PARTITIONING OF CHARGED LOCAL-ANESTHETICS INTO MODEL MEMBRANES FORMEDBY CATIONIC SURFACTANT - EFFECT OF HYDROPHOBICITY OF LOCAL-ANESTHETICMOLECULES

The partitioning of five hydrochloride salts of local anesthetics, dib ucaine (DC . HCl), tetracaine (TC . HCl), bupivacaine (BC . HCl), lido caine (LC . HCl), and procaine (PC . HCl), into surface-adsorbed films and micelles formed by decylammonium chloride (DeAC) was studied by t he surface tension of aqueous solutions of DeAC-local anesthetic mixtu res. Thermodynamic quantities of the partitioning of the anesthetics, total surface density, and compositions of the anesthetics in the surf ace-adsorbed film and micelle were evaluated by applying thermodynamic equations to the surface tension data. The quantities of the anesthet ics partitioned into the molecular aggregates of DeAC were determined from the phase diagrams of surface adsorption and micelle formation. T he phase diagrams of surface adsorption and micelle formation showed t hat the local anesthetic partitioned into the surface-adsorbed film an d micelle of DeAC decreases in the order of DC . HCl, TC . HCl, BC . H Cl, LC . HCl, and PC . HCl. A good correlation was seen between the pa rtitioning order and anesthetic potency of these local anesthetics. Th e phase diagrams of DeAC-BC . HCl, DeAC-LC . HCl and DeAC-PC . HCl sys tems behaved peculiarly that the compositions of these systems in the adsorbed film and micelle had negative values. The results suggested t hat weakly hydrophobic anesthetics such as BC . HCl, LC . HCl, and PC . HCl did not partition into the hydrophobic environment of the adsorb ed film and micelle of DeAC. Strongly hydrophobic local anesthetics su ch as DC . HCl and TC . HCl partitioned into the aggregates of DeAC. T he difference is attributable to the hydrophobicity of their molecules . By comparing the compositions of micelle with those of surface-adsor bed film at the critical micelle concentration, it was shown that the partitioning of the anesthetics was also influenced by the geometry of the aggregates into which the anesthetics were partitioned.