Structure and dynamics of forsterite-scCO₂/H₂O interfaces as a function of water content

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• 作者：Sebastien Kerisit^a ; ^{sebastien.kerisit@pnnl.gov} ; John H. Weare^b ; Andrew R. Felmy^a
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2012
• 期刊代码：27_00167037
• 类别：ear
• 出版时间：1 May, 2012
• 卷：84
• 期：Complete
• 页码：137-151
• 文件大小：1941 K

摘要

Molecular dynamics (MD) simulations of forsterite surfaces in contact with supercritical carbon dioxide (scCO₂) fluids of varying water content were performed to determine the partition of water between the scCO₂ fluid and the mineral surface, the nature of CO₂ and H₂O bonding at the interface, and the regions of the interface that may be conducive to formation. Calculations of the free energy of the associative adsorption of water onto the (0 1 0) forsterite surface from the scCO₂ phase indicated that the formation of a water film up to three-monolayer thick can be exothermic even for water contents below the water saturation concentration of the scCO₂ fluid. In MD simulations of scCO₂/H₂O mixtures in contact with the (0 1 0) forsterite surface, H₂O was found to readily displace CO₂ at the surface and, therefore, CO₂ directly contacted the surface only for water coverages below two monolayers. For thicker water films, a two-monolayer hydration layer formed that CO₂ could not penetrate. The MD simulations thus suggest that, in the presence of sufficient water, formation occurs in the water films and not via direct reaction of CO₂ with the forsterite surface. Simulations of the hydroxylated (0 1 0) surface and of the (0 1 1) surface suggested that this conclusion can be extended to forsterite surfaces with different surface structures and/or compositions. The density, diffusion, and degree of hydration of CO₂ as well as the extent of CO₂/H₂O mixing at the interface were all predicted to depend strongly on the thickness of the water-rich film, i.e., on the water content of the scCO₂ fluid.