Water-rich microemulsions

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. 

Figure 19 Double-oil diffusion experiment with nonionic surfactant, (a) Self-diffusion coefficients and (b) diffusion coefficient ratio A" as a function of temperature in a water-rich microemulsion with nonionic surfactant. A transition from oil-in-water droplets to a bicontinuous microstructure occurs with increasing temperature (decreasing spontaneous curvature of the C12E5 surfactant film). The maximum in K indicates that an attractive interaction between the micelles is operating prior to the formation of a bicontinuous structure. Kq = 1.69 is the diffusion coefficient ratio in the pure oil mixture and is indicated as a broken line in (b). Note that the initial decrease of the self-diffusion coefficients shows that the droplets grow in size before the bicontinuous transition. The phase boundary at 25.7 C is indicated as a vertical broken line. (Data from Ref 43.)...

Sixteen different water-rich microemulsion compositions from Fig. 13.1 were tested as mobile phases on a 15x0.46 cm Spherisorb 5pm C18 column (SFCC, France). The alkylbenzene homologous series, from toluene to decylbenzene, was used as the test solute [5]. 

At constant pressure, it is convenient to present the composition-temperature phase behaviour in terms of a phase prism, as illustrated in Figure 17.1. A sequence of constant temperature sections is illustrated in Figure 17.2. Applied to nonionics, phase diagram (a) would correspond to the lowest temperature (< Tq) and (i) to the highest temperature (> To). At lower temperatures (a), a water-rich microemulsion is stable. We note that the phase boundary corresponding to the maximum solubility limit of oil is essentially a straight... 

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. 

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... 

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-... 

From (62) and (70) it follows that the Lifshitz and tricritical points coincide giving the Lifshitz tricritical point [18,66] for 7 = 27/4. 7 = 27/4 can be considered, as a borderline value between the weak (7 <27/4) and the strong (7 >27/4) surfactants. For the weak surfactants the tricritical point is located at the transition between the microemulsion and the coexisting uniform oil- and water-rich phases, whereas for the strong surfactants the tcp is located at the transition between the microemulsion and the liquid-crystal phases. The transition between the microemulsion and the ordered periodic phases is continuous for p < Ps < Ps and first order for p > p[. 

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 ... 

A position to the right of the microemulsion area means the presence of a lamellar liquid crystal as has been repeatedly demonstrated by Ekwall (19). The temporary appearance of liquid crystals when W/0 microemulsions are brought into contact with water have, with this result, been given a satisfactory explanation. The faster transport of the surfactant into the aqueous layers gives rise to temporarily higher surfactant concentrations and the stability limits for the water rich W/0 microemulsions phase are exceeded towards the liquid crystalline phase in water. 

Fig. 5 shows a hypothetical phase diagram with representation of microemulsion structures. At high water concentrations, microemulsions consist of small oil droplets dispersed in water (o/w microemulsion), while at lower water concentrations the situation is reversed and the system consists of water droplets dispersed in oil (w/o microemulsions). In each phase, the oil and water droplets are separated by a surfactant-rich film. In systems containing comparable amounts of oil and water, equilibrium bicontinuous structures in which the oil and the water domains interpenetrate in a more complicated manner are formed. In this region, infinite curved channels of both the oil and the water domains extend over macroscopic distances and the surfactant forms an interface of rapidly... 

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