电解质吸附对水基带电表面黏着的影响

为研究界面黏着，利用表面力仪在高离子强度电解质水溶液中测量负电云母表面之间的法向力.在纯水中，因范德华吸引，两表面跳跃至直接接触(间距0?)，分离两表面时测得界面黏着力为-46.7 m N/m.在0.1 mol/L K₂SO₄中，因K₊离子牢固吸附于云母，表面间距稳定于5?，黏着力仅-2.9 m N/m；在0.1 mol/L Ca(NO₃)₂中，云母表面吸附的Ca₂⁺离子产生显著短程水合排斥，但在较低载荷下解吸附，导致两表面直接接触，黏着力高达-40.7 m N/m.加入Ca(OH)₂于0.1 mol/L K₂SO₄仅产生微弱短程排斥，黏着状态几乎与纯K₂SO₄水溶液中相同.聚电解质PCE从0.1 mol/L K₂SO₄中吸附于云母表面且诱导远程空间位阻排斥，但在中等载荷下解吸附，导...

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 ? 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 ?, 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 ?. 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 ? 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 ? 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 ? 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.