Pharmaceutical suspensions: relation between zeta potential, sedimentation volume and suspension stability.

The effect of added surface-active agents of various ionic types on the sedimentation volume of drug suspensions of betamethasone, griseofulvin, nalidixic acid and thiabendazole has been investigated, and the results correlated with previously measured zeta potentials. Study of the zeta potential/sedimentation volumes versus concentration plots showed that apparently only coagulated, deflocculated or sterically stabilized systems were formed. In most cases the sterically stabilized systems were produced from mixtures of ionic/non-ionic surfactants. These are examples of controlled coagulation, although non-ionic surfactant alone conferred stability against caking. Secondary minimum flocculation was not apparent but this may have been due to the method of examination of suspensions. The work confirmed that the DLVO theory of colloid stability and its modification to include a steric term can be applied to coarse suspension systems.