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

Graphical The counterion specificity of polyelectrolyte brushes is determined by specific ion-pairing interactions, modulating the osmotic pressure and hydrophobicity of the brushes. 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 (ClO 4 − ), 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 ClO 4 − 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.