Study of the Interaction between Light Anhydrous Silicic Acid and Drugs

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Abstract
Benzoic acid and aspirin crystals were mixed with various kinds of light anhydrous silicic acid (LASA), having different surface areas, and the state of the medicinals in the mixtures was investigated by using differential scanning calorimetry, powder X-ray diffractometry and infrared spectroscopy. It was found that benzoic acid and aspirin became amorphous in the mixtures with LASA at a low concentration level of the medicinals, and that the larger the specific surface area, the higher the ratio of an amorphous state to a crystalline state. The adsorption of the medicinal molecules on the surface of LASA is considered to be a possible mechanism for this phenomena. Hydrogen bonding between medicinal molecules and Si-OH groups of LASA may play an important role on the adsorption. The stability of aspirin in the mixtures was also investigated at 0 and 79% relative humidity at 50.degree. C. The decomposition of aspirin in the mixtures was faster than that of the aspirin crystal. Aspirin in the silica gel mixture had the fastest decomposition rate among the tested samples, due to the adsorption and condensation of aspirin molecules in capillaries of silica gel.