Cation identity in clay-polyelectrolyte self-assembled hydrogels: Rheological and NMR study of the polyitaconate-counterion interactions

Mechanical properties of self-assembled nanocomposite hydrogels based on cationic clay (Laponite®) nanoparticles and high molecular weight polyelectrolyte molecules, herein bio-based poly(itaconic acid), are directly related to the clay-polymer ionic interactions, which are affected i.e. by the type of polyelectrolyte counterions. The effect of monovalent cations (herein Li+, Na+, and K+) can be predicted according to the Hofmeister series, which relates specific ion effects with the physicochemical phenomena such as self-assembly via ordering of cations from cosmotropic to chaotropic (Li+ < Na+ < K+). Thus, the analogical relation between the effect of poly(itaconic acid) counterions and mechanical properties of resulting hydrogels was expected. Surprisingly, our study revealed the anomaly of the Hofmeister series concerning the mechanical strength of hydrogels indicating the following cations order Na+ < Li+ < K+. Therefore, we focused on the anomalous effect of poly(itaconic acid) counterions on the mechanical properties (strength and toughness) of final Laponite®-polyitaconate hydrogels, which became the essence of this paper. For this purpose, we combined oscillatory rheological characterization and multinuclear 1D, spin-spin relaxation times T2, and diffusion NMR experiments (1H, 7Li, 13C, 23Na, 39K) for an in-depth understanding and explanation of the role of the polyitaconate-counterion interactions in the Laponite®-polyitaconate hydrogels. Our study revealed that the properties of final hydrogels are a result of initial polyitaconate-counterion interactions in the aqueous solutions of corresponding polyitaconates. In particular, the anomaly of the cation order was successfully correlated to their analogical effect on the mobility of polyelectrolyte and water molecules, effective counterion size, and strength of polyelectrolyte-water interactions.