Temperature dependence of the lateral hydrogen bonded clusters of molecules at the free water surface

Molecular dynamics simulation of the liquid-vapour interface of SPC/E water have been performed on the canonical (N,V,T) ensemble in twelve temperatures ranging from 300 to 500 K using systems of two different sizes in order to locate the percolation threshold of the lateral hydrogen bonding network of surface waters. This infinite network, present at ambient conditions, is found to break up at about 450 K, i.e., 200 K below the critical temperature of the model, and hence the breakup of this two dimensional hydrogen bonding network at the liquid surface well precedes that of the three dimensional hydrogen bonding network in the bulk phase. We found that surface percolation can be described as a random bond percolation, i.e., all water molecules can form up to 3 lateral hydrogen bonds with their neighbours in the surface layer, and the formation or breaking of any of these possible hydrogen bonds is independent of that of all the other ones.