Lateral Interactions Among Phospholipid Head Groups at the Heptane/Water Interface

The strong lateral repulsions among phosphatidylcholine head groups in monolayers and bilayers, and their large positive dependence on temperature, have long been considered puzzling. Here we analyze the careful and extensive experiments of Taylor et al. on the lateral pressure-area isotherms of phosphatidylcholine molecules at low density at the heptane/water interface to determine the second and third virial coefficients. We investigate the possible origins of these repulsions through consideration of the in-plane and out-of-plane dipole interactions among the zwitterions and among the carbonyl groups. We show that spectroscopic and thermodynamic experiments are consistent only with a model in which the principal dipolar repulsion arises from a small average out-of-plane displacement of the N⁺ end of the head group, surprisingly directed back toward the oil phase, rather than into the water phase, and increasing with temperature. The repulsion between head groups is enhanced significantly by out-of-plane dipole fluctuations.