Investigation of the self-association of an amphiphilic drug in aqueous solution

Abstract

The pattern of association of the antihypertensive drug, trimetaphan camphorsulphonate, in water and aqueous electrolyte solution has been examined using light scattering techniques. Light scattering curves exhibited no significant discontinuity attributable to a critical micelle concentration and could not be simulated using the mass action model for micellar association. The applicability of several one- and two-parameter models of stepwise association has been examined. The association was most satisfactorily described using a two-parameter model in which the equilibrium constant for dimerization was considered as an independent variable and higher stepwise association constants were identical. This model of association successfully predicted the number-average degree of association of the system as determined by vapour pressure osmometry. The concentration dependence of the surface activity at the air/solution interface was also consistent with stepwise association.