Effects of Hofmeister Anions on DPPC Langmuir Monolayers at the Air−Water Interface

In this work we investigated the effect of sodium salts of different monovalent anions belonging to the Hofmeister series on Langmuir monolayers of DPPC (1,2-dipalmitoyl phosphatidylcholine). The salts used were NaCl, NaBr, NaNO3, NaI, NaBF4, NaClO4, and NaSCN. The monolayer phase behavior and the morphology and structure of the lipid phases were studied by surface pressure−area isotherms, Brewster Angle Microscopy (BAM), Grazing Incidence X-ray Diffraction (GIXD), and Infrared Reflection−Absorption Spectroscopy (IRRAS). The presence of electrolytes in the subphase was found to increase the surface pressure at a fixed area per molecule, indicating a stabilization of the liquid-expanded phase of the monolayer. This increase is different for different anions and different electrolyte concentrations. X-ray diffraction and infrared spectroscopy experiments show that moderate concentrations of chaotropic anions, such as I-, do not significantly change the conformation and packing properties of the hydrocarbon chains. The lattice parameters and the ordering of the lipid molecules in the liquid-condensed phase remain essentially unaffected, even at quite high electrolyte concentrations. This finding suggests that anions partition into or bind to the looser liquid-expanded phase, thus providing entropic stabilization of that phase, but do not penetrate into or bind to the domains of the liquid-condensed phase. This intriguing result is discussed in terms of the possible modes of interaction of anions with lipid interfaces.