Landau theory of two-component phospholipid dispersions

Improving the Priest model (Priest, R.G., 1980, Mol. Cryst. Liq. Cryst. 60: 167-184) not only the phase transition temperatures and phase diagrams of one- and two-component lecithin homologues but also the calorimetrical spectra and cooperative unit sizes have been determined. In the case of one-component systems, the obtained cooperative unit sizes are higher, by a 1,5-2 factor, than the values derived from the generally used simple, but restrictly valid, van't Hoff enthalpy to the calorimetrical enthalpy (DHvH/DHcal) formula. Concerning the two-component systems, we can state that: 1) The shapes of the calorimetrical spectra depend strongly on the values of the cooperative unit sizes. 2) By means of the model the experimentally obtained calorimetrical spectra can be reproduced. The cooperative unit size is the only model parameter. 3) The mixing of the components results in the drastic decrease of the cooperative unit size. 4) During the mixing, the greater the chain length difference between the components, the stronger is the decrease of the cooperative unit size. 5) The lower limit of the cooperative unit size is about ten.