Solvation of Na+ and Cl- at the water-platinum (100) interface

The structural, energetic, and dynamical aspects of the solvation of Na+ and Cl− at the water–plantinum (100) interface are investigated by molecular-dynamics computer simulation. Although the structure of interfacial water is significantly different from that of bulk water, the structure of the ion–water solvation complex at the interface closely resembles that in the bulk. The free energy of adsorption is calculated as a function of the distance from the metal. It is nonmonotonic and is qualitatively very different for Na+ and Cl−. The shape of the free-energy curve can be explained in terms of solvation structure and the local perturbation of the interfacial water structure. The reorientation dynamics of water near the ion show that the structure-breaking effect of Cl− at the interface is much more significant than in the bulk, but that Na+ slows down water reorientation both in the bulk and at the interface. Collective solvent dynamics, as measured by equilibrium fluctuations of solvent–ion electrostatic energy, are surprisingly similar in the bulk and at the interface, in marked contrast with what continuum models predict. The mobility of ions near the interface is much less than it is in the bulk, which is also in disagreement with models which do not take into account the existence of a strongly adsorbed water layer.