Crown ether-modified polyelectrolytes and their interactions with cations – A QCM study

In this study, the build-up of polyelectrolyte multilayers (PEMs) containing 15-crown-5 (CE) groups and their interactions with various cations were studied by using quartz crystal microbalance with dissipation monitoring (QCM-D). First, poly(allylamine hydrochloride) (PAH) was modified with 4’-carboxybenzo-CE via carbodiimide chemistry. CE was chosen as its complexes with Na+ and K+ are reported to be more stable compared to those with other cations. The resulting functionalized polyelectrolyte (PAHCE) and poly(4-styrene sulfonic acid) (PSS) were used in the layer-by-layer build-up of a multilayer onto gold-coated quartz resonators, enabling their characterization with QCM-D. Compared to (PAH/PSS)4, (PAHCE/PSS)4 resulted in slightly thicker layers based on Voigt (65 ± 5 vs. 57 ± 3 nm) and Sauerbrey (45 ± 2 vs. 38 ± 3 nm) modelling of the QCM-D data. The same trend was found for the optical, dry thickness, as obtained with ellipsometry (15 ± 0.3 vs. 13 ± 1 nm). Next, the QCM-D characteristics of these PEMs were monitored in situ when exposed to various aqueous salt solutions (LiCl, NaCl, KCl, CsCl, RbCl, and MgCl2). Starting from Cs+, the frequency change of the (PAHCE/PSS)4 system upon changing to K+ and Na+ solutions was found to be ≈ 3 times larger than for (PAH/PSS)4. With a polycation (PAHCE) as the outermost PEM layer, the salt-exchange behavior was less visible due to increased charge rejection of cations. Therefore, we also modified a bio-based polyanion, pectin with 4’-aminobenzo-CE and built (PAH/pectinCE)4. Also in this case, the addition of CE increased the PEM layer thickness compared to (PAH/pectin)4, both in a wet state (Sauerbrey modelling, 447 ± 19 vs. 314 ± 17 nm) and when dry (115 ± 4 vs. 66 ± 3 nm). Again, we observed the largest QCM-D responses for K+ and Na+ solutions (≈ 6 and 12 times larger, respectively) compared to (PAH/pectin)4. The effect of CE is more prominent in pectin-based PEMs due their relatively higher thickness. Given the large toolbox of available polyelectrolytes and ionophores, we anticipate that functionalized PEMs can facilitate the further development of ion separation applications