Conformation of single polyelectrolyte chains vs. salt concentration: Effects of sample history and solid substrate

Using AFM single polymer molecule experiments we compare conformations of adsorbed on the mica substrate (in the isoelectric point) polyelectrolyte molecules (PE) of poly(methacryloyloxyethyl dimethylbenzylammonium chloride) at different concentrations of Na3PO4 salt. We compare the conformations at solid-liquid interface (1) with the conformations of dry samples obtained if the samples of the adsorbed PE were slowly dried (2) and if the PE chains were rapidly deposited and dried on the surface (3). The experiments demonstrate very similar conformations for the cases (1) and (2), while the case (3) differs substantially. The results suggest that the conformations (1) and (2) are formed under strong influence of the substrate. In all cases we observed very pronounced transitions from the extended coil conformation to the pearl necklace-like globule with an increase of salt concentration. The necklace-globules in the cases (1) and (2) appear at a higher salt concentration as compared to the case (3). The strong van der Waals interactions with the solid substrate stabilize the PE globule.