Donnan Contribution and Specific Ion Effects in Swelling of Cationic Hydrogels are Additive: Combined High-Resolution Experiments and Finite Element Modeling

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.

Design and optimization of superabsorbent hydrogels based on acrylic acid/2‐acrylamido‐2‐methylpropane sulfonic acid copolymers

“…Third, the crosslinker mass range was increased to 20–40 mg of MBAA to explore whether the more crosslinked SAHs identified as having better AULs in Cycle 1 maintained comprehensive superabsorbency properties with the simultaneous measurement of CRC. Finally, the mass of APS initiator was considered as an additional factor given the large impact that the incorporation of the sulfate‐containing AMPS comonomer has on swelling; since the APS initiates polymerization by via a sulfate radical that is incorporated into the gel network, we hypothesized that higher initiator amounts may generate a hydrogel with a higher concentration of sulfated chain ends, with both the charge and enhanced entropy of the chain ends promoting swelling 30 . The resulting experimental design, and the corresponding measured AUL and CRC values acquired, are shown in Table 3.…”

Superswelling Microneedle Arrays for Dermal Interstitial Fluid (Prote)Omics

“…It was previously reported that an increase in ionic strength decreases the sorption capacity and the swelling rate of acrylate hydrogels due to the Donnan effect. [50][51][52] A macroscopic structural collapse was observed for dextrancomplemented H2, sodium poly(acrylate)-based H7 and reference PVME/MA hydrogels after the 30 min immersion. These formulations also display the fastest swelling rate, and their tendency to promptly absorb liquids may explain this phenomenon.…”

“…It was previously reported that an increase in ionic strength decreases the sorption capacity and the swelling rate of acrylate hydrogels due to the Donnan effect. [ 50–52 ]…”

Polymeric Nanoparticles in Brain Cancer Therapy: A Review of Current Approaches

“…These nanogels have the potential to add NP-based advantages, such as the ability to easily cross the BBB, be internalized by cells, and efficiently encapsulate drugs, to hydrogel systems [ 160 ]. Combining these with hydrogel swelling capabilities, or ability to expand in aqueous solution [ 161 ], and hydrophilicity makes nanogels a powerful tool for drug delivery [ 158 ]. Many recent in vivo studies have confirmed the ability of various nanogels to cross the BBB and selectively target tumor cells [ 162 , 163 , 164 ]; however, further investigation is required to determine whether these systems can be safely and effectively incorporated into clinical practice.…”