Computer Simulation Study of <i>tert</i>-Butyl Alcohol. 2. Structure in Aqueous Solution

Abstract

MD simulations of tert-butyl alcohol (TBA), dissolved in water, are carried out. Two aqueous alcohol solutions are studied, a dilute 2 mol % and a more concentrated, 8 mol % solution. Two new potential models are used for TBA, a simple rigid three-site model and a flexible all-atom 15-site model. In solution, the flexible TBA model is dissolved with flexible SPC water molecules, while the rigid tert-butyl alcohol is simulated in rigid SPC/E water. This study principally focuses on the hydration structure around TBA and a possible self-association of TBA due to the strong amphiphilic character of this bulky molecule. In the more concentrated solutions and for both TBA models, small aggregates of a few alcohol molecules are formed spontaneously, persisting several tens of picoseconds. Tail-to-tail pairwise configurations of alcohols are preferred. In the dilute solution, little tendency to aggregation is observed during the simulation. The diffusion of the alcohols is reduced considerably as the concentration of the solution increases. Water structure is significantly enhanced upon the addition of TBA. The hydration structure of both models of TBA is markedly different from that of methanol, indicating that the steric bulk of a tert-butyl group can have large, though indirect, effect on hydrogen bonding.