Polyelectrolyte brushes were formed at the interface between hydrophobized mica and water by adsorbing amphiphilic diblock copolymers consisting of a hydrophobic and a charged block. We measured force-distance profiles with the surface force apparatus (SFA) for several polyelectrolyte brushes of different lengths as a function of aqueous 1:1 electrolyte concentration. Long-ranged electrostatic doublelayer forces were not observed in the force curves, and our results suggest that more counterions than predicted by the Manning theory are condensed onto the polymer backbones inside the brush. The brush height observed with the SFA follows the predictions of the mean field theory: at low ionic strength the height is independent of the external salt concentration c s, whereas at high external ionic strength the height shrinks with a weak power law L0 ∝ cs -1/3 . All force vs separation data for three different brushes, with various salt concentrations, collapse with reasonable accuracy into two reduced plots of data in the osmotic and salted brush regimes, respectively, the variables of which are developed in the scope of the mean-field brush theory.

We obtained mean-field expressions for the compression normal forces between two identical opposing quenched polyelectrolyte brushes in the presence of monovalent salt. The brush elasticity is modeled using the entropy of ideal Gaussian chains, while the entropy of the microions and the electrostatic contribution to the grand potential is obtained by solving the nonlinear Poisson–Boltzmann equation for the system in contact with a salt reservoir. The interplay between these distinct contributions upon variations of the ionic strength determines the brush thickness. For the polyelectrolyte brush we considered both an isotropic charged slab as well as a longitudinally heterogeneous charge profile obtained using a self-consistent field theory. Using the Derjaguin approximation, we related the planar-geometry results to the realistic two-crossed cylinders experimental setup. Theoretical predictions are compared to experimental measurements of the salt dependence of the compression normal forces between two quenched polyelectrolyte brushes formed by the adsorption of diblock copolymers poly(tert-butyl styrene)-sodium poly(styrene sulfonate) onto an octadecyltriethoxysilane hydrophobically modified mica surface.

This study investigates the structures of layers of amphiphilic diblock copolymers of poly(t-butyl styrene)-poly-(styrene sulfonate) (PtBS-PSS) adsorbed on both the bare mica surface (hydrophilic) and an octadecyltriethoxysilane (OTE)-modified mica surface (hydrophobic). When the surface is rendered hydrophobic, the nonsoluble block exhibits stronger interaction with the surface and higher adsorbed masses are achieved. Interaction forces between two such adsorbed layers on both substrates were measured using the surface forces apparatus. The effect of salt concentration (C s ) and molecular weight (N) on the height of the self-assembled layers (L 0 ) was examined in each case. The resulting scaling relationship is in good agreement with predictions of the brush model, L 0 ∝ N 1.0 in the low-salt limit and L 0 N -1 ∝ (C s /σ) -0.32 in the salted regime, when adsorption takes place onto the hydrophobized mica surface. For adsorption on the bare mica surface, L 0 N -0.7 ∝ C s -0.17 agrees with the scaling prediction of the sparse tethering model. The results suggest that, on the hydrophilic bare mica surface, the adsorbed amount is not high enough to form a brush structure and only very little intermolecular stretching of the tethered chains occurs; in contrast, the presence of the hydrophobic OTE layer increases the tethering density such that the polyelectrolyte chains adopt a brush conformation.