Pressure Induced Phase Transition of an Oil External Microemulsion

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The critical phenomenon of a c-component oil external microemulsion system containing pure normal decane, double distilled water, and an anionic surfactant, sodium dimethyl hexyl sulfosuccinate, was studied as a function of applied external pressure for the first time using laser light scattering. It was found that the divergence of the scattered intensity and static correlation length can be described in terms of the critical exponents as a function of ΔP = |P-Pc|. Furthermore, the critical exponents of ΔP, v and γ associated with the scattered intensity and static correlation length respectively, are the same as those obtained from previous studies with other thermodynamic variables, i.e. temperature, carbon chain length of oil, etc. Thus pressure can induce a critical type phase transition in this oil external microemulsion. Prior studies have shown that the attractive potential between droplets is most likely the driving force for the critical points of this microemulsion, and that increasing the alkane number of the oil increases the attractive potential. The pressure, in a sense, contributes to this attractive force by increasing the density of the oil, i.e., increasing the effective alkane number. Furthermore, we have measured the thermal pressure coefficient, (dP/dT)v, to begin to understand the thermodynamic properties of this oil external microemulsion.