AZEOTROPIC TRANSFORMATION IN MIXED MONOLAYERS OF ETHYL HEPTADECANOATE AND TETRADECANOIC ACID

The surface pressure of mixed monolayers of ethyl heptadecanoate and tetradecanoic acid has been measured as a function of mean area at various compositions and temperatures. It has been observed that the phase transition takes place from the expanded to condensed monolayers over the whole range of compositions and the transition surface pressure has a maximum at a composition rich in tetradecanoic acid. With the aid of the thermodynamic method developed previously, the phase diagram has been constructed which is characterized by a positive azeotropy. Further, the molar entropy and energy changes associated with the phase transition have been calculated by applying the improved thermodynamic treatment. It has been clarified that the positive azeotropic transformation in the mixed monolayers is brought about by the mutual interaction between ethyl heptadecanoate and tetradecanoic acid molecules which is smaller than the interactions between the same kind of constituent molecules. © 1979.