Interaction of Poly(acrylic acid) Brushes with Multivalent Cations: An In Situ Study

Weak polyelectrolyte brushes represent a versatile class of responsive surface coatings. The adjustment of their charges by modifying the pH value and the salt content of the surrounding solution has a considerable effect on the elongation of the brush, the interaction between the chains, and the degree of swelling. Recent studies have extensively demonstrated the impact of singly charged cations through theoretical and experimental means. Analyses of the interaction between polyacids and multicharged cations remain underrepresented. Therefore, the influence of multivalent cations (Ca²⁺ and Ce³⁺) on the swelling behavior of poly(acrylic acid) (PAA) brushes is investigated as a function of salt concentration and pH. Swelling measurements show a behavior that differs significantly from the well-known case of monovalent cations. To elucidate this anomalous behavior, this study employs a combination of in situ spectroscopic ellipsometry, atomic force microscopy (AFM), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The investigations reveal a highly intricate interaction behavior, which can be ascribed to the multivalency of the cations utilized. The FTIR measurements indicate the impact of the cations on the p<i>K</i>_a value of the brush and the dissociation behavior. This study provides insights into the charging properties of PAA brushes, highlighting the decisive additional influence of the valency of the cations in combination with their concentration. Multivalent ions have been observed to substitute for the protons of the carboxyl groups through an entropically driven process, resulting in degrees of dissociation, α > 0.5, even at a pH of 3, where PAA brushes are expected to be fully undissociated. This phenomenon leads to an increased degree of swelling at very low salt concentrations and a reduction at increasing ionic strength. Evidence was found for different types of ion/carboxylate coordination, with bridging coordination between the carboxyl groups of PAA and the cations.