Incorporation of the local anesthetic procaine hydrochloride (PC·HCl) into surface-adsorbed films and micelles formed by decylammonium chloride (DeAC) was investigated by measuring the surface tensions of aqueous solutions of a DeAC-PC·HCl mixture. The thermodynamic quantities of the miscibility in these molecular aggregates, the total surface density, and the compositions of PC·HCl in the surface-adsorbed film and micelle were calculated by applying thermodynamic equations to the surface tension data. The phase diagrams of surface adsorption and micelle formation showed a peculiar feature; namely, the compositions of PC·HCl in the adsorbed film and micelle showed negative values over the whole composition range measured. The negative values were caused by the exclusion of the PC·HCl molecules from the adsorbed film and the micelle of DeAC. PC·HCl was not incorporated into such hydrophobic environments. In contrast, tetracaine hydrochloride (TC·HCl) was incorporated into the adsorbed film and micelle of DeAC at all DeAC-TC·HCl compositions. The difference in incorporation between TC·HCl and PC·HCl is attributable to the hydrophobicity of their molecules.
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