Part 2. Crossover from depletion attraction to adsorption: Polyethylene glycol induced electrostatic repulsion between lipid bilayers

Using the surface forces apparatus, we have measured the static and hydrodynamic forces between two substrate-supported lipid bilayers in water and semidilute, aqueous solutions of poly(ethylene glycol) (PEG), a neutral polymer for which water is a good solvent. In part 1 (previous paper in this issue), we presented detailed measurements of depletion attraction in the presence of 8000 molecular weight PEG. In part 2 (this work), we address the behavior of higher molecular weight 20 000 PEG. Specifically, the higher molecular weight PEG adsorbs to the surfaces and both a steric as well as an unexpected electrostatic repulsion were measured. We determined that higher MW PEG chelates metal cations and thereby induces an apparent electrostatic charge on the bilayer surfaces upon adsorption.