Use of ion-exchange membranes to measure transfer free energies of charged local anesthetics: correlation to anesthetic potency.

Ion-selective electrodes, sensitive to local anesthetic cations, were prepared with carboxylated poly(vinyl chloride) (PVC) membranes. Three plasticizers with varying degrees of polarity were used to adjust the hydrophobicity of the membrane. The affinity of the drug to the ion-exchange membrane was measured by the electromotive force of the cation-selective electrodes. The difference in the transfer free energies of the anesthetics for the membrane was estimated in reference to dibucaine. The values correlated to their clinical potencies. By comparing drugs with similar structures, the transfer free energy per methylene moiety linked to the hydrophilic domain was found to be -1.7 kJ.mol-1, and that of Cl linked to the hydrophobic domain was -3.1 kJ.mol-1. Interferences from Na+ and K+ were estimated as the selectivity coefficients against dibucaine. The values were 2.6 x 10(-5) for Na+ and 1.2 x 10(-4) for K+. The ion-exchange membrane appears to mimic the surface properties of cell membranes. These cation-selective electrodes have potential applicability in measuring charged local anesthetic concentrations (activities) in biological materials under limited conditions.