Oil-rich microemulsions

Shioi, A., Harada, M., and Tanabe, M. (1995), Static light scattering from oil-rich microemulsions containing polydispersed cylindrical aggregates in sodium bis(2-ethylhexyl) phosphate system,/. Phys. Chem., 99,4750 1756. 

Light scattering measurements and theoretical treatment strongly support the idea that attractive interactions between inverse micelles play an important role in the stability of oil rich microemulsions. In the system containing pentanol, attractions between (i)/o micelles can be sufficient to give rise to a phase separation between two microemulsion phases. 

Figure 1.7 Vertical sections T(wb) and 7 (wA) through the phase prism which start at the binary water-surfactant (wb = 0) and the binary oil-surfactant (wA = 0) corner, respectively. These sections have been proven useful to study the phase behaviour of water- and oil-rich microemulsions, (a) Schematic view of the sections T wg) and T(wA) performed at a constant surfactant/fwater + surfactant) mass fraction ya and at a constant surfactant/(oil + surfactant) mass fraction 7b, respectively, (b) T(wb) section through the phase prism of the system FhO-n-octane-CioEs at ya = 0.10. Starting from the binary system with increasing mass fraction of oil wg, the oil emulsification boundary (2- 1) ascends, while the near-critical phase boundary (1 - 2) descends, (c) T(wA) section through the phase prism of the system EbO-n-octane-QoEs at 7b = 0.10. The inverse temperature behaviour is found on the oil-rich side With increasing fraction of water wA the water emulsification boundary (1 - 2) descends, whereas the near-critical phase boundary (2 —> 1) ascends.

Whereas an ethoxylated alcohol with dodecyl tails (e.g. C12E5) forms middle-phase microemulsions, ionic surfactants with dodecyl tails, such as sodium dodecyl sulfate (SDS) or dodecyltrimethylammonium bromide (DTAB), are too hydrophilic for formation of middle-phase microemulsions. Simply increasing the length of the hydrocarbon tail to compensate for the high hydrophilicity of the ionic head-groups favours the formation of viscous liquid crystal line phases rather than fluid microemulsion phases (36, 37). However, increasing the hydrophobicity by adding double tails to the surfactant, as for example with didodecyldimethylam-monium bromide surfactant (DDAB), suppresses some of the tendency to form liquid crystals, and allows for formation of oil-rich microemulsions (38). However, this surfactant is too hydrophobic, and is far from the... 

Without addition of salt, Aerosol OT forms a much-studied region of oil-rich microemulsions (41). However, the even more hydrophilic single-tailed surfactants such as sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) are so far from the optimal hydrophilic-lipophilic balance that addition of salt alone is not enough for the formation... 

Lattice models have been studied in mean field approximation, by transfer matrix methods and Monte Carlo simulations. Much interest has focused on the occurrence of a microemulsion. Its location in the phase diagram between the oil-rich and the water-rich phases, its structure and its wetting properties have been explored [76]. Lattice models reproduce the reduction of the surface tension upon adsorption of the amphiphiles and the progression of phase equilibria upon increasmg the amphiphile concentration. Spatially periodic (lamellar) phases are also describable by lattice models. Flowever, the structure of the lattice can interfere with the properties of the periodic structures. 

Shioi A and Flarada M 1996 Model for the geometry of surfactant assemblies in the oil-rich phase of Winsor I microemulsions J. Chem. Eng. Japan 29 95... 

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. 

The function /[0(r)] has three minima by construction and guarantees three-phase coexistence of the oil-rich phase, water-rich phase, and microemulsion. The minima for oil-rich and water-rich phases are of equal depth, which makes the system symmetric, therefore fi is zero. Varying the parameter /o makes the microemulsion more or less stable with respect to the other two bulk uniform phases. Thus /o is related to the chemical potential of the surfactant. The constant g2 depends on go /o and is chosen in such a way that the correlation function G r) = (0(r)0(O)) decays monotonically in the oil-rich and water-rich phases [12,13]. This is the case when gi > 4y/l +/o - go- Here we take, arbitrarily, gj = 4y l +/o - go + 0.01. 

In the real space the correlation function (6) exhibits exponentially damped oscillations, and the structure is characterized by two lengths the period of the oscillations A, related to the size of oil and water domains, and the correlation length In the microemulsion > A and the water-rich and oil-rich domains are correlated, hence the water-water structure factor assumes a maximum for k = k 7 0. When the concentration of surfac-... 

Here scalar order parameter, has the interpretation of a normalized difference between the oil and water concentrations go is the strength of surfactant and /o is the parameter describing the stability of the microemulsion and is proportional to the chemical potential of the surfactant. The constant go is solely responsible for the creation of internal surfaces in the model. The microemulsion or the lamellar phase forms only when go is negative. The function/(<))) is the bulk free energy and describes the coexistence of the pure water phase (4> = —1), pure oil phase (4> = 1), and microemulsion (< ) = 0), provided that/o = 0 (in the mean-held approximation). One can easily calculate the correlation function (4>(r)(0)) — (4>(r) (4>(0)) in various bulk homogeneous phases. In the microemulsion this function oscillates, indicating local correlations between water-rich and oil-rich domains. In the pure water or oil phases it should decay monotonically to zero. This does occur, provided that g2 > 4 /TT/o — go- Because of the < ), —<(> (oil-water) symmetry of the model, the interface between the oil-rich and water-rich domains is given by... 

This equation defines the internal surfaces in the system. The model has been studied in the mean held approximation (minimization of the functional) [21-23,117] and in the computer simulations [77,117,118], The stable phases in the model are oil-rich phase, water-rich phase, microemulsion, and ordered lamellar phase. However, as was shown in Refs. 21-23 there is an infinite number of metastable solutions of the minimizahon procedure ... 

Oil rich phase Oil rich phase Water in oil microemulsion... 

It appears that the role of increasing salinity is to change the mean curvature of the surfactant sheetlike structure from a value favoring closure on the oil-rich regions (swollen inverted micelles). In between, in bicontinous microemulsion having comparable amounts of oil and water, the preferred mean curvature must be near zero. 

Figure 2. Phase diagram of an. 4077water/supercritical propane system in the oil-rich comer of the temaiy phase diagram. The regions to the right of the solid lines are the one-phase, clear microemulsions. The various IV value lines ([H2O]/ [AOT]) are also indicated. The temperature of the system is 103°C. (Compositions are given in weight percent.)...

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. 

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