Types of colloidal systems

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Chapter: Pharmaceutical Drugs and Dosage: Colloidal dispersions

On the basis of the type and extent of molecular interactions of the dis-persed phase with the dispersion medium, colloidal systems can be classi-fied into three groups: lyophilic, lyophobic, and association colloids.


Types of colloidal systems

On the basis of the type and extent of molecular interactions of the dispersed phase with the dispersion medium, colloidal systems can be classi-fied into three groups: lyophilic, lyophobic, and association colloids.


1. Lyophilic colloids

A lyophilic colloid (solvent loving) is a system in which the dispersed phase has an affinity for the dispersion medium. Depending on the type of disper-sion medium (solvent), both lipophilic (i.e., lipid-loving, which represents the same characteristics as hydrophobic or water-hating) and hydrophilic (i.e., water-loving, which represents the same characteristics as lipophobic or lipid-hating) colloids can be lyophilic (solvent-loving). Thus, lipophilic colloids are a dispersion of the lipophilic or hydrophobic material in an organic solvent. Hydrophilic colloids are a dispersion of hydrophilic mate-rial in an aqueous medium.

Owing to their affinity for the dispersion medium, lyophilic materials form colloidal dispersions with relative ease. Examples of lyophilic colloids include gelatin, acacia, proteins (such as insulin), nucleic acids, albumin, rubber, and polystyrene. Of these, the first five produce lyophilic colloids in aqueous dispersion medium and are called hydrophilic colloids. Rubber and polystyrene form lyophilic colloids in organic solvents and are thus referred to as lipophilic colloids.


2. Lyophobic colloids

Lyophobic (solvent-hating) colloids are composed of materials that have little attraction, if any, for the dispersion medium. Lyophobic colloids are intrinsically physically unstable. These are formed by the mismatch of pre-ferred molecular interactions of the dispersed phase and the dispersion medium. For example, water and hydrophilic molecules prefer stronger hydrogen bonding, dipole–dipole interactions, and electrostatic interac-tions, while lipids and hydrophobic molecules prefer weaker van der Waals interactions. Examples of lyophobic colloids are gold, silver, arsenous sul-fate, and silver iodide. Thus, dispersion of hydrophobic molecules, par-ticles, or material in an aqueous medium results in hydrophobic colloids. Special methods and energy input are required to prepare stable lyophobic colloids, as they do not form spontaneously.

Differences in the properties of hydrophilic and hydrophobic colloids are summarized in Table 9.1.


3. Association colloids

Association, or amphiphilic colloids are formed by the grouping or self-association of the dispersed phase, which is amphiphilic (e.g., surface-active agents). These molecules exhibit both lyophilic and lyophobic properties. At low concentrations, amphiphiles exist separately as molecular dispersions or true solutions and do not form a colloid. However, at higher concentra-tions, self-association of several monomers, or individual molecules occurs leading to micelle formation. The concentration at which micelles are formed is known as the critical micelle concentration (CMC). 

Table 9.1 Differences in properties of hydrophilic and hydrophobic colloids


The number of monomers that aggregate to form a micelle is called the aggregation number. As with lyophilic colloids, formation of association colloids is spontaneous, provided that the concentration of the amphiphile in solution exceeds the CMC.

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