Copolymer polyelectrolyte adsorption onto titanium dioxide

We study the adsorption of a polyelectrolyte, which is a hydrolysed styrene maleic anhydride (hSMA) copolymer, on titanium dioxide. When dissolved in aqueous solutions, at basic pH, this polyelectrolyte presents an apolar par t, styrene, and a polar part, two carboxylic functions. Our experiments sho w that the adsorption isotherms are dependent on the pH and on the total io nic strength of the solution which can be varied both by increasing the con centration of the polyelectrolyte and by addition of an external monovalent salt-potassium nitrate. The adsorption isotherms display a plateau followe d by an increasing adsorption at small salt concentrations. A detail study of the ionic strength effect show that this increasing adsorption cannot be attributed to polydispersity nor to successive adsorption layers. At high salt concentrations, the adsorption model is Langmuirian. The observed adso rption may be explained by interactions taking place: between the substrate and the copolymer segments. inside the adsorbed layer. We proposed that th e interactions near the solid substrate are electrostatic between the discr ete positive sites of the surface and the hSMA copolymers and nonelectrosta tic such as hydrogen bonds. The interactions inside the adsorbed layer are nonelectrostatic corresponding to hydrophobic bonds between the preadsorbed hSMA copolymers and the free hSMA copolymers in solution via their phenyl groups. The interactions near the solid surface are predominant for small i onic strengths, whereas the interactions inside the adsorbed layer are pred ominant for high ionic strength as the repulsion between the hSMA copolymer segments is quantitatively diminished. (C) 1999 Elsevier Science B.V. All rights reserved.