Molecular Transport Across Oil-Brine Interfaces Impacts Interfacial Tension: Time-Effects In Buoyant And Pendant Drop Measurements

The buoyant drop method is a ubiquitous tool for addressing phenomena at the liquid-liquid interface via the determination of the interfacial tension (IFT) between two immiscible phases. Here, the focus is on how electrolytes (in an aqueous phase) and carboxylic acids (in a decane phase) impact the interfacial layer between the two phases. The IFT measurement provides a single number, which is not fulfilling when it comes to deducing information about a complex multiparameter system. Furthermore, the temporal evolution of IFT does not always reach a steady-state value on a time scale, which is realistic to use for comparative studies. We have investigated the temporal evolution of IFT in a series of experiments with varying compositions of the decane-carboxylic acid phase and the brine phase. The results show that there are at least two opposing effects in play. For water-soluble acids, the IFT initially increases with time until a turnover point is reached from where there is a gradual decay. The IFT at the turnover point is close to that of the pure water-decane system. For a poorly water-soluble acid, the IFT shows a much smaller increase and the turnover happens much faster. For a water-soluble acid, there is a high degree of sensitivity toward the electrolyte; it determines the position (in time) of the IFT peak and the steepness of the subsequent decay. Now, if the phases are reversed, that is, by placing a drop of brine in the decane-surfactant phase, the IFT decreases with time regardless of the acid and with little impact of the electrolyte and its concentration in the brine. We propose an explanation for the observed behavior (supported by COSMO-RS calculations), which is based on diffusion in and out of the two phases, solubility, and interfacial reactivity (i.e., aggregation between electrolytes and carboxylic acids).