Thermodynamics of water-lipid interaction in presence and absence of salts

Extents of hydration of different lipids such as cholesterol, lecithin, lysolecithin, gangliosides, hydroxy cerebrosides, sphingomyelin, Dalda (hydrogenated fat), coconut oil and sunflower oil in the presence and absence of some inorganic electrolytes, sucrose and urea have been studied using the isopiestic vapour pressure technique. Except triglycerides, the shape of the water vapour adsorption isotherms for other lipids are in agreement with type II or type III BET isotherms respectively. The overall shape of each curve for triglycerides does not agree with any of the shapes of five types of BET isotherms. At water activity (1) approaching unity, maximum number of moles of water bound (delta n1zero) per mole of cholesterol are 1.6 and 2.6 at 28 degrees C and 37 degrees C, respectively. Values of delta n1zero at 23 degrees C stand in the following order: Ganglioside > lysolecithin > sphingomyelin > hydroxy cerebrosides. For triglycerides n1 increases with increase of a1 and reaches maximum in between 0.95 to 0.98 beyond which it sharply decreases to zero as a1 approaches unity. Standard free energy changes (delta Gzero) and enthalpy changes (delta Hzero) for cholesterol and lysolecithin have been evaluated from the experimental data. The excess hydration of cholesterol in the presence of several inorganic salts, urea and sucrose have been estimated from isopiestic experiments and free energies of excess hydration have been evaluated.