Waals interactions in a medium

As we already discussed in Section 2.5.3 for excess polarizabilities of molecules dissolved in a solvent, the London dispersion interactions between molecules in a solvent medium may be very different from those of isolated molecules in free space. The intrinsic permanent dipole moment, p, and polarizability of an isolated gas molecule, a, may be different in the liquid state or when dissolved in a medium, and this can only be determined by experiment. 

Equations (92) and (93) show that the presence of a solvent medium other than a free space much reduces the magnitude of van der Waals interactions. In addition, the interaction between two dissimilar molecules can be attractive or repulsive depending on refractive index values. Repulsive van der Waals interactions occur when n3 is intermediate between nx and n2, in Equation (92). However, the interaction between identical molecules in a solvent is always attractive due to the square factor in Equation (93). Another important result is that the smaller the n - nj) difference, the smaller the attraction will be between two molecules (1) in solvent (3) that is the solute molecules will prefer to separate out in the solvent phase which corresponds to the well-known like dissolves like rule. However there are some important exceptions to the above explanation, such as the immiscibility of alkane hydrocarbons in water. Alkanes have nx = 1.30-1.36 up to 5 carbon atoms, and water has a refractive index of n = 1.33, and very high solubility may be expected from Equation (93) since the van der Waals attraction of two alkane molecules in water is very small. Nevertheless, when two alkane molecules approach each other in water, their entropy increases significantly because of the very high difference in their dielectric constants and also the zero-adsorption frequency contribution consequently alkane molecules associate in water (or vice versa). This behavior is not adequately understood. 

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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. 

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