Phospholipid monolayers at the n-heptane/water interface. Part 2.-Dilute monolayers of saturated 1,2-diacyl-lecithins and -cephalins

Surface pressure against area isotherms for dilute monolayers of a homologous series of 1,2-diacyl-lecithins spread at the oil/water interface are presented as a function of temperature and salt concentration. Comparative results for di-C₁₄-cephalin monolayers are used to distinguish head-group effects. Over the whole range of areas studied the isotherms are independent of chain length and salt concentration but vary with head-group, with lecithins giving the higher pressures. From conventional Amagat-type plots the monolayers are non-ideal. Apparent co-areas for both lecithins and cephalins are large with the lecithins showing a marked positive temperature coefficient. Negative entropies of compression calculated from the lecithin data are larger than those for single chain ionised molecules spread at the oil/water interface. These phospholipids show little evidence of cluster formation at areas higher than the phase transitions reported in part 1, but undergo complex configurational changes on compression which depend on medium range dipolar interactions and steric interaction of the chains.