Colloidal dispersions

Colloidal dispersions

Introduction

Dispersed systems consist of one phase, known as the dispersed phase, distributed throughout a continuous phase or dispersion medium. The dispersed systems range in size from particles of atomic and molecular dimensions to visible particles, which can be up to several millimeters in diameter. On the basis of the size of the dispersed phase, dispersed systems are classified into the following types:

1. Molecular dispersions (<1 nm): Molecular dispersions are true solu-tions of one component in another. They are visibly homogeneous. The size of dispersed phase of molecular dispersions is typically less than 1 nm in diameter. True solutions do not scatter light and are clear or colored.

2. Colloidal dispersions (1 nm to 0.5 μm): Colloidal dispersions have the dispersed-phase size larger than the molecular dimensions of the dispersed phase in true solutions, while being much smaller than the particles that would be visible to the naked eye. The size of dispersed phase of molecular dispersions is typically 1 nm to 0.5 μm in diameter. Colloidal dispersions scatter light and appear turbid. Many natural systems, such as suspensions of microorgan-isms, blood, and isolated cells in culture, are colloids. Some hydro-philic colloids can be used as blood plasma substitutes to maintain osmotic pressure.

3. Coarse dispersions (>0.5 μm): Coarse dispersions have a particle size significantly larger than molecular and colloidal dispersions, such that the dispersed phase rapidly and spontaneously segregates if appropriate stabilization strategies are not utilized. The size of the dispersed phase of coarse dispersions is typically greater than 0.5 μm in diameter. Coarse dispersions scatter light and are visually cloudy/milky. Emulsions and suspensions are examples of coarse dispersions.

Colloidal solutions are preferred for pharmaceutical applications, where maximizing the surface area of the dispersed phase is important. Some examples of colloids used as pharmaceuticals are as follows:

·           Colloidal kaolin is used for toxin absorption in the gastrointestinal (GI) tract.

·           Colloidal aluminum hydroxide is used for neutralizing excess acid in stomach.

·           Colloidal dispersion of amphotericin B and sodium cholesteryl sulfate (Amphocil^®) is used as an antifungal agent.

·           Colloidal silver chloride, silver iodide, and silver protein are effective germicides. They do not cause irritation, which is characteristic of ionic silver salts.

·           Colloidal copper has been used in the treatment of cancer.

·           Colloidal gold as a diagnostic agent for paresis and colloidal mercury for syphilis.

·           Psyllium hydrophilic colloid (Metamucil^®) is used as an oral laxative.