Solvent effects on molecular interactions

The presence of solvent molecules between solute molecules affects all the interaction energies. We have seen that the presence of solvent molecules changes the molecular polarizabilities of solutes in Section 2.5.3 and decreases the strength of van der Waals interactions (Section 2.6.4) and the total intermolecular pair potential energies (Section 2.7.4). When two molecules interact in a condensed liquid medium, there are many solvent molecules interfering in this interaction. Now, this becomes a many-body interaction and we have to consider some important new effects  

3 Solute-solvent interactions can change the properties of the solute molecules, such as their dipole moment, polarizability and degree of ionization. Consequently, the properties of any solute are different in different condensed media. 

5 If molecules (1) and (2) are in complete contact in liquid (3), the hard-core repulsion interaction must also be considered. This sometimes gives rise to positive values in potential expressions. 

These effects are interrelated and can be collectively termed as solvent effects or medium effects. In addition, the van der Waals forces between (1-1), (2-2) and (1-2) pairs are reduced because of the dielectric screening of the medium (3), which is particularly important for liquid solvents with high dielectric constants. The attraction force is decreased by a factor of the medium s for Keesom and Debye interactions, and by a factor of for London dispersion interactions. This strong reduction in the attractive pair potential means that the contributions of molecules further apart tend to be relatively minor, and each interaction is dominated only by its nearest neighbors. 

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Uruska, I. and Koschmidder, M. (1986) Solvent effects on molecular interactions in 2-amino- and 3-aminopyridine-molecular iodine-solvent systems. J. Chem. Soc., Perkin Trans. 

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