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

Abstract

The LCST phase-transition of aqueous PNIPA solutions in rising concentrations of the strong chaotropic salt KSCN was studied microcalorimetrically by DSC and apparently for the first time by ITC. An endothermic (entropy driven) binding of KSCN onto PNIPA was observed, explained by electrostatic perturbation of hydrophobic hydration by adsorbed ions. A good fit was found for the one-typeof-sites binding model, and the binding affinity increased with rising temperature from 15 to 20 °C but decreased at 25 °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-toglobule collapse with increased heterogeneity along the polymer chain, caused by salt adsorption. A hysteresis was 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 on polymers and biopolymers in aqueous solutions.

Study of the temperature-induced aggregation of polyoxometalate-poly(N,N-diethylacrylamide) hybrids in water

“…Such a gradual, broad transition along with the presence of a shoulder has formerly been attributed to the presence of salts. 25,26 Because an excess of NH 4 PF 6 was used in the synthesis of the NH 4 + , some salt remained in the samples. However, when equivalent quantities of NH 4 PF 6 were added to polymer sample PDEAAm-C 12 P55 no change of either the T onset or the width at half maximum was observed ( Figure SI-3).…”

Super-porous nanocomposite PNIPAm hydrogels reinforced with titania nanoparticles, displaying a very fast temperature response as well as pH-sensitivity

“…Between 5 and 10 mol% SMA the swelling behavior strongly changes: Both Q max (25°C) and Q min (50°C) strongly increase (Q min up to values of [23][24][25][26][27][28][29][30][31][32][33][34][35], which means that the gels become strongly hydrophilic. The increased hydrophilicity can be explained in the context of Hofmeister effects [63,64]: The carboxylate anions can be considered as chaotropic (singly charged with charge delocalization), and as such support the hydration and unfolding of the polymer to which they are bonded ''salting-in effect'', via disrupting the hydrogen bridging of water and via increasing the entropy of H 2 O, which both favor increased hydrogen bridging between H 2 O and the polymer's amido groups. The sodium carboxylate groups thus create a local environment highly favorable to the polymer dissolution inside the gel (The medium-sized sodium cations generally display only very small Hofmeister effects in comparison to the anions).…”

Preparation of thermosensitive hydrogels by means of tandem physical and chemical crosslinking

“…On the other hand, thermosensitive poly(N-isopropylacrylamide) (PNIPA) exhibits a unique soluble-insoluble change at its lower critical solution temperature (LCST). [22][23][24] Its LCST is about 32 o C, which is near mammalian physiological temperatures and easy to be adjusted by copolymerization with other molecules. [25][26][27][28] The special properties of PNIPA make it a good candidate material for injectable hydrogels.…”