The mixing behavior of hydrated dimyristoylphosphatidylcholine and its partially fluorinated analogue, di(nonafluorotetradecanoyl)phosphatidylcholine, where all hydrogens of the terminal n-butyl segments in two hydrophobic chains are replaced by fluorines, has been investigated by calorimetry and X-ray diffraction. The analysis of the calorimetric data by using the regular solution theory has revealed that the phase diagram of the mixtures is a eutectic type with a miscibility gap. The non-ideality parameters of the gel and liquid-crystalline phases are estimated to be 5.7 +/- 0.1 and 4.2 +/- 0.1 kJ/mol, respectively. These large values indicate that the mixing is significantly non-ideal. X-ray diffraction has shown that a phase separation occurs in the miscibility gap. (c) 2013 Elsevier B.V. All rights reserved.