Adsorption of nile red by poly(N-isopropylacrylamide) gels in binary water/tetrahydrofuran mixtures

The adsorption of Nile Red by poly(N-isopropylacrylamide) (PNIPAM) gels in binary water/tetrahydrofuran solutions was investigated using absorption spectrophotometry as a function of the volume fraction of THF, φ. Due to the cononsolvency abilities of such binary solvent mixtures, deswelling of the PNIPAM gels is observed in water-rich mixtures, 0 [less-than or equal to] φ [less-than or equal to] 0.6, while the gels reswell for φ > 0.6. The position of the absorption band of Nile Red before and after equilibration with the PNIPAM gels indicates that the composition of the external solvent mixture is not practically influenced by the swelling process. On the other hand, it is found that the gels can effectively adsorb Nile Red in water-rich mixtures (φ < 0.6), whereas no significant adsorption was observed in tetrahydrofuran-rich mixtures. In fact, about 80% of the dye is adsorbed by the PNIPAM gels, at φ = 0.2. Under these conditions, the rather shrunk PNIPAM gel offers a more convenient less polar environment for Nile Red than the water-rich bulk solvent.