Surface forces in aqueous polyvinylamine solutions. I. Glass surfaces

A noninterferometric surface force apparatus has been used to measure inter actions between glass spheres in dilute aqueous polyvinylamine solutions at two different salt concentrations. Close to the substrate charge neutraliz ation point, an attractive interaction is present mainly because of bridgin g of the extending polymer tails. Additional adsorption leads to an overcom pensation of the glass negative surface charge, and the interaction at this point is dominated by a long-range double-layer repulsion. The results fro m fitting the Derjaguin-Landau-Verwey-Overbeek theory to the measured force curves demonstrate that the degree of overcompensation increases with poly electrolyte concentration and increasing ionic strength of the solution (ad dition of indifferent electrolyte). An increase in ionic strength results i n the screening of the electrostatic forces which leads to: (i) a reduced f ree energy cost of creating a charged interface, (ii) a decreased repulsion between protonated amine groups along the polymer backbone and a correspon ding increased chain flexibility, (iii) a reduced electrostatic attraction between the polyelectrolyte and the surface. The first effect is apparently is the most important in the present case.