Propane-Induced Inversion of the Pressure Dependence of Highly Efficient Microemulsions: Phase Behavior and Its Prediction via HLD

The influence of pressure on microemulsion properties is generally weak given the components’ low compressibility. However, pressure can become decisive if short-chain alkanes are present, e.g., in enhanced oil recovery, where the impact of live oils contributes to formulation design optimization. Supporting the transfer from laboratory to oil field, we investigated the pressure-dependent phase behavior of application-relevant brine/butyldiglycoln-decane/propanealkyl alkoxy sulfate/alkyl sulfonate microemulsions. Equal amounts of water/oil were solubilized into a one-phase microemulsion using only a few wt % of surfactant. In n-decane-rich microemulsions, pressure leads to an inversion from water-in-oil to oil-in-water microemulsions, resulting from stronger surfactant headgroup hydration, whereas in propane-rich microemulsions, curvature changes from o/w to w/o, caused by better interactions between the compressible propane and the surfactant tails at elevated pressures. These effects lead to an inversion of the sign of the hydrophilic–lipophilic deviation pressure coefficient β and compensate each other with roughly 80 mol % of propane in the oil mixture, where the microemulsion is pressure-independent, i.e., β = 0 at 25 °C.