Oscillatory packing and depletion of polyelectrolyte molecules at an oxide-water interface

Total internal reflection microscopy (TIRM) has been used to study the interactions between a 5 μm borosilicate glass sphere and a silica slide in the presence of a nonadsorbing polyelectrolyte, sodium (polystyrene sulfonate) (NaPSS). The effect of the polymer concentration, within the dilute solution regime, on the observed interactions was investigated. In all cases, the interactions displayed a short-range electrostatic repulsion followed immediately, at larger separations, by a decaying oscillatory interaction that is attributed to structuring of the polyelectrolyte in solution. The periodicity of the oscillations, as a function of concentration, indicates that at large surface separations the polymer chains are ordered as a nonintermixing, space-filling, latex. At polymer concentrations of between 200 and 1000 ppm, a transition to a system of ordered rods, parallel to the interface was seen for the final layer of polymer molecules.