Effects of surfactants on the aqueous stability and solubility of beta-lactam antibiotics.

Studies were undertaken to elucidate the interaction between beta-lactam antibiotics and surfactant micelles and to examine the effects of surfactants on their aqueous stability and solubility. The apparent binding constant of the micelle-antibiotic complex was determined as a function of the solution pH at 37 degrees and mu = 0.15 by the dynamic dialysis method and hydrolysis study. In the interaction with nonionic and anionic micelles of polyoxyethylene-23-lauryl ether (I) and sodium lauryl sulfate (II), large differences were noted in the binding constants between the undissociated and ionized species of penicillins. However, the cationic surfactants, cetyltrimethylammonium bromide (III), showed no significant difference in the binding constants for both species. Acid degradation of penicillins was protected in micellar solutions of I and III but was facilitated in micelles of II. The surfactants exerted no influence on the neutral degradation of the antibiotics used. The solubilization of penicillin V acid by micelles of I was studied at pH 2.0 and 35 degrees. The solubility increased threefold in the presence of 10 mM I.