Adsorption of Cationic Polyelectrolyte on Quartz from Aqueous Background Electrolyte Solution at pH 3

Adsorption isotherm of cationic polyelectrolyte, poly(styrene-co-dimethylaminopropylmaleimide), (molecular mass is 2 × 10⁴) on the surface of fused quartz in aqueous 10^–4 M KCl solution at pH 3 was measured by the method of capillary electrokinetics. The limiting coverage of adsorption layer corresponds to surface charge ₀ = 0.82 C/mol that exceeds the value obtained earlier at pH 6.5. However, if one takes into account the higher charge of a macromolecule at pH 3, the values of packing density of copolymer molecules in completely filled adlayers appeared to be close: 7.88 × 10¹⁰ at pH 3 and 7.27 × 10¹⁰ cm^–2 at pH 6.5. The average binding energy of the molecules and the quartz surface calculated by the Langmuir equation is equal approximately to 21kT and lies between the values of the energy of electrostatic (25.4kT) and hydrophobic (17.7kT) adsorption at pH 6.5 calculated earlier. It can be assumed that, at pH 3, charged units of a macromolecule form ion–dipole bonds with silanol groups, while uncharged groups form hydrophobic bonds with siloxane surface sites.