Atomistic computer simulation of the clay-fluid interface in colloidal laponite

Monte Carlo and molecular dynamics computer simulations have been used to s tudy the structure and dynamics of the interlayer aqueous solution in a col loidal sodium laponite clay at 277 K. The system studied has a clay-clay sp acing of 34.06 Angstrom, and contains 1200 interlayer water molecules and 2 4 sodium counterions. The density profiles for interlayer species show two distinct layers of surface water as one moves away from the clay particles. The innermost of these layers is strongly oriented to form hydrogen bonds to the surface oxygen atoms. Radially averaged pair distributions have been calculated as a function of distance from the clay surfaces, and show that throughout our system the water structure is significantly perturbed from the bulk. In particular, we observe an increase in the second nearest-neigh bor oxygen-oxygen distance, similar to that reported for low-density water at 268 K [A. K. Soper and M. A. Ricci, Phys. Rev. Lett. 84, 2881 (2000)]. T he majority of the sodium counterions are fully hydrated by six water molec ules. These hydrated ions have a strong tendency to remain close to the sol id surfaces, as so-called "outer-sphere" complexes. However, we also observ e cations further from the clay sheets, in the diffuse layer. Diffusion of water and cations in the plane of the clay sheets is comparable to that in the bulk, but is significantly reduced normal to the clay sheets. (C) 2001 American Institute of Physics.