Molecular Dynamics Study of Kinetic Hydrate Inhibitors: The Optimal Inhibitor Size and Effect of Guest Species

We propose a model for slowing down of clathrate hydrate formation caused by kinetic hydrate inhibitors (KHIs) on the basis of the Gibbs-Thomson effect. The residence time of inhibitor molecules bound to the hydrate surface and the intrinsic growth rate of the clathrate hydrate without KHIs are key ingredients of the model. The binding free energies of the monomer, dimer, tetramer, and octamer of a KHI, polyvinylcaprolactam (PVCap), are calculated using molecular dynamics simulations to estimate the residence times, which are far beyond the feasible simulation time. Our model accounts for the kinetic inhibition mechanism while reproducing experimental data of the size dependence of PVCap very well. We demonstrate that this model explains why blends of high and low-molecular-weight polymers show better performance than the KHI with a unimodal molecular weight distribution and why quaternary ammonium cations are good KHIs for tetrahydrofuran hydrate although they cannot inhibit formation of natural gas hydrates.