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.     

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.     

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.     

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.     

Glyme-lithium salt phase behavior

Phase diagrams are reported for glyme mixtures with simple lithium salts. The glymes studied include monoglyme (DME), diglyme, triglyme, and tetraglyme. The lithium salts include LiBETI, LiAsF6, LiI, LiClO4, LiBF4, LiCF3SO3, LiBr, LiNO3, and LiCF3CO2. The phase diagrams clearly illustrate how solvate formation and thermophysical properties are dictated by the ionic association strength of the salt (i.e., the properties of the anions) and chain length of the solvating molecules. This information provides critical predictive capabilities for solvate formation and ionic interactions common in organometallic reagents and battery electrolytes.     

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.     

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.     

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).     

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.     

Crystals from concentrated glyme mixtures. The single-crystal structure of LiClO4

A procedure for the preparation of high-quality single crystals from concentrated glyme mixtures is presented. Anhydrous single crystals of LiNO(3) and LiClO(4) were prepared in this manner, and the single-crystal structure of LiClO(4) (orthorhombic, Pnma, a = 8.6447(12) A, b = 6.8512(10) A, c = 4.8254(7) A, Z = 4) was determined as an example. This procedure is expected to be widely applicable for not only salts but also a wide range of other materials solvated by glymes.     

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.     

Physicochemical studies on microemulsions 9. Conductance percolation of AOT-derived W/O microemulsion with aliphatic and aromatic hydrocarbon oils

Both volume- and temperature-induced percolation of conductance of w/o microemulsions formed with AOT in cyclic aliphatic and aromatic oils (cyclohexane, cyclohexanone, toluene, and xylenes), and volume percolation of water/AOT/oil systems using linear aliphatic hydrocarbons (n-hexane, n-heptane, n-octane, i-octane, and n-decane) have been studied. The effect of additives, viz. sodium cholate (NaC), sodium deoxycholate (NaDC), cholesterol, n-butanol, and t-butanol, toluene, and xylenes (o, m, and p) on the temperature-induced percolating processes using the oils cyclohexane and cyclohexanone has been examined. The percolation results have been analyzed in the light of scaling equation, and the energy of activation of the ion-transport phenomenon has been evaluated for both pre- and postpercolation stages. From the percolation data, the diameter of the microdispersed water droplets, their population, and surface area have been estimated. The enthalpy of dispersion of water in AOT/oil medium has been determined from isothermal titration calorimetric (ITC) measurements.     

Intermolecular Interactions in Li+‐glyme and Li+‐glyme–TFSA− Complexes: Relationship with Physicochemical Properties of [Li(glyme)][TFSA] Ionic Liquids

The stabilization energies (ΔE_form) calculated for the formation of the Li⁺ complexes with mono‐, di‐ tri‐ and tetra‐glyme (G1, G2, G3 and G4) at the MP2/6‐311G** level were ?61.0, ?79.5, ?95.6 and ?107.7 kcal mol^?1, respectively. The electrostatic and induction interactions are the major sources of the attraction in the complexes. Although the ΔE_form increases by the increase of the number of the O???Li contact, the ΔE_form per oxygen atom decreases. The negative charge on the oxygen atom that has contact with the Li⁺ weakens the attractive electrostatic and induction interactions of other oxygen atoms with the Li⁺. The binding energies calculated for the [Li(glyme)]⁺ complexes with TFSA^? anion (glyme=G1, G2, G3, and G4) were ?106.5, ?93.7, ?82.8, and ?70.0 kcal mol^?1, respectively. The binding energies for the complexes are significantly smaller than that for the Li⁺ with the TFSA^? anion. The binding energy decreases by the increase of the glyme chain length. The weak attraction between the [Li(glyme)]⁺ complex (glyme=G3 and G4) and TFSA^? anion is one of the causes of the fast diffusion of the [Li(glyme)]⁺ complex in the mixture of the glyme and the Li salt in spite of the large size of the [Li(glyme)]⁺ complex. The HOMO energy level of glyme in the [Li(glyme)]⁺ complex is significantly lower than that of isolated glyme, which shows that the interaction of the Li⁺ with the oxygen atoms of glyme increases the oxidative stability of the glyme.     

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.     

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.     

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。     

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.     

Simultaneous effect of microemulsions and phase-transfer agents on aminolysis reactions

The catalytic effect of triethylene glycol dimethyl ether (glyme) on the butylaminolysis of 4-nitrophenylcaprate (NPC) in water/AOT/chlorobenzene microemulsions has been studied. Experimental results show the existence of four simultaneous reaction pathways. One of them takes place at the microemulsion interphase where the rate-determining step of butylaminolysis is the formation of the addition intermediate, T+/-. The locus of the other three pathways is the continuous medium of the microemulsion. These three pathways consist of the decomposition of the addition intermediate catalyzed by butylamine, by glyme, and by both of them. The kinetic model allows us to obtain the value of every rate and distribution constant involved in the overall reaction mechanism. We must emphasize that the reactions located in the continuous medium exhibit a kinetic behavior similar to the corresponding one found in pure chlorobenzene. On the basis of the pseudophase model, the percentage of reaction in each of the microdomains of the microemulsion has been calculated. Likewise, changes in the loci of reaction from the interphase to the continuous medium as a function of catalyst concentration have been proved.     

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.     

混合反胶束的电导与微结构研究

反胶束是两亲分子在非极性溶剂中形成的一种有序组合体，在医药、化工、采油、胶束催化及酶催化等领域中有重要应用．与胶束溶液相比，人们对反胶束的形成与结构的了解至今仍不充分．特别是对于由混合表面活性剂形成的反胶束的研究几乎无人涉及．本文采用动态光散射、电导及荧光光谱等手段对阴离子表面活性剂AOT与非离子表面活性剂形成的混合反胶束进行了研究，旨在探讨利用表面活性剂的复配来调节和控制反胶束的结构和性能．亚实验部分二异辛基磺化琉璃酸钠（AOT，Sigma公司）；Brij30为含4个氧乙烯基（EO基）的十二碳醇（AcrosOrgani…