Solvation structure around aqueous alcohols

Molecular dynamics simulations have been used to characterize the structure of water around n-alcohols (methanol to butanol) in dilute aqueous solution. A number of different analyses have been used to examine the extent to which the alcohol modifies the behavior of neighboring water molecules. These include radial distribution functions, specific order parameters designed to probe for ice- and clathrate-like local structures, and velocity and orientational time correlation functions. The static structure of water around the hydrophobic end of the alcohol was found to be essentially the same as that found in bulk water; in particular, there was no evidence of clathrate-like cages around the hydrophobic end of the alcohol. Some enhancement of the water structure was found in the vicinity of the alcohol hydroxyl group, with the hydrogen bonding network being closer to tetrahedral in the solvation shell than in bulk water; however, this enhancement was unaffected by the length of the associated hydrophobic chain. In contrast, the dynamic behavior of water was considerably modified near all parts of the alcohols, with both translational and rotational motion being slower for water in the solvation shell than in the bulk. Only in the case of water solvating the hydroxyl group was the dynamical behavior found to vary between the different alcohols, with the dynamics getting slower along the series from methanol to n-butanol. This was found to be largely due to inertial effects and not due to variations in the strength of the hydrogen bonding networks.