Forces between mica surfaces bearing adsorbed polyelectrolyte, poly-L-lysine, in aqueous media

We have measured the forces F as a function of surface separation D acting between two smooth curve mica sheets, immersed in aqueous electrolyte medium (potassium nitrate), both in the absence and in the presence of polyelectrolyte (poly-L-lysine, M= 90 000) adsorbed onto the surfaces from solution. Forces between the bare surfaces are typical of double-layer electrostatic interactions and in reasonable accord with DLVO theory. Following adsorption of the polyelectrolyte we find a monotonically increasing, long-range repulsion on a first approach (or compression) of the surfaces, commencing at D≈ 120 nm. On decompression, as well as on subsequent compression/decompression cycles, we find the long-range repulsion has irreversibly disappeared, to be replaced by a reversible much shorter-range repulsion (D≲ 20 and 70 nm, for 10–1 and 10–3 mol dm–3 KNO3 electrolyte, respectively). An analysis of the F against D profiles suggests that a combination of DLVO-type electrostatic repulsion and steric factors is primarily responsible for the observed interactions. The adsorbance of poly-L-lysine onto the mica surfaces has also been determined, via refractive-index measurements, and is 2 ± 0.5 mg m–2 of mica surface.