Effect of Electrolyte Adsorption on Adhesion between Charged Surfaces in Aqueous Conditions

In order to investigate interfacial adhesion behaviors, normal interaction forces between negatively charged mica surfaces were directly measured in various electrolyte aqueous solutions at high ionic strength with the well-established surface forces apparatus technique, which had distance and force measuring resolutions of down to 1 Å and ~10 nN, respectively. In pure water, a weak but long-range electrostatic repulsion force between the surfaces was detected and this interaction force gradually increased as the surfaces were slowly approached. When approaching the surfaces to a separation of less than 50 Å, the lower mica surface supported by a cantilever spring suddenly jumped to the fixed upper mica surface because of the van der Waals attraction between the mica substrates, resulting in direct contact of the two mica surfaces at separation of 0 Å. Upon subsequently separating the two surfaces, an interfacial adhesive force of about −46.7 mN/m was measured when the lower mica surface jumped out of contact from the upper mica surface. In comparison, the electrostatic repulsion effect between the mica surfaces was totally screened in 0.1 mol/L K₂SO₄ solution due to high ionic strength and no interaction force was detected before the surfaces came into the similar jump-into contact. Since K⁺ ions electrostatically adsorbed onto the negatively charged mica surfaces robustly, the separation between the surfaces maintained at around 5 Å even though the surfaces came into hard wall contact, and the adhesive force measured at the jump-out moment was remarkably lowered to about −2.9 mN/m. In 0.1 mol/L Ca(NO₃)₂ solution, Ca²⁺ ions also bound onto mica because of electrostatic attraction, but the adsorption of Ca²⁺ ions induced a significant short-range hydration repulsion between the two surfaces, rather than electrostatic repulsion. Such hydration interaction force was not detected in the monovalent K₂SO₄ solution. The binding strength of Ca²⁺ ion hydration layers on mica was weak so that the adsorbed Ca²⁺ ions completely desorbed in relatively low load conditions, resulting in direct contact between the mica surfaces with adhesive force up to around −40.7 mN/m. The addition of Ca(OH)₂ into 0.1 mol/L K₂SO₄ solution merely yielded a weak short-range repulsion and the adhesion status between the mica surfaces was virtually the same to that in the pure K₂SO₄ solution. Robustly adsorbed K⁺−Ca²⁺ mixed layers with strong hydration repulsion did not form on the mica surfaces in the 0.1 mol/L K₂SO₄ solutions mixed with Ca(OH)₂. Polyelectrolyte poly(carboxylate ether) (PCE) adsorbed onto mica from 0.1 mol/L K₂SO₄ solution and the adsorption layer was around 50 Å thick. Strong long-range steric repulsion was detected between the opposing PCE adsorption layers, but the adsorbed PCE molecules were completely squeezed out from between the confined mica surfaces in moderate compression conditions, leading to weak interfacial adhesive contact as observed in the pure 0.1 mol/L K₂SO₄ solution. In contrast, poly(naphthalene sulfonate) (PNS) exhibited high adsorption strength on mica without any desorption even at hard wall contact level in 0.1 mol/L K₂SO₄ solution. The thickness and the maximum compression rate of the PNS adsorption layer were about 57 Å and 43.9%, respectively. The steric repulsion from the opposing PNS adsorption layers completely prevented the two mica surfaces from coming into the van der Waals attraction range, thereby thoroughly avoiding adhesive contact. Robust binding/adsorption is a necessary condition for adsorbed electrolytes to tune interfacial adhesion.