Mixing of Partially Fluorinated Carboxylic Acids and Their Hydrocarbon Analogues with Dipalmitoylphosphatidylcholine at the Air−Water Interface

The interaction of partially fluorinated carboxylic acids with a biologically relevant surfactant, dipalmitoylphosphatidylcholine (DPPC), was investigated at the air−water interface. The compression isotherms of mixtures of three partially fluorinated carboxylic acids ( 1 − 3 ) and their hydrocarbon analogues ( 4 − 6 ) with DPPC were recorded at various compositions on hydrochloric acid (pH = 1.9, 32 ± 2 °C) as a subphase. The mixing behavior was assessed by analyzing the concentration dependence of the average molecular area at constant film pressure (area/mole fraction or A − X diagram). All six carboxylic acids ( 1 − 6 ) show a negative deviation from ideal behavior at surface pressures between 3 and 25 mN/m, which is indicative of an attractive interaction with DPPC in the mixed monolayer at the air−water interface. With the exception of nonafluorpentadecanoic acid ( 1 ), all carboxylic acids investigated show a concentration dependence of the breakpoint of the phase transition from the liquid-expanded to the liquid-condensed state, which supports the interpretation of the A − X diagrams.