Non-electrostatic contributions

Classification of continuum models that include both electrostatic and non-electrostatic contributions. 

The second method of estimation which has so far been developed is based on consideration of those AC values which were available in 1967 when Helgeson developed it (74). This method essentially separates electrostatic and non-electrostatic contributions to AS° and AC and Helgeson has compared the predictions of a number of different assumptions concerning AC° with published high temperature equilibrium constants. He concluded that the best one to make is that ACp is proportional, at each... 

More recently, Abraham and Johnston26 have attempted to put the above comparisons on a quantitative basis. Following the procedure of Alfenaar and de Ligny27, they divided AG values for transfer from methanol to aqueous methanol into a non-electrostatic contribution and an electrostatic contribution, viz. 

The electrostatic solvation energy is only a part of the total solvation energy. Cavitation, dispersion and repulsion terms must be added. We show below that the MPE method leads to similar electrostatic energies than the polarizable continuum model (PCM) of Tomasi and co-workers [10], provided the same cavities are used. Therefore, non-electrostatic terms in these methods may be computed using the same computational strategies [15]. We emphasize the fact that accurate non-electrostatic contributions are often difficult to compute since they are based on parameterized formulae that cannot be directly compared to experiment. The obtained data must therefore be used with prudence, especially if they are expected to play a major role in the process under study. Fortunately, in many circumstances, non-electrostatic terms are small and/or vary little, so that they can be neglected. Tunon et al. [80] developed a parameterized expression for the MPE method using an expression of the type... 

Previous studies have shown that there is a correlation between the enthalpy of hydration of alkanes and their accessible surface area [30,31] or related magnitudes. Moreover, relationships between the hydration numbers calculated from discrete simulations for hydrocarbons and both the free energy and enthalpy of hydration of these molecules have also been reported [32] and have been often used to evaluate solvation enthalpies. Analysis of our results, illustrates the existence of a linear relationship between A//n eie and the surface of the van der Waals cavity,. SVw, defined in MST computations for the calculation of the non-electrostatic contributions (Figure 4-1). In contrast, no relationship was found for the electrostatic component of the hydration enthalpy (A//eie data not shown). Clearly, in a first approximation, one can assume that the electrostatic interactions between solute and solvent can be decoupled from the interactions formed between uncharged solutes and solvent molecules. 

AG (H ) or AG (C1 ). Later on, Alfenaar and de Ligny determined the non-electrostatic contribution to AG from the solubilities of analogous uncharged species of the same radius (as e.g. noble gases) and extrapolated the free energy of transfer of an electrolyte conected by the non-electrostatic contribution of the variable counterion as function of the reciprocal radius to r = 0. The authors attempted, furthermore, to improve the method by adding to the electrostatic and non-electrostatic terms of AG further terms proportional to r and which account for ion-dipol and ion-quadrupole interactions ... 

The electrostatic part of the solvent shift was determined by performing the SVPE calculations at the HF/6-31-i-G(d) level, whereas the non-electrostatic contribution was determined by the PCM calculation... 

Total contribution of short-range non-electrostatic solute-solvent interactions Gibbs free energy barrier in aqueous solution calculated as the AG(gas) value plus the electrostatic solvent shift determined by the SVPE calculation, without or with the non-electrostatic contributions determined by the PCM calculation... 

As seen in Table 2, the solvent effects are crucial for calculating realistic free energy barriers and, not surprisingly, the calculated solvent shifts are dominated by the solute-solvent electrostatic interactions. The estimated short-range non-electrostatic contributions to the free energy barriers are negligible compared to the electrostatic contributions to the solvent shifts. 

The two terms for the non-electrostatic contribution for the contact value pi(R/2) are slightly different from the corresponding expressions (29) and (30). However, for the low ion concentrations this difference is not so important. The principal defect of the HAB approach is connected with the difficulty to reproduce the electrostatic contribution which for the contact value should give the expression corresponding to (31). 

Low-coverage volumetric isotherm data [41, 42] were used to extract the isosteric heat of adsorption extrapolated to zero coverage, the gas-solid virial coefficient, and the two-dimensional second virial coefficient. It is found that fitting the two-dimensional virial coefficient obtained from the measurements in the vicinity of the surface in terms of Lennard-Jones inter-molecular potentials reduces the well depth obtained from the bulk by about 20%. In addition, the effective quadmpole moment of CO needs to be significantly reduced [42] by as much as about 50%. These fitted parameters are believed to account for various substrate mediation effects in some effective way (see Refs. 41 and 42 for more details concerning these param-eterizations). It is also concluded [41] that the asymmetric empirical parameterization of Ref. 238 should be replaced by models in which the similarity between the isoelectronic CO and N2 molecules is exploited for the non-electrostatic contributions as in Ref. 17. 

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