The Pharmaceutics and Compounding Laboratory
Colloidal Dispersions

Stability of Colloids

Since a uniform dispersion of particles is important for the diagnostic and therapeutic effectiveness as well as the safety of administration of pharmaceutical colloids, stability against settling or coprecipitation is an important consideration. The addition of a hydrophilic colloid to a hydrophobic one causes the hydrophilic colloid to adsorb onto and completely surround the hydrophobic particles which then take on some of the properties of the hydrophilic colloid. The hydrophilic colloid shields the hydrophobic system from the destabilizing effects of electrolytes; thus the hydrophilic substance is called a "protective colloid". Stability of such a system is enhanced, because in order to precipitate the hydrophobic colloid, both the protective solvent sheath surrounding it and the electric charge must be removed. Gelatin and methylcellulose derivatives are commonly used as protective colloids.

Sometimes in pharmaceutical formulations buffer salts are added to maintain a pH required for product stability. Occasionally such buffers may contribute to potential instability by forming insoluble salts with metallic ions. This problem may occur especially with phosphate buffers since most heavy metal phosphates are insoluble. If an insoluble phosphate salt precipitates from a colloidal dispersion, it may co-precipitate the colloidal particles along with it. To prevent this phenomenon from occurring, chelating agents may be used that will preferentially complex the metal ions, and thus prevent them from reacting with the phosphate. Alternatively, nonphosphate buffers may be substituted for phosphate buffers, when feasible, to prevent the instability.