The structure of the halite-brine interface inferred from pressure and temperature variations of equilibrium dihedral angles in the halite-H 2OCO 2 system
The equilibrium fluid-solid dihedral angle has been determined for H 2OCO 2 fluids and Ar in halite polycrystals at 1-2000 bars and 300-600°C. The H 2O-halite angle is a sensitive function of pressure and temperature and decreases monotonically with both parameters. The CO 2-halite angle is higher than the H 2O-halite angle at all pressures and temperatures examined, shows no significant pressure dependence, and is indistinguishable within error from the argon-halite angle. Simple thermodynamic analysis of the variation of dihedral angle with pressure and temperature shows that the surface activity of H 2O molecules on halite is significant and results in a negative excess entropy for the halitebrine interface. The variation of H 2O-halite dihedral angle with pressure shows that the excess thickness of the interface is negative in the vapour field and positive in the liquid field. From this we constrain the adsorption density of H 2O on halite to between 1.0-2.4 molecules of H 2O per surface unit cell of halite. This is in agreement both with possible hydration numbers for NaCl under these conditions and with observations of the adsorption density of H 2O on halite surfaces at low temperatures in a partial vacuum.