Effects of salinity on the viscosity and birefringence of a microemulsion system

Abstract

Oil/water/surfactant systems form complex equilibrium phases which are sensitive to a number of parameters, including amount and concentration of cosurfactant (often an alcohol), salinity, and temperature. If one of these variables is changed systematically as, for example, the salinity, an interesting transition may be observed in which at low salinities a microemulsion is in equilibrium with an excess oil phase, at moderate salinities a middle phase microemulsion is in equilibrium with both excess oil and excess water phases, and at higher salinities brine is in equilibrium with a microemulsion phase. To help elucidate the structure of the microemulsion, studies of viscoelasticity and streaming birefringence in oscillatory shear flow have been conducted of a middle phase-forming system as a function of salinity. It is found that the viscoelastic properties of the microemulsions are unchanged for shear rates varying from 0.1 to 100 sec −1. Both the birefringence and the viscosity maximize near the salinity marking the transition from lower phase to middle phase microemulsion. Further inflections in these properties occur at a salinity marking the midrange of the middle phase microemulsion. For all cases the dominent relaxation time is near 3 to 5 msec while the birefringence changes by two orders of magnitude. The birefringence is a sensitive indicator of the elastic structure of the microemulsion.