Effect of increasing pressure on excess volumes for cavities in TIP4P water

NPT simulation results on excess volumes computed by the direct method are presented for spherical cavities in TIP4P water. The cavity is created by defining an exclusion volume for water-oxygen. This volume gives a well defined contribution to the excess volume, while the contribution arising from the coupling of all interactions in the system depends on pressure and temperature. This partition is in agreement with Kirkwood-Buff integrals, which provide a useful analysis of excess volumes in terms of the cavity-solvent distribution function. Two main effects of increasing pressure along the isotherm of 298 K are investigated. One refers to comparison of qualitatively different behaviours observed when increasing the exclusion volume at a constant high pressure in comparison to atmospheric pressure. For a nanometric sized cavity, these lead to extrapolate positive and negative adsorption at the cavity surface, respectively at 8000 and 1 atm. A simple radial dependence of excess volumes is able to reproduce these features. The other effect concerns the variation of excess volume under the increasing of pressure in a wide range up to 10,000 atm at a fixed cavity radius. Results are presented for two cases corresponding to cavities that can host spherical solutes whose size are as large as a water molecule and slightly larger than a fullerene molecule. Curves obtained by fitting with heuristic models previously tested on pressure dependence of water density enable estimation of the slope. Except for low pressures, these estimated values appear to be generally consistent with those obtained from simulation results of compressibility.