Investigation of physico-chemical behaviour of local anaesthetics in aqueous SDS solutions

The behaviour of local anaesthetics tetracaine (TCA), procainamide (PAM) and novocaine (NOV) in aqueous sodium dodecyl sulfate (SDS) solutions has been investigated by steady-state fluorescence emission spectroscopy, electrical conductance and reaction kinetics. A sharp increase of fluorescence emission of local anaesthetics (LA) in aqueous surfactant solutions at concentrations close to the critical micelle concentration (cmc) was observed due to the formation of premicellar aggregates. Plots of the specific conductance, k, against [SDS], measured in the presence of a fixed amount of LA, clearly had two break points. Rates of the alkaline hydrolysis of the ester function of LA were dramatically reduced in the presence of SDS at concentrations close to the cmc (similar to 8.3 mM). Both the increase in fluorescence emission intensity and the reduction of hydrolysis reaction rates occur within a very narrow range of SDS concentration near the cmc and then level off/down. The strongest incorporation into negatively charged micellar aggregates was observed for TCA, the anaesthetic having the strongest hydrophobicity, and the weakest incorporation was observed for PAM, the least hydrophobic molecule. This result is important in the correlation of the anaesthetic potency of LA with their hydrophobicity.