Landau theory of one and two-component phospholipid bilayers: II. Excess heat capacity functions and cooperative unit sizes

Improving the Priest model not only the phase transition temperature and the phase diagrams of one and two-component lecithin homologues but also the excess heat capacity curves and cooperative unit sizes are determined. Here we used a new method to determine the cooperative unit size. It is pointed out that this method and the generally used expression, the ratio of van’t Hoff enthalpy to the calorimetric enthalpy (DHvH/(DHcal) result in the same values for the cooperative unit size if the partition function can be approximated by means of the maximum-term method. This condition does not fulfil for one-component lecithin membranes, consequently the cooperative unit sizes obtained by the two methods differ by a 1.5-2 factor. A great advantage of this new method that one can determine the cooperative unit size of multicomponent systems too. As regards to the two-component systems, we can state the following: 1- The shapes of the excess heat capacity curves depend strongly on the values of the cooperative unit sizes. 2- By means of the model, the experimentally obtained excess heat capacity curves can be reproduced. 3- The mixing of the components results in the drastic decrease of the cooperative unit size. 4- During mixing, the greater the chain length difference between the components is, the stronger the decrease of the cooperative unit size.