Phase Behavior and Interfacial Tension for Oil-Water-Surfactant Systems

PHASE BEHAVIOR AND INTERFACIAL TENSION FOR OIL-WATER-SURFACTANT SYSTEMS 

Let us consider mixtures contatning a few percent of an anionic surfactant, approximately equal volumes of oil and NaQ brine, and usually some short-chain alcohol used as a cosurfactant or cosolvrait. If only the salinity is varied, a general pattern of phase behavior has berai observed (Reed and Healy, 1977) 

At low salinities, an oU-in-water microranulsion coexists with nearly pure oil, as shown in Figpre 4.24. Solubilization of oil and hence microemulsion drop size increase with increasing salinity (i.e., as repulsion betweai the charged head groups decreases). The interfacial tension between phases is low by ordiriary standards, but is normally greater than 0.01 mN/m. This combmadon of phases is sometimes called Winsor type 1 behavior, after an early researcher on microemulsion phase behavior (Winsor, 1954). 

At high salinities, the situation is reversed and an oil-continuous micro-emulsion coexists with excess brine, as the figure shows. Drop size increases with decreasing salinity (i.e., with increasing repulsion between head groups). 

FIGURE 4.24 Phase behavior as a function of the salinity for a system containing a petroleum sulfonate surfactant, a short-chain alcohol, and approximately equal volumes of oil and brine. 

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