Effect of hydrotropes on the volume phase transition in poly(N-isopropylacrylamide) hydrogel

With a view to understand the complex molecular forces involved in the volume phase transition (VPT) exhibited by poly(N-isopropylacrylamide) hydrogels the effect of hydrotropes on VPT was monitored. Hydroxybenzenes, hydroxybenzoates, and benzenesulfonates, which constitute the class of hydrotropes, are capable of solubilizing a variety elf nonpolar organic molecules in water. Hydrotropes permit an assessment of subtle molecular and structural features on the VPT because ortho, meta, and para isomers differ in their hydrotropy. Among the compounds studied, in a selected concentration range we observed that dihydroxybenzoic acid is the most efficient in lowering the transition temperature and salicylate anion raises the transition temperature while m-hydroxybenzoate offers minimum perturbation. The results are indicative of entropic cooperativity of many interactions stabilizing a conformation as the driving force for the volume phase transition.