Counterion Specificity of Polyelectrolyte Brushes: Role of Specific Ion-Pairing Interactions

We demonstrate here that the properties of poly (2-(methacryloyloxy) ethyl trimethylammonium chloride) brushes can be tuned by counterion species. When the brushes are exposed to external chloride (Cl^- ) counterions, obvious dehydration and collapse are only observed at high salt concentrations. In the presence of very strongly chaotropic perchlorate (ClO₄^- ), the brushes strongly dehydrate and collapse at a very low salt concentration. For the strongly chaotropic thiocyanate ion (SCN^- ), the changes in hydration and conformation of the brushes are similar to those observed for ClO₄^- but at a smaller extent at very low salt concentrations. With the addition of kosmotropic acetate (Ac^- ), hydration of the brushes increases, accompanied by a swelling of the brushes in the low-salt-concentration regime. In contrast, the brushes dehydrate and collapse with increasing concentration of Ac^- in the high-salt-concentration regime. The counterion specificity of the brushes demonstrated here is determined by specific ion-pairing interactions through modulating the osmotic pressure within the brushes and the hydrophobicity of the ion pairs.