(57c) Role of Solvent Additives in the Hydrophobic Hydration and Association of Methane Molecules

In the hydrophobic hydration of molecules such as methane, often the picture is one of  a clathrate-like cage forming around the hydrophobic solute. Here we evaluate this picture in the context of additives such as tetrahydrofuran that are known to encourage gas-hydrate formation. Indeed, the ability of THF as a facile promoter of methane-hydrate formation has made THF-induced  enclathration of much utility in studying gas-hydrates.  To explore the molecular role of such additives, we calculate the hydration free energy and potential of mean force between two methane molecules in pure water and in aqueous solutions containing various additives such as THF, methanol, urea, tetramethylamine n-oxide (TMAO), and mixtures of urea and TMAO. All the calculations are founded on a newly developed framework of obtaining thermodynamic properties of hydration based on regularizing the solute-system binding energy distribution. This approach gives a transparent accounting of the contributions from the solute-medium interactions within a defined local domain (here approximately the size of a hydrate cage), the packing contributions accounting for evacuating the local domain in a solution but without the solute, and long-range interactions of the solute with the material outside the local domain.  Results from this analysis will be discussed.