Application of total intermolecular pair potentials in a liquid medium

4 Application of total intermolecular pair potentials in a liquid medium 

In condensed phases such as liquids, two molecules are not isolated but have many other molecules in their vicinity. If these two molecules are dissolved in another solvent medium then the situation may be very complicated, and only by introducing simplifying assumptions is it possible to construct a simple theory of dense media and a solution using Lennard-Jones or similar total intermolecular pair potentials. Some important facts, which should be taken into account before applying any simplifying assumptions, are given below  

1 As we have already discussed in Section 2.5.3 for excess polarizabilities of molecules dissolved in a solvent, and in Section 2.6.4 for van der Waals interactions in a medium, when two molecules 1 and 2 are dissolved in a medium 3, the van der Waals forces between them are reduced because of the dielectric screening of the medium. This reduction is particularly important for liquids with high dielectric constants. The attraction force is decreased by a factor of the medium s er for Keesom and Debye interactions and by a factor of e] 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 contributions from its nearest neighbors. 

2 The dielectric constant (relative permittivity) is a macroscopic property. If molecules 1 and 2 approach each other so closely, there will be no room for a solvent molecule between them. Then we may question the use of the medium s r in the denominator of the related expressions, because if any polarization takes place, this may be due to a much smaller effective value of r. On the other hand, some associated solvents apply forces on solute molecules, which are determined mainly by the orientation of the molecules of 1 and 2 in the medium 3, so that the resultant distribution functions are not only functions of r but also dependent on the orientation angle. 

3 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. 

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