Non-intrinsic contribution to the partial molar volume of cavities in water

In general the partial molar volume of a cavity in a liquid or that of a solute in a solvent can be dissected in an intrinsic contribution and a non-intrinsic one. It is shown that, if the intrinsic contribution is given by the van der Waals volume of the cavity, the non-intrinsic contribution is always a positive quantity in water, increasing with cavity size. This result has a general validity, being in line with the basic notion that there is a lot of void volume in liquids, and water, in particular, has a small volume packing density. In addition, it is pointed out that the recent claim [F.M. Floris, J. Phys. Chem. B 108 (2004) 16244] that the non-intrinsic contribution changes sign, passing from negative to positive values for sufficiently large cavities, is simply due to the choice of using the excluded volume of the cavity instead of the van der Waals volume as the intrinsic contribution.