Cholesterol-dipalmitoyl cephalin and cholesterol-dilauroyl cephalin monolayers spread on silicic acid substrates

Mixed cholesterol-dipalmitoyl cephalin and cholesterol-dilauroyl cephalin monolayers are slightly more expanded on silicic acid substrates than on silica-free substrates. Plotting the mean molecular area of the mixed monolayers against the mole fraction of cephalin shows that cholesterol produces condensation of the cephalin monolayer whether or not the substrate contains silicic acid, and the more expanded the pure phospholipid film, the greater is the condensation produced. These phenomena have been tentatively interpreted in terms of hydrophobic interactions between hydrocarbon chains and electrostatic interactions between the horizontally oriented polar groups of the cephalin molecules.