Interactions of a Dye and a Solvent with Poly(N-isopropylacrylamide) Gel in Relation to Its Phase-Transition Behavior

The binding of a dyestuff and the sorption of a solvent to a polymer gel have been studied in relation to the phase-transition behavior of the gel. Binding or sorption isotherms of a dye (erythrosine B) and a solvent (benzyl alcohol) to a polymer gel (poly(N-isopropylacrylamide) : NIPA) were measured and found first to be Langmuir- and BET-type curves, respectively. A single Langmuir-type adsorption isotherm and the superposition of the two isotherms of the dye were fit for the collapsed (40 °C) and swollen states (25 °C) of the gel, respectively. The phase-transition behavior of the NIPA gel was not affected at all by binding of the dye, but was highly altered by the addition of benzyl alcohol. A reentrant-type phase transition of the NIPA gel was observed with increasing concentration of benzyl alcohol at 25 °C. It is quite interesting to note that the gel shrinks when the concentration of the solvent inside the gel attains its solubility limit (ca. 0.4 mol dm-3), and swells again when it reaches the solubility in the bulk solution phase (outside the gel). At the stage of re-swelling, the solution in the gel interior consists of almost pure benzyl alcohol. These results indicate that the NIPA gel shows a selective sorption of benzyl alcohol, and separates it from its aqueous solutions.