Mixed Monolayer Properties of Tetradecanoic Acid with n-Perfluorocarboxylic Acids with 10, 12, 14, 16, and 18 Carbon...

Surface pressure (pi) and surface potential (DeltaV)-area (A) isotherms were obtained for mixed monolayers of different perfluorocarboxylic acids (FCs) with a hydrocarbon fatty acid, tetradecanoic acid (HC14), on a substrate solution of pH 1.0 as a function of compositions in the mixture by employing the Langmuir method and the ionizing electrode method. Data for the mixed monolayers were analyzed in terms of the additivity rule. Judging from Matuo's classification of two-dimensional phase diagrams, our phase diagrams might be classified into three types. The first is a positive azeotrope type; the combinations of tetradecanoic acid (HC14) with perfluorodecanoic acid (FC10) and with perfluorododecanoic acid (FC12) seem miscible with each other. The second is a completely immiscible type; the combination of perfluorotetradecanoic acid (FC14) and tetradecanoic acid. The third might be a eutectic type, to which the combinations of HC14 with FC16 and with FC18 are assigned; they are miscible in the expanded state but immiscible in the condensed state. Furthermore, the data on the mean molecular area, the surface dipole moment, and the phase diagrams constructed enabled us to estimate the molecular orientation of the respective acids in the mixed monolayer state.