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-forming local anesthetic dibucaine and tetracaine salts having different halide ions as counterions from each other (dibucaine hydrochloride (DC·HCl), hydrobromide (DC·HBr) and hydroiodide (DC·HI), and tetracaine hydrochloride (TC·HCl) and hydrobromide (TC·HBr)) were measured as a function of the molality at 298.15 K under atmospheric pressure. The thermodynamic quantities of adsorption and volume for the anesthetics were evaluated numerically based on the experimental results. The surface densities of the anesthetics increased in the order of chloride, bromide, and iodide salts. This behavior may result from differences in the degree of three factors, that is, a hydration interaction between water molecules and counterions, the counterion distribution in the surface-adsorbed film, and ion-pair formation in the adsorbed film between anesthetic cations and counterions. It was further shown from the surface pressure vs. area per adsorbed molecule curves that the adsorbed films of the anesthetics undergo a phase transition from a gaseous state to an expanded state at low concentrations. On the other hand, the volumes of micelle formation for DC·HCl and DC·HBr have been found to be almost equal to each other. This fact means that the effect of these halide counterions on the volume of micelle formation is considerably small in contrast to the results of surface adsorption.