The adsorption of dodecylammonium chloride (DAC), which is a cationic surfactant, at water/hexane interface has been studied by measuring the interfacial tension as functions of temperature and pressure at various concentrations of DAC in the aqueous solution. By applying the thermodynamic treatment developed previously to the experimental results, the thermodynamic quantity changes associated with interface formation and corresponding partial molar quantity changes of DAC have been evaluated. These values have been compared with those of sodium dodecyl sulfate (SDS), which is an anionic surfactant. It has been observed that the dependence of the entropy change on temperature and that of the volume change on pressure are remarkable. It has also been observed that DAC and SDS differ appreciably in the values of the partial molar entropy and energy of adsorption while they have similar values in the partial molar volume change of adsorption under atmospheric pressure.
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