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 solutio n. A number of different analyses have been used to examine the extent to w hich the alcohol modifies the behavior of neighboring water molecules. Thes e include radial distribution functions, specific order parameters designed to probe for ice- and clathrate-like local structures, and velocity and or ientational time correlation functions. The static structure of water aroun d the hydrophobic end of the alcohol was found to be essentially the same a s that found in bulk water; in particular, there was no evidence of clathra te-like cages around the hydrophobic end of the alcohol. Some enhancement o f the water structure was found in the vicinity of the alcohol hydroxyl gro up, with the hydrogen bonding network being closer to tetrahedral in the so lvation 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 t he solvation shell than in the bulk. Only in the case of water solvating th e hydroxyl group was the dynamical behavior found to vary between the diffe rent alcohols, with the dynamics getting slower along the series from metha nol 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.