Statics and dynamics of free and hydrogen-bonded OH groups at the air/water interface

We use classical atomistic molecular dynamics simulations of two water models (SPC/E and TIP4P/2005) to investigate the orientation and reorientation dynamics of two subpopulations of OH groups belonging to water molecules at the air/water interface at 300 K: those OH groups that donate a hydrogen bond (called "bonded") and those that do not (called "free"). Free interfacial OH groups reorient in two distinct regimes: a fast regime from 0 to 1 ps and a slow regime thereafter. Qualitatively similar behavior was reported by others for free OH groups near extended hydrophobic surfaces. In contrast, the net reorientation of bonded OH groups occurs at a rate similar to that of bulk water. This similarity in reorientation rate results from compensation of two effects: decreasing frequency of hydrogen-bond breaking/formation (i.e., hydrogen-bond exchange) and faster rotation of intact hydrogen bonds. Both changes result from the decrease in density at the air/water interface relative to the bulk. Interestingly, because of the presence of capillary waves, the slowdown of hydrogen-bond exchange is significantly smaller than that reported for water near extended hydrophobic surfaces, but it is almost identical to that reported for water near small hydrophobic solutes. In this sense water at the air/water interface has characteristics of water of hydration of both small and extended hydrophobic solutes.