The interaction of 11-(perfluorohexyl)-undecanoic acid, a partially fluorinated carboxylic acid, with five phosphatidylcholines (PCs) of chain length varying from C14 to C18 was investigated in monolayers at the air-water interface and in fully hydrated bilayers. For the monolayer studies, the compression isotherms of mixtures of the acid with the respective PC were recorded at various compositions on hydrochloric acid (PH = 1.9, 32 +/- 2 degreesC) 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) and, where applicable, the concentration dependence of the breakpoint of the phase transition from the liquid-expanded to the liquid-condensed state. All five binary mixtures showed a negative deviation from ideality at low surface pressures (5 mN/m). At higher surface pressures, a negative deviation was observed for the even-numbered-chain PCs, whereas the odd-numbered-chain PCs showed nearly ideal behavior (greater than or equal to 10 mN). The phase transition of the odd-numbered-chain PCs as well as the C16 PC (D16PC) was concentration dependent. The collapse pressures are in general concentration dependent over the entire concentration range with D17PC and D18PC being an exception. Mixtures of these two long-chain phospholipids display a concentration-independent collapse pressure at mid-to-high concentrations of the respective phospholipids. These observations suggest (partial) miscibility of the components at the air-water interface. For the bilayer studies, differential scanning calorimetry thermograms of fully hydrated acid-PC mixtures of varying mole fraction (X-PC ranging from 1 to 0.6) were recorded and the influence of the acid on the pre- and main transition of the PC was analyzed. At no mole fraction was a peak corresponding to excess acid (melting point = 62.5 degreesC) observed, suggesting that the acid is incorporated into the lipid bilayer, that is, not phase separated from the phospholipid. Similar to perhydrocarbon acids, the fluorinated acid eliminates the pretransition of all five PCs. However, in contrast to perhydrocarbon acid-PC mixtures, the temperature of the main phase transition hardly increases in the presence of the fluorinated acid although some peak broadening can be observed. The general differences in the phase behavior are attributed to a less favorable packing of the hydrophobic tails in the mono- and bilayer. The biological implications of the observed phase behavior of the fluorocarbon acid-PC mixtures are currently unknown, but immiscibilities in both mono- and bilayers suggest the presence of domains with high acid concentrations which may (adversely) impact the function of biological mono- and bilayers.