(178u) A Theoretical Study of the Hydration of Methane

The formation of methane hydrates represents a problem for natural gas production, transportation, and processing, as it may clog up the pipelines, with potentially catastrophic consequences. Two types of inhibitors are used[1] to prevent the occlusion: thermodynamic, that shift the liquid-hydrate boundary on the phase diagram by altering the chemical potential, and kinetic, that are designed either to delay the nucleation or to maintain aceptable rheological properties with the crystals that may grow. The choice of an inhibitor for a specific situation, and eventually its chemical modification or design, requires a better understanding of the gathering of water molecules to encage individual gas molecules and of the accretion of the crystal. Whereas the shapes and the number of water molecules of the gas-containing cavities in the crystal structures of gas hydrates are well established, the same is not true for the ordering of water molecules around non-polar solutes in aqueous solution. Though the number of water molecules around the methane molecule has been determined to be 20 from a MAS-NMR study[2], an analysis of a large number of hydration shells obtained from numerical simulations[3], it was concluded that the probability of occurrence of a 5¹² cage around methane in aqueous solution is much less than 10⁷. Because the results from numerical simulations depend on the force-field employed, in this work a similar analysis is presented, but using a novel four-site water model (TIP4Q) combined with three different models for CH₄, one of them being a united-atom model. The all-atom models are used to estimate the rotational barrier of CH₄ within the dodecahedral cage. Furthermore, the study is extended to analyze the interactions between two hydrated methane molecules as a function of their distance.