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106 CHAPTER 2

for positive ions and

–1
for negative ions (values in kilocals (gram ion) .

2.15.6. Ion–Induced Dipole Interactions in the Primary Solvation
Sheath

At this level of sophistication, one wonders whether there are other subtle
interactions that one ought to consider.
For instance, when the water molecule is in contact with the ion, the field of the
latter tends to distort the charge distribution in the water molecule. Thus, if the ion is
positive, the negative charge in the water molecule tends to come closer to the ion, and
the positive charge to move away. This implies that the ion tends to induce an extra
dipole moment in the water molecule over and above its permanent dipole moment.
For small fields, one can assume that the induced dipole moment is proportional
to the inducing field X

where the proportionality constant, is known as the deformation polarizability and
is a measure of the “distortability” of the water molecule along its permanent dipole
axis.
Thus, one must consider the contribution to the heat of formation of the primary
solvated ion (i.e., step 3 of the cycle used in the theoretical calculation presented
earlier), which arises from interactions between the ion and the dipoles induced in the
water molecules of the primary solvent sheath. The interaction energy between a dipole
and an infinitesimal charge dq is or, since is the field dX due to this
charge, the interaction energy can be expressed as Thus, the interaction energy
between the dipole and an ion of charge exerting a field can be found by,