Surface tension study on the molecular-aggregate formation of local anesthetic dibucaine hydrochloride

Abstract

The formation of molecular aggregates of local anesthetic dibucaine hydrochloride (DC·HCl) was investigated by measuring the surface tension of aqueous DC·HCl solutions as a function of temperature at various concentrations around the critical micelle concentration (CMC). The thermodynamic quantities concerning the aggregate formation of DC·HCl, surface density, entropy and energy changes of adsorption from the monomeric and micellar states, and entropy change of micelle formation, were calculated. They were compared with the corresponding results of straight-chain cationic surfactants, decylammonium chloride (DeAC) and decyltrimethylammonium chloride (DeTAC). From the surface pressure versus area per adsorbed molecule curves, the surface-adsorbed film became more expanded in the order of the geometry of their molecular structures, DeAC<DeTAC<DC·HCl, at a high surface pressure. The entropy and energy changes of adsorption showed that the surface adsorption of DC·HCl roughly resembles DeTAC rather than DeAC. On the other hand, the behavior of entropy change of micelle formation versus temperature curves suggested that the micelle formation of DC·HCl may be different from those of DeAC and DeTAC.