Quantifying the interactions of non-polar solutes in water is essential for developing a molecular understanding of several colloidal and biological self-assembly processes, including colloidal crystallization, protein folding, and molecular recognition. Macroscopically, the hydrophobicity of a surface is characterized by a droplet contact angle. However, this approach is not feasible at the nanoscale (e.g. to characterize a protein surface), and molecular measures of hydrophobicity are needed. Further, in a variety of interesting cases, both polar and non-polar groups are presented on the surface, and when this chemical heterogeneity or pattern is nanoscopic, interfacial properties are non-additive. To address these challenges, we have developed a novel technique that uses biased molecular dynamics simulations to show that water density fluctuations (and not the average local density), specifically, the ease with which water can be displaced from the vicinity of a surface, is a quantitative measure of its hydrophobicity. Using this fluctuations-based measure, we investigate how the hydrophobicity of a surface changes due to chemical patterning, curvature and proximity to other surfaces. We further demonstrate how the properties of water at interfaces influence the thermodynamic driving force as well as the kinetics of self-assembly. Our results provide a computational tool for mapping the hydrophobicity of complex particles, with relevance to developing predictive strategies for their binding and aggregation.

Characterizing hydrophobicity at the nanoscale

246th National Meeting (2013)

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