Cavitation free energy

To the quantum mechanical contribution to the solvation free energy (4) it is also necessary to add the work required to create the cavity in which the solute is placed this quantity, namely the cavitation free energy, is calculated with the Pierotti-Claverie formula [16, 17]. 

Since Pierotti s theory was developed for solutes with spherical shape, its implementation to molecular-shaped cavities is performed by using the procedure proposed by Claverie (Eq. 4-7), [21] where the cavitation free energy of a given atom i is determined from the contribution of the isolated atom, AGcav,i, and a weighting factor, Wj, determined from the ratio between the surface of such an atom and the total surface of the sphere generated by that atom ... 

Another approach consists in assimilating the solvent to a macroscopic continuum which is characterized by some v ell-defined temperature dependent macroscopic quantities such as the dielectric permittivity at a given frequency m (a)), or the surface tension tj. In principle, the latter quantity would be used to evaluate the cavitation free energy as soon as the geometry of the cavity is defined 11 Nevertheless, the validity of the continuum model for such a determination is questionable since at the molecular level an averaged macroscopic quantity is somewhat meaningless. 

The solvation energy of the solute is, at least in part, due to the energy associated with the formation of such a cavity. This cavitation free energy is essentially the product of the area times a parameter having the dimensions of energy per unit area. Thus, the parameter is, in principle, a surface tension or interfacial tension, but on a molecular scale, and not necessarily equal to the macroscopic quantities. Such macroscopic concepts as area and surface tension have proven useful, even if not completely Justified, on the molecular level. 

As said at the beginning of this section, the simplest and most diffuse case where this method can be used is that of liquid crystals, in particular in their nematic phases. Such phases are generally approximated by rigid, rod-like particles. This picture should also allow the evaluation of a further contribution to solvation free energy in addition to the electrostatic one briefly described above, the cavitation free energy Ccav (see Section 3). Actually, its calculation in terms of standard Pierotti formulas leads to serious errors, as this formulation assumes a rigid spherical shape for solvent molecules. This difficulty has been overcome in the anisotropic versions of PCM cited above with an alternative formulation of the SPT in terms of particles with sphero-cylindric shapes. To the best of our knowledge, this PCM formulation is the only one ever done for this problem. 

Classic nucleation theory must be modified for nucleation near a critical point. Observed supercooling and superheating far exceeds that predicted by conventional theory and McGraw and Reiss [36] pointed out that if a usually neglected excluded volume term is retained the free energy of the critical nucleus increases considerably. As noted by Derjaguin [37], a similar problem occurs in the theory of cavitation. In binary systems the composition of the nuclei will differ from that of the bulk... 

Part of the motivation behind so straightforward an approach derives from its ready application to certain simple systems, such as the solvation of alkanes in water. Figure 11.8 illustrates the remarkably good linear relationship between alkane solvation free energies and their exposed surface area. Insofar as the alkane data reflect cavitation, dispersion, and the hydrophobic effect, this seems to provide some support for the notion that these various terms, or at least their sum, can indeed be assumed to contribute in a manner proportional to solvent-accessible surface area (SASA). 

Fuciarelli AF, Sisk EC, Thomas RM, Miller DL (1995) Induction of base damage in DNA solutions by ultrasonic cavitation. Free Radical Biol Med 18 231-238 Fulford J, Bonner P, Goodhead DT, Flill MA, O Neill P (1999) Experimental determination of the dependence of OH radical yield on photon energy a comparison with theoretical simulations. J Phys Chem A 103 11345-11349... 

Inspection of the free energy components points out the dominant role of AGeie in water, which amounts to around 145% of the experimental hydration free energy (Table 4-1). In turn, the non-electrostatic term, AGn-eie, gives rise to an unfavorable contribution to the hydration of these compounds, which reflects the larger magnitude of the cavitation term compared to the van der Waals one in water [15], Overall, except for hydrocarbons, the transfer of polar solutes from the gas phase to water is a favorable process, which mainly originates from the electrostatic interactions between solute and water molecules. 

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