Adsorption of Sulfobetaine Polyampholyte on Silica Surfaces from Aqueous Salt Solutions

Adsorption of poly( N -3-sulfopropyl- N -methacrylooxyethyl- N, N -dimethyl ammonium betaine) (polysulfobetaine abbreviated as PSB), which is a neutral- and zwitterion-type polyampholyte, onto a silica surface from aqueous NaCl solutions was studied by adsorbance measurement and ellipsometry. Adsorbance A increases with decrease of added salt concentration C s 0, while the thickness of adsorbed layer L decreases with the decrease of C s 0 . Comparison of L with the radius of gyration of PSB in NaCl solutions reveals the existence of collapsed (pancake)-, normal (fence)-, and elongated (pole)-like conformational regimes of PSB at the surfaces depending on the salt concentration. The salt distribution between the adsorbed layer and the bulk solution was estimated, and it was found that at low C s 0 positive ad(b)sorption of NaCl occurs while high C s 0 results in negative absorption of NaCl. The distribution of NaCl among adsorbed layers and bulk solution could be explained by Debye−Hückel theory, which takes into account the electrostatic attractive interaction among salt ions and zwitterion groups on PSB chain. It was also found that the polymeric osmotic and deformation pressure of adsorbed layer Δπ is related to the thickness of adsorbed layer L as Δπ ≈ L - 9/4 .