Microemulsion phase behavior

Upadhyaya A, Acosta EJ, Scamehorn JF, Sabatini DA (2006) Microemulsion phase behavior of anionic-cationic surfactant mixtures Effet of tail branching. J Surfact Deterg 9 169-179... 

Effects of Polymers, Electrolytes, and pH on Microemulsion Phase Behavior... 

The effect of polymers on microemulsions phase behavior has been reported by Hesselink and Faber (8). They have described the surfactant-polymer phase separation in terras of the incompatibility of two different polymers in a single solvent, considering the microemulsion as a pseudo-polymer system. The effect of polymers on the phase behavior of micellar fluids has been recently studied by Pope et al. (9) and others (10,11). 

The effect of 750 ppm Xanthan gum on the microemulsion phase behavior is shown in Figure 5(b). The observed phase behavior is similar except that the extent of the three-phase region is widened. Thus at both 0.8 and 1.0 gm/dl salt concentrations there exists a polymer-containing brine phase in equilibrium with the microemulsion phase. When no polymer is present, the microemulsion phase is in equilibirum with only excess oil. The volumes of the polymer phases are small and the interface between the polymer phase and microemulsion is diffipult to detect in Figure 5(b). However, phase separation is clearly visible in Figure 5(c), which illustrates the oil-equilibrated phase behavior at a higher polymer concentration of 1500 ppm. 

Figure 8. Microemulsion phase behavior in polarized light of 5 gm/dl TRS 10-410, 3 gm/dl IBA/n-hexadecane systems with and without 750 ppm Xanthan (Pfizer). Flat bottom tubes with polymer and round bottom tubes without polymer. Salinities are 0.4, 1.0, 1.6 and 2.2 gm/dl NaCl from left to right. T=22°C.

This chapter covers the fundamentals of surfactant flooding, which include microemulsion properties, phase behavior, interfacial tension, capillary desaturation, surfactant adsorption and retention, and relative permeabilities in surfactant flooding. It provides the basic theories for surfactant flooding and presents new concepts and views about capillary number (trapping number), relative permeabilities, two-phase approximation of the microemulsion phase behavior, and interfacial tension. This chapter also presents an experimental study of surfactant flooding in a low-permeability reservoir. 

So far, we have described quantification of microemulsion phase behavior. The procedures described here can be coded in a small program or even in an Excel spreadsheet. More practically, we can use a sample UTCHEM simulation file called batch.txt to simulate phase behavior pipette tests. The idea of the batch.txt file is to treat a pipette as a core plug with porosity 1.0 and a very high permeability (e.g., 1,000,000 darcies). When we inject many pore volumes of water, oil, and surfactant whose compositions are the same as those in the pipette test, the flow becomes a steady-state. In such a steady-state flow, the component concentrations from the simulation should be the same as their respective volumetric fractions in the pipette test. 

Hirasaki et al. (1983) assumed that if an excess water phase wets preferentially to a microemulsion phase and a microemulsion phase preferentially to an oleic phase, then (1) in the absence of an excess water phase, the microemulsion is the wetting phase (2) in the absence of an excess oil phase, the microemulsion is the nonwetting phase and (3) when all the three phases are present, the microemulsion is a spreading phase between the excess oil and excess water. Hirasaki et al. (2008) further pointed out that the current understanding of microemulsion phase behavior and wettability is that the system wettability is likely to be preferentially water-wet when the salinity is below the optimal salinity (Winsor I) and is likely to be preferentially oil-wet when the salinity is above the optimal salinity (Winsor II), even in the absence of alkali. Their view is supported by Nelson et al. (1984), Israelachvili and Drummond (1998), and Yang (2000). 

In doing so, their formnlation provides a prediction for microemulsion phase relative permeabihties from the water and oil phase data. If no oil exists in a simulation block (e.g., in an aquifer block), then Sqt is 0. However, it is not necessarily true that k n can be represented by kjo. If Eqs. 7.142 through 7.146 are used to dehne microemulsion phase relative permeabihty, the microemulsion phase behavior should depend on composition, and we propose that co is dehned by the component fractions in the microemulsion phase such that... 

A general pattern of microemulsion phase behavior exists for systems containing comparable amounts of water and a pure hydrocarbon or hydrocarbon mixture together with a few percent surfactant. For somewhat hydrophilic conditions, the surfactant films tend to bend in such a way as to form a water-continuous phase, and an oil in water microemulsion coexists with excess oil. Drops in the microemulsion are spherical with diameters of order 10 nm. Both drop size and solubilization expressed as (VJVX the ratio of oil to surfactant volume in the microemulsion, increase as the system becomes less hydrophilic. At the same time interfacial tension between the microemulsion and oil phases decreases. Just the opposite occurs for somewhat lipophilic conditions. That is, a water in oil microemulsion coexists with excess water with drop size and solubilization of water (VJV,) increasing and interfacial tension decreasing as the system becomes less lipophilic. When the hydrophilic and lipophilic properties of the surfactant films are nearly balanced, a bicontinuous microemulsion phase coexists with both excess oil and excess water. For a balanced film (VJV,) and (VJV ) in the microemulsion are nearly equal, as are 7, 0 and... 

The general pattern of microemulsion phase behavior described above is seen when the amounts of water and hydrocarbon present are comparable. However, a hydrocarbon-free mixture of surfactant and water (or brine) near optimal conditions is typically not a simple micellar solution but either the lamellar liquid crystalline phase or a dispersion of this phase in water. Starting with such a mixture and adding hydrocarbon, we sometimes find that the system passes through several multiphase regions before reaching the microemulsion/oil/water equilibrium characteristic of optimal conditions. 

Burauer, S., Sachert, T., Sottmann, T. and Strey, R. (1999) On microemulsion phase behavior and the monomeric solubility of surfactant. Phys. Chem. Chem. Phys., 1, 4299-4306. 

Aveyard, R., Binks, B.P. andFletcher, P.D.I. (1990) Surfactant molecular geometry within planar and curved monolayers in relation to the microemulsion phase behavior. In D.M. Bloor and E. Wyn-Jones (eds), The Structure, Dynamics and Equilibrium Properties of Colloidal Systems. Kluwer Academic Publishers, Dordrecht/Boston/London, pp. 557-581. 

Baran, J.R., Pope, G.A., Wade, W.H. and Weerasooriya, V. (1996) Water/chlorocarbon winsor I III II microemulsion phase behavior with alkyl glucamide surfactants. Environ. Sci. Technol, 30(7), 2143-2147. 

Shinoda, K., Araki, M., Sadaghiani, A., Khan, A. and Lindman, B. (1991) Lecithin-based microemulsions Phase behavior and microstructure. /. Phys. Chem., 95, 989-993. 

Ghosh O, Miller CA. Liquid crystalline and microemulsion phase behavior in alcohol-free aerosol-OT/oil/brine systems. J Phys Chem 1987 91 4528-4535. 

Kellay H, Binks BP, Hendrikx Y, Lee LT, Meunier J. Properties of surfactant monolayers in relation to microemulsion phase behavior. Adv Colloid Interface Sci 1994 49 85-112. 

Benett KE, Phelps CHK, Davis HT, Scriven LE. Microemulsion phase behavior—observations, thermodynamic essentials, mathematical simulation. Soc Petroleum Eng J 1981 21 747-762. 

This chapter will focus on a simpler version of such a spatially coarse-grained model applied to micellization in binary (surfactant-solvent) systems and to phase behavior in three-component solutions containing an oil phase. The use of simulations for studying solubilization and phase separation in surfactant-oil-water systems is relatively recent, and only limited results are available in the literature. We consider a few major studies from among those available. Although the bulk of this chapter focuses on lattice Monte Carlo (MC) simulations, we begin with some observations based on molecular dynamics (MD) simulations of micellization. In the case of MC simulations, studies of both micellization and microemulsion phase behavior are presented. (Readers unfamiliar with details of Monte Carlo and molecular dynamics methods may consult standard references such as Refs. 5-8 for background.)... 

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