Surface spectroscopy of specific cation effects at liquid/liquid interfaces: An unconventional approach to self-assembly

Functional oil/aqueous interfaces stabilized by amphiphiles have received extensive interest due to their role in a range of modern technologies, including chemical separations, biological membranes, and neuromorphic computing. Knowledge of the molecular structure and associated tunability at buried oil/aqueous interfaces is thus of significant importance in the design of new interfaces with emergent and targeted functionality. While the assembly of cationic surfactants at interfaces has been studied for some time with attention paid to anions as moderators in the assembly, aqueous cations are thought to play a negligible role because of their weak surface activities and repulsion by the positively charged headgroups. Here, we will discuss an unexpected specific cation effect that moderates the self-assembly of amphiphilic molecules at oil/aqueous interfaces via direct and indirect interactions. These results provide new insight into the competition between ion-pairing, hydration and van der Waals interactions in moderating self-assembly.