Swimming in Threes

Abstract

Chemistry Understanding a liquid at the molecular level is a rather daunting prospect—even small droplets contain a number of molecules too vast for most people to easily imagine, let alone analyze. It turns out, though, that you can get a decent sense of many important properties by thoroughly analyzing the interaction of just a pair of molecules, and then extrapolating that interaction across the whole sample. Pieniazek et al. point out one instance where, although this approach falls short, consideration of three molecules' mutual interaction affords a much better match to experimental data. Specifically, they simulate the structure of a water surface and its associated response to a highly surface-selective vibrational probing technique (sum frequency generation). Past experimental studies in this vein, using isotopically labeled HOD-in-D2O solutions to simplify spectral interpretation, have produced puzzling features attributed to distinct ice-like and bulk liquid–like local arrangements at the interface. The three-body simulations reproduce the experimental data reasonably well and suggest no need to invoke ice-like ordering. J. Am. Chem. Soc. 133 , 10.1021/ja2026695 (2011).