Preferential partitioning of uncharged local anesthetics into the surface-adsorbed film

The surface tension and pH of aqueous solutions of three hydrochloric acid (HCl) - uncharged anesthetic (mepivacaine (MC), bupibacaine (BC) and dibucaine (DC)) mixtures were measured as a function of total molality and composition of local anesthetic in order to investigate the competitive surface-adsorption of uncharged and charged local anesthetics. The behavior of the surface tension versus total molality and pH versus total molality curves remarkably changed at the composition corresponding to an equimolar mixture. The pH measurements showed that uncharged and charged forms coexisted only at compositions more than the equimolar mixture. The partitioning quantities of respective uncharged and charged anesthetics into the surface-adsorbed film were estimated from their surface densities calculated thermodynamically. The greater quantity of uncharged anesthetics existed in the adsorbed film at the coexisting composition, that is, the uncharged anesthetics adsorbed more preferentially than charged ones. The relative ease with which uncharged anesthetics transferred into the surface-adsorbed film was proportional to the hydrophobicities and well correlated the anesthetic potencies. At compositions in the vicinity of physiological pH (ca. 7.4), the bulk solution is more abundant in charged anesthetics than uncharged ones, whereas the uncharged molecules is conversely more abundant in the surface region. The present results clearly imply that the surface-active molecule of local anesthetic in the physiological pH is the uncharged form and the partitioning is greatly dependent on the hydrophobicity among the anesthetics.