Effects of Divalent Cations on Phase Behavior and Structure of a Zwitterionic Phospholipid (DMPC) Monolayer at the Air−Water Interface

Effects of divalent cations (Ca²⁺, Mg²⁺, Ni²⁺, and Zn²⁺) on a zwitterionic phospholipid monolayer at the air−water interface are investigated by surface pressure−area isotherms and in situ X-ray scattering. Divalent cations lower the surface pressure for the fluid (LE) to condensed (L₂) phase transition in a strongly ion-specific manner. Surprisingly, the two-dimensional lattice dimensions and the tilt of the lipids’ alkyl tails in the L₂ phase show a nearly ion-nonspecific dependence on the excess surface pressure above the transition pressure. An empirical “universal” relationship was found between the tail tilt and the excess pressure, with the tails in the L₂ phase always displaying a tilt of 29° at the transition. A practical implication of these results is that, regardless of the divalent cation present, the microscopic details of the lipid tail packing in the L₂ phase can be deduced at any surface pressure once the transition pressure is obtained from isotherms.