Hydration force between phosphatidylcholine surfaces in aqueous electrolyte solutions

Abstract

The effects of varying the concentration of monovalent salts on the van der Waals and hydration forces acting between surfaces made with the phospholipid dipalmitoylphosphatidylcholine were studied using the osmotic pressure technique of Rand and Parsegian (D. LeNeveu et al. Nature (London) 259, 601–603 (1976)). Applied osmotic pressure versus lamellar separation relationships for the same lipid in different monovalent salt solutions (10 m M, 100 m M, or 1 M NaCl or KCl) were compared. Our results indicate that monovalent salts affect the force character between lipid surfaces in a manner different from that observed for mica surfaces (R. M. Pashley J. Colloid Interface Sci. 80, 153–162 (1981)) which has a different mechanism for adsorbing charges. Specifically, we found a qualitative decrease in both the net repulsive force and van der Waals force between DPPC bilayer surfaces as the concentration of the monovalent salt in solution increased. We infer that the decrease in hydration force is due to decreased structuring of the solvent between the lipid bilayers as the ionic concentration increases.