Water ion adsorption dominates charging at nonpolar polymer surfaces in multivalent electrolytes.

We systematically applied microslit electrokinetic experiments to explore the role of various electrolytes (KCl, CaCl(2), K(2)SO(4) and LaCl(3)) in the pH dependent charging of nonpolar poly(tetrafluoroethylene-co-2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole) (Teflon(R) AF) thin films. The results impressively reveal the dominating role of hydroxide and hydronium ions for the charging of water/hydrophobic wall interfaces even in the presence multivalent electrolyte ions. In any system the charging primarily reflected the preferential adsorption of hydroxide over hydronium ions while the electrolyte ions were concluded to determine the magnitude of the surface charge according to their hydration characteristics.