Microemulsions mixtures

Calculations of the small-angle x-ray scattering expected from a disordered array of reverse micelles (whose dimensions can be accurately determined for this system since the interfacial area and volume fractions are well known) differ markedly from measured scattering spectra, except in the most water-rich microemulsion mixtures. Only at the highest water contents which form microemulsions alone, are conductivity and X-ray spectra consistent with water-filled reverse micelles embedded within an oil continuum. 

In most cases, it appears possible to interpret the critical behavior of microemulsion mixtures as a liquid/gas-like critical point [113-116]. Several light- and neutron-scattering studies on oil-rich ternary and quaternary microemulsions have clearly demonstrated that the structure of these media can be described as a solution of interacting water-in-oil droplets. As first shown by Calje et al. [117], the droplets may behave essentially as hard spheres. However, in many systems an attractive contribution to the interactions exists. It has been established that the strength of attractions between W/0 micelles is strongly dependent on the micellar size and on the chain lengths of both the alcohol and oil molecules. In particular, attractions have been found to increase when the micellar radius increases or the alcohol chain length decreases and the molecular volume of the oil increases [114, 115, 118-120]. 

One particular advantage of using mixtures of nonionic and ionic surfactants as microemulsifiers is the formation of temperature-insensitive microemulsions (17). Recall that the temperature-dependence of the phase behaviour of balanced microemulsion mixtures with ionic surfactants such as Aerosol OT (see Figure 4.10) is opposite to that found for ethoxylated alcohols (see Figure 4.5). Upon raising the temperature of Aerosol OT mixtures, a hydrophilic shift occurs (2-3-2), although with hoxylated alcohols, a lipophilic shift occurs (2-3-2). Intuitively, upon mixing ionic and nonionic surfactants, the temperature dependence should cancel at a particular ratio (5) of the two surfactants. 

A good example was brought by Moniruzzaman et al. which have used ionic liquid-in-oil (IL/o) microemulsions to enhance the topical and transdermal delivery of acyclovir (ACV). The microemulsion was composed by a blend of nonionic surfactants, namely polyoxyethylene sorbitan monooleate (Tween-80) and sorbitan laurate (Span-20), isopropyl myristate (IPM) as an oil phase, and IL [Cimim](CH30)2P02 (dimethylimidazolium dimethylphosphate) as a pseudophase. The solubility of ACV on the microemulsion system significatively increased in the presence of IL, which act as a drug reservoir during the process of delivery. Moreover the transdermal delivery was only achieved when the IL was present in the microemulsion mixture. 

Cationic surfactants may be used [94] and the effect of salinity and valence of electrolyte on charged systems has been investigated [95-98]. The phospholipid lecithin can also produce microemulsions when combined with an alcohol cosolvent [99]. Microemulsions formed with a double-tailed surfactant such as Aerosol OT (AOT) do not require a cosurfactant for stability (see, for instance. Refs. 100, 101). Morphological hysteresis has been observed in the inversion process and the formation of stable mixtures of microemulsion indicated [102]. 

Lattice models for bulk mixtures have mostly been designed to describe features which are characteristic of systems with low amphiphile content. In particular, models for ternary oil/water/amphiphile systems are challenged to reproduce the reduction of the interfacial tension between water and oil in the presence of amphiphiles, and the existence of a structured disordered phase (a microemulsion) which coexists with an oil-rich and a water-rich phase. We recall that a structured phase is one in which correlation functions show oscillating behavior. Ordered lamellar phases have also been studied, but they are much more influenced by lattice artefacts here than in the case of the chain models. 

When comparable amounts of oil and water are mixed with surfactant a bicontinuous, isotropic phase is formed [6]. This bicontinuous phase, called a microemulsion, can coexist with oil- and water-rich phases [7,1]. The range of order in microemulsions is comparable to the typical length of the structure (domain size). When the strength of the surfactant (a length of the hydrocarbon chain, or a size of the polar head) and/or its concentration are large enough, the microemulsion undergoes a transition to ordered phases. One of them is the lamellar phase with a periodic stack of internal surfaces parallel to each other. In binary water-surfactant mixtures, or in... 

In the latter the surfactant monolayer (in oil and water mixture) or bilayer (in water only) forms a periodic surface. A periodic surface is one that repeats itself under a unit translation in one, two, or three coordinate directions similarly to the periodic arrangement of atoms in regular crystals. It is still not clear, however, whether the transition between the bicontinuous microemulsion and the ordered bicontinuous cubic phases occurs in nature. When the volume fractions of oil and water are equal, one finds the cubic phases in a narrow window of surfactant concentration around 0.5 weight fraction. However, it is not known whether these phases are bicontinuous. No experimental evidence has been published that there exist bicontinuous cubic phases with the ordered surfactant monolayer, rather than bilayer, forming the periodic surface. 

The model has been successfully used to describe wetting behavior of the microemulsion at the oil-water interface [12,18-20], to investigate a few ordered phases such as lamellar, double diamond, simple cubic, hexagonal, or crystals of spherical micelles [21,22], and to study the mixtures containing surfactant in confined geometry [23]. 

In this section we characterize the minima of the functional (1) which are triply periodic structures. The essential features of these minima are described by the surface (r) = 0 and its properties. In 1976 Scriven [37] hypothesized that triply periodic minimal surfaces (Table 1) could be used for the description of physical interfaces appearing in ternary mixtures of water, oil, and surfactants. Twenty years later it has been discovered, on the basis of the simple model of microemulsion, that the interface formed by surfactants in the symmetric system (oil-water symmetry) is preferably the minimal surface [14,38,39]. 

A. Ciach. Statistical mechanics of ternary surfactant mixtures including microemulsions. Pol J Chem 66 1347-1387, 1992. 

K. Chen, C. Ebner, C. Jayaprakash, R. Pandit. Microemulsions in oil-water-surfactant mixtures Systematics of a lattice-gas model. Phys Rev A 55 6240, 1988. 

A. Ciach, J. S. Hoye, G. Stell. Microscopic model for microemulsion. II. Behavior at low temperatures and critical point. J Chem Phys 90 1222-1228, 1989. A. Ciach. Phase diagram and structure of the bicontinuous phase in a three dimensional lattice model for oil-water-surfactant mixtures. J Chem Phys 95 1399-1408, 1992. 

Aughel and coworkers [63] studied the phase behavior of hydrocarbon-water mixtures in the presence of alkyl(aryl)polyoxyethylene carboxylates for enhanced oil recovery and found good salt tolerance with an alkyl ether carboxy-late (C13-C15) with 7 mol EO and a good microemulsion forming effect with the 3 EO type. 

Different methods are used in microemulsion formation a low-energy emulsification method by dilution of an oil surfactant mixture with water and dilution of a water-surfactant mixture with oil and mixing all the components together in the final composition. These methods involve the spontaneous formation of microemulsions and the order of ingredient addition may determine the formation of the microemulsion. Such applications have been performed with lutein and lutein esters. ... 

High pressure homogenization may also be used to form microemulsions but the process of emulsification is generally inefficient (due to the dissipation of heat) and extremely limited as the water-oil-surfactant mixture may be highly viscous prior to microemulsion formation. ... 

The XRD and TEM showed that the bimetallic nanoparticles with Ag-core/Rh-shell structure spontaneously form by the physical mixture of Ag and Rh nanoparticles. Luo et al. [168] carried out structure characterization of carbon-supported Au/Pt catalysts with different bimetallic compositions by XRD and direct current plasma-atomic emission spectroscopy. The bimetallic nanoparticles were alloy. Au-core/Pd-shell structure of bimetallic nanoparticles, prepared by co-reduction of Au(III) and Pd(II) precursors in toluene, were well supported by XRD data [119]. Pt/Cu bimetallic nanoparticles can be prepared by the co-reduction of H2PtClg and CuCl2 with hydrazine in w/o microemulsions of water/CTAB/ isooctane/n-butanol [112]. XRD results showed that there is only one peak in the pattern of bimetallic nanoparticles, corresponding to the (111) plane of the PtCu3 bulk alloy. 

Holt studied the Diels-Alder reaction in a mixture of water, 2-propanol, and toluene as microemulsions.33 The endo/exo ratio between the reaction of cyclopentadiene and methyl methacrylate was enhanced with increasing amount of water in the presence of a surfactant. 

Graciaa A. et alii, The Partitioning of Nonionic and Anionic Surfactant Mixtures Between Oil/Microemulsion/Water Phases , n° SPE 13 030, Houston, 1984. 

In a biodesulfurization process, there are actually three phases. For a liquid mixture containing the three phases - liquid fossil fuel, water, and the biocatalyst, more than one filter would be required. One filter will preferentially collect either the liquid fossil fuel or aqueous phase as the filtrate. The retentate will then flow to the second filter, which will collect the component not removed before. The remaining retentate, containing the biocatalyst, can then, preferably, be recycled. The process can be used to resolve an emulsion or microemulsion of the liquid fossil fuel and aqueous phase resulting from a... 

The Landau-Ginzburg model of a ternary mixture of oil, water, and surfactant studied here was proposed by Teubner and Strey [115] on the basis of the scattering peak in the microemulsion phase. Later it was refined by Gompper and Schick [116]. Its application to various bulk and surface phenomena is described in detail in Ref. 117. 

The window in the product profile of the ECP, where microgels are exclusively formed, also comprises the compositions of the reaction mixture in which microemulsions are formed. 

Because the presence of an electrolyte increases the dimensions of micelles and microemulsion droplets [115], it may be expected that in presence of ions the size of microgels is also increased. This expectation could be confirmed external electrolyte increases Mw (Fig. 21) as well as dz and [r ] (Fig. 22) up to the limit of the emulsion stability. Therefore, the addition of an external electrolyte to the reaction mixture for the ECP of EUP and comonomers is a means to vary the molar mass, the diameter and the intrinsic viscosity of microgels from EUP and comonomers deliberately. 

These microdroplets can act as a reaction medium, as do micelles or vesicles. They affect indicator equilibria and can change overall rates of chemical reactions, and the cosurfactant may react nucleophilically with substrate in a microemulsion droplet. Mixtures of surfactants and cosurfactants, e.g. medium chain length alcohols or amines, are similar to o/w microemulsions in that they have ionic head groups and cosurfactant at their surface in contact with water. They are probably best described as swollen micelles, but it is convenient to consider their effects upon reaction rates as being similar to those of microemulsions (Athanassakis et al., 1982). 

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