Electrokinetic measurements reveal interfacial charge at polymer films caused by simple electrolyte ions

The interfacial charge on spin-coated films of poly(tetrafluoroethylene-co-2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole) (Teflon AF) was studied by streaming potential and streaming current measurements in diluted aqueous solutions of potassium chloride, potassium hydroxide, and hydrochloric acid. xi potential and surface conductivity were derived from electrokinetic data determined at varied concentrations of the electrolytes by means of the novel microslit electrokinetic setup (ref 1: J. Colloid Interface Sci. 1998, 208, 329). The results obtained revealed the high relevance of unsymmetrical (preferential) adsorption of ions as the origin of charge formation at unpolar polymer materials in aqueous environments. The preferential adsorption of hydroxide ions (OH-) was found to predominate as compared to the adsorption of hydronium ions (H3O+) at similar concentrations, i.e., in solutions of neutral pH. No effect of preferential adsorption was induced by chloride (Cl-) and potassium (K+) ions. For the first time xi potential and surface conductivity data were evaluated to quantify the charge density of the inner layer at the polymer-water interface. The results indicate the presence of both cations and anions in the stagnant layer in all analyzed cases. The charge density and the total ion concentration in this inner part of the electrical double layer were found to increase with increasing ionic strength of the solution. However, the total ion densities of the stagnant layer remained rather low. This suggests that the ions are localized in one interfacial plane. With regard to the origin of the characterized charging process we conclude that the OH- and H3O+ ions capability to form hydrogen bonds and the difference in the structures of the hydration shells of these two ions are of highest importance to explain the observed phenomena.