Abstract:
The equilibrium fluid-solid dihedral angle has been determined for H2O-CO2 fluids and Ar in halite polycrystals at 1-2000 bars and 300-600°C. The H2O-halite angle is a sensitive function of pressure and temperature and decreases monotonically with both parameters. The CO2-halite angle is higher than the H2O-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 H2O molecules on halite is significant and results in a negative excess entropy for the halite-brine interface. The variation of H2O-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 H2O on halite to between 1.0-2.4 molecules of H2O 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 H2O on halite surfaces at low temperatures in a partial vacuum.