Formation of a middle-phase from a lower or upper-phase microemulsion

Abstract

The equilibrium theory of lower- and upper-phase microemulsions earlier derived is extended to obtain the conditions for the stability of such microemulsions. Consideration of stability is aimed at understanding how lower- and upper-phse microemulsions separate into a middle-phase and an excess phase, and what are the requirements for the compatibility between a lower-phase microemulsion and a polymer added to it. Under the assumption that the microstructure of microemulsion remains as spherical drops of constant radius, the lower- and upper-phase microemulsions are shown to become unstable and separate into a “middle-phase” and an “excess” phase, as the optimal salinity is approached from either a low or a high salinity. This happens due to the increase in the van der Waals attraction among the drops. The presence of polymer is shown to enhance the phase separation, when the steric repulsion between the microemulsion drops and polymer molecules occurs.