On the cavity size distribution in water and n-hexane

The cavity size distribution functions in water and n-hexane were determined by Pohorille and Pratt, in a series of important works, from molecular dynamics simulations. These functions are considered as experimental data. In the present investigation the ability of scaled particle theory in reproducing such distributions is tested. In the case of water the scaled particle theory results compare favorably with the experimental distribution if a proper choice of the size to be assigned to water molecules is performed. Specifically, a slight size increase from 2.70 to 2.80 A is necessary to reach agreement for the largest cavities detected by Pohorille and Pratt. In the case of n-hexane the scaled particle theory results do not agree with the experimental distribution especially in the region of small cavities. This deficiency is because a n-hexane molecule cannot be realistically treated as a single spherical exclusion volume. The implications of such findings are analyzed and discussed in depth.