Mixed Langmuir monolayers of cholesterol and 'essential' fatty acids

Langmuir monolayers of cholesterol and various fatty acids, such as stearic , oleic, linoleic, alpha -linolenic, and arachidonic acids. spread at the a ir/water interface are investigated. The system of cholesterol and stearic acid is found to be immiscible. with only one collapse, occurring at the sa me surface pressure for all composition range. However, surface pressure (p i) - area (A) isotherms of cholesterol/unsaturated fatty acids show a chara cteristic course with two collapse states. The pressure of the first collap se varies with the proportion of the components in the mixture. while the s econd collapse, occurring at the surface pressure characteristic of cholest erol alone, is independent of mole fraction of the investigated fatty acid. The application of the surface phase rule indicates that the unsaturated f atty acids/cholesterol mixtures are miscible up to the surface pressure cor responding to the first collapse. Negative values of the excess free energy of mixing in all composition ranges prove that the mixtures are stable. Th e interactions existing in mixtures of cholesterol and unsaturated fatty ac ids possessing even numbers of double bonds are strongest in the lower regi on of fatty acid proportion, and the results are consistent with the minimu m values of the excess free energy of mixing, indicating the most stable mi xtures. For cholesterol and unsaturated fatty acids with odd numbers of dou ble bonds the behavior is different, and the strongest interactions occur i n both low and high regions of mole fraction of an acid.