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, NO, and SCN are found. For solutions containing mixtures of NO 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.