Langmuir monolayer miscibility of single-chain partially fluorinated amphiphiles with tetradecanoic acid

The surface pressure (pi)-molecular area (A) and surface potential (DeltaV)-A isotherms have been measured for monolayers of tetradecanoic acid (myristic acid: MA), partially fluorinated amphiphiles [single-chain (perfluorooctyl)pentanol (F8C5OH) and single-chain (perfluorooctyl)pentylphosphocholine (F8C5PC)], and their two-component combinations in order to investigate their miscibility at the air/water interface. The data for these systems were analyzed in terms of an additivity rule and excess Gibbs free energy. An interaction parameter and an interaction energy between the two components were calculated from the Joos equation, which allows description of collapse pressures of miscible monolayers. Two-dimensional phase diagrams for the binary systems were constructed and found to be a positive azeotropic type. These results indicate that the two-component MA/F8C5OH and MA/F8C5PC monolayers are miscible in the monolayer state. To confirm their miscibility and phase behavior upon compression, morphological observations with fluorescence microscopy (FM), Brewster angle microscopy (BAM), and atomic force microscopy (AFM) have been performed. These observations show that the addition of F8C5OH or F8C5PC to MA makes MA ordered domains in the monolayer region fluidize very effectively and that a fern-like network is formed as a 3-D structure by over-compression beyond the monolayer collapse. The present paper systematically clarifies the miscibility between MA and F8C5OH or F8C5PC within the monolayer and indicates that these fluorinated chemicals may have a possibility of biomedical uses and applications.