Ion-specificity: from air-water interface to the free energetics of hydrophobic association

Abstract

Chemistry occurring at or near the aqueous interfaces influences various fields of processes and applications. With the development of computational resources, molecular dynamics (MD) simulations are widely used to complement the experimental results with the microscopic scale. The heart of this work is to study the stabilities of ions at different types of surface using MD. This work begins with examining the single-ion potential of mean force (PMF) at the air-water interface with various force fields. Surface stable ions, characterized as such by minima in PMF, induce larger interfacial fluctuations, conferring more covariance entropy approaching the interface. Smaller anions show no interfacial PMF minima, nor are the enhanced interfacial fluctuation. The difference is traced to hydration shell properties of the anions, and the coupling of these shells with distant solvent. The behavior of molecular guanidinium cation at the air-water surface is investigated next. Resulting PMF indicates that parallel orientations of the guanidinium plane (relative to the Gibbs dividing surface) exhibit pronounced minima, with significantly greater interfacial fluctuations, while the perpendicular orientations exhibit no discernible surface affinity. At the interface between a hydrophobic region of HFBII and the aqueous solvent, we find similar ion-specific signature at the liquid-vapor interfaces. Where strong local interactions are not dominant, as in the case of protein's hydrophobic surfaces that inherently hold higher fluctuations, the anions tend to differentiate themselves based on their charge density (or "hydrophobicity"). In the case of hydrophilic interfaces, the strong charge-dipole and charge-charge interactions dominate and equalize the stabilities and interface perturbing effects of both anions. This work next explores hydrophobic interaction between nonpolar solutes with the addition of inorganic salts. Iodide anion directly stabilizes the contact state to a much greater extent than chloride anion. Water is driven further away from the tube surface because of iodide anions segregating to the surface, which results in destabilizing contribution to the hydrophobic association. Results of this work highlight how impurities induce long-ranged solvent perturbations that propagate to different interfaces, along with the connections between induced surface fluctuations and the impurities' surface affinities/stabilities. Future extension of this work and additional applications is suggested.