INTERMOLECULAR FORCES AND INTERACTION ENERGY 217 



substance and a protein involves breaking of the previous hydrogen bonds 

 between protein and water and between the substance and water, this 

 mobility may be of some significance in the rates of such reactions. 



The entropy change resulting from the formation of a single hydrogen 

 bond is usually between — 10 and — 20 cal/mole/degree in the gas or liquid 

 phases. However, in aqueous solution the JS and JH for hydrogen bond 

 formation may be quite different from the values that are generally given 

 for the pure state. Let us assume that two molecules, A and B, associate 

 through a hydrogen bond. Before this bond is formed, each molecule in 

 solution has, to some extent, formed a hydrogen bond with a neighboring 

 water molecule; following the association of A and B, the water molecules 

 released will tend to form hydrogen bonds with other water molecules in 

 the aqueous structure. Thus the over all reaction may be written as: 



A-H-w + B-H-it; -> A-H-B + w-H-ir 



where iv represents either H2O or OH, depending on the origin of the hy- 

 drogen atom in the bond. The over-all thermodynamic changes are, there- 

 fore, not as great as in the pure phase. There is probably still a small de- 

 crease in the entropy during hydrogen bond formation in aqueous solution 

 (Lumry, 1959, p. 157). 



Ion-Induced Dipole Interactions 



All molecules in an electric field become polarized and this results in a 

 dipole whose moment is dependent on the strength of the field and the pro- 

 perties of the molecule. In the most general case the total polarization is 

 given by: 



P = P, + P,+ P„, (6-22) 



Pg is the electronic polarization which is due to the displacement of the 

 electrons in the field. P^ is the atoinic polarization and is due to the displace- 

 ment of atoms or groups in the molecule. P,^ is the orientation polariza- 

 tion and results from the preferential orientation of a molecule with a per- 

 manent dipole in the field. In a molecule without a permanent dipole the 

 polarization induced by the field is P^ + P^ which may be called the in- 

 duced polarization P,. It is a polarization that is dependent on the inherent 

 flexibility of movement of the electrons and atoms in a molecule, that is, 

 the polarizability of the molecule. Such induction will, of course, occur in 

 a molecule with a permanent dipole, but in this section we shall be concerned 

 only with electronic and atomic polarizations; the polarization of a perma- 

 nent dipole in the field of an ion has been discussed in a previous section. 

 The distinction between P^ and P^ is important only in alternating fields, 

 with which we are not concerned, and therefore the resultant induced 



