EFFECT OF COUNTERIONS ON THE SURFACE-ADSORPTION AND VOLUME BEHAVIOR OF MICELLE-FORMING LOCAL-ANESTHETICS

The surface tension and densities of aqueous solutions of micelle-form ing local anesthetic dibucaine and tetracaine salts having different h alide ions as counterions from each other (dibucaine hydrochloride (DC . HCl), hydrobromide (DC . HBr) and hydroiodide (DC . HI), and tetrac aine hydrochloride (TC . HCl) and hydrobromide (TC . HBr)) were measur ed as a function of the molality at 298.15 K under atmospheric pressur e. The thermodynamic quantities of adsorption and volume for the anest hetics were evaluated numerically based on the experimental results. T he surface densities of the anesthetics increased in the order of chlo ride, bromide, and iodide salts. This behavior may result from differe nces in the degree of three factors, that is, a hydration interaction between water molecules and counterions, the counterion distribution i n the surface-adsorbed film, and ion-pair formation in the adsorbed hi m between anesthetic cations and counterions. It was further shown fro m the surface pressure vs. area per adsorbed molecule curves that the adsorbed films of the anesthetics undergo a phase transition from a ga seous state to an expanded state at low concentrations. On the other h and, the volumes of micelle formation for DC . HCl and DC . HBr have b een found to be almost equal to each other. This fact means that the e ffect of these halide counterions on the volume of micelle formation i s considerably small in contrast to the results of surface adsorption.