A systematic study is conducted to describe the transient interfacial tension (IFT) behavior of crude-oil/caustic interfaces. The variation in this behavior with variation in the composition of the aqueous phase and other parameters is the focus of this study. It is found that at low pH values, the experimentally observed behavior is similar to that reported earlier. At high pH values, however, a maximum in IFT is observed, followed by a minimum at a late time. Sodium chloride is shown to have no significant effect on the shape of the transient IFT curve, but the absolute values of IFT are at  lower NaCl concentrations. Divalent ions dramatically increase the IFT. 

A chemical model that takes into account the kinetics and the detailed chemisry of the process is used to explain the observed phenomena. The phenomenological surface phase approach is used to model the interface. The resultant set of ordinary differential equations is linearized by making certain justifiable, simplifying assumptions. The analytical solution to the set of equations yields the variation of the various concentrations with time. The IFT is then related to the interfacial concentration of the surface-active species, A_s–, by the Gibbs equation. The variation in the transient IFT behavior on changing the kinetic constants, phase volumes, and ionic concentrations is discussed. Finally, it is shown that the extension of this model to a two-component model would successfully explain all the experimentally observed phenomena.