Determination of the aggregation properties of weakly self-associating systems by NMR techniques : the self-association of propranolol hydrochloride in aqueous electrolyte solution Juan M. Ruso,a Pablo Taboada,a David Attwood,b Mosqueraa and Sarmiento*a V•  ctor Fe  lix a Grupo de de Coloides y Departamento de Aplicada y Departamento de F•  sica Pol•  meros, F•  sica de la Materia Condensada, Facultad de Universidad de Santiago de Compostela, F•  sica F•  sica, Spain. E-mail : fsarmi=usc.es b School of Pharmacy and Pharmaceutical Sciences, University of Manchester, Manchester, UK M13 9PL Received 29th November 1999, Accepted 20th January 2000 An analytical method for the determination of critical aggregation concentrations (cac) from the concentration dependence of chemical shifts from 1H nuclear magnetic resonance data for weakly self-associating systems is proposed. Application to solutions of the amphiphilic drug propranolol hydrochloride in the presence of added electrolyte (0.00, 0.05, 0.10, 0.20, 0.30 and 0.50 mol kg~1 NaCl) at 293.15 K has yielded cac values which are independent of the proton selected for analysis and in reasonable agreement with values derived from other experimental techniques. Aggregation numbers calculated by the application of mass action theory to the chemical shift data, and the standard Gibbs energy of aggregation calculated from the cac data, are in agreement with previously reported values. 1 Introduction The presence of changes in physical properties of some amphi- philic drugs in aqueous solution is assumed to be indicative of self-aggregation and the discontinuity has been identiÐed with the critical concentration of the systems.1 In some drug solu- tions, it has been possible to detect premicellar aggregates at low drug concentrations2h5 and the discontinuity in these systems marks a transition to larger aggregates rather than the monomerÈmicelle transition assumed by the mass action theory of micelle formation. Nevertheless, the application of this theory to data for a wide variety of amphiphilic drugs using a variety of experimental techniques has generated ther- modynamic parameters of aggregation in good agreement with those determined experimentally.6,7 Of the many antihypertensive drugs with b-adrenoceptor blocking action whose properties have been examined,8,9 only propranolol hydrochloride shows any signiÐcant association in the absence of electrolyte. We have previously reported a study of the aggregation of propranolol hydrochloride as a function of temperature10 and also used light scattering tech- niques to quantify the interaction between aggregates in the presence of added electrolyte and to determine aggregate size.11 The aim of the present study is to determine the aggre- gation properties of propranolol hydrochloride in aqueous electrolyte solutions (0.00 to 0.50 mol kg~1 NaCl) from nuclear magnetic resonance measurements. Proton NMR has been used in the determination of chemical shift of selected protons of propranolol during dilution over a wide concentra- tion range and the critical aggregation concentration (cac) and aggregation number have been determined through the appli- cation of mass action theory to these data. We have pre- viously demonstrated the applicability of this technique in the determination of the aggregation properties of several amphi- philic penicillins12 and the mode of association of pheno- thiazine molecules.13 A problem encountered in the determination of the cac from chemical shift data on systems of low aggregation number is the lack of an abrupt inÑection and the consequent difficulty of assigning a precise value for the cac. In this paper we present a method of data analysis based on the Phillips deÐnition of the critical micelle concen- tration (cmc)14 which addresses this problem. 2 Experimental Propranolol [1-isopropylamino-3-1-(naphthyloxy)-2-propra- nol] hydrochloride was obtained from Sigma Chemical Co. (No. P-0884). Sodium chloride (Fluka) was of analytical grade. 1H NMR was recorded on a JEOL EX270 270 MHz spectrometer at 293 ^ 0.1 K. The chemical shifts of selected peaks were accumulated using a peak pick facility. All spectra were compared with sodium 3-(trimethylsilyl)propionate (TSP) which acted as an internal standard. Assignment of spectral lines was by comparison with bibliographic data.15 h18 3 Results and discussion Chemical shifts obtained for the protons 1, 2, 3 and 4 (expressed in parts per million, ppm) situated at the aromatic, alcohol, amino and methyl groups respectively (see Scheme 1) are shown in Figs. 1 to 4 as a function of the reciprocal of total drug concentration. Similar plots were obtained for elec- trolyte concentrations of 0.00, 0.05, 0.10, 0.20, 0.30 and 0.50 Scheme 1 DOI : 10.1039/a909407e Phys. Chem. Chem. Phys., 2000, 2, 1261È1265 1261 This journal is The Owner Societies 2000 (