Dissociation equilibrium between uncharged and charged local anesthetic lidocaine in a surface-adsorbed film

Hitoshi Matsuki, Michio Yamanaka, Hiroshi Kamaya, Shoji Kaneshina, Issaku Ueda

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10 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)512-520
Number of pages9
JournalColloid and Polymer Science
Issue number5
Publication statusPublished - Feb 1 2005

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Polymers and Plastics
  • Colloid and Surface Chemistry
  • Materials Chemistry


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