Classical Molecular Dynamics on Composite Systems of Clathrate Methane-Water-Kinetic Inhibitor

摘要

Classical molecular dynamics simulations are performed for chlathrate-methane-water-inhibitor systems using different inhibitor/water ratios and a 51262cavity with its water constituents 'frozen' at their initial positions. Vinylpyrrolidone (VP), Vinylvalerolactam (Vval), L-Proline (LP), 1-Formylpyrrolidine (1FP), Dimethylaminoethyl methacrylate (DMAEMA) are used as inhibitors. The goal is to get some insights on the behavior of the systems in the early stages of chlathrate formation where the inhibitor is thought to perform its main inhibition effect. The Dreiding Force Field is used for methane and the inhibitor and SPC/E for both free water and water constituting a chlathrate. Before performing the simulations for the chlathrate systems, evidence was searched to find if the combination of force field used reproduced certain phenomena such as the cage effect for the methane-water system, and the agglomeration of water in chlathrate-like structures at the concentrations used with the 'frozen' cavity. The decision of fixing the crystal was based upon Hartree-Fock simulations where the inhibitor was found to adsorb on the cavity without altering it. The systems are analyzed via visualization of the trajectories, examination of the radial distribution functions and of the mean square displacements. In most cases the inhibitors are found to adsorb on the cavity via hydrogen bonds between pendant hydrogen atoms of the cavity and the carbonyl group of the inhibitor. While adsorbed on the crystal, the inhibitor also forms hydrogen bond with the initially free water showing evidence of the possible action of inhibitor via disrupting the chlathrate network and becoming part of the network itself. No waters are found to locate around the hydrophobic moiety, suggesting the hypothesis of a delay in the growth onset caused by the presence of the adsorbed inhibitor covering faces of the incipient cry' stals which are in this way refrained to keep nucleating by contact to other crystals.