How polyelectrolyte adsorption depends on history: A combined Fourier transform infrared spectroscopy in attenuated total reflection and surface forces study

We present a systematic study of how adsorption history affects the thickne ss, surface forces, and interfacial rheology of a model cationic polymer. T he polymer was quaternized poly-4-vinylpyridine, QPVP (weight-average degre e of polymerization n(w) = 325 and 98% quaternized with ethyl bromide). The main comparisons concerned one-step adsorption from solution at a variable salt concentration up to 0.5 M NaCl, versus two-step adsorption (initial a dsorption from buffer solution without added salt, then NaCl added later). The aqueous solutions were buffered at pH = 9.2 such that the surfaces (mic a in the case of surfaces forces (SFA) experiments, oxidized silicon in the case of in situ infrared (FTIR-ATR) experiments) in each case carried a la rge negative charge. The SFA and FTIR-ATR experiments gave consistent estim ates of the amount of polymer adsorbed, confirming the expectation that ads orption should be driven by electrostatic attraction to the surface of larg e opposite charge. The adsorbed amount showed little dependence on path, va lidating the common assumption of equilibration in this respect. However th e layer thickness measured by surface forces, the shear nanorheology respon se at a given surface force, and the dichroism of pendant side groups of th e polymer all showed a pronounced dependence on the path to reach the adsor bed state. We interpret the measurements to suggest that two-step adsorptio n produces an inhomogeneous layer comprised of a dense layer of segments cl osest to the solid surface and a sparse outer layer. In particular, two-ste p adsorption produced thicker layers and a greater tendency to decouple she ar forces from those that resist compression in the normal direction, there by lessening the shear forces at a given level of normal force.