Marangoni Instability Driven by Adsorption and Association of Oppositely Charged Surfactants at an Oil–Water Interface

When an aqueous buffer solution of the cationic surfactant tetradecyltrimethylammonium bromide near neutral pH is contacted with a solution of the fatty acid surfactant palmitic acid in tetradecane, convective flows appear spontaneously at the interface. This coincides with turbidity formation in the aqueous phase, consistent with aggregation between the headgroups of the two surfactants. Motivated by these experimental observations, a mathematical model is developed for transport, adsorption, and fluid flow in two immiscible contacting fluid phases, each containing a distinct surfactant. Linear stability analysis reveals that gradients generated by dynamic adsorption alone cannot drive instability, which only becomes possible if there is a process by which adsorbed surfactant is removed from the interface, as modeled by an interfacial reaction representing formation and desorption of aggregates in the experiments. A full numerical simulation shows that asymmetry in surfactant transport properties can inhibit convection due to the accumulation of surfactants on the interface.