Equilibrium of a Microemulsion That Coexists With Oil or Brine

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Abstract
When salinity, or an equivalent variable, is increased, microemulsions generally undergo orderly transitions from a lower- to middle- to upper-phase. Even though the Significance of such multiphase behavior has been well recognized in the design of surfactant flood processes, their quantitative nature in terms of the molecular structures of the surfactant lipophile, hydrophile, and the oil and brine salinity has not been fully understood. A theory of lower- and upper-phase microemulsions that gives reasonable predictions of their interfacial tensions (IFT's) and phase behavior is presented. In the theory, the Surfactant monomers adsorbed at oil/brine interface cause the interface to bend as a result of an imbalance between the hydrophile/brine interaction on the one hand and lipophile/oil interaction on the other. With sufficient imbalance, high local curvature causes small drops of one phase to disperse into the other. In addition, interactions between these drops are taken into account for the microemulsion equilibrium. The theory also offers a possibility of being able to describe the hydrophile/lipophile-balanced state (optimal salinity state of Healy and Reed1) in terms of the tendency of surfactant layer at the oil/brine interface to bend.

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