MISCIBILITY OF BINARY PHOSPHOLIPID MIXTURES UNDER HYDRATED AND UNHYDRATED CONDITIONS .1. PHOSPHATIDIC ACIDS WITH DIFFERENT ACYL CHAIN-LENGTH

The miscibility of binary mixtures of phosphatidic acids (PA) with different acyl chain length (n = 14, 16 and 18) was examined in the absence of and presence of added water, based on phase diagrams constructed by DSC measurements. The solidus and liquidus curves obtained with unhydrated systems exhibited a minimum isoconcentration point, indicating the non-ideal mixing in both solid and liquid phases. The non-ideality of mixing increased with the chain length difference between the two components. The phase diagrams for hydrated PA mixtures showed a horizontal solidus line, which demonstrates the complete demixing of the two lipid species in solid-gel phase over the whole composition range. In the liquid-crystalline phase of hydrated PA bilayers, the lateral phase separation was suggested by the appearance of a horizontal portion in the liquidus line. The phase diagrams obtained with unhydrated systems were analyzed according to the thermodynamic equations based on the Bragg-Williams approximation. The difference in the mixing behavior was interpreted qualitatively in terms of the pair-interaction in hydrated and unhydrated mixtures.