Two important long-range forces between molecules

2 Two important long-range forces between molecules 

We first discuss two very important long-range forces, the Coulombic and those originating fi om 

The relative permittivity is a measure of the extent of reduction of electric fields by a given medium and consequently of the reduced strengths of electrostatic interactions in a (polar) medium. A high relative permittivity means that charges are easily maintained in the medium. The relative permittivity of water is about 80, a high value that allows ion dissociation to occur, whereas in air and in non-polar liquids, e.g. hexane or benzene (e of about 2), we would expect no dissociation. For this reason, most solid dispersions are studied in aqueous media. The permanent dipoles in the wato molecules stabilize the charges on ions. 

Relative permittivity values are useful in many respects. For example, if we plot the solubilities of sodium chloride and glycine in solvents of different relative permittivities at 25 °C a linear relationship is obtained. Hydrogen bonding solvents are effective solvents and high relative permittivity values imply high solubilities. For example, the solubility of NaCl in acetone is 4 X 10 kg NaCl per kg solvent while that of glycine in acetone is 2 x 1(T kg NaCl per kg solvent 

The very strong Coulombic forces partially explain the difficulties associated with constructing suitable theories for electrolyte solutions (Prausnitz, Lich-tenthaler and de Azevedo, 1999). They also form the basis of the electrostatic repulsive forces between colloidal particles, which are discussed in Chapte 10, as the colloids are typically charged in aqueous media. 

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