In order to show that thermodynamic treatment is useful in the study on the monolayer penetration, the surface tension has been measured with the system consisting of pentadecanoic acid monolayer and dodecylammonium chloride solution. Applying the thermodynamic equations to the experimental data, the surface density of dodecylammonium chloride, and entropy and energy of penetration have been estimated numerically. It has been found that the surface density, entropy, and energy all decrease with increasing the concentration of film-forming substance. Therefore, it has been concluded that the penetration of monolayer of this system is caused by the decrease of the energy of penetration which compensates the decrease of the entropy of penetration.
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