The influence of electrolyte ions on the interaction forces between polystyrene surfaces

The forces between a flat polystyrene film and a polystyrene sphere have been measured in water and solutions of KCl and KOH. All force-distance curves show at distances >5-50 nm an electrical double-layer repulsion whose range and strength depend on the electrolyte concentration. The electrical double-layer potential at the polystyrene surface coincides with its zeta potential. An attractive force of variable range and strength dominates at shorter distances in water, KCl concentrations up to 10−2 M and a 10−5 M KOH solution. We attribute it to the existence of air nanobubbles on the polymer surface. It is not affected by the KCl concentration. In KOH concentrations ≥10−4 M, the attraction disappears. Instead, an additional “soft” steric repulsion occurs at distances <5-20 nm.