Phospholipid interactions in model membrane systems. I. Experiments on monolayers

We study the lateral headgroup interactions among phosphatidylcholine (PC) molecules and among phosphatidylethanolamine (PE) molecules in monolayers and extend our previous models. In this paper, we present an extensive set of pressure-area isotherms and surface potential experiments on monolayers of phospholipids ranging from 14 to 22 carbons in length at the n-heptane/water interface, over a wide range of temperature, salt concentration, and pH on the acid side. The pressure data presented here are a considerable extension of previous data (1) to higher surface densities, comprehensively checked for monolayer loss, and include new data on PE molecules. We explore surface densities ranging from extremely low to intermediate, near to the main phase transition, in which range the surface pressures and potentials are found to be independent of the chain length. Thus, these data bear directly on the headgroup interactions. These interactions are observed to be independent of ionic strength. PC and PE molecules differ strongly in two respects: (a) the lateral repulsion among PC molecules is much stronger than for PE, and (b) the lateral repulsion among PC molecules increases strongly with temperature whereas PE interactions are almost independent of temperature. Similarly, the surface potential for PC is found to increase with temperature whereas for PE it does not. In this and the following paper we show that these data from dilute to semidilute monolayers are consistent with a theoretical model that predicts that, independent of coverage, for PC the P-N+ dipole is oriented slightly into the oil phase because of the hydrophobicity of the methyl groups, increasingly so with temperature, whereas for PE the P-N+ dipole is directed into the water phase.