Molecular self-assembly of surfactants at liquid/liquid interfaces is a key process that has direct relevance to a wide range of fields including chemical separations, nano material synthesis, nuclear waste remediation, and even neuromorphic computing. Understanding the mechanisms that drive self-assembly is key to the design of such soft materials with complex or emergent functionality. However, probing the molecular structures at buried liquid/liquid interfaces has proven challenging because of the overwhelming signals from the bulk phases that conceal the interfacial species. Surface-specific spectroscopies, such as sum frequency generation (SFG), are ideally suited to investigate these interfacial species, structures and dynamics. Here, we present an SFG study of the self-assembly of an oligomer consisting of an oligodimethylsiloxane (ODMS) tail covalently attached to a positively charged methyl imidazolium (MIM<sup>+</sup>) headgroup at buried oil/aqueous interfaces. We find that ions in the aqueous phase interact with the oligomer headgroup, alter the interfacial water organization and hydrogen-bond network, and subsequently modify the conformation of the ODMS tails in the oil phase. Molecular dynamics simulation is not only in good agreement with the SFG data but also shows evidence for ion pairing interactions and changes in head group solvation. These findings provide insight into the molecular organization at liquid/liquid interfaces and demonstrate the possibility to precisely control the interfacial properties, and therefore functions, through aqueous phase compositions.

Aqueous ions regulate self assembly of oligomers at liquid/liquid interfaces

ACS Fall 2021

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