(342f) Molecular-Level Order Modulates the Hydrophobic Interactions between Nonpolar Self-Assembled Monolayers

Understanding water-mediated interactions between functionalized interfaces is vital to the design of new, biologically relevant materials. Of particular interest is understanding the relationship between hydrophobic interactions – the water-mediated interactions that drive the association of nonpolar materials – and material parameters. Conventionally, the magnitude of hydrophobic interactions between extended material interfaces is attributed to interfacial chemical properties, such as the amount of interfacial solvent-exposed nonpolar surface area. However, it was recently experimentally demonstrated that the hydrophobic interactions between alkanethiol self-assembled monolayers (SAMs) depend on the length of the alkanethiol chains. This result is surprising because the monolayers are uniformly nonpolar, suggesting that the physical properties of the monolayers, and particularly ligand order, contribute to hydrophobic interactions acting between the surfaces. In this work, we use molecular dynamics simulations to investigate the effect of molecular-level order (crystallinity) on the hydrophobic interactions between nonpolar alkanethiol SAMs to explain the experimental observations. We observe that fluctuations of the alkanethiol ligands strongly correlate with ligand order measured by both simulations and experiments, and in agreement with experiments we measure an increase in the strength of hydrophobic attraction as SAM order increases. We explain these trends by showing that the molecular order of the SAM interface impacts the nanoscale structure of nearby water molecules, leading to new predictions regarding the temperature-dependence of the hydrophobic interactions that are validated experimentally. This study elucidates how hydrophobic interactions can be influenced by interfacial physical properties which may inspire the design synthetic materials with fine-tuned interfacial hydrophobicity.