Combining molecular dynamics simulations and transition state theory to evaluate the sorption rate constants for decanol at the surface of water

Molecular dynamics simulation of a decanol molecule near the surface of water is used to investigate T the adsorption and desorption kinetics of decanol at the interface. The decanol molecule, modeled using the OPLS potential functions, is simulated in a film consisting of 700 SPC/E water molecules. The change in potential of mean force (PMF) during transfer of decanol from the bulk of the aqueous phase to the air/water interface is calculated to be -9.5 kcal/mol. In addition, the PMF reveals the existence of an activation free energy barrier of 2.1 kcal/mol for the adsorption process, The orientational distribution function for decanol and the concentration profiles of the atoms within the molecule reveal that decanol is constrained in its average orientation as it inserts itself onto the surface of water. At the peak of the activation barrier, decanol is, on average, constrained to lie parallel to the surface due to an effective repulsion between the decanol and near-surface water molecules. By using transition state theory and the PMF, we calculate a desorption rate constant of 9.7 x 10(2) s(-1). Fluctuations in the interactions of the decanol with the water, however, lead to a recrossing of the activation free energy barrier, thus resulting in a substantial reduction in the overall desorption rate. A value of 0.046 was calculated for the Grote-Hynes transmission coefficient. The overall desorption rate is calculated to be 45 s(-1), in surprisingly good agreement with experimental estimates (12-220 s(-1)). The adsorption rate constant is calculated to be 3.4 x 10(8) s(-1).