The dissociation equilibrium between uncharged local anesthetic lidocaine (LC) and charged local anesthetic LC (LC•H+) in a surface-adsorbed film was investigated by measuring the surface tension and pH of aqueous solutions of a mixture of hydrochloric acid and LC. The surface tension values decreased slightly with increasing total molality mt at 0 ≤ X2 ≤ 0.5, where X2 is the mole fraction of LC in the mixture, while they decreased rapidly with increasing mt at 0.5 < X2 < 1. It was shown from the pH measurements that almost all LC molecules were changed into LC•H+ ions by protonation at 0 < X2 ≤ 0.5 and both forms coexisted only at 0.5 < X2 ≤ 1. The quantities of the respective LC and LC•H+ transferred from the aqueous solution to the adsorbed film, i.e., their surface densities, were calculated by applying the thermodynamic equations derived to the surface tension and pH data. A greater quantity of LC than LC•H+ existed in the adsorbed film at the coexisting composition. The partitioning behavior of LC and LC•H+ in the adsorbed film was characterized by three composition regions: (1) slight partitioning of low surface-active LC•H+ in the region at 0 ≤ X2 < 0.5, (2) preferential partitioning of LC at 0.5 < X2 < around 0.7, and (3) negative partitioning of LC•H+ at around 0.7 ≤ X2 < 1. The present results clearly indicate that uncharged local anesthetics transfer into hydrophobic environments such as cell membranes more than charged ones.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Polymers and Plastics
- Colloid and Surface Chemistry
- Materials Chemistry