PAC3241 Study Guide - Final Guide: Viscosity, Brownian Motion, Crystal Growth
23 Jun 2018
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The desired properties of pharmaceutical dispersed formulations.
●Safe
●Efficacious
●Microbially, chemically and physically stable
●Constantly delivery uniform dosages
●Not settle rapidly
●Easily redispersible upon shaking
●Homogenous after shaking
●Not cake upon storage
●Appropriate particle size
●Appropriate rheological properties- good flow, easy to pour and measure, ease of administration
●Elegant- smooth, non-gritty, not ugly looking
Types of colloidal dispersed systems and the factors affecting the stability and dosage
uniformity of these systems:
Lyophilic colloidal dispersion
Lyophilic colloidal particles (e.g. acacia, hypromellose (HPMC) and gelatin) interact well with solvent
molecules to produce particle-solvent interactions. Therefore, the particles are readily dispersed in the
solvent medium.
To increase the stability of the system, the affinity of the solvent sheath for individual particle has to be
increased. The solvent sheath affinity is determined by 3 factors:
1. Change of solvent
2. Addition of electrolytes
3. Addition of solutes
Salting out occurs where added solute or electrolytes (ions) compete for water of hydration and
dehydrate the particles in dispersed phase. For example, gelatin will precipitate out by competitive
desolvation upon addition of NaCl or hydrophilic solvent (e.g. ethanol, sweeteners or colouring
agents).
Good interaction between the particle surface and the solvent results from solvent sheath formation,
leading to solvation of particles which prevents particle-particle interaction.
Lyophobic colloidal dispersion
There is no solvent sheath formation, resulting little or no interaction between solvent and dispersed
particles. This leads to potential for particle-particle interaction and therefore particle aggregation,
precipitation and caking.
Factors affecting the stability and dosage uniformity included added excipients, particle size and
method of preparation. For example, excipients like sweeteners and buffer salts have greater affinity
for solvents as compared with drug particles and indirectly strip the solvent sheath formation which
eventually leads to precipitation (i.e competitive desolvation).
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