Potential of Mean Force for Ca2+ at the Cleaved Mica-Water Interface

Monte Carlo simulations reveal that in the water coverage range of 0-28 H2O/Ca2+, Ca2+ is specifically adsorbed above tetrahedral substitutions of Si by Al at distances of 1.53-1.72 A from the cleaved mica surface. A random distribution of Ca2+ on mica is found to be strongly unfavorable as compared to the regular one. At the highest simulated water coverage, inner-sphere adsorption of Ca2+ at 1.72 angstrom is more favorable by 121 kJ/mol than its outer-sphere adsorption at 3.99 angstrom. The activation energy for Ca2+ desorption from the mica surface is calculated to be 144 kJ/mol, which is lower by similar to 20 kJ/mol and higher by similar to 60 kJ/mol than recently reported values for Mg2+ and K+ desorption, respectively.