1980 Volume 53 Issue 8 Pages 2217-2220
The surface pressure of mixed monolayers of octadecylammonium chloride and tetradecanoic acid has been measured as a function of mean area per film molecule at various compositions and temperatures. It has been observed that the transition pressure from expanded to condensed state of either constituent decreases with the addition of the other one. This behavior has been called a negative azeotropy in the two-dimensional state in contrast to the positive azeotropy observed for the ethyl heptadecanoate–tetradecanoic acid system. By applying the thermodynamic treatment, the phase diagram has been constructed and the apparent molar entropy and energy associated with the phase transition have been calculated. The results support the view that the negative azeotropic transformation in the mixed monolayer is brought about by the mutual interaction between octadecylammonium chloride and tetradecanoic acid molecules which is stronger than that between the same kind of molecules.
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