Adsorption of a Sodium Ion on a Smectite Clay from Constrained Ab Initio Molecular Dynamics Simulations

We have performed ab intitio molecular dynamics (MD) simulations to study the adsorption of a sodium ion onto the surface of a smectite clay. In a previous study using Car-Parrinello MD simulations [Boek and Sprik, J. PhYs. Chem. B 2003, 107, 3251], spontaneous adsorption was not observed. To study this process in more detail, we have used constraint MD simulations to sample regions of the phase space inaccessible to standard MD methods. We have chosen a reaction coordinate and constrained the value over a certain range to force adsorption. Using this method, we have calculated potentials of mean force for the process. We also have obtained, for the first time, the free-energy differences between a fully solvated "outer sphere" complex, and the sodium that is bound to the surface (an "inner sphere" complex). These complexes have been identified in previous experiments and simulations using classical potentials. We found that the lowest free-energy region exists at a distance of 6.1 angstrom away from the center of the clay layer, between the "inner-sphere" and outer-sphere" complexes. In this state, the sodium is bound to only one surface oxygen atom. The stabilization of this region is greater when the sodium adsorbs onto a Si5Al ring.