Potential determining salts in microemulsions: interfacial distribution and effect on the phase behavior

In this work we consider potential determining salts, also referred to as phase transfer agents for a future objective of electrochemistry at the oil-water interface in microemulsions. We have studied these salts, composed of a hydrophilic and a hydrophobic ion, in microemulsion stabilized by nonionic surfactants with an oligo ethylene oxide headgroup. NMR measurements show that the salts preferentially dissociate across the surfactant interface between the oil and water domains, and hence create a potential drop across the surfactant film, and back to back diffuse double layers in the oil and water phases. These observations are also supported by Poisson-Boltzmann calculations. This adsorption like event stabilizes the microemulsion. Repulsive long-range interactions between thin lamellae of surfactant and water lamellae in oil were observed using SAXS, thus confirming the presence of electrostatic forces mediated through the oil domain. We also observed that reversing the charges on the potential determining salts had opposite effects on the phase inversion temperature.