Strong dipole parameter

An alternative approach uses the polarity index, P proposed by Snyder. This is based upon experimentally determined gas chromatographic retention of three test solvents on a large number of stationary phases. The test solvents selected are ethanol, 1,4-dioxane and nitromethane. As well as an overall polarity index (P), three other parameters are calculated, Xe (a proton acceptor parameter), xproton donor parameter) and x (a strong dipole parameter). 

Hansen [137-139], and later van Krevelen [114] proposed the generalization of the solubility parameter concept to attempt to include the effects of strong dipole interactions and hydrogen bonding interactions. It was proposed that the cohesive energy density be written as the sum of three terms, viz. 

Xi will represent a selectivity parameter for measuring the proton donor character i = d), the proton acceptor character i = e), and the strong dipole character (i = n). The sum of the three parameters is thus normalized to 1. Polarity index and selectivity parameters for some solvents are listed in Table 2. Using this approach, solvents that are chemically similar yield similar selectivity parameters, but solvents that are chemically different may appear in a single selectivity group. Some discrepancies can occur between the solvent classification according to the solubility parameters and to the polarity... 

Lennard-Jones parameters not given because the strong dipole-dipole terms are treated separately (see text). 

Although the potential energy functions can be made to reproduce thermodynamic solvation data quite well, they are not without problems. In some cases, the structure of the ion solvation shell, and in particular the coordination number, deviates from experimental data. The marked sensitivity of calculated thermodynamic data for ion pairs on the potential parameters is also a problem. Attempts to alleviate these problems by introducing polarizable ion-water potentials (which take into account the induced dipole on the water caused by the ion strong electric field) have been made, and this is still an active area of research. 

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