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The layer-by-layer deposition method to prepare multilayers of polyelectrolytes of alternating charge has been followed in situ by means of optical reflectometry experiments. It turns out that in solutions containing both polyelectrolyte and appropriate salts up to a certain concentration, the regular build up of multilayers is modified and becomes an adsorption/redissolution process. We explain this by taking into account (i) that during the regular multilayer formation process the macromolecules cannot equilibrate, (ii) that the added salt plasticizes the multilayer to a state where the molecules are sufficiently mobile to enable them to equilibrate between the layer and the surrounding solution, and (iii) that the presence of excess polyelectrolyte brings the system to a one-phase region of the polyelectrolyte complex phase diagram, implying that polyelectrolyte complexes must dissolve under these conditions.

The adsorption of polyelectrolytes on oppositely charged surfaces is investigated by numerical calculations based on a recent model for polyelectrolyte adsorption by Bóhmer et al.,1 which is an extension of the self-consistent-field theory of Scheutjens and Fleer for adsorption of uncharged homopolymers. First, the effects of salt concentration, segment charge, and surface charge density are described for the case of pure electrosorption. It appears that the adsorption always decreases with increasing salt concentration. We call this the screening-reduced adsorption regime. The conformation of adsorbed polyelectrolytes with moderate segment charge is rather extended, since they also gain adsorption energy for segments not attached to the surface, due to the long range character of the electrostatic interaction. The polyelectrolyte can be displaced from the surface at a certain critical salt concentration, cK, which depends on the segment charge t, and the surface charge density , according to cM = ( )10/11 following Muthukumar.2 Our numerical data agree rather well with this. The force balance is changed if the segments also have a specific (short-range, nonelectrostatic) interaction with the surface. Now, the screening-reduced adsorption regime occurs only for very low segment charge and sufficiently high surface charge density. The majority of the cases constitutes the screening-enhanced adsorption regime, where the adsorption increases with increasing ionic strength. A transition from the screening-reduced adsorption regime to the screening-enhanced adsorption regime, or vise versa, can take place if the strength of the nonelectrostatic attraction or the electrostatic interaction, by means of the segment charge or the surface charge density, is varied. In the screening-enhanced adsorption regime, a maximum in the adsorbed amount as a function of the salt concentration can occur if the counterions also have a specific interaction with the surface. The theoretical results agree qualitatively with reported experimental results.

The adsorption of bovine serum albumin (BSA) in a planar poly(acrylic acid) (PAA) brush layer has been studied by fixed-angle optical reflectometry. The influence of polymer length, grafting density, and salt concentration is studied as a function of pH. The results are compared with predictions of an analytical polyelectrolyte brush model, which incorporates charge regulation and excluded volume interactions. A maximum in adsorption is found near the point of zero charge (pzc) of the protein. At the maximum, BSA accumulates in a PAA brush to at least 30 vol %. Substantial adsorption continues above the pzc, that is, in the pH range where a net negatively charged protein adsorbs into a negatively charged brush layer, up to a critical pH value. This critical pH value decreases with increasing ionic strength. The adsorbed amount increases strongly with both increasing PAA chain length and increasing grafting density. Experimental data compare well with the analytical model without having to include a nonhomogeneous charge distribution on the protein surface. Instead, charge regulation, which implies that the protein adjusts its charge due to the negative electrostatic potential in the brush, plays an important role in the interpretation of the adsorbed amounts. Together with nonelectrostatic interactions, it explains the significant protein adsorption above the pzc.

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