Surface Adsorption and Volume Behavior of Local Anesthetics

The surface tension and densities of the aqueous solutions of six hydrochloride salts of local anesthetics in clinical use, tetracaine (TC·HCl), procaine (PC·HCl), dibucaine (DC·HCl), bupivacaine (BC·HCl), mepivacaine (MC·HCl), and lidocaine (LC·HCl), were measured as a function of the molality at 298.15 K under atmospheric pressure. It was observed that the surface tension vs molality and density vs molality curves of DC·HCl and TC·HCl have distinct break points at concentrations corresponding to the critical micelle concentration (cmc) and the surface tension vs molality curves of DC·HCl, TC·HCl, and BC·HCl have other break points at lower concentrations than the cmc. The surface densities of anesthetics were evaluated numerically by applying the thermodynamic equations to the surface tension data. They decreased in the order of DC·HCl, TC·HCl, BC·HCl, MC·HCl, LC·HCl, and PC·HCl in accordance with anesthetic potency. Further, the adsorbed films of DC·HCl, TC·HCl, and BC·HCl were turned out to occur phase transition from a gaseous state to an expanded state by examining the surface pressure vs area per adsorbed molecule curves. On the other hand, by analyzing the density data, volume properties of anesthetics in aqueous solution were evaluated and the volume parameters of micelle formation of DC·HCl and TC·HCl were also determined. The surface adsorption and volume behavior of these anesthetics were revealed on the basis of the thermodynamic quantities.