Stabilizing Dipolar Interactions Drive Specific Molecular Structure at the Water Liquid-Vapor Interface

QA Besford and MY Liu and AJ Christoffersonk, JOURNAL OF PHYSICAL CHEMISTRY B, 122, 8309-8314 (2018).

DOI: 10.1021/acs.jpcb.8b06464

Using molecular dynamics simulations we probe the structure and interactions at the water liquid-vapor (LV) interface. In the interfacial region, strong ordering of dipole moments is observed, where water molecules exhibit "frustrated" orientations. By selectively analyzing the dipolar potential of mean force between these frustrated molecules and other molecules, we find a significant enhancement of dipolar interactions across the interfacial region. This interaction is derived in terms of a component of the surface tension, with a temperature-dependent magnitude of similar to-20 mN m(-1), representing a stabilizing interaction at the interface. This stabilization has the same magnitude, but opposite sign, to the surface tension of alkanes and short-chain alcohols. Our results highlight a mechanism by which interfacial waters recover lost free energy from an absence of van der Waals interactions in the vapor region and likely explains the driving force for specific water structure at the LV interface.

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