Equilibrium Aspects of Polycation Adsorption on Silica Surface:  How the Adsorbed Layer Responds to Changes in Bulk Solution

Abstract

Adsorption of cationic high molecular weight polyacrylamides (CPAM) (M(w) is about 800 kDa) with different fractions of cationic units tau = 0.09 and tau = 0.018 onto silica surface was studied over a wide range of pH (4-9) and KCl concentration (c(s) = 10(-3)-10(-1) M) by in-situ null ellipsometry. We discuss how the adsorbed layer depends on the bulk conditions as well as kinetically responds to changes in solution conditions. The adsorbed amount Gamma of CPAM increases with pH for all studied electrolyte concentrations until a plateau Gamma is reached at pH > 6. At low pH we observed an increase in adsorbed amount with electrolyte concentration. At high pH there is no remarkable influence of added salt on the values of the adsorbed amount. The thickness of adsorbed polymer layers, obtained by ellipsometry, increases with electrolyte concentration and decreases with pH. At low c(s) and high pH the polyelectrolyte adsorbs in a flat conformation. An overcompensation of the surface charge (charge reversal) by the adsorbed polyelectrolyte is observed at high c(s) and low pH. To reveal the reversibility of the polyelectrolyte adsorption with respect to the adsorbed amount and layer thickness, parameters such as polyelectrolyte concentration (c(p)), c(s), and pH were changed during the experiment. Generally, similar adsorbed layer properties were obtained independent of whether adsorption was obtained directly to initially bare surface or by changing pH, c(s), or the concentration of polyelectrolyte solution in the presence of a preadsorbed layer, provided that the coverage of the preadsorbed layer was low. Once a steady state of the measured parameters (Gamma, d) was reached, experimental conditions were restored to the original values and corresponding changes in Gamma and adsorbed layer thickness were recorded. For initially low surface coverage it was impossible to restore the layer properties, and in this case we always ended up with higher coverage than the initial values. For initial high surface coverage it was usually possible to restore the initial layer properties. Thus, we concluded that polyelectrolyte appears only partially reversible to changes in the solution conditions due the slow rearrangement process within the adsorbed layer.

Continuous polyelectrolyte adsorption under an applied electric potential

“…6, we show OWLS measurements of PLL adsorption at various NaCl concentrations, for a substrate potential of 1.5 V. Generally, we observe an increased asymptotic rate with [NaCl] ( Table 1) . Nonetheless, increased salt probably leads to thicker initial adsorbed layers, as observed in many other systems (5,11,12). Our results show thicker initial layers to be correlated with increased rate and onset mass of continuous adsorption.…”

“…Adsorption is usually spontaneous, with electrostatic interactions naturally playing a key role. These interactions, and therefore the adsorption process itself, may be controlled through solution variables such as salt concentration and pH (5)(6)(7)(8)(9)(10)(11)(12). However, there generally exists an upper limit to the extent of polyelectrolyte adsorption.…”

Dissolved and Colloidal Substances (Dcs) and the Charge Demand of Papermaking Process Waters and Suspensions: A Review

“…Similar experimental results were later obtained by Samoshina et al (2005). These results can be interpreted in terms of a three-dimensional conformation of the adsorbed polyelectrolyte, i.e.…”

Theoretical and Experimental Investigations of Polyelectrolyte Adsorption Dependence on Molecular Weight

“…However, in contrast to our previous work, 29 we will assume that the surface is fully charged at all salt concentrations. This is motivated by our results using a titrating surface below (model II), and also by experimental results by Samoshina et al 15 These results suggest that, in the presence of a highly charged polymer, a silica surface will become fully charged irrespective of the salt concentration (at pH 9). Still, as we will show below, the modification of our previous model, in which the charge density increased with salt, does not lead to a substantial change of the predictions, provided that we are allowed to make a similar modification of the non-electrostatic potential (γ).…”