism, which considers the distribution of the ketone between the continuous medium and the interface and assumes that reaction takes place only at the interface, has enabled us to estimate rate constants at the interface of the reverse micellar systems. At W(0) < 10, it was considered that NaBH(4) is wholly at the interface and, at W(0) >/= 10, where there are free water molecules, also the partitioning between the interface and the water pool was taken into account. The results were used to evaluate CAF and NaBH(4) distribution constants between the different pseudophases as well as the second-order reaction rate constant of the reduction reaction in the micellar interface.

Reverse Micellar Aggregates: Effect on Ketone Reduction. 1. Substrate Role

The reduction of three aromatic ketones, acetophenone (AF), 4-methoxyacetophenone (MAF), and 3-chloroacetophenone (CAF), by NaBH(4) was followed by UV-vis spectroscopy in reverse micellar systems of water/AOT/isooctane at 25.0 degrees C (AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate). The first-order rate constants, k(obs), increase with the concentration of surfactant due to the substrate incorporation at the reverse micelle interface, where the reaction occurs. For all the ketones the reactivity is lower at the micellar interface than in water, probably reflecting the low affinity of the anionic interface for BH(4)(-). Kinetic profiles upon water addition show maxima in k(obs) at W(0) approximately 5 probably reflecting a strong interaction between water and the ionic headgroup of AOT; at W(0) < 5 by increasing W(0) BH(4)(-) is repelled from the anionic interface once the water pool forms. The order of reactivity was CAF >> AF > MAF. Application of a kinetic model based on the pseudophase formalism, which considers distribution of the ketones between the continuous medium and the interface, and assumes that reaction take place only at the interface, gives values of the rate constants at the interface of the reverse micellar system. At W(0) = 5, we conclude that NaBH(4) is wholly at the interface, and at W(0) = 10 and 15, where there are free water molecules, the partitioning between the interface and the water pool has to be considered. The results were used to estimate the ketone and borohydride distribution constants between the different pseudophases as well as the second-order reaction rate constant at the micellar interface.

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 ( W 0: 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 W 0 and became minimum when W 0 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 W 0 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 W 0 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 W 0 range ( W 0 < 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.

The role of the cosurfactant in the CTAB/water/n-pentanol/n-hexane system: Pentanol effect on the phase equilibria and mesophase structure

We report on the effect of 1-pentanol loading on the phase behaviour of mixtures of CTAB/water/n-hexane at fixed mole ratios 1∶80∶47. The cosurfactant induces changes in the interfacial film curvature. By increasing the pentanol/CTAB mole ratio, the system evolves from oil-in-water to water-in-oil structures. For very large 1-pentanol loading some water is expelled from the reverse micelles resulting in a L2plus water equilibrium (emulsification failure). In the range of compositions investigated most of the phase equilibria reveals the coexistence of liquid crystals and hexane/pentanol solutions. In these cases, we estimate the interface composition by assuming a constant 1-pentanol concentration in all the oil domains, and by tacking into account the alcohol solubility in water. In the case of single-phase sample the interfacial composition was determined by means of turbidimetric titration. The adsorption of cosurfactant at the interface of direct micelles, planar lamellae, bicontinuous microemulsions, and spherical reverse micelles follows the same adsorption isotherm (independently from the curvature of the interface). Moreover, the results obtained unambiguously show that the interface composition dictates the spontaneous curvature of interfacial film. Actually positive, null, and negative curvatures correspond to different compositions of the interfacial film. Once the influence of cosurfactant on the spontaneous curvature of the interface is understood, the appearance of the emulsification failure upon pentanol loading can be rationalised within the framework of the flexible surface model.

Microencapsulation of n-Hexadecane as a Phase Change Material in Polyurea

For thermal energy storage application, polyurea microcapsules about 2.5 mum in diameter containing phase change material were prepared using interfacial polycondensation method. in the system droplets in microns are first formed by emulsifying an organic phase consisting of a core material ( n-hexadecane) and an oil-soluble reactive monomer, toluene-2, 4-diisocyanate (tdi), in an aqueous phase. by adding water-soluble reactive monomer, diamine, monomers tdi and diamine react with each other at the interface of micelles to become a shell. ethylenediamine (eda), 1, 6-hexane diamine (hda) and their mixture were employed as water-soluble reactive monomers. the effects of diamine type on chemical structure and thermal properties of the microcapsules were investigated by ft-ir and thermal analysis respectively. the infrared spectra indicate that polyurea microcapsules have been successfully synthesized; all the tg thermographs show microcapsules containing n-hexadecane can sustain high temperature about 300 degreesc without broken and the dsc measurements display that all samples possess a moderate heat of phase transition; thermal cyclic tests show that the encapsulated paraffin kept its energy storage capacity even after 50 cycles of operation. the results obtained from experiments show that the encapsulated n-hexadecane possesses a good potential as a thermal energy storage material.