Volume study on the micelle formation of local anesthetic dibucaine hydrochloride and dodecyltrimethylammonium chloride mixtures

Densities of aqueous solutions of mixtures of dibucaine hydrochloride (DCHC), a local anesthetic, and dodecyltrimethylammonium chloride (DTAC) were measured as functions of total molality. The apparent molar volumes of the mixtures were calculated from the density data. The molar volumes in the monomeric and micellar states were evaluated from the dependence of the apparent molar volume on the total molality. The critical micelle concentration (cmc) of the mixtures was determined from the break point of the apparent molar volume vs. the total molality curves, while the micellar composition was evaluated from the dependence of the cmc on the mole fraction of DTAC in the mixture. The composition diagram of micellization has a swollen cigar shape, slightly distorted in the composition range near the DTAC component; such a distortion indicates slightly non-ideal mixing between the hydrophobic groups of DCHC and DTAC molecules in the micelle. However, the molar volume of micellar state varies depending upon the micellar composition along a straight line connecting the molar volumes of DCHC and DTAC. Moreover, the volume of micelle formation is smaller for DCHC, which has larger monomer and micellar volumes than DTAC, and increases linearly with increasing DTAC composition in the micelle. Therefore, the bulky structure of the hydrophobic part of the DCHC molecule affects both the miscibility and the volume behavior in the DCHC-DTAC mixed micelles.