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).     

Water-AOT-alkylbenzene microemulsions: influence of alkyl chain length on structure and percolation behavior

We study the percolation behavior of the water-in-oil (w/o) droplet phase of AOT (sodium bis[2-ethylhexyl] sulfosuccinate)-based microemulsions with different alkylbenzenes (toluene, ethylbenzene, butylbenzene or octylbenzene) as oil phase. We use microemulsions of varying composition with molar water to surfactant ratios 0≤W≤ 50 and droplet (water plus surfactant) volume fractions 10%≤φ≤50%. Using dielectric spectroscopy, a percolation transition is observed in w/o microemulsions with butylbenzene or octylbenzene. With increasing molecular weight of the alkylbenzene, the percolation temperature T(P) decreases. The structure of the microemulsions is determined by small angle X-ray scattering (SAXS). With increasing molar weight of the alkylbenzene, the stability range of the L(2) droplet phase extends to higher W. The larger amount of solubilizable water can be related to variable oil penetration of the AOT monolayer, which affects the spontaneous curvature of the surfactant shell.     

On the temperature percolation in a w/o microemulsion in the presence of organic derivatives of chalcogens

The course of temperature percolation in a w/o microemulsion system comprising water/bis(2-ethylhexyl) sulfosuccinate sodium, AOT/isooctane affected by the presence of additives has been investigated. Additives, viz., organic derivatives of chalcogens including dipyridyl diselenide (Py2Se2), diphenyl diselenide (Ph2Se2), and dipyridyl ditelluride (Py2Te2), have been assimilated in the reverse micellar system. Formulations have been studied in terms of (i) the concentration variation of additives, (ii) the change in omega (= [H2O]/[AOT]), and (iii) the change in the nonpolar continuum, S (= [oil]/[AOT]). Phenyl derivatives hinder the percolation, whereas the pyridyl derivative in moderate amounts favors the phenomenon. The estimated values of the critical exponents are lower than those predicted by the dynamic percolation theory. The association model has been implemented to access the thermodynamic parameters of droplet clustering. Pyridyl compounds are expected to alter the rigidity of the surfactant monolayer, which could help to promote the attractive interdroplet interaction. FT-IR spectroscopy has been used to elucidate the changes occurring in the core water in the presence of organic derivatives of chalcogens as the droplet size is increased. Results have been rationalized in terms of the alteration in the physicochemical behavior of the water/AOT/isooctane microemulsion in the presence of additives.     

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.     

甲苯对水/AOT/醇反相微乳液体系电导行为的影响

采用电导法研究了以二(2-乙基己基)琥珀酸酯磺酸钠(AOT)为表面活性剂、庚醇和癸醇分别与甲苯的混合溶剂为油相的反相微乳液体系在有无添加剂氯化钠时的电导行为。 结果表明,甲苯对水/AOT/癸醇体系电导渗滤有明显的抑制作用,而对水/AOT/庚醇体系没有影响;添加氯化钠对水/AOT/庚醇（癸醇）/甲苯体系的电导率基本无影响。     

Physicochemical studies of octadecyl-trimethyl-ammonium bromide: a critical assessment of its solution behavior with reference to formation of micelle, and microemulsion with n-butanol and n-heptane

Octadecyl-trimethyl-ammonium bromide (C18TAB) is a much less studied representative in the alkyltrimethylammonium halide surfactant series. A comprehensive study of its normal and reverse micelle (microemulsion) formation has been herein conducted by the methods of conductometry, tensiometry, fluorimetry, and microcalorimetry. The energetics of its air/liquid interfacial adsorption and self-association in aqueous solution have been examined. The phase behavior of its combinations with water, n-butanol, and n-heptane in the formation of microemulsions have been investigated with identification of a variety of phases. The energetics of formation of water dispersion in oil (w/o) has been evaluated from dilution experiments conducted at different temperatures. From the results, structural parameters of the droplets have been determined at different [water]/[surfactant] mole ratios (omega) and temperatures. The w/o dispersions have evidenced both volume- and temperature-induced conductance percolation. The results have been treated in light of the Scaling equations, and the associated parameters for the process have been determined. The activation energies for the temperature-induced percolation process of the w/o dispersion have been evaluated and assessed.     

AOT/H2O/油微乳液体系的浊度、密度和微观结构

建立了通过精密测量密度、折射率和浊度研究微乳液滴微观性质的方法,获得水与表面活性剂的摩尔比r分别为10.5和12的AOT/H2O/甲苯微乳液体系液滴的微观结构及相互作用参数,得到AOT分子的长度L=1.07 nm,用液滴间的相互作用讨论了相变温度与r的关系.用L=1.07 nm,通过密度测量得到AOT/H2O/甲苯和AOT/H2O/环己烷两个微乳液体系不同r值下液滴的微观结构参数,与文献报导的数据吻合得很好.发现在微乳液滴中的水的密度明显大于自由水的密度,并随溶剂变化,而AOT分子的构型不变.     

AOT/H_2O/油微乳液体系的浊度、密度和微观结构

建立了通过精密测量密度、折射率和浊度研究微乳液滴微观性质的方法,获得水与表面活性剂的摩尔比r分别为10.5和12的AOT/H2O/甲苯微乳液体系液滴的微观结构及相互作用参数,得到AOT分子的长度L=1.07nm,用液滴间的相互作用讨论了相变温度与r的关系.用L=1.07nm,通过密度测量得到AOT/H2O/甲苯和AOT/H2O/环己烷两个微乳液体系不同r值下液滴的微观结构参数,与文献报导的数据吻合得很好.发现在微乳液滴中的水的密度明显大于自由水的密度,并随溶剂变化,而AOT分子的构型不变.     

On Negative Birefringence in Water-in-Oil Microemulsions

The occurrence of a negative birefringence in water-in-oil (W/O) microemulsions has been substantiated and analyzed. The analysis is based on the well-established aqueous nanodroplet model of W/O microemulsions. In the particularly investigated water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/aliphatic oil systems, the AOT monolayer exhibits a negative induced intrinsic birefringence, which is brought about by the pronounced polarizability of the alkyl moiety of the AOT molecule approximately perpendicular to the direction of its permanent dipolement.     

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.     

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.     

STUDY ON PERCOLATION OF MIXED REVERSE MICELLES

The temperature-induced percolation behaviors of AOT reverse micelles in the presence of nonionic surfactants have been studied. The effects of water content, solvent and concentration of electrolyte in solubilized water have also been investigated. It was found that the percolation temperature of AOT reverse micelles was decreased by adding nonionic surfactants, and more pronounced effects were observed with the increase of EO chain length and content of nonionic surfactants. The increase of molecular volume of the solvent and the increase of concentration of the added NaCl electrolyte have shown assisting and resisting effects on the process, respectively. The apparent hydrodynamic diameter of droplets of different mixed reverse micelles has been measured using dynamic light scattering, by which the percolating mechanism of mixed reverse micelles was discussed in combination with the results obtained from conductivity measurements.     

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.     

What is so special about aerosol-OT? Part IV. Phenyl-tipped surfactants

Properties are reported for new phenyl-tipped anionic surfactants, which are aromatic chain relatives of the normal aliphatic aerosol-OT (AOT, sodium bis(2-ethyl-1-hexyl)sulfosuccinate). Variations in chain length and branching with these aromatic surfactants have important effects on aqueous and water-in-oil (w/o) microemulsion phase properties. In dilute aqueous systems, chain structure affects the cmc and surface tension behavior: compared to linear chain analogues, the branched-chain surfactants display lower surface tensions but also reduced packing as measured by molecular area at the cmc a(cmc). Owing to the phenyl-tipped structure, water-in-oil microemulsions were stabilized with aromatic toluene as an oil but not with aliphatic heptane; the latter is commonly used with normal AOT. Contrast variation small-angle neutron scattering (SANS) was used to characterize the microemulsion aggregates and adsorbed films. These SANS data show that water-in-toluene microemulsions stabilized by aromatic-AOTs contain mildly polydisperse spherical nanodroplets of similar structure to those found in systems containing normal AOT. Molecular areas at the air-water and toluene-water interfaces are found to be of similar magnitude and follow a trend that correlates with variations in surfactant chain structure. The new results with aromatic surfactants build on extensive studies of aliphatic AOT analogues (Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733. Nave, S.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2002, 16, 8741. Nave, S.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. 2002, 18, 1505), suggesting that the versatility of normal AOT originates from an optimized head and chain spacer group rather than from any specific effects of the 2-ethyhexyl chain structure.     

A shear-induced viscoelastic system through addition of a crown-ether to AOT w/o-microemulsions

 Addition of the crown-ethers 15-crown-5 (CE5) and 18-crown-6 (CE6) to Aerosol OT (AOT)-stabilized w/o microemulsion leads to drastic rheological changes. A mixture of 20 wt% solution of a CE in water with an oil stock solution of AOT is biphasic but can be transformed into a homogeneous, transparent, viscoelastic solution by simple shaking. This gelly phase demixes again after hours up to several days. In addition, anomalous percolation properties are found for mixtures containing small amount of CE. Both effects are interpreted taking into account the complexing ability of the CEs with respect to the sodium counterion of AOT. Received: 2 October 1997 Accepted: 11 March 1998     

Effect of alpha-lactalbumin on aerosol-OT phase structures in oil/water mixtures

The ability of water-soluble, globular proteins to tune surfactant/oil/water self-assemblies has potential for the formation of biocompatible microemulsions and also plays a role in protein function at biological interfaces. In this work, we examined the effect of the protein alpha-lactalbumin on Aerosol-OT (AOT) phase structures in equivolume mixtures of oil and 0.1 M brine. In this pseudo-ternary system, surfactants are free to move to either oil or water phase to adopt phase structures close to the spontaneous curvature of the surfactants. Using small-angle X-ray scattering, we observed that addition of this protein changed the spontaneous curvature of the surfactant monolayer substantially. In the absence of protein, AOT adopted a negative spontaneous curvature to form spherical w/o microemulsion droplets. When less than 1 wt % of alpha-lactalbumin was added into the system, the w/o droplets became nonspherical and larger in volume, corresponding to an increase in water uptake into the droplets. As the protein-to-surfactant ratio increased, protein, surfactant, and oil increasingly partitioned toward the aqueous phase. There the protein triggered the formation of o/w microemulsions with a positive spontaneous curvature. These protein-containing structures exhibited significant interparticle attraction. We also compared the influence of two oil types, isooctane and cyclohexane, on the protein/surfactant interactions. We propose that the more negative natural curvature of the AOT/cyclohexane monolayer in the absence of protein prevented protein incorporation within organic phase structures and consequently pushed the system self-assembly toward aqueous aggregate formation.     

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.     

Refractive index of water-AOT-n-heptane microemulsions

Refractive index measurements on water/AOT/n-heptane microemulsions as a function of the volume fraction of the dispersed phase (water plus AOT) and of the water/AOT molar ratio R have been performed at 25°C. The refractive index was found to vary monotonically with without any change in rate during the crossover of the percolation threshold. Such a behavior suggested that, well above the percolation threshold, the water-AOT-n-heptane microemulsions are still formed by water-containing AOT reversed micelles dispersed in the oil phase. The analysis of the experimental data allowed an evaluation the fraction of the water molecules bonded to the AOT head group as a function of R.     

The Conductance Percolation and Droplets Dimension of AOT in Alkanol Systems

The conductance behaviors of AOT in alkanol (hexanol, heptanol, octanol, and decanol) reverse microemulsions have been investigated. The percolation phenomenon induced by water is observed in the water/AOT/decanol system at 15°C and 30°C, and the water/AOT/octanol system at 15°C. The percolation phenomenon of water/AOT/alkanol systems is discussed from the interaction between the hydroxy group of alkanol and the polar group of AOT, droplets diffusion coefficient, and the rate constant for droplets collision. The droplets size and diffusion coefficient of the water/AOT/alkanol systems have also been studied by modifying the water concentration. The results show that hydrodynamic diameter of droplets decreases and diffusion coefficient increases with the increasing of water content, which may be explained by the polarity of alkanol phase.     

Characterizing interfacial friction in bis(2-ethylhexyl) sodium sulfosuccinate reverse micelles from photoisomerization studies of carbocyanine derivatives

Photoisomerization of two carbocyanine derivatives has been examined in bis(2-ethylhexyl) sodium sulfosuccinate (AOT) reverse micelles to understand the factors that govern this process in the interfacial region of organized assemblies. To this effect, fluorescence lifetimes and quantum yields of 3,3(')-diethyloxadicarbocyanine iodide and merocyanine 540 have been measured in AOT∕isooctane∕water and AOT∕cyclohexane∕water reverse micellar systems as a function of the mole ratio of water to the surfactant, W. The nonradiative rate constants, which have been identified as the rates of photoisomerization for these solutes, were obtained from the experimentally measured parameters. The steady rise and subsequent saturation observed in the nonradiative rate constants upon increasing W has been rationalized in terms of micellar packing. An inverse correlation has been obtained between the nonradiative rate constants and the critical packing parameter, indicating that the interfacial friction experienced by the solute molecule is essentially described by this parameter.