Self-association of local anaesthetic drugs in aqueous solution.

The aggregation characteristics of a series of local anaesthetic drugs in water and electrolyte solution have been examined using total intensity light scattering, photon correlation spectroscopy and vapour pressure osmometry. The association of cinchocaine hydrochloride was micellar. An appreciable increase in the effective diffusion coefficient as solutions of cinchocaine were diluted close to the critical micelle concentration was observed and has been discussed. The association of amethocaine hydrochloride could be described using a co-operative stepwise association model. No association could be detected in water or 0.1 mol dm-3 electrolyte for butacaine hemisulphate and the hydrochlorides of procaine, proparacaine, mepivacaine, lignocaine, bupivacaine and prilocaine at drug concentrations of less than 0.2 mol dm-3.