Delayed Swelling and Dissolution of Hydrophobically Associated Hydrogel Coatings by Dilute Aqueous Surfactants

Noncovalently cross-linked hydrogels can exhibit toughness and mechanical adaptability typically associated with biological tissues, which make them promising for a variety of applications. However, molecules in the environment can interact to significantly alter the properties of these hydrogels, which could adversely impact their performance. Here, we illustrate how two common ionic surfactants, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), influence the swelling and rheological properties of hydrogel films cross-linked by hydrophobic associations based on a random copolymer of poly­(N,N-dimethylacryalmide-co-2-(N-ethylperfluorooctanesulfonamido)­ethyl acrylate (DMA–FOSA) using a quartz crystal microbalance with dissipation (QCM-D). The effect of the surfactants on the swelling and dissolution of these physically crosslinked hydrogels is contrasted with the effect of aqueous 2-propanol (IPA), which can dissolve the copolymer readily. The addition of IPA, SDS, and CTAB at low concentrations increases the swelling of the hydrogel film, decreases the elastic modulus, and increases the rheological phase angle (more fluidlike). A transition from swelling to (partial) dissolution occurs at higher concentrations with a threshold of approximately 7 wt % IPA, 0.1·cmc for SDS, and 0.5·cmc for CTAB to promote partial dissolution. With IPA, initial swelling is always observed immediately after the solvent is added, whereas the initial swelling can be significantly delayed with long incubation times to dissolution, on the order of hours, at low concentrations of surfactant. These results illustrate that simple ionic surfactants can dissolve these hydrophobically cross-linked hydrogels, but identification of the maximum concentration of surfactant that will not dissolve the hydrogels may be challenged by the long incubation times that increase as the concentration decreases. The long times to dissolve these thin films (initial thickness <200 nm for copolymer) suggest that stability of bulk physically cross-linked hydrogels in complex aqueous environments may be challenging to accurately assess if surfactant diffusion is the limiting factor.