(346bt) Utilizing Molecular Dynamics Simulations to Predict the Modulation of Interfacial Hydrophobicity By Chemical and Physical Properties

Authors: 
Dallin, B. C., University of Wisconsin-Madison
Van Lehn, R. C., University of Wisconsin-Madison
Predicting hydrophobic interactions is a complex challenge because of the cooperative and competing effects on hydrophobicity that arise from the interfacial chemical and physical properties of functionalized materials. The current understanding of hydrophobicity at the nanoscale relates hydrophobic interactions, surface properties, and the structure of interfacial water molecules. Developing experimentally validated simulation models that accurately capture these relationships is necessary to predict how surface properties modulate hydrophobic interactions. This type of predictive framework would be valuable for the design of materials that control processes driven by hydrophobic interactions, such as amphiphile self-assembly, nano-bio interactions, and peptide-surface binding.

In this study, we utilized classical molecular dynamics (MD) simulations with enhanced sampling techniques to explain the experimentally observed modulation of interfacial hydrophobicity by the physical and chemical properties of self-assembled monolayers (SAMs). First, we used this methodology to explain how the molecular-level order of uniformly nonpolar SAM alters hydrophobic interactions by perturbing interfacial water structure. Next, we applied this methodology to SAMs with polar and nonpolar groups mixed on the surface (chemically heterogeneous SAMs) to understand how specific chemical groups modulate hydrophobicity. This study revealed how each group uniquely perturbs the interfacial water structure and hydrogen bonding network, resulting in changes to the hydrophobicity of the SAMs. Finally, using the insights gained from the previous two studies, we modeled small (< 5 nm radius) SAM-protected gold nanoparticles (AuNP) to understand how both physical and chemical heterogeneities cooperatively and/or competitively influence hydrophobicity. The physical heterogeneity arises from the curvature of the AuNP whereas the chemical heterogeneity arises from the different chemical groups decorating the surface of the AuNP. We developed an analysis technique to map hydrophobicity onto the complex geometries of the AuNPs to understand how these features perturb the interfacial water structure and hydrophobicity. The results from our study provide new insights into the relationship between interfacial chemical and physical properties and hydrophobicity that can be used to guide the design of functionalized materials.