Effects of sodium salts of lyotropic anions on low-temperature, ordered lipid monolayers

Electrolytes are known to impart considerable disorder to lipid assemblies, including monolayers at the air-water interface, bilayers, and vesicles. In the present work, we have investigated the disordering effect of sodium salts of monovalent anions that span the lyotropic series on the monolayers of 1,2-dipalmitoyl-sn-glycero-phosphocholine (DPPC) at 12 °C. Pressure-area isotherms, Brewster-angle microscopy (BAM), grazing-incidence X-ray diffraction (GIXD), and infrared absorption-reflection spectroscopy (IRRAS) were used to investigate in complementary ways the salt effects on lipid monolayers. At 12 °C these effects were found to be quite dramatic, a major finding being that the liquid-expanded phase, which is not present at this temperature on a pure water subphase, reappears and dominates in the presence of electrolytes. Salts expand the monolayer, destroy the ordered phase that exists at zero pressure, and affect the ordering of the lipid chains and their tilt angle in the liquid condensed phase. Finally, very chaotropic anions force DPPC lipids to adopt an untilted conformation in the condensed phase, an unprecedented finding for nonmixed Langmuir monolayers of this phospholipid. A distinctly different behavior of very chaotropic anions from that of normal chaotropic ones thus emerges. The effect of the former is not just a limited perturbation of the lipid assembly but a major disruption of the structure, which arises from competition between the lipids and the ions for interfacial sites.