Hydrogels based on PAAm network with PNIPAAm included: hydrophilic–hydrophobic transition measured by the partition of Orange II and Methylene Blue in water

This work describes the transition from hydrophilic to hydrophobic in hydrogels of Polyacrylamide (PAAm) having Poly(N-isopropylacrylamide) (PNIPAAm) included. The transition was measured through the partition coefficient, K, Orange II and Methylene Blue dyes at several temperatures, using different amount of Acrylamide (AAm), Methylene-bis-acrylamide (MBAAm), as cross-linking agent, and PNIPAAm. The dye concentration in the gel and the solution were measured using UV–Vis spectroscopy. Values of partition coefficient, K, were determined as the ratio of the dye concentration in hydrogel relative to water. Methylene Blue, the less hydrophilic dye, showed higher K values when compared to Orange II, more hydrophilic one. Value of K depends on the temperature and on the PNIPAAm content. The PNIPAAm chains are solvated by water and randomly distributed at temperatures below 32 °C, the LCST of PNIPAAm, but collapse near or above the LCST. The collapsed PNIPAAm chains induce a more hydrophobic environment that increases the solubility of Methylene Blue and decreases the solubility of Orange II in the hydrogels of PAAm with PNIPAAm included. These hydrogels show potential application concerning the separation processes, where the temperature and/or the hydrophilicity control the diffusion.