Unification of angle-averaged induction interactions

If no ion is present in the system (q, = 0) then the above expression gives the sum of the Keesom-dipolar orientation and Debye-induction contributions to the total van der Waals forces between two molecules. A third contribution to van der Waals forces is also present, 

Equation (62) can be used to compare the relative magnitudes of ionic, dipolar, and induction contributions to a molecular interaction, and a ratio of 800 3 1 was calculated, respectively, when qx = 1 e, = fi2 = 1D, a, = a2 = (47t 0)3 x 10 30m3, and a molecular separation distance of r = 0.5 nm at T = 300 K, were selected for an indicative example. This shows the importance of the presence of an ion in a medium. Equation (62) also shows the molecular volume effect such that the efficiency of a dipolar interaction depends on (fd2lr6 (fd/V)2). rather than the absolute value of the dipole moment. 

The unified expression can be used while commenting on very practical situations in solvents. Assume that two spherical solute molecules 1 and 2 having dielectric constants of j and e2 and radii of and a2, respectively, are present in a solvent medium having a dielectric constant of r. What will be the net resultant interaction If we combine Equation (62) with the excess total polarizability expression (Equation (48)), given for molecules dissolved in a solvent medium, we obtain 

1 The interaction type between solute molecules dissolved in a solvent medium may be attractive, repulsive or zero depending on the relative magnitudes of e1( e2 and ,. 

2 The interaction between any two identical uncharged molecules ( = and flj = a2) is always attractive regardless of the nature of the solvent medium. (Note that we must use the square of the parentheses including j and e2 in Equation (63) for this case.) 

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