Microcalorimetric Study of the Effects of a Chaotropic Salt, KSCN, on the Lower Critical Solution Temperature (LCST) of Aqueous Poly(N-isopropylacrylamide) (PNIPA) Solutions

The LCST phase-transition of aqueous PNIPA solutions in rising concentrations of the strong chaotropic salt KSCN Wits Studied microcalorimetrically by DSC and apparently for the first time by ITC. An endothermic (entropy driven) binding of KSCN onto PNIPA wits observed, explained by electrostatic perturbation of hydrophobic hydration by adsorbed ions. A good fit was found for the one-type of-sites binding model, and the binding affinity increased with rising temperature From 15 to 20 degrees C but decreased at 25 degrees C. DSC Measurements emphasized the lowering and broadening of the endothermic peak of PNIPA phase-transition with rising KSCN concentration, explained by reduced cooperativity of coil-to-globule collapse with increased heterogeneity along the polymer chain, Caused by salt adsorption. A hysteresis wits observed between heating and cooling DSC peaks, which decreased asymptotically with rising KSCN concentration. further supporting that binding occurs. This work provides new insights into the mechanisms of chaotropic salt effects oil polymers and biopolymers in aqueous solutions.