Tuning the self-assembled structure of ionic oligomers at buried liquid/liquid interfaces using aqueous phase ions

Functional oil/aqueous interfaces stabilized by amphiphiles have applications in numerous fields of science and engineering, including chemical separations, neuromorphic computing, nanomaterial synthesis, and nuclear waste remediation. Changing conditions in the bulk phase can lead to a dynamic response of the interface, which is the key to achieving the diversity of functions commonly found in nature. Designing a new function into the interface requires a mechanistic understanding of the interfacial chemistry, which relies on the knowledge of the molecular composition and conformation at the monolayer level. In this work, we use vibrational sum frequency generation (SFG) combined with molecular dynamics (MD) simulations to investigate the self-assembly of ionic oligomers at buried hexadecane/aqueous interfaces. The oligomer consists of an oligodimethylsiloxane (ODMS) tail covalently attached to a positively charged imidazolium (MIM⁺) headgroup. We show that the presence of salts in the aqueous phase significantly changes the conformation and packing of the ODMS tails by forming ion-pairs with the headgroup as well as, surprisingly, directly interacting with the tails. At the same time, dramatically different H-bonding structures emerge at the interface, indicative of unique interfacial solvation. These effects on the self-assembled structures are further found to strongly depend on the ionic strength and ion identity, a notion supported by MD simulations, which reveal how the surface activity of the ions plays a key role in regulating the interfacial behavior of the oligomers. Our findings describe key mechanistic principles needed to tune the interfaces for a range of applications and have implications for the design of novel functional interfaces.