(286b) Predicting Perturbations to Water Structure By Chemically Heterogeneous Self-Assembled Monolayers to Understand Context-Dependent Hydrophobicity

Predicting hydrophobic interactions between chemically heterogeneous surfaces (i.e., surfaces with polar and nonpolar functional groups in close proximity) is a significant challenge due to cooperative and competing effects by the specific chemical properties of surface functional groups. These effects have led to hydrophobic interactions being labeled as context-dependent interactions, meaning that both the specific chemical groups and their spatial relation to one another dictate the strength of hydrophobic interactions. Extensive experimental and theoretical efforts have focused on understanding the underlying relationship between the chemical properties, interfacial water structure, and hydrophobic interactions. Recently, experiments using an atomic force microscope (AFM) measured the hydrophobic force between self-assembled monolayers (SAMs) with mixed nonpolar and polar functional groups, creating chemically heterogeneous surfaces. This study found that amide functional groups reduced the magnitude of hydrophobic interactions to a greater extent than amine functional groups. Understanding how these specific chemical groups uniquely perturb the surrounding interfacial water molecules would inform how these groups change the hydrophobicity of the material.

In this study, we utilized classical molecular dynamics (MD) simulations to investigate how amine, amide and hydroxyl functional groups influence the hydrophobicity of mixed SAMs. We used umbrella sampling to directly measure hydrophobic forces between mixed SAMs with compositions that mimic the experiments. Using this technique, we reproduced the qualitative trends of the AFM experiments. We further applied indirect umbrella sampling to measure the hydrophobicity of mixed SAMs with the same functional groups but different surface patterns. We then leveraged the atomistic resolution of MD to relate the changes in hydrophobicity to perturbations in interfacial water structure caused by the specific chemical groups decorating the SAMs. We were able to relate changes in the tetrahedral behavior and hydrogen bond network of the water molecules at the SAM interface to changes of hydrophobicity. This new understanding describing how specific chemical groups perturb interfacial water structure provides important information about how materials tune hydrophobic interactions and could be used to inform the design of new materials.