Mechanism of adsorption of clindamycin and tetracycline by montmorillonite.

IR and X-ray analyses of the interaction of clindamycin with montmorillonite indicate that clindamycin is adsorbed by a cation-exchange mechanism under pH conditions favoring the cationic form of the drug and by physical adsorption when the unionized drug is present. This physical adsorption is relatively weak since the drug is readily desorbed by alkaline washing. Tetracycline is adsorbed by cation exchange at low pH values where the +00 species predominates. Complexation with divalent interlayer cations contributes significantly to adsorption at higher pH values where the +-0 and +-- species exist. In a strongly alkaline solution, the 0-- species was not adsorbed in the interlayer space of montmorillonite but rather produced an external calcium-tetracycline complex. This study illustrates the utility of X-ray and IR analyses in elucidating the mechanisms responsible for clay-drug interactions.