Solute-Solvent interactions

Solubility depends on chemical, electrical & structural effects that lead to mutual interactions between the solute and the solvent.

-In pre-or early formulation, selection of the most suitable solvent is based on the principle of “like dissolves like”. That is, a solute dissolves best in a solvent with similar chemical properties. i.e.

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• Polar solutes dissolve in polar solvents. E.g salts & sugar dissolve in water .

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• Non polar solutes dissolve in non polar solvents. Eg. naphtalene dissolves in benzene.

-To explain the above rule, consider the forces of attraction between solute and solvent molecules.

•If the solvent is A & the solute is B, and

•the forces of attraction are represented by A-A, B-B and A-B,

•one of the following conditions will occur:

1. If A-A >> A-B  ÞThe solvent molecules will be attracted to each other & the solute will be excluded. Example: Benzene & water, where benzene molecules are unable to penetrate the closely bound water aggregates.

2. If B-B >> A-B  Þ The solvent will not be able to break the binding forces between solute molecules. Example: NaCl in benzene, where the NaCl crystal is held by strong electrovalent forces which cannot be broken by benzene.

3. If A-B >> A-A or B-B, or the three forces are equal Þ The solute will disperse & form a solution. Example: NaCl in water.