Introduction

When two different substances are mixed together so that they mingle intimately, a two component system is produced. When one component is distributed uniformly throughout the second, the first component is called the dispersed phase and the second, the dispersion medium or continuous phase. Either phase may be solid, liquid, or gas.

Frequently in pharmacy a solid substance is dispersed in a liquid, usually water, and the resulting product may have the characteristics of either a molecular dispersion (true solution), a colloidal dispersion, or a coarse dispersion depending on the particle size of the dispersed solid. In true solutions the dispersed particles are ions or small molecules having particle size less than 1 nanometer (nm). In colloidal dispersions the particles are either single, large molecules of high molecular weight (macromolecules) or aggregates of smaller molecules with diameters between 1 nm and 500 nm in size (0.001 - 0.5µ). In coarse dispersions the particles are greater than 500 nm in diameter. Note that the particle size ranges given are not rigid and overlap does occur between each dispersion class.

The dispersed phase of a colloidal dispersion may be classified as being either lyophilic (solvent-loving) or lyophobic (solvent-hating). If the solvent is water these classifications are termed by hydrophilic and hydrophobic, respectively.

Molecules of a hydrophilic colloid have an affinity for water molecules and when dispersed in water become hydrated. Hydrated colloids swell and increase the viscosity of the system, thereby improving stability by reducing the interaction between particles and their tendency to settle. They may also possess a net surface electrical charge. The charge sign depends on the chemical properties of the colloid and the pH of the system. The presence of a surface charge produces repulsion of the charged particles and thus reduces the likelihood that the particles will adhere to one another and settle. Some examples of hydrophilic colloids used in pharmacy are acacia, methylcellulose, and proteins, such as gelatin and albumin.

A hydrophobic colloid has little or no affinity for water molecules in solution and produces no change in system viscosity. The particles may carry a charge, however, due to the adsorption of electrolyte ions from the solution. The dispersion of such particles is due to mutual repulsion of like charges and Brownian movement.

Neutralization of the particle charge may occur by addition of ions of opposite charge. The neutralized particles, which possess high surface free energy, cling together resulting in a precipitate. Most important is the influence of charge type and valence of the ions added. Their effect is summed up in the Schulze-Hardy Rule which states, first, that the effectiveness of an electrolyte is determined primarily by the nature of the ion opposite in charge to the colloidal particles, and second, as the valence of this ion increases, the effectiveness of the electrolyte increases markedly. Thus, for a negatively charged hydrophobic colloid such as arsenous sulfide, aluminum chloride is about 10 times more effective than magnesium chloride, and 500 times more effective than sodium chloride in causing the colloid to precipitate. Other examples of substances which form hydrophobic colloids in water are silver iodide, hydrated ferric oxide, sulfur, and gold.