Free Energy Profile of Small Solute Molecules at the Free Surfaces of Water and Ice, as Determined by Cavity Insertion Widom Calculations
The solvation free energy profiles of 15 different small solutes, i.e., Ar, HF, H2O, H2S, NH3, CH4, CH2F2, CH2Cl2, CHCl3, methanol, formaldehide, formic acid, CO2, acetone, and acetonitrile have been calculated across the water/vapor and ice/vapor interfaces by means of Monte Carlo simulations and Cavity Insertion Widom (CIW) calculations. The CIW calculations have been performed using a new, efficiently parallelized algorithm, which can provide linear speedup on a large number of processors in a distributed memory system.

All the solutes considered show preference for being adsorbed at the surface of both liquid water and ice. The free energy gain of this adsorption relative to the isolated state of the solute molecule is found to be independent of the state (i.e., liquid vs. frozen) of the condensed phase, and it is larger for solutes that are able to form stronger or more hydrogen bonds with the water molecules at the surface. On the other hand, the free energy gain of the adsorption from the bulk liquid phase is found to be stronger for solutes exhibiting stronger amphiphilic character. The solvation free energy of all the solutes studied is found to be considerably higher in ice than in bulk liquid water. The results obtained have several implications on the chemistry of the atmosphere, which are also discussed in the paper.