Colloidal properties of benzylpenicillin: Comparison with structurally-related penicillins

The self-association of sodium benzylpenicillin (penicillin G) in aqueous solution as a function of temperature has been examined. Critical concentrations were determined by conductivity measurements in water over the temperature range 288–313 K and by static and dynamic laser light scattering measurements at 298 K. Both techniques indicate the existence of two critical concentrations: the lowest critical premicelle concentration (cpc) and the highest critical micelle concentration (cmc). Aggregation numbers and effective charges of the aggregates were obtained from static light scattering according to the Anacker and Westwell and the Anacker and Jacobs treatments. The thermodynamic properties of the self-association were derived by application of the modified mass action model for systems with low aggregation number. From the analysis of the thermodynamic parameters, a two-step aggregation model has been proposed by which the dimers created at the cpc form micelles at the cmc. A comparative study of the thermodynamics of structurally-related penicillins, ampicillin, oxacillin, cloxacillin, and dicloxacillin has been carried out, including the enthalpy–entropy compensation phenomenon. A compensation temperature of (1/303) K−1 was found for all penicillins.