Abstract
Finite element modeling applied to analyze experimentally determined hydrogel swelling data provides quantitative description of the hydrogel in the aqueous solutions with well-defined ionic content and environmental parameters. In the present study, we expand this strategy to analysis of swelling of hydrogels over an extended concentration of salt where the Donnan contribution and specific ion effects are dominating at different regimes. Dynamics and equilibrium swelling were determined for acrylamide and cationic acrylamide-based hydrogels by high-resolution interferometry technique for step-wise increase in NaCl and NaBr concentration up to 2 M. Although increased hydrogel swelling volume with increasing salt concentration was the dominant trend for the uncharged hydrogel, the weakly charged cationic hydrogel was observed to shrink for increasing salt concentration up to 0.1 M, followed by swelling at higher salt concentrations. The initial shrinking is due to the ionic equilibration accounted for by a Donnan term. Comparison of the swelling responses at high NaCl and NaBr concentrations between the uncharged and the cationic hydrogel showed similar specific ion effects. This indicates that the ion non-specific Donnan contribution and specific ion effects are additive in the case where they are occurring in well separated ranges of salt concentration. We develop a novel finite element model including both these mechanisms to account for the observed swelling in aqueous salt solution. In particular, a salt-specific, concentration-dependent Flory–Huggins parameter was introduced for the specific ion effects. This is the first report on finite element modeling of hydrogels including specific ionic effects and underpins improvement of the mechanistic insight of hydrogel swelling that can be used to predict its response to environmental change.
Highlights
Responsive hydrogels, referred to as “smart”, “intelligent”, stimuli-responsive, stimuli-sensitive or environmentally sensitive hydrogels, are three-dimensional hydrophilic network structures of polymer chains capable of changing their properties in response to environmental parameters such as ionic strength, temperature, pH, light, magnetic and electric fields
We focus on experimental determination of swelling properties of uncharged and weakly charged cationic hydrogels in an extended range of salt concentrations and combine this with finite element modeling of the swelling
This technique offers a resolution of about 2 nm for hemispherical hydrogels [39], and have been employed for quantitative characterization of swelling of e.g., anionic hydrogels [39], glucose sensing hydrogels [4,15], hybrid DNA-acrylamide hydrogels [10,40,41] or zwitterionic hydrogels [42]
Summary
Responsive hydrogels, referred to as “smart”, “intelligent”, stimuli-responsive, stimuli-sensitive or environmentally sensitive hydrogels, are three-dimensional hydrophilic network structures of polymer chains capable of changing their properties in response to environmental parameters such as ionic strength, temperature, pH, light, magnetic and electric fields. Gels 2020, 6, 31 hydrogel responsive properties are exploited as a basis for designing soft materials responding to enzymes or biomolecules by integrating specific recognition moieties that transform the recognition to a response. Features such as high water content, elastic nature, biocompatibility and the possibility to tailor-make physical and chemical properties by incorporation of different moieties, contribute to their wide application range, predominantly in the biomedical and bioanalytical fields [1,2,3,4,5,6,7,8,9,10]. The hydrogel equilibrium swelling and kinetics are important in selecting the appropriate composition of the hydrogel for the target application and predicting its behavior
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