(247c) Spatially Varying Chemical and Physical Properties Modulate Interfacial Hydrophobicity of Self-Assembled Monolayers

Water, while simple in structure, contains many unusual physicochemical properties such as a distinct polar charge distribution which leads to the formation of hydrogen bonds. In solutions, these strong water-water interactions lead to relatively weak or unfavorable interactions with nonpolar solutes which indirectly drives the association of hydrophobic materials. This indirect driving force is known as a hydrophobic interaction (HI) which is critical to protein folding/misfolding, ligand-substrate binding, and colloidal self-assembly. Understanding the relationship between HIs and the properties of functionalized surfaces is vital to the design of new materials that enhance control over complex self-assembly and adsorption in aqueous environments. Conventionally, the strength of HIs between surfaces is attributed to interfacial chemical properties, such as the available nonpolar solvent-accessible surface area. However, recent experiments demonstrated that HIs between functionalized self-assembled monolayers (SAMs) with similar areas depended on specific chemical and physical properties, such as functional group chemistry, surface patterning, and SAM order (crystallinity).

Experiments using an atomic force microscope (AFM) revealed specific chemical and physical properties have unexpected effects on the strength of HIs. In this work, we used molecular dynamics simulations to investigate with atomistic resolution the origin of those effects. We found that the hydrophobicity of SAMs can be altered by changing interfacial properties (e.g., alkyl chain length, functional group chemistry, and ligand composition). Specifically, we observed functional group chemistry modulates the hydrophobicity of the adjacent nonpolar domains in mixed SAMs. Further, we investigated the roles of molecular fluctuations and SAM order to determine the key component influencing nonpolar SAM hydrophobicity. We find that spatially varying SAM order leads to interfaces that appear chemically heterogeneous. We identified that perturbations to the nanoscale structure of nearby water molecules can explain the effects of interfacial properties on hydrophobicity. These perturbations were then quantified to provide a mechanism to explain how spatially varying interfacial properties modulate hydrophobicity. These physical insights can then be applied to new experimental systems to predict interfacial hydrophobicity.