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

Abstract

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, NaNO 3 , NaI, NaBF 4 , NaClO 4 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 This intriguing result is discussed in terms of the possible modes of interaction of anions with lipid interfaces.

Effects of Sodium Salts of Lyotropic Anions on Low-Temperature, Ordered Lipid Monolayers

“…SCN -is known to have a strong effect on DPPC monolayers at higher temperatures. [1][2][3][4][5] The partition coefficient of SCN -between the monolayer and bulk water at 22 C is among the highest calculated, implying in fact a specific interaction of the ion with the phospholipids. 4 Two separate populations of SCN -ions have been detected under DPPC monolayers using vibrational sum frequency generation spectroscopy.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In the last decade, the mechanism of specific ionic action in several systems has been considerably clarified through intensive research effort in this area (see recent reviews, journal issues, and books on the topic [6][7][8][9][10][11][12][13][14][15][16][17][18] ). It has been established that specific ion action involves a fine balance of interactions between water (the usual solvent), ions, and a solute or a surface, depending on the system.…”

“…[1][2][3][4][5] Our monolayer work focuses primarily on anions. We formerly worked at room temperature (22 ºC), and used a variety of simple sodium salts and the classical phospholipid 1,2-dipalmitoyl-sn-glycero-phosphocholine (DPPC).…”

“…3,4,14 The ions create disorder, favoring the liquid-expanded phase over the liquid condensed phase. 1 The partitioning chemical potentials of several anions between DPPC monolayers and bulk water were evaluated with an appropriate electrostatic model. They were found to favor interfacial solubilization for the chaotropic anions of the lyotropic series.…”

“…We have used NaCl, NaBr, NaNO 3 , NaSCN, NaClO 4 , NaPF 6 , and sodium tetraphenylborate (NaTPB) as electrolytes, to span the greatest possible range of the lyotropic series. Chloride was found to have a very marginal interaction with DPPC monolayers at room temperature, [1][2][3][4] and is in fact found in many phenomena to exhibit "indifferent" behavior and to be at the border between kosmotropic and chaotropic anions. [6][7][8][13][14][15] At the other end of the scale tetraphenylborate is not usually considered to belong to the lyotropic series.…”