Dynamic Water Vapor Sorption in Crosslinked Polycationic Hydrogels: Thermodynamic and Kinetic Insights

The sorption behaviour of neutral and anionic hydrogels has been studied extensively, while polycationic systems received much less attention.[1] Polycationic hydrogels based on diallylammonium derivatives represent a promising class of materials with a number of potential applications[2] and tuneable water vapor sorption properties. Chemical and physical crosslinking, as well as the potential to introduce ionic interactions by means of copolymerisation with anionic molecules such as acrylates, enables precise control over sorption kinetics and equilibrium behaviour. This study systematically investigates the thermodynamic and kinetic properties of crosslinked polycationic hydrogels using dynamic vapor sorption (DVS) analysis, focusing on the influence of crosslinking density, counterions, and monomer structures.

Five hydrogel formulations, synthesized via radical copolymerisation of diallyldimethylammonium chloride (DADMAC) and N,N-diallylpiperidine bromide (DAPip), were analysed under isothermal and temperature-dependent conditions. The thermodynamic parameters revealed positive enthalpy (ΔH = 2.17–4.10 kJ/mol) and entropy (ΔS = 10.7–18.6 J/(mol·K)), confirming endothermic sorption processes driven by increased system disorder. Activation energies (EA) for adsorption ranged from 29.32 kJ/mol to 41.83 kJ/mol, suggesting a combination of physical and chemical sorption mechanisms.

Kinetic analyses identified diffusion-limited adsorption and multi-stage desorption, highlighting the role of steric hindrance and electrostatic interactions in sorption dynamics. Copolymer hydrogels containing additional charge-charge interactions with e. g. sodium acrylate, exhibited higher sorption capacities compared to purely chemically crosslinked networks, which demonstrated reduced swelling but maintained structural integrity. DAPip-based hydrogels contain sterically hindered ammonium groups, which reduce water uptake but accelerate sorption kinetics, particularly enhancing desorption rates. As a result, a directional sorption behaviour can be observed.

These findings offer fundamental insights into the structure-function relationships governing sorption in polycationic hydrogels. By tailoring crosslinking strategies and ionic interactions, these materials can be engineered for humidity regulation, moisture barriers, and water harvesting systems. The results highlight the potential of polycationic hydrogels as versatile sorption-active materials bridging fundamental polymer research with industrial and environmental applications.[2,3]

1.         Bashir, S.; Hina, M.; Iqbal, J.; Rajpar, A.H.; Mujtaba, M.A.; Alghamdi, N.A.; Wageh, S.; Ramesh, K.; Ramesh, S. Fundamental Concepts of Hydrogels: Synthesis, Properties, and Their Applications. Polymers (Basel) 2020, 12, doi:10.3390/polym12112702.

2.         Jung, A.; Weichold, O. A 3-in-1 alkaline gel for the crack injection in cement-based materials with simultaneous corrosion protection and re-passivation of crack-crossing steel rebars. Constr Build Mater. 2022, 344, doi:10.1016/j.conbuildmat.2022.128092.

3.         Mrohs, T.B.; Weichold, O. A Simple Preparation of Crosslinked, Highly Alkaline Diallyldimethylammonium Hydroxide Hydrogel Particles via Inverse Static Anion Exchange. Gels 2024, 10, doi:10.3390/gels10110743.