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.

-

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

•

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

Types of Solutions

•Saturated

ØSolvent holds as much solute as is possible at that temperature.

ØDissolved solute is in dynamic equilibrium with solid solute particles.

•Unsaturated

ØLess than the maximum amount of solute for that temperature is dissolved in the solvent.

•Supersaturated

ØSolvent holds more solute than is normally possible at that temperature.

ØThese solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask.

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How Does a Solution Form?

  As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them.

  If an ionic salt is soluble in water, it is because the ion-dipole interactions are strong enough to overcome the lattice energy of the salt crystal.

Solutions

•Solutions are homogeneous mixtures of two or more pure substances.

•In a solution, the solute is dispersed uniformly throughout the solvent.

Non-Polar Solvents

•Non-Polar Solvents (Carbon tetrachloride, Benzene, Mineral Oil) have:

  --Low dielectric constant

  --Cannot reduce attraction, break covalent bonds, nor

  ionize weak electrolytes

  - They are unable to form hydrogen bonds with non electrolytes.

  - Non polar solvents can dissolve non polar solutes through weak van der Waals forces

•Dielectric Constant:

  --Measure of polarizability of a molecule

  --Polarizability (Increase DC--Increase Polarizability)

Co-Solvents

•Used to increase aqueous solubility of weak electrolytes and non-polar molecules

•Solute more soluble in mixture of solvents (alcohol/water mixture)

Polarity

•Solubility due in large part to polarity of solvent

•Three categories:

  1-   Polar (water) acts as a solvent according to the following mechanisms:

  a) Dielectric constant:

 due to their high dielectric constant, polar solvents reduce the force of attraction between oppositely charged ions in crystals.

Example: water possessing a high dielectric constant (> = 80) can dissolve NaCl, while chloroform (> = 5) & benzene (> = 2) cannot. Ionic compounds are practically insoluble in these 2 solvents.

Expression of Solubility

•Percent solubilities:

  --%w/v  g/100 ml solution

  --%v/v  ml/100 ml solution

  --%w/w  g/100 g solution