Simulation of Adsorption Isotherms of Water on Ionic Surfaces

Abstract

The intention of this work is to aid the construction of a water model capable of describing bulk water in contact with ionic surfaces. The model should be sufficiently simple to be useful for simulating large systems. Here, the extended simple point charge (SPC/E) water model, which is one of the most commonly used descriptions of bulk water, was tested in a strongly heterogeneous system. Simulations were carried out on the NaCl(100) and the calcite(10¯14) surfaces, described in terms of Lennard-Jones and Coulomb potentials. On the NaCl surface Molecular Dynamics simulations show that SPC/E water forms clusters due to its high permanent dipole moment. Better agreement with the experimental adsorbate structure was achieved by reducing the water dipole moment to its gas phase value. On the calcite surface adsorption isotherms were simulated using a Molecular Dynamics-Monte Carlo hybrid technique, which consists of a normal Molecular Dynamic simulation with randomly chosen insertion and removal Monte Carlo steps repeated after a constant time period. The method is tested by calculating adsorption isotherms for two Lennard-Jones systems, i.e. methane in contact with the graphite basal plane and with the zeolite silicalite (ZSM5), for which we obtain excellent agreement with the experimental isotherms. For calcite, however, the simulated adsorption isotherms again show a strong dependence on the choice of the dipole moment. The SPC/E value yields a strongly exaggerated coverage, whereas the gas phase value severely underestimates the coverage in comparison with the experiment. From our results we draw the conclusion that a fixed charged model is not recommendable for the simulation on ionic surfaces, and that it is highly desirable to include variable polarization.