Selective adsorption of poly(ethylene oxide) onto a charged surface mediated by alkali metal ions

Using a surface force balance, we have measured normal and shear interactions between mica surfaces across pure water and across 0.1 M aqueous solutions of LiNO₃, NaNO₃, KNO₃, and CsNO ₃, both prior to adding polymer and following addition of 1.5 × 10^-4 w/w poly(ethylene oxide) (PEO, M_w = 170 kD) and overnight incubation. Our results reveal that while the PEO adsorbs strongly from the KNO₃ and CsNO₃ solutions, unexpectedly it does not adsorb at all from the LiNO₃ and NaNO₃ salt solutions. We attribute this to the different nature of the hydration layers about the alkali metal ions: these favor liganding to the negatively charged mica surface of the etheric -O- group on the ethylene oxide monomer for the case of the more weakly hydrated K⁺ and Cs⁺, but not for the case of Na⁺ or Li⁺ with their more strongly bound water. A simple model relating the electrostatic energy changes occurring upon such liganding to the experimentally measured hydration energies of the different alkali metal ions supports this attribution.