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⁻.