The Stability of Drug Adsorbates on Silica

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Abstract
Colloidal and porous silicas are used as carriers in solid, semisolid and liquid dosage forms. Adsorption of active ingredients onto their large surface areas can be used to regulate drug release or for the uniform distribution of drug in single dose units with a very low drug content. In the adsorbates the contact between drug and carrier surface on the molecular level can be of great importance for the chemical stability of drug preparations. This is demonstrated by the following examples: Hydrolytic degradation of acetyl salicylic acid in dry silica adsorbates is mainly determined by alkaline impurities of the carrier and strongly adsorbed water on the silica surface. The “catalytic” action of silicas is, therefore, directly dependent on the preparation technique of the carrier. Propantheline a cationic ester compound is adsorbed on silica from aqueous solution. In aqueous silica suspensions and in dry adsorbates the ester hydrolysis is controlled by the pH, the neutral salt content, and buffer substances, due to different adsorption mechanisms. The oxidative degradation of butylhydroxyanisole in silica adsorbates was also found to be enhanced in the presence of alkaline impurities. The oxidation of linoleic acid methylester in oleogels of colloidal silica proved to be influenced both by carrier impurities and the specific adsorption of intermediates (peroxides) onto the surface.