Electrostatic enthalpy mixing

The Case of g < 1 The task is now to obtain an expression for the (electrostatic) enthalpy change upon mixing ng moles of polymeric salt (in the presence of ng j moles of simple salt) with an additional amount of simple salt, ng Q(] (j, so that the ratio of (total) simple salt to polymer equivalents passes from... 

It has been proposed that this mixing enthalpy is of purely electrostatic origin representing a slight modification of the environment near the ions. We can, for instance, envisage dissolution of silver iodide in silver phosphate schematically as follows... 

The last term in the formula (1-196) describes electrostatic and Van der Waals interactions between atoms. In the Amber force field the Van der Waals interactions are approximated by the Lennard-Jones potential with appropriate Atj and force field parameters parametrized for monoatomic systems, i.e. i = j. Mixing rules are applied to obtain parameters for pairs of different atom types. Cornell et al.300 determined the parameters of various Lenard-Jones potentials by extensive Monte Carlo simulations for a number of simple liquids containing all necessary atom types in order to reproduce densities and enthalpies of vaporization of these liquids. Finally, the energy of electrostatic interactions between non-bonded atoms is calculated using a simple classical Coulomb potential with the partial atomic charges qt and q, obtained, e.g. by fitting them to reproduce the electrostatic potential around the molecule. 

For alkali metal cations—with the one exception of the Li" ion, whose solvation is a mix between electrostatic and covalent interactions—classical electrostatic calculations can account quite well for the observed enthalpies of gas-phase solvation. For instance, hydration can be split up into energy contributions from ion-dipole induced dipole dispersion (E g), and repulsive (E - p) contributions (18) (Table 8). Clearly, the dominant term is the Coulombic attraction between the cation and the water dipoles, which decreases in magnitude as the ion gets bigger. 

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