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Show COLLEGE OF SCIENCE UNDERGRADUATE RESEARCH ABSTRACTS Jesse Spencer Riccardo Baron Valeria Molinero CROSSING THE LINE: THERMODYNAMICS OF METHANE HYDRATION TRAVERSING THE LIQUID-VAPOR INTERFACE Jesse Spencer (Riccardo Baron, Valeria Molinero) Departments of Chemistry and Medicinal Chemistry University of Utah The role of water in chemical and physical processes has been historically discounted as a diffusive medium in which other molecules interact while water produces little effect on them. Only recently has water become formally recognized as an active participant in such processes. Our study focuses on the thermodynamics of hydrophobic hydration of methane. This prototypical small hydrophobic molecule assembles with water to form clathrate hydrate crystals and it is also a model for hydrophobic biological ligands. Hydration of methane is special interest to the oil and gas industry as unwanted formation of methane clathrates in oil pipelines is associated with high revenue loses. Hydrophobic interactions are also important in biology. The process of binding of hydrophobic ligands to the hydrophobic site of an enzyme in solution may be decomposed into processes, which involve evacuation of water from the cavity, disruption of the water surface formed adjacent to the cavity and expelling the solute from the solution. Previous work shows that for methane this process is exothermic with a free energy minimum where methane is within the cavity. In this study w e address the contribution of the cavity toward dehydration of methane. Using molecular dynamics simulations of a monatomic water model proposed by Molinero et al. w e computed the complete profile of thermodynamic functions -enthalpy, entropy and free energy of hydration- as a function of the distance of the methane to the water/vapor interface. This computational experiment demonstrates that a monatomic model, which does not include electrostatics, can reproduce the hydration free energy of methane in excellent agreement with experimental values. Furthermore we show that methane will reside just outside the liquid surface, within the vapor/vacuum interface. In agreement with atomistic studies w e show that the process of hydrophobic hydration is exothermic and orders the structure of the surrounding water, decreasing its entropy. |
