Dilute surfactant induced swelling and dissolution of hydrophobically crosslinked hydrogels*

Non-covalently crosslinked hydrogels can exhibit toughness and mechanical adaptability associated with biological tissues, but interactions with molecules in the environment can significantly alter their properties. Here,the swelling and rheological properties of hydrogel films crosslinked by hydrophobic associations based on a random copolymer of poly(N,N-dimethylacryalmide-co-2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate (DMA-FOSA) from dilute sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) solution are quantified with quartz crystal microbalance with dissipation (QCM-D). At concentrations much less than the CMC, the DMA-FOSA hydrogel swells signficiantly with a decrease in the elastic modulus and increased tan δ. At 0.1x CMC for SDS and 0.5x CMC for CTAB partial dissolution occurs, but the kinetics are slow with limited swelling occuring over the first 30 min and no signatures of dissolution until after 1000 min of exposure in some cases. The long incubation times, order of hours, associated with dissolution of thin hydrogel films from introduction of two simple ionic surfactants may challenge identification of long-term instabilities with physically crosslinked hydrogels from exposure to complex aqueous environments.