Self-Association of Amphiphilic Penicillins in Aqueous Electrolyte Solution: A Light-Scattering and NMR Study Pablo Taboada, David Attwood, † Juan M. Ruso, Felix Sarmiento, and Vı ´ctor Mosquera* Grupo de Fı ´sica de Coloides y Polı ´meros, Departamento de Fı ´sica Aplicada y Departamento de Fı ´sica de la Materia Condensada, Facultad de Fı ´sica, Universidad de Santiago de Compostela, E-15706 Santiago de Compostela, Spain, and School of Pharmacy and Pharmaceutical Sciences, University of Manchester, Manchester M13 9PL, U.K. Received October 26, 1998. In Final Form: January 11, 1999 The self-association of the penicillins cloxacillin, dicloxacillin, and flucloxacillin in water and in the presence of added electrolyte (0.025-0.40 mol kg -1 NaCl) at 30 °C has been examined by light-scattering and NMR techniques. Inflections in the data from both techniques were observed at a single critical concentration for solutions of cloxacillin and at two critical concentrations for dicloxacillin and flucloxacillin. Aggregation numbers and effective micellar charges were calculated from the static light-scattering data for the stable aggregates formed at the first critical concentration. Application of the valance-generalized light-scattering theory for multicomponent systems to data at concentrations above the second critical concentration provided an estimate of the aggregate size of the associated species present at high solution concentration. The interaction between aggregates was interpreted from diffusion data from dynamic light-scattering using DLVO theory. Micellar properties have been determined by the application of mass action theory to the concentration dependence of 1 H NMR chemical shifts, confirming the results obtained by the light-scattering technique. Introduction The study of the properties of surface active drugs in solution provides an opportunity to investigate the influ- ence of the molecular structure of the hydrophobe on the association characteristics of amphiphilic molecules. 1,2 The penicillin drugs selected for study form an interesting series of molecules in which the only variation in the molecular structure is the number and nature of the substituents on the aromatic ring of the hydrophobe (see Chart 1). The penicillins under investigation are cloxacillin (X ) H), dicloxacillin (X ) Cl) and flucloxacillin (X ) F). Interest in the colloidal properties of penicillins extends back to the late 1940s and includes studies by McBain and co-workers, 3 Hauser et al., 4 and Few and Schulman. 5 These early investigations, mainly on penicillin G, were adversely affected by surface active impurities. A more recent study 6 has reported the micellar properties of several synthetic penicillins (including flucloxacillin and cloxacillin) both in water and in the presence of 0.15 M NaCl. The present study extends this work and considers the influence of electrolyte on the mode of association, the micellar properties, and the intermicellar interactions of the selected series of penicillins using static and dynamic light-scattering techniques and NMR. The measurements were carried out at higher penicillin concentration than previously examined in order to detect any second critical concentration, which is a characteristic feature of some drugs. 2,7,8 To quantify the interaction between the ag- gregates, the data have been interpreted using the Corti and Degiorgio 9 treatment of diffusion data based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloid stability. 10 Experimental Section Materials. Sodium cloxacillin monohydrate ([5-methyl-3-(o- chlorophenyl)-4-isoxazolyl]penicillin) and sodium dicloxacillin monohydrate ([3-(2,6-dichlorophenyl)-5-methyl-4-isoxazolyl]- penicillin) were obtained from Sigma Chemical Co. Sodium flucloxacillin monohydrate ([3-(2-chloro-6-fluorophenyl)-5-meth- yl-4-isoxazolyl]penicillin) was a generous gift from Smithkline Beecham Pharmaceuticals. Sodium chloride was of Analar grade. Water was double-distilled, deionized, and deaerated before use. Light-Scattering Measurements. Static light-scattering measurements were performed at 30 ( 0.1 °C using a Malvern 7027 laser light-scattering instrument equipped with a 2-W argon * To whom correspondence should be addressed at the Uni- versidad de Santiago de Compostela. † University of Manchester. (1) Attwood, D.; Florence, A. T. Surfactant Systems; Chapman and Hall: London, 1983; Chapter 4. (2) Attwood, D. Adv. Colloid Interface Sci. 1995, 55, 271. (3) McBain, J. W.; Huff, H.; Brady, A. P. J. Am. Chem. Soc. 1949, 71, 373. (4) Hauser, E. A.; Marlow, G. J. J. Phys. Colloid. Chem. 1950, 54, 1077. (5) Few, A. V.; Schulman, J. H. Biochim. Biophys. Acta 1953, 10, 302. (6) Attwood, D.; Agarwal, S. P. J. Pharm. Pharmacol. 1984, 36, 563. (7) Attwood, D.; Doughty, D.; Mosquera, V.; Perez Villar, V. J. Colloid Interface Sci. 1991, 141, 316. (8) Attwood, D.; Blundell, R.; Mosquera, V. J. Colloid Interface Sci. 1993, 157, 50. (9) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981, 85, 711. (10) Verwey, E. J. W.; Overbeek, J. T. G. In Theory of the Stability of Lyophobic Colloids, Matijevic, E., Ed.; Wiley: New York, 1948. Chart 1 2022 Langmuir 1999, 15, 2022-2028 10.1021/la981501p CCC: $18.00 © 1999 American Chemical Society Published on Web 02/24/1999