Influence of anionic surfactants on the electric percolation of AOT/isooctane/water microemulsions

A study was carried out concerning the influence of sodium alkyl sulfonates on the electric percolation of AOT/isooctane/water microemulsions ([AOT] = 0.5 M and W = [H2O]/[AOT] = 22.2). An important effect was observed with regard to the percolation temperature caused by the addition of small quantities of alkyl sulfonates (rho = [alkyl sulfonate]/[AOT] = 0.01). The short chain alkyl sulfonates (C3-C5) cause an increase in the percolation temperature, which in turn is reduced as we increase the chain length of the additive until we obtain a percolation temperature which is lower than that which is observed in the absence of an additive (C6-C8). For hydrocarbon chains of a greater length we can observe a new increase in the percolation temperature (C10-C18). This behavior has been explained as a consequence of (i) the incorporation of the additives at the interphase of the microemulsion and (ii) the geometric parameters of the different surfactants added to the microemulsion.     

Temperature-induced percolation behavior of AOT reverse micelles affected by poly(ethylene glycol)s

The influence of poly(ethylene glycol)s additives viz. mono- (EG), di- (DEG), tri- (TEG), tetra- (TeEG) and poly(ethylene glycol)-400 (PEG-400) on temperature-induced electrical percolation of water/AOT/isooctane microemulsion system has been investigated. The composition of microemulsion systems has been kept constant to omega=22 and [additive] = 0.1 M w.r.t. dispersion medium. The effect of increase in the non-polar continuum (S= [Oil]/[AOT]) is indicated by increase in the percolation threshold, theta(c). The findings have been elaborated in terms of validity of scaling laws in the light of the dynamic percolation theory. The activation energy of the process, DeltaEp, has been estimated from Arrhenius plots. Pseudophase concept of the micellar aggregation has been utilized to assess the thermodynamics of clustering of the nanodroplets. The state of trapped water in the micellar core and the corresponding interactions with the AOT head group has been visualized through 1H NMR and FTIR analysis. Results show that at higher omega (>16.0), encapsulated water behaves like free or the bulk water.     

Effects of Alkylamines on the Percolation Phenomena in Water/AOT/Isooctane Microemulsions

We carried out an investigation on the influence of several alkylamines, frequently present in reactions carried out in microemulsions, on the properties of the water/AOT/isooctane system. The presence of alkylamines has an important effect on the electrical percolation phenomena. This effect of amines on the electrical percolation of microemulsions of AOT/isooctane/water can be explained by taking into account the ability of these substrates to associate with the AOT film in the microemulsion, the basicity of the amine, and the different solubility of the amine in the three pseudophases of the system. Copyright 2000 Academic Press.     

Influence of polyethylene glycols on percolative phenomena in AOT microemulsions

The influence of different polyethylene glycol (PEG) on the percolation of the ternary system composed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) + isooctane + water has been studied. The additives used were chosen on the basis of its chain length (the number of polymeric units). In all cases, we observed a decrease in the percolation threshold on increasing the amount of PEG added to the AOT microemulsions. We observed a correlation between the effect exerted by the additive upon the percolation temperature and its chain length. Moreover, a relationship between the percolation temperature and the additive partition coefficient between 1-octanol and water (logP) was found. Both of them proved the importance of the inclusion of the additives into the microemulsion interface to explain their influence upon the percolative phenomenon. Such inclusion modified the properties of the AOT film, facilitating the exchange of matter between droplets.     

Kinetics of the Oxidation of Phenylhydrazine by [Fe(CN)6]3− in Water‐in‐Oil Microemulsions

The oxidation reaction of phenyl hydrazine (Phh) by hexacyanoferrate ([Fe(CN)₆]^3?) has been studied in water‐in‐oil (w/o) microemulsion media. The kinetic profile of the reaction was investigated as a function of [Phh], droplet size, and droplet concentration. Comparison of the kinetic profiles of the reaction in microemulsion, water‐urea, and water‐AOT‐urea media indicates that the kinetic profile of the reaction in microemulsion shows a behavior similar to that of the reaction in water‐AOT‐urea medium at 4 M urea. An initial increase and then a decrease in k_obs is observed with increasing molar ratio, W_o(=[H₂O]/[AOT]) at constant [AOT] (=0.4 M), whereas k_obs decreases upon increasing the AOT concentration at constant molar ratio.     

Water in oil microemulsions as reaction media for a Diels-Alder reaction between N-ethylmaleimide and cyclopentadiene

The Diels-Alder reaction between N-ethylmaleimide and cyclopentadiene in water/AOT/isooctane microemulsions, where AOT denotes sodium bis(2-ethylhexyl)sulfosuccinate, was studied. The rate of the reaction was found to be higher than that obtained in pure isooctane, irrespective of the particular microemulsion composition used. The efficiency of this catalytic action ranged from a factor of 3 at low water contents (viz., W = [H2O]/[AOT] = 2) to 15 at W = 35. On the basis of these results, the reaction takes place simultaneously in the continuous medium and at the microemulsion interface. The favorable arrangement of the reactants at the interface results in more than 95% of the reaction occurring in this microenvironment. The kinetic analysis revealed the rate constant at the microemulsion interface to change with the water content. For small W values a bimolecular rate constant at the interface close to that observed in hexane was obtained. This value increases with W and for W > 20, a value close to that obtained in ethanol was found. This can be ascribed to the absence of hydrogen bonding at the microemulsion interface as well as the accelerating effects due to enforced hydrophobic interactions.     

The Dynamics and Energetics of Percolation of Water/AOT/Isooctane Microemulsions upon the Addition of Tween Nonionic Surfactants

The temperature‐induced percolation of water/AOT/isooctane microemulsions was studied in the presence of the Tween series of polyoxyethylated nonionic surfactants employing conductometry. Percolation temperatures were determined utilizing the Sigmoidal‐Boltzmann equation procedure. The results were analyzed in terms of the percolation temperature, scaling parameters, activation energy, and thermodynamics of the clustering process. It was observed that the Tween series of surfactants aided the percolation process, and the percolation parameters were found to be independent of the hydrophobic chain length of these additives. The percolation temperature was found to be dependent on the concentration of the Tweens. It was concluded that the influence of the Tween series of surfactants on the percolation phenomenon was due to the number of ethylene oxide moieties in the head groups region of the additive. The effects of these additives was described in terms of modifications in the microemulsion's interfacial layer, outer oil layer, viscosity of the water micropool, and interactions between the anionic head groups of AOT.     

Influence of n-alkyl acids on the percolative phenomena in AOT-based microemulsions

A study was carried out on the influence of the n-alkyl acid addition on the electric percolation of AOT/iso-octane/water microemulsions ([AOT] = 0.5 M and W= [H(2)O]/[AOT] = 22.2). The observed influence has been explained taking into account the organic nature of these molecules and, hence, their capacity of disturbing the structure of the AOT-film. For these reasons, relationships with their molecular structure (chain length) were analysed.     

Phase Behavior of Nonaqueous Microemulsions of n‐Methyl‐2‐Pyrrolidinone/Diisooctylsodium Sulfosuccinate (AOT)/Alkanes

The phase behavior of n‐methyl‐2‐pyrrolidinone (NMP) microemulsions formed by the combination of NMP and oils (hexane, heptane, octane, and isooctane) in the presence of diisooctylsodium sulfosuccinate (AOT) were studied. The ternary plots were constructed and found to be similar in nature and consisted of a gel, a clear, or microemulsion, and a two phase region. Effects of varying amounts of water added to NMP and varying the chain length of n‐alkane (as oil) on the microemulsion region were investigated. Fluorescence probes such as Auramine‐O and the sodium salt of aniline‐1‐ napthalenesulfonic acid were employed to investigate the nature of the microemulsion region. Volume and temperature induced percolation studies have indicated the absence of percolation process in these microemulsion systems.     

Oil-continuous microemulsions mixed with an amphiphilic graft copolymer or with the parent homopolymer Polymer–droplet interactions as revealed by phase behavior and light scattering

Polymer–droplet interactions have been studied in AOT/water/isooctane oil-continuous microemulsions mixed with an amphiphilic graft copolymer, or with the parent homopolymer (AOT = sodium bis(2-ethylhexyl) sulfosuccinate). The graft copolymer has an oil-soluble poly(dodecyl methacrylate) backbone and water-soluble poly(ethylene glycol) side chains. Pseudo-ternary polymer/droplet/isooctane phase diagrams have been established for both the parent homopolymer and the graft copolymer, and the two types of mixture display entirely different phase behavior. The homopolymer–droplet interaction is repulsive, and a segregative phase separation occurs at high droplet concentrations. By contrast, the graft copolymer–droplet interaction is attractive: the polymer is insoluble in the pure oil, but dissolves in the microemulsion. A comparatively high concentration of droplets is required to solubilize even small amounts of polymer. Static and dynamic light scattering has been performed in order to obtain information on structure and dynamics in the two types of mixture. For optically matched microemulsions, with a vanishing excess polarizability of the droplets, the polymer dominates the intensity of scattered light. The absolute intensity of scattered light increases as phase separation is approached owing to large-scale concentration fluctuations. Dynamic light scattering shows two populations of diffusion coefficients; one population originates from “free” microemulsion droplets and the other from the polymer (for homopolymer mixtures) or from polymer–droplet aggregates (for mixtures with the graft copolymer). The graft copolymer forms large polymer–droplet aggregates with a broad size distribution, which coexist with a significant fraction of free droplets.     

Behavior of acetyl modified amino acids in reverse micelles: a non-invasive and physiochemical approach

The well-characterized, monodisperse nature of reverse micelles formed by sodium bis-(2-ethylhexyl)sulfosuccinate/water/isooctane and their usefulness in assimilating compounds of varied interests have been exploited to investigate the effect of acetyl modified amino acids (MAA) viz., N-acetyl-L-glycine (NAG), N-acetyl-L-aspartic acid (NAA) and N-acetyl-L-cysteine (NAC), on the water pool and physiochemical properties. Non-invasive techniques such as FTIR and UV-vis absorption spectroscopy have been employed to analyze the interactions of MAA with core water and the AOT headgroup. The micropolarities on both sides of AOT interface have further been investigated by UV-vis absorption probes, methyl orange (MO) and methylene blue (MB). The dynamics of water and temperature induced percolation process have also been studied. The MAA molecules have been found to assist the process with the increase in water content where as a contrary behavior has been observed with the increase in temperature. Conductivity results have been further rationalized in terms of scaling equations, which delineate the dynamic nature of the percolation process. The results have also been analyzed in the light of activation energy of the percolation process and thermodynamics of droplet clustering.     

Influence of glymes upon percolative phenomena in AOT-based microemulsions

A study was carried out on the influence of different polyethylene glycol dimethyl ethers (glymes) on the conductance percolation of AOT/isooctane/water microemulsions. The glymes used were chosen on the basis of this chain length (the number of polymeric units). In all cases we observed a decrease in the percolation threshold on increasing the amount of a glyme added to the microemulsion. We observed a correlation between the effect exerted by the glyme and its chain length, which shows the importance of including them in the interface for the percolative phenomenon. Such inclusion modifies the properties of the AOT film, facilitating the exchange of matter between droplets.     

反相微乳液中苯酚硝化反应选择性的研究

采用气相色谱内标法研究了苯酚在AOT/异辛烷/水、CTAB/正癸醇/异辛烷/水、DBSA/异辛烷/水3种反相微乳液中进行硝化反应的选择性;考察了表面活性剂种类、反应时间、反应温度以及反相微乳液的含水量等因素对反应选择性的影响.研究结果表明,苯酚在微乳液体系中的硝化反应具有明显的邻位选择性,阴离子表面活性剂DB-SA体系的邻位选择性最高,这与它同时具有微乳催化和酸催化作用有关.     

Quenching of fluorescence of 1-hydroxypyrene-3,6,8-trisulfonate (HPTS) by Cu2+, Co2+, Ni2+, I-, and cetylpyridinium (CP+) ions in water/AOT/heptane microemulsion

The quenching of the fluorescence of HPTS (1-hydroxypyrene-3,6,8-trisulfonate) by Cu(2+), Ni(2+), Co(2+), I(-), and CP(+) (cetylpyridinium cation) has been studied in the w/o microemulsion medium formed with water, AOT [sodium salt of bis (2-ethylhexyl) sulfosuccinic acid], and heptane as components at two [H(2)O]/[AOT] ratios (omega), 6 and 20. The quenching process has been found to be dynamic in nature. The lifetimes of HPTS in the microemulsion medium in the absence and in the presence of quencher have been determined. The analysis of the results has been performed in terms of the Stern-Volmer equation and the quenching sphere of action model. The Poisson distribution equation has been also used in the analysis of the probability of quencher distribution in the microemulsion compartment. The quenching of HPTS has been found to be much lower in microemulsion than in bulk water.     

Study on the Activity and Kinetic Characteristics of Glucoamylase in W/O Microemulsion Systems

The activity and kinetic characteristics of glucoamylase has been investigated in W/0 microemulsion systems of AOT/isooctane/buffer, CTAB/isooctane/l-pentanol/buffer and TX-100/ibooctane/I-pentanol/buffer, and compared with that in aquous solution. The effect of various parameters, such as pH optimum, To ptimum and water content, on the activity of the enzyme in microemulsion was determined. The results obtained show that the structure of the microemulsion has strong effect on enzyme activity compared to the k_cat, in aquous solution, the apparent turnover number k'_cat values were decreased in AOT, TX- 100 based system, and in creased in CTAB based system.     

The effect of additives on the water solubilization capacity and conductivity in n-pentanol microemulsions

The influence of additives such as sodium salicylate and sodium chloride on the water solubilization capacity of AOT in n-pentanol solutions has been investigated. The water solubilization capacity is enhanced by sodium salicylate and decreased by sodium chloride. The percolation behavior of the water/AOT/n-pentanol system is studied by modifying the water concentration and temperature. No percolation threshold induced by water or temperature is detected either in the absence or in the presence of additives. The values of ln sigma have a linear correlation with temperature in the range of 5-40 degrees C. The activation energy is also estimated and discussed.     

Effect of added alpha-lactalbumin protein on the phase behavior of AOT-brine-isooctane systems

We have found that the presence of <1 wt% of the globular protein alpha-lactalbumin has a significant impact on the equilibrium phase behavior of dilute sodium bis(ethylhexyl) sulfosuccinate (AOT)/brine/isooctane systems. Nuclear magnetic resonance (NMR), Karl Fischer titration, and ultraviolet spectroscopy were used to determine the surfactant, oil, water, and protein content of the organic and aqueous phases as a function of the total surfactant and protein present. As a small amount of alpha-lactalbumin is added to the mixture, there is a substantial increase (up to 80%) in the maximum water solubility in the water-in-oil microemulsion phase. Dynamic light scattering measurements indicate that this increase is due to a decrease in the magnitude of the (negative) spontaneous curvature of the surfactant monolayer, as droplets swell in size. As the molar ratio of alpha-lactalbumin to AOT surpasses approximately 1:300, the partitioning of water, protein, and surfactant shifts to the excess aqueous phase, where soluble assemblies with positive curvature are detected by dynamic light scattering. Significant amounts of isooctane are solubilized in these aggregates, consistent with the formation of oil-in-water microemulsion droplets. Circular dichroism studies showed that the tertiary structure of the protein in the microemulsion is disrupted while the secondary structure is increased. In light of these findings, the protein most likely expands to a molten-globule type conformation in the AOT interfacial environment, but does not substantially unfold to become an extended chain.     

Switching electrical conductivity in an AOT-isooctane-water microemulsion through photodimerization of solubilized N-methyl-2-quinolone

The electrical conductivity of microemulsions composed of Aerosol OT (AOT), isooctane and water as a function of temperature was studied in the absence and presence of N-methyl-2-quinolone (NMQ), and consequences of an in situ photodimerization of NMQ were investigated. A conductivity increase upon raising the temperature of a water-in-oil microemulsion indicates percolation. Percolation temperatures (Tp) can be influenced specifically by the addition of certain substances. Small amounts of solubilized N-methyl-2-quinolone induce lower Tp (by up to 9 K), and photodimerization of NMQ shifts Tp back to higher values. Consequently, the microemulsion can be switched from conducting to non-conducting without varying temperature or composition by exposing samples to UV light at lambda > 310 nm. The effect can be reverted in part by irradiation at lambda = 250 nm. Coumarin derivatives--structurally related to NMQ--show much slighter effects.     

Percolation phenomenon in mixed reverse micelles: the effect of additives

The conductivity of AOT/IPM/water reverse micellar systems as a function of temperature, has been found to be non-percolating at three different concentrations (100, 175 and 250 mM), while the addition of nonionic surfactants [polyoxyethylene(10) cetyl ether (Brij-56) and polyoxyethylene(20) cetyl ether (Brij-58)] to these systems exhibits temperature-induced percolation in conductance in non-percolating AOT/isopropyl myristate (IPM)/water system at constant compositions (i.e., at fixed total surfactant concentration, omega and X(nonionic)). The influence of total surfactant concentration (micellar concentration) on the temperature-induced percolation behaviors of these systems has been investigated. The effect of Brij-58 is more pronounced than that of Brij-56 in inducing percolation. The threshold percolation temperature, Tp has been determined for these systems in presence of additives of different molecular structures, physical parameters and/or interfacial properties. The additives have shown both assisting and resisting effects on the percolation threshold. The additives, bile salt (sodium cholate), urea, formamide, cholesteryl acetate, cholesteryl benzoate, toluene, a triblock copolymer [(EO)13(PO)30(EO)13, Pluronic, PL64], polybutadiene, sucrose esters (sucrose dodecanoates, L-1695 and sucrose monostearate S-1670), formamide distinctively fall in the former category, whereas sodium chloride, cholesteryl palmitate, crown ether, ethylene glycol constitute the latter for both systems. Sucrose dodecanoates (L-595) had almost marginal effect on the process. The observed behavior of these additives on the percolation phenomenon has been explained in terms of critical packing parameter and/or other factors, which influence the texture of the interface and solution properties of the mixed reverse micellar systems. The activation energy, Ep for the percolation process has been evaluated. Ep values for the AOT/Brij-56 systems have been found to be lower than those of AOT/Brij-58 systems. The concentration of additives influence the parameters Tp and Ep for both systems. A preliminary report for the first time on the percolation phenomenon in mixed reverse micelles in presence of additives has been suggested on the basis of these parameters (Tp and Ep).     

The effect of changing the microstructure of a microemulsion on chemical reactivity

A kinetic study was carried out on various solvolytic reactions in water/ NH4OT /isooctane microemulsions. The NH4OT surfactant is a derivative of the sodium salt of bis(2-ethylhexyl) sulfosuccinate (NaOT or AOT), where the Na+ counterion has been replaced by NH4+. The counterion substitution effects the phase diagram of the system, and therefore, NH4OT-based microemulsions with high water content reaching values of W = 350 (W = [H2O]/[NH4OT]) can be obtained. The presence of high W values suggests a transition in the microemulsion microstructure from water-in-oil (w/o) to oil-in-water (o/w), as was confirmed by conductivity and 1H NMR self-diffusion measurements. The interpretation of the kinetic studies in terms of pseudophase formalism allows us to analyze the effect of the microemulsion on chemical reactivity, regardless of its microstructure. It has been confirmed that the values of the solvolytic rate constants at the interphase of oil-in-water microemulsions are similar to those obtained for aqueous SDS systems, showing that the hydration degree of the interphase of the oil-in-water microemulsions is independent of W. The influence of the surfactant counterion on the solvolytic rate constants was analyzed by comparing HOT-, NaOT-, and NH4OT-based microemulsions. An important influence on the rate constants caused by the changes in the structural properties of water has been observed as was confirmed by the water 1H NMR signals.