Interactions between silica particles and poly(2-vinylpyridine) brushes in aqueous solutions of monovalent and multivalent salts

AFM colloidal probe technique, scratch tests, and spectroscopic ellipsometry are employed to study the conformation of a poly(2-vinyl pyridine) brush grafted to a planar surface and its interaction with microsized silica spheres in solutions containing monovalent (Cl^-) and multivalent counterions (SO₄^2- and PO₄^3-) at pH 2.5. During approach of the sphere, steric repulsion is observed with all salts at any concentration. The approach force-distance curves are fitted according to the Alexander-de Gennes model in order to calculate the equilibrium brush thickness L. These data are compared to the brush thickness determined by ellipsometry and AFM scratch tests. Different values are obtained but all of them decrease with increasing salt concentration. This effect is enhanced by counterions of higher valence because they have a stronger screening effect and ion correlation due to their greater charge per unit volume. With NaCl solutions, a reswelling of diluted P2VP coils is observed at Cl^- concentrations >1 M. When the sphere is retracted, weak adhesion forces occur at Cl^- concentrations >1.3×10^-2 M and at all concentrations of SO₄^2- and PO₄^3-.