WATER-VAPOR SORPTION BY CEPHALOSPORINS AND PENICILLINS

The sorption of water vapor by the sodium salts of ampicillin, nafcill in, ceftriaxone, cefazolin, piperacillin, cefotaxime and cefuroxime at 24, 34, 44 and 54 degrees C was studied at vapor pressures between 5 and 100 torr by a new method (Hansen et al., 1996). Piperacillin absor bs water at all temperatures and vapor pressures studied. Cefuroxime a nd cefotaxime did not react. All the other compounds exhibited behavio r consistent with formation of a new phase at some critical water vapo r pressure, (PH2O)-H-c. Enthalpy changes for the water sorption reacti on are calculated from a van't Hoff treatment of the (PH2O)-H-c values . The thermodynamics of phase changes resulting from reactions of thes e compounds with water is discussed. The rate of water sorption is a l inear function of the water vapor pressure at partial pressures above the equilibrium vapor pressure over the hydrated phase. Kinetics of wa ter sorption as a function of temperature were measured. The activatio n energy for water sorption is approximately zero.