Swelling and Dye-Adsorption Characteristics of an Amphoteric Superabsorbent Polymer

Amphoteric superabsorbent polymers (SAPs) based on the anionic monomer sodium acrylate (SA) and the cationic monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) were synthesized by solution polymerization using N,N'-methylenebisacrylamide as a cross linking agent The ratio of anionic to cationic repeat units was varied to obtain anionic, cationic, and amphoteric SAPs. The synthesized SAPs were characterized by Fourier transform infrared spectroscopy. The equilibrium swelling capacity of the SAPs was found to be dependent on the nature and extent of the net charge on the SAPs but independent of pH. The equilibrium swelling capacity was lowest for the SAP whose ratio of anionic to cationic repeat units was unity. The equilibrium swelling capacity increased as this ratio deviated from unity. The adsorption of an anionic dye (orange G) and a cationic dye (methylene blue) was carried out from the individual solution as well as from their mixture. The adsorption of the dyes was found to be dependent on the nature and amount of net charge on the SAPs but independent of pH. The amount of the dye adsorbed decreased as the net charge on the amphoteric SAPs decreased. The amphoteric SAPs with net negative or positive charge selectively adsorbed oppositely charged dyes from the mixture, but the amounts adsorbed were lower than those adsorbed from the individual dye solutions.