Combined Experimental and Theoretical Study of Weak Polyelectrolyte Brushes in Salt Mixtures

The swelling behavior of a hydrophobic poly(2diisopropylamino)ethyl methacrylate (PDPA) brush immersed in aqueous solutions of single and mixed salts has been investigated using ellipsometry and numerical self-consistent field (nSCF) theory. As a function of solution ionic strength, the osmotic and salted brush regimes of weak polyelectrolyte brushes as well as substantial specific anion effects in the presence of K⁺ salts of Cl^-, NO₃^-, and SCN^- are found. For solutions containing mixtures of NO₃^- and Cl^-, the brush swelling is the same as one would expect on the basis of the concentration-weighted average of the brush behavior in the single salt solutions. However, in mixtures of SCN^- and Cl^-, the swelling response is more complicated and substantial divergence from ideal behavior is observed. Mean-field theory shows excellent qualitative agreement with the ellipsometry findings. nSCF reveals that for the SCN^-/Cl^- cases the swelling behavior of the PDPA brush most likely arises from the predominant localization of the weakly hydrated SCN^- within the brush compared to the more strongly hydrated Cl^-.