Evaluating the terminal group switching of responsive monolayer films: examining structural rearrangements via molecular simulations and experimental wetting measurements

Abstract

Monolayers with terminal groups that are driven by thermodynamics to respond to changes in solvent are designed with the Molecular Simulation Design Framework (MoSDeF). Specifically, atomistic simulations of amorphous silica surfaces functionalized with alkylsilane chains terminated with competing hydrophobic and hydrophilic moieties are studied. We show that such moieties can achieve a dynamic switching response through molecular reorientation of the individual hydrophobic/hydrophilic groups when changing from a polar (water) to a non-polar (n-hexane) solvent. This responsiveness occurs more strongly with half-functionalized monolayer films containing a second, shorter alkylsilane chain that provides film stability and free volume for the terminal groups to transition between bistable states via the switching mechanism. Experiments are performed to measure the contact angles of polar and non-polar liquids on a subset of synthetically viable versions of the broader simulated data set. The simulations and experiments show that the interface is responsive to the nature of the solvent, and that the degree of switchability is affected by both the relative length of the terminal groups and the identity of the polar group.