title: Charge regulation and energy dissipation while compressing and sliding a cross-linked chitosan hydrogel layer.
abstract: Interactions between a silica surface and a surface coated with a grafted cross-linked hydrogel made from chitosan/PAA multilayers are investigated, utilizing colloidal probe atomic force microscopy. Attractive double-layer forces are found to dominate the long-range interaction over a broad range of pH and ionic strength conditions. The deduced potential at the hydrogel/aqueous interface is found to be very low. This situation is maintained in the whole pH-range investigated, even though the degree of protonation of chitosan changes significantly. This demonstrates that pH-variations change the concentration of counterions within the hydrogel to keep the interior close to uncharged, which is similar to what has been observed for polyelectrolyte brushes. Changes in pH and ionic strength affect the adhesion force and the friction force between the silica surface and the hydrogel layer, but not the friction coefficient. This suggests that the main energy dissipation mechanism arises from processes occurring within the hydrogel layer, rather than at the silica/hydrogel interface, and we suggest that it is related to stretching of polymer chains between the cross-linking points. We also find that an increased cross-linking density, from 40% to 100%, in the hydrogel reduces the friction coefficient.
authors.affiliation: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden.
authors.forename: Chao
authors.initials: C
authors.lastname: Liu
authors.affiliation: Technical University of Denmark, Department of Chemistry, Kemitorvet 207, DK-2800 Kgs. Lyngby, Denmark. Electronic address: esth@kemi.dtu.dk.
authors.forename: Esben
authors.initials: E
authors.lastname: Thormann
authors.affiliation: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden.
authors.forename: Eric
authors.initials: E
authors.lastname: Tyrode
authors.affiliation: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden; SP Chemistry, Materials and Surfaces, P.O. Box 5607, SE-114 86 Stockholm, Sweden.
authors.forename: Per M
authors.initials: PM
authors.lastname: Claesson
mesh_terms: D048271:Chitosan / Q000737:chemistry*; D003432:Cross-Linking Reagents / Q000737:chemistry*; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate / Q000737:chemistry*; D018625:Microscopy, Atomic Force; D009994:Osmolar Concentration; D011108:Polymers / Q000737:chemistry*; D012822:Silicon Dioxide / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D003432:Cross-Linking Reagents; D011108:Polymers; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate; D012822:Silicon Dioxide; D048271:Chitosan
keywords: Chitosan; Friction; Hydrogel; Ionic strength; Surface force; pH