Insights into Microscopic Diffusion Processes at a Solid/Fluid Interface under Supercritical Conditions: A Study of the Aqueous Calcite (10(1)over-bar4) Surface

CY Lu and D Perez and DD Hickmott and AF Voter, JOURNAL OF PHYSICAL CHEMISTRY C, 116, 25934-25942 (2012).

DOI: 10.1021/jp310391d

The diffusion of adsorbed molecules at a solid/fluid interface is a phenomenon of great fundamental interest and technological importance. We use molecular dynamics and kinetic Monte Carlo simulations to investigate the diffusion processes at an aqueous calcite (10 (1) over bar4) interface under various geologically relevant supercritical conditions 101.3 MPa (1000 atm), 300-800 K. Between 600 and 700 K, the adsorption configuration changes from an outer- to inner-sphere surface complex, and the desorption free energy barrier, Delta G(d), increases greatly. The ratio Delta G(d)/k(B)T takes a minimum value at 600 K, making it the temperature with the greatest desorption tendency. Consequently, the temperature dependence of the mean lateral diffusivity of a finite bulk system reverses at 600 K. More interestingly, the system exhibits typical characteristics of bulk fluid-mediated surface diffusion. For example, the mean-squared displacement shows different scaling properties with changes in temperature or bulk thickness. The optimized bulk thickness which maximizes the effective surface diffusivity was also found to vary with temperature.

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