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.