Miscibility and phase behavior of DPPG and perfluorocarboxylic acids at the air–water interface

The miscibility and phase behavior of two components of phospholipids and perfluorocarboxylic acids [FCn; perfluorododecanoic acid (FC12), perfluorotetradecanoic acid (FC14), perfluorohexadecanoic acid (FC16), and perfluorooctadecanoic acid (FC18)] have been systematically investigated using Langmuir monolayer technique. Dipalmitoylphosphatidylglycerol (DPPG) is utilized as a phospholipid component in biomembranes. Surface pressure (π)–molecular area (A) and surface potential (ΔV)–A isotherms have been measured for the DPPG/FCn systems on 0.15 M NaCl (pH 2.0) at 298.2 K. From the isotherm results, two-dimensional phase diagrams are constructed and classified into miscible and immiscible patterns. Furthermore, the phase behavior of the DPPG/FCn systems has been morphologically examined using fluorescence microscopy (FM) and atomic force microscopy (AFM). These images indicate different phases among the four systems. In particular, specific phase morphology is observed in the middle molar fraction range for the DPPG/FC14 system; FC14 is selectively excluded from mixed DPPG–FC14 monolayers to be concentrated in the phase boundary as surface pressure increases. Then DPPG is refined as a patched film. Moreover, the data obtained here are compared to those in the previous systems in which different kinds of phospholipids were treated. Through a series of the miscibility investigations, it is proposed that combinations of hydrophobic chain lengths and of polar headgroups contribute to the monolayer miscibility between phospholipids and perfluorocarboxylic acids.