Depletion, adsorption, and structuring of sodium poly(acrylate) at the water-silica interface. 1. An atomic force microscopy force study

An atomic force microscope has been used to measure the force between a spherical silica probe and a flat silica surface in the presence of solutions of sodium polyacrylate molecules at pH 7. Two different molecular weight sodium poly(acrylate) samples, of low polydispersity, were used over a wide range of polymer concentrations (0-3.5% w/w). The effects of added simple electrolyte (sodium nitrate) upon the polymer adsorption/depletion behavior were also explored. The measured forces indicate that in the absence of added electrolyte, sodium polyacrylate molecules deplete from the silica-water interface at pH 7, leading to an attractive depletion force at close approach of the surfaces and weak liquidlike structural force oscillations at larger surface separations. These oscillations have a scaling length approximately proportional to c(p)(-0.50), which was independent of polymer molecular weight and in good agreement with the predictions of both scaling theory and computer simulation studies. The depletion layer thickness scaled according to c(p)(-0.6). In 1 x 10(-4) M NaNO3 solution, the observed forces where similar to those in the absence of salt at low polymer concentrations, although at higher polymer concentrations the force curves indicated polymer adsorption. Adsorption occurred at a lower polymer concentration for the higher molecular weight polymer sample. At higher polymer concentration, the force oscillations vanished as the polymer solution became homogeneous and the polymer chains presumably entangled.