CLASSIFICATION OF COLLOIDAL SYSTEMS

The classification of colloidal dispersions as advanced by Oswald is based on the affinity or interaction between the dispersed phase and the dispersion medium. Three categories are discernible in this classification, namely lyophilic, lyophobic and association colloids.

Lyophillic dispersions: [=solvent loving]

·         These are dispersions in which there is considerable attraction between dispersed phase and dispersion medium (or extensive solvation).

·         Because of high affinity, lyophilic colloids form spontaneously when the dispersed phase (e.g. solid) come onto contact with dispersion medium(e.g. liquid).

·         The lyophilic colloids are thermodynamically stable.

·         They are reversible

When the dispersion medium is water the system is called hydrophilic. Examples of hydrophilic colloid include bentonite, acacia, gelatin and starch. All these disperse spontaneously in water.

Substance that have high affinity for oils are called lipophilic or oleophilic. Examples of oleophilic substances include gum, rubber, and polystyrene which disperse spontaneously in benzene, activated charcoal forms dispersion in all oils, and aluminium stearate disperse readily in cottonseed oil.

LYOPHOBIC DISPERSIONS

Colloids with little attraction , if any, for the dispersion medium are called lyophobic colloids. (lyophobic = solvent hating). Hydrophobic dispersions include oleophilic materials such as polystyrene, steroids, paraffin wax, and lipophilic drugs all of which are not hydrated by water. Gold, silver and sulfur are all hydrophobic.

Lyophobic dispersions

·         Are not easily prepared. They do not form spontaneously as lyophilic colloids. Special methods must be applied to prepare lyophobic colloids. These include dispersion methods and condensation methods. (the methods will be discussed later on).

·         Are intrinsically unstable. Therefore they must be stabilized.

·         Are irreversible that is if the dispersion medium is separated from the disperse phase, the dispersion cannot be reconstituted.

ASSOCIATION COLLOIDS

Certain molecules or ions are characterized by having two distinct regions of opposing solution affinities within  the same molecule or ion. Such molecules are called amphiphiles. In low concentration amphiphiles exist as separate molecules and their size4 is too small to form colloids.(subcolloids). When the concentration is increased, a critical concentration shall be reached when the molecules aggregate (up to 50 or more monomers) t form micelles that are large enough (around 0.5 μm) to lie within the colloidal range. The concentration at which the micelles begin to form is called critical micelle concentration (CMC). The number of monomers that aggregate to form a micelle is known as the aggregation number of the micelle. Because micelles are large enough to qualify as colloidal particles; such compounds are called association colloids.

As with lyophobic colloids,

·         Formation of association colloids is spontaneous, as long as the concentration of amphiphiles in solution exceeds the CMC.

·         The association colloids are stable

Compare and contrast lyophobic, lyophilic and association colloids. (See Martin, Physical pharmacy).

PROPERTY	LYOPHOBIC SOL	LYOPHILIC SOL	ASSOCIATION COLLOIDS
Particle size	Inorganic particles eg gold, silver	Large organic molecules	Aggregates of small organic molecules (micelles)
Solvation	Little, if any interaction (salvation) occurs between particles and dispersion medium	Dispersed molecules are solvated (interaction with dispersion medium).	Hydrophobic or hydrophilic portion of the molecule is solvated depending on whether the dispersion medium is water or oil
Ease of formation	Molecules disperse spontaneously to form colloidal solution	Molecules disperse spontaneously to form colloidal solution	Colloidal aggregates are formed spontaneously when CMC is reached
Addition of electrolytes	Stabilized by small amounts. Slight excess lead to coagulation	Stable in presence of electrolytes. Destabilized by high conc. of electrolytes (salted out) may be due to desolvation by electrolytes	In aqueous medium, CMC is reduced by by addition of electrolytes. Salting out may occur at higher concentration of electrolytes.
Reversibility	Not easily redispersed upon evaporation of the medium.	Redispersible	Redispersible (reversible)
Faraday-Tindal effect	Intense	diffuse
Electrical charge	Essential to stabilize	Not essential. Can be adjusted by pH.
Viscosity	Viscosity is not greatly  increased by presence of lyophobic colloidal particles. It may be similar to dispersion medium	Very viscous. At sufficient concentration, they form a gel	Viscosity increases as the concentration of amphiphille increases as micelles ncrease in number and become asymmetric.