Dissolution Process

•Influenced by:

–Free surface energy (surfactant)

–Shape of particles (surface area)

–Temperature

–Agitation (type and degree; diffusion layer)

–Amount already in solution (near saturation)

–Viscosity, volume of solvent

Solubility of solids in liquids

What does it mean if something dissolves?

•The dissolving process involves the sugar molecules being pulled away from each other.

•When the sugar molecules are pulled apart (dissolved) they remain as sugar molecules and do not break down into the elements that make up sugar

(C, H, O)

Solubility and Rate of Dissolution

Solubility of liquids in liquids

 Liquid-liquid systems may be divided into 2 categories:

1)Systems showing complete miscibility such as alcohol & water, glycerin & alcohol, benzene & carbon tetrachloride..

2) Systems showing Partial miscibility as phenol and water;

two liquid layers are formed each containing some of the other liquid in the dissolved state.

Complete miscibility occurs when:

The adhesive forces between different molecules (A-B) >> cohesive forces between like molecules (A-A or B-B).

Partial miscibility results when:

Cohesive forces of the constituents of a mixture are quite different, e.g. water (A) and hexane (B). A-A » B-B.

The non polar molecules (B) will be squeezed out by the powerful attractive forces existing between the molecules of the polar liquid.

The term miscibility refers to the mutual solubility of the components in liquid-liquid systems.

Solubility of liquids in liquids

Preparation of pharmaceutical solutions involves mixing of 2 or more liquids (alcohol & water to form hydroalcoholic solutions, volatile oils & water to form aromatic waters, volatile oils & alcohols to form spirits …)

•General rule – Like dissolves Like

•

•Polar solvents are miscible with polar solutes

•Non-polar solvents are miscible with non-polar solutes

Henry’s Law

Sg = kPg

C= σ p

where

•Sg  (C) is the solubility of the gas;

•k  (σ ) is the Henry’s law constant for that gas in that solvent;

•Pg is the partial pressure of the gas above the liquid.

Works best for dilute solutions of gases that don’t interact with solvent

Example: CO2 has a Henry's law constant of 1.64x103 atm.

What is the mole fraction of CO2 dissolved in water when the pressure of CO2 above the solution is 1.35atm?

Effects on Solubility

•Pressure: increases solubility of gases (Henry’s law)

•Temperature: solubilty of most gases decreases, solubility of liquids and solids may be affected both ways

•Co-solvents: increase solubility by changing the solvent polarity.

•Salting out: elaboration of gases from solutions by electrolytes (NaCl) and highly polar nonelectrolytes (glucose)

Solubility of gas in liquids

•Examples of pharmaceutical solutions of gases include: HCl, ammonia water & effervescent preparations containing CO2 maintained in solution under pressure.

• The solubility of a gas in a liquid is the concentration of dissolved gas when it is in equilibrium with some of the pure gas above the solution.

• The solubility depends on the pressure, temperature, presence of salts & chemical reactions that sometimes the gas undergoes with the solvent

Gas Solubility

• The solubility of a gas/liquid mixture   may be expressed as the amount of   free gas that can go into   solution at a given temperature and   pressure.

• In general, solubility will increase as   the pressure increases, and as the   temperature decreases.

Types of solution

Solutions of pharmaceutical importance include:

- gases in liquids

- liquids in liquids

- solids in liquids

Factors Affecting Solubility

•Pharmacist use the axiom “like dissolves like”:

ØPolar substances tend to dissolve in polar solvents.

ØNonpolar substances tend to dissolve in nonpolar solvents.

  The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.