CHEMICAL-REACTION VOLUMES IN MODEL FLUID SYSTEMS .1. HARD-SPHERE SOLVATION AND DIATOMIC DISSOCIATION PROCESSES

Analytical equation of state and cavity distribution function models are used to explore the thermodynamics of solvation and diatomic dissociation in hard-sphere fluid systems. Calculated free energies of solvation and dissociation are shown to agree with available computer simulation measurements. Derivatives of these free energies with respect to pressure yield partial molar solvation and reaction volumes as a function of solute size and solvent density over the entire vapor-liquid density range. The resulting volumes are found to depend significantly on pressure. At low pressure, excess partial molar volumes (relative to an ideal gas) are equal to the volume excluded to solvent centers by the solute. At high pressure these partial molar volumes decrease significantly, approaching, but never reaching, the solute volume (or volume change, for a dissociation reaction). Solvent structure is found to play a prominent role in determining solvation and reaction volumes. Hard-sphere model reactions display the same sort of pressure-dependent reaction volume behavior often found in experimental high-pressure chemical reactivity studies.