Sodium Chloride-Induced Phase Transition in Nonionic Poly(N-isopropylacrylamide) Gel

A nonionic poly(N-isopropylacrylamide) hydrogel, which exhibits a temperature-dependent volume phase transition (lower critical solution temperature, LCST) behavior in aqueous solution, demonstrates an unusual sodium chloride-induced phase transition phenomenon as well. The gel collapses sharply at critical sodium chloride concentration, which depends on the temperature. Other salts tested show no such behavior, and typical salting-out behaviors (the gradual gel collapse with increasing their concentration) were observed. It appears that chloride ions play a major role in this phase transition.