An improvement in the theory of regular solutions

In constructing the theory of regular, nonpolar, binary solutions, the partial molal energy of mixing has been expressed by the square of difference between "solubility parameters," usually evaluated by the molal energy of vaporization per cubic centimeter, (DeltaE(v)/V)(1/2) = delta, at approximately 25 degrees C. We have found, however, that liquids whose molecules contain methyl groups do not conform to these parameters; to account for their solvent power for iodine requires parameters increased in proportion to the number of methyl groups per molecule. The anomaly may be attributed to the absence of outer nonbonded electrons able to exert London attractive force between neighboring molecules. Parameters derived from solubility for iodine in both methyl-containing liquids and fluorochemicals greatly improve the predictive power of the theory of regular solutions.