Lipid membranes have been widely used in areas ranging from fundamental research to industrial applications. However, inherent physicochemical properties of lipids (i.e., low mechanical robustness, chemical instability, etc.) hinder their performance in many applications. New lipid analogues with tunable materials properties have been developed to address these problems. Previously, we synthesized a new type of charged, amphipathic oligomer – oligodimethylsiloxane imidazolium cation (ODMS-MIM<sup>+</sup>), a lipid mimic consisting of a positively charged ionic liquid headgroup and a hydrophobic tail and tracked in real time its self-assembly at the air/water and hexadecane/water interfaces. Here, we employ vibrational sum frequency generation (vSFG) spectroscopy to study the influence of a series of simple inorganic anions as counterions in aqueous solution to the positively-charged methylimidazolium headgroups at the interface. The anions studied exhibited a range of surface activities and ion pairing with the methylimidazolium cations at the interface, which affected the ordering and packing of the covalently attached hydrophobic ODMS tails in the hydrophobic phase. The addition of a Langmuir trough in conjunction with the SFG measurements allows for independent control of surface pressure and area per molecule of the oligomer while mapping structural and H-bonding changes in the interfacial monolayer. Paired with molecular dynamics (MD) simulations we provide new insight into dynamical processes occurring at the interface during membrane assembly and how ion interactions can be used to impact assembly of oligomers into functional interfaces.

Insight into the influence of anions on ionic oligomer membrane formation and structural change

ACS Fall 2021

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