Ultrafast dynamics of water in cationic micelles

The effect of confinement on the dynamical properties of liquid water is investigated for water enclosed in cationic reverse micelles. The authors performed mid-infrared ultrafast pump-probe spectroscopy on the OH-stretch vibration of isotopically diluted HDO in D(2)O in cetyltrimethylammonium bromide (CTAB) reverse micelles of various sizes. The authors observe that the surfactant counterions are inhomogeneously distributed throughout the reverse micelle, and that regions of extreme salinity occur near the interfacial Stern layer. The authors find that the water molecules in the core of the micelles show similar orientational dynamics as bulk water, and that water molecules in the counterion-rich interfacial region are much less mobile. An explicit comparison is made with the dynamics of water confined in anionic sodium bis(2-ethythexyl) sulfosuccinate (AOT) reverse micelles. The authors find that interfacial water in cationic CTAB reverse micelles has a higher orientational mobility than water in anionic AOT reverse micelles.     

Determination of pH in reversed micelles

The pH in the reversed micellar system of di(ethylhexyl) sodium sulfosuccinate (Aerosol OT, AOT) / phosphate buffer solutions/octane was determined by a P-NMR technique, and pHs in the reversed micelles containing buffer solutions other than the phosphate buffer solution were measured by the spectrophotometric method with the aid of Phenol Red. pHs in reversed micelles were found to be substantially determined by the buffer capacity of buffer solutions solubilized into the systems. By means of both the methods, pKa of Phenol Red in the systems was found to be 7.7, which is almost consistent with that in water. Analysis of Na-NMR spectra indicates that the mobility of the sodium ion of AOT is independent of the molar ratio of water to AOT when the ratio is above 7 and is restricted strongly by the interaction with the sulfonate group of AOT. The relationship between pH and the mobility of the sodium ion was discussed on the basis of the data of Na-NMR spectra.     

Some dielectric properties of reversed micelles in the system sodium octanoate/water/n-decanol

We have found it possible to estimate the dielectric constant of the aqueous micellar core by means of quite simple calculations. The presented method requires assumptions about the particle shape and about the distribution of the different species in order to obtain the right order of magnitude. The presented calculations deal only with limited conditions in the model system sodium octanoate/water/decanol. Here we find that a spherical micelle model can be applied to surprisingly broad concentration intervals. At the alcohol-poor phase boundary other kinds of aggregates might occur. It is our purpose to develop the presented model to involve other particle shapes and also other model systems.     

A “microscopic” model for the dynamics of water

A “reduced” model for the dipolar relaxation of water taking hydrogen-bond dynamics into account is built up. Via proper diffusional assumptions th     

用磷酸二异辛酯/正辛醇反胶束从面包酵母粗提液中萃取谷胱甘肽

采用磷酸二异辛酯（P204）/正辛醇反胶束液从面包酵母粗提液中萃取还原型谷胱甘肽(GSH),研究了各种因素对萃取率的影响以及水相pH值和P204浓度与GSH萃取率关系的数学模型。正交实验确定,GSH的最佳萃取条件是水相pH值2.0,K+浓度0.1 mol•L-1,萃取时间15 min,P204浓度0.16 mol•L-1,GSH单次的萃取率为33.1%,2次萃取率达到49.7%。     

Reorientation dynamics of nanoconfined water: power-law decay, hydrogen-bond jumps, and test of a two-state model

The reorientation dynamics of water confined within nanoscale, hydrophilic silica pores are investigated using molecular dynamics simulations. The effect of surface hydrogen-bonding and electrostatic interactions are examined by comparing with both a silica pore with no charges (representing hydrophobic confinement) and bulk water. The OH reorientation in water is found to slow significantly in hydrophilic confinement compared to bulk water, and is well-described by a power-law decay extending beyond one nanosecond. In contrast, the dynamics of water in the hydrophobic pore are more modestly affected. A two-state model, commonly used to interpret confined liquid properties, is tested by analysis of the position-dependence of the water dynamics. While the two-state model provides a good fit of the orientational decay, our molecular-level analysis evidences that it relies on an over-simplified picture of water dynamics. In contrast with the two-state model assumptions, the interface dynamics is markedly heterogeneous, especially in the hydrophilic pore and there is no single interfacial state with a common dynamics.     

Volumetric and transport properties of aerosol-OT reversed micelles containing light and heavy water

Densities and viscosities of sodium bis(2-ethylhexyl) sulfosuccinate (AOT in-heptane system containing light and heavy water, as a function of the molar ratio R (R=[H₂O or D₂O]/[AOT]) were measured at 0, 5, 25 and 40°C. At low R values, the apparent molar volume of deuterium oxide is smaller than that of light water. The difference is related to the strength of the hydrogen bonding H₂O and D₂O. The viscosities of both H₂O-AOT-n-heptane and D₂O-AOT-n-heptane systems were explained in terms of intermicellar interactions mainly governed by hydration of the head groups of AOT.     

Photochemistry of a cyanine dye in reversed micelles

The effect of microenvironment on the existing state and spectral properties of a cyanine dye in different systems were investigated. Due to the space limitation and the polarity evolution of the water cell of reversed micelles, the optical behavior of the dye in reversed micelles was very different from in water and alcohol. The effect of surfactants with different charge on the interaction of a cyanine dye with AgCl nanoparticles in reversed micelles were also researched. The adsorption state of the dye on AgCl nanoparticles in reversed micelles was discussed.     

The influence of water on the photophysical and photochemical properties of Piroxicam in AOT/iso-octane/water reversed micelles

The photophysical properties of Piroxicam , a nonsteroidal anti-inflammatory drug (NSAID), were investigated at different pH_ext values in reversed micelles of Aerosol-OT (AOT) in iso -octane, using both steady-state and picosecond time-resolved fluorescence spectroscopy. In contrast with the very complex data obtained in aqueous media, where several prototropic species are in equilibrium, the reversed micellar system essentially favors two species. The absorption spectra shows only one isosbestic point at λ= 348 nm. Excited-state intramolecular proton transfer (ESIPT), also detected in water, is promoted at low water pool contents measured by ω₀= [H₂O]/[AOT]. A strongly shifted (λ_em= 470 nm) tautomeric emission is found. Upon the gradual increase of ω₀, striking differences with pH_ext are found. At pH_ext= 4, the drug preferentially locates itself in the interfacial region partitioning between a hydrophobic and a hydrophilic domain. Global analysis was applied to the decay data and the results were interpreted by the "two-state excited-state" formalism. At pH_ext= 7, the anionic species is prevalent and the probe locates itself deeper inside the water core of the reversed micelles. Thus, a strong dependence on water content is detected, approaching a behavior similar to that observed in free aqueous solutions.     

Conformational studies of the oligomers of L-lysine in reversed micelles

The effect of the chain length on the conformation of oligo-L-lysines (Lys-n, n= 9, 12 and 15) was examined in the reversed micelles of bis(2-ethylhexyl)sodium sulfosuccinate (AOT) in octane by the circular dichroism (CD) measurements. These oligomers seem to take a-structure in these systems. The structure-inducing effect of the reversed micelles is enhanced as the molar ratio of water to AOT (w₀=[H₂O]/[AOT]) becomes smaller. On the other hand, in the aqueous solutions the oligomers having 12 and 15 residues show the conformational transition from random coil to-helical structure by the addition of AOT, but the short oligomer of 9 residues does not show such a conformational transition.     

Calorimetric study of the alpha-tocopherol solubility in reversed AOT micelles

The experimental data of heat of mixing (Q) for heterogeneous system alpha-tocopherol/AOT/n-heptane with and without water at 25 degrees C are presented. The Q dependence on AOT (sodium bis (2-ethylhexyl) sulfosuccinate) concentration, and R parameter defined as R=[H2O]/[AOT] with flow calorimetric method were investigated. Using the D'Aprano model (which is formally identical to that used earlier by Magid et al.) the binding constant (K), the distribution constant of alpha-tocopherol (K distr) between hydrocarbon and the micellar phase, and the standard enthalpy of transfer (DeltaH tr 0) of alpha-tocopherol from the hydrocarbon to AOT reversed micelles were calculated. The solubility of alpha-tocopherol in AOT reversed micelles explored with the calorimetric technique was compared to the literature data obtained respectively with UV spectrophotometry for reversed micelles and by other techniques for the phospholipid bilayer.     

Solubilized states of water and formation of reversed micelles in polyoxyethylated nonylphenyl ethers in cyclohexane media

The limiting amounts of solubilization of water in the concentration range 0–200 mmol kg^–1 polyoxyethylene (6 and 10) nonylphenyl ethers (NP-6 and NP-10)/cyclohexane solutions were measured by the Karl-Fischer method at 25°C. Utilizing NMR, ESR, and near infrared spectroscopic techniques, the states of water in the solubilization region obtained were examined as a function of the molar ratio of water to surfactants at various surfactant concentrations. In NP-6 system, three solubilized states of water, i.e., water interacted directly with the oxyethylene moiety of surfactant, bound water next to the hydrated oxyethylene moiety, and bulk-like water were built up. However, in NP-10 system only directly interacted water was present. It was found that the directly interacted water is distributed between monomeric surfactants and reversed micelles, and others are distributed to swollen micelles and W/O microemulsions. In addition, the minimum amounts of water required to form reversed and swollen micelles were calculated.     

FT-IR investigation of the acetamide state in AOT reversed micelles

The state of acetamide nanoparticles encapsulated in the hydrophilic core of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reversed micelles and dispersed in CCl ₄ has been investigated by Fourier transform infrared spectroscopy. The analysis of the vibrational spectra reveals even at the higher acetamide to AOT molar ratio some changes of the typical H-bonded structure of solid acetamide ascribable to their small size, confinement effects, and acetamide-AOT head group interactions. The stretching modes of acetamide CO and AOT sulfonate groups indicate unambiguously specific acetamide-AOT head group interactions.     

Native and modified glucose oxidase in reversed micelles

The enzymatic activity of the native and modified glucose oxidase (GOx) from Aspergillus niger in the system of reversed micelles of Aerosol OT in octane was investigated. Two forms of the modified enzyme were studied: a hydrophobized form obtained by the attachment of palmitic chains to lysine amino groups by the reaction with palmitic acid ester of N-hydroxysuccinimide and a glycosylated (hydrophilized) form obtained by the attachment of the cellobiose moieties. The native glucose oxidase and its derivatives, while incorporated into micelles in a surfactant concentration range from 0.05 to 0.3 M, display an enzymatic activity, which is comparable with the activity in aqueous solution. The dependence of the enzymatic activity on hydration degree of surfactant (the molar ratio of water to surfactant, W₀) does not indicate the formation of qualitatively new associated forms of the enzyme subunits inside the micelles. The apparent size of Aerosol OT micelles obtained by dynamic light scattering gradually increases from 10±3 nm at low W₀ up to 25±5 nm at high W₀. Incorporation of the native and hydrophobized glucose oxidase into micelles does not affect their mean size. Kinetic analysis shows that the enzyme specificity is about an order of magnitude greater in the system of reversed micelles as compared with aqueous solution.     

Synthesis of mixed silver halide nanocrystals in reversed micelles

The specifics of the synthesis of silver halide nanocrystals of mixed composition and the core-shell structures in reversed micelles were experimentally studied. It was shown that homogeneous AgBr _x I_{1 ? x} nanocrystals of ～5 nm in size with the iodide concentration up to 70%, as well as the core-shell structures AgI/AgBr and AgBr/AgI, can be synthesized by the micellar synthesis. It was found that the relation of the crystalline structures of the core and shell materials plays an important role in the shell formation. The shell of γ-AgI alone is formed on the AgBr nanocrystals with a close lattice type, whereas β-AgI with the hexagonal lattice forms an individual phase of nanoparticles, rather than the shell.     

Fluorescence in microemulsions and reversed micelles : A Review and New Results

The use of fluorescence to study physicochemical structures of alcohol/surfactant/water systems, microemulsions and reversed micelles is reviewed, and the application of these media in analytical fluorescence spectroscopy is discussed. The sodium dodecylsulfate/1-pentanol/heptane/ water system is studied by using pseudo-ternary diagrams. Wide areas of existence of thermodynamically stable and optically clear phases (Winsor IV and two liquid crystals) were found both in the absence and presence of sodium sulfate (0.2 M). The influence of the composition of media on the fluorescence characteristics of pyrene, benzo [e]pyrene, 2-naphthol and p-amino-benzoic acid is studied.     

Preparation of cadmium sulfide nanoparticles in self-reproducing reversed micelles

Octyl octanoate (O-OL) underwent hydrolysis in sodium octanoate (NaOA) reversed micelles in 85:15 = isooctane:octanol (OL) (v/v), containing w = [H2O]/[NaOA] = 40. The products of the hydrolysis, octanoic acid (OA) and octanol (OL), lead to the formation of additional (albeit smaller) reversed micelles; hence the process is considered to be self-reproducing. Self-reproduction was found to be catalyzed by lithium hydroxide, solubilized in the water pools, as well as by hydrogen sulfide, added to the solution of the reversed micelles. Addition of hydrogen sulfide to cadmium perchlorate containing self-reproducing reversed micelles resulted in the formation of cadmium sulfide (CdS) nanoparticles. Diameters of the CdS containing nanoparticles could be altered from 5.4 to 1.8 nm by changing the [Cd2+]/[H2S] ratios from 0.25 to 10. The CdS nanoparticles formed were capped by mercaptopropionic acid, isolated as solids, and could be repeatedly redispersed in water without changing their sizes. Additional CdS nanoparticles were generated in the supernatants removed from the precipitated capped CdS nanoparticles.