Polyelectrolyte Adsorption on Solid Surfaces: Theoretical Predictions and Experimental Measurements

Abstract

This work utilizes a combination of theory and experiments, to explore the adsorption of two different cationic polyelectrolytes onto oppositely charged silica surfaces, at pH 9.Both polymers, Poly(diallyldimethylammonium chloride), PDADMAC, and Poly(4-vinyl Nmethylpyridinium iodide), PVNP, are highly charged, and highly soluble in water. Another important aspect is that a silica surface carries a relatively high surface charge density, at this pH level. This means that we have specifically chosen to investigate adsorption under conditions where electrostatics can be expected to dominate the interactions. Of specific focus in this work is the response of the adsorption to the addition of simple salt, i.e. a process * To whom correspondence should be addressed 1 where electrostatics is gradually screened out. Theoretical predictions from a recently developed correlation-corrected classical density functional theory for polyelectrolytes, are evaluated by direct quantitative comparisons with corresponding experimental data, as obtained by ellipsometry measurements. We find that, at low concentrations of simple salt, the adsorption increases with ionic strength, reaching a maximum at intermediate levels (about 200 mM). The adsorption then drops, but retains a finite level even at very high salt concentrations, indicating the presence of non-electrostatic contributions to the adsorption. In the theoretical treatment, the strength of this relatively modest, but otherwise largely unknown, non-electrostatic surface affinity, was estimated by matching predicted and experimental slopes of adsorption curves at high ionic strength. Given these estimates for the non-electrostatic part, experimental adsorption data are essentially captured with quantitative accuracy by the classical density functional theory.

Theoretical and Experimental Investigations of Polyelectrolyte Adsorption Dependence on Molecular Weight

“…The refractive index increment measurements are provided in the Supporting Information of our previous work. 29 The optical models employed to determine the silica substrate and adsorbed layer properties, were based on three (with polymer) or four (without polymer) layers: Si-SiO 2 -Bulk or Si-SiO 2 -Polymer film-Bulk. The optical properties and thickness of the SiO 2 layer can be determined by measuring the Ψ and ∆ values in two different media, air and water.…”

“…28,29,42 Here we shall use the approximations suggested by Fors-man and Nordholm. 42 Consider a system with a pairwise additive interaction potential φ (r).…”

“…Specifically, we have set d w = d = 0.25nm, which is identical to the value used in ref. 29 Adjusting γ such that the slope of the adsorption curves for long-chain PVNP and PDADMAC are reproduced (as in ref. 29 ) leads to γ = 2 and γ = √ 1.6 for PVNP and PDAD-MAC, respectively.…”

“…For polymers with a high molecular weight, on the other hand, polydispersity effects are predicted to be small (see ref. 29 for details). In model II, the surfaces are allowed to titrate.…”

Classical Density Functional Theory of Ionic Solutions

“…• Extending the theory beyond the PBA to include ion correlations (which in some systems are of vital importance) can often be carried out within the DFT, using phenomenological approaches that are remarkably accurate [1,[5][6][7][8].…”