Insights into Ion Specificity in Water–Methanol Mixtures via the Reentrant Behavior of Polymer

In the present work, we have for the first time systematically investigated the ion specific reentrant behavior of poly(N-isopropylacryamide) (PNIPAM) in water–methanol mixtures. Turbidity measurements demonstrate that SCN– and ClO4– depress the reentrant transition, whereas other anions enhance the transition. As the anion changes from chaotropic to kosmotropic, the minimum critical phase transition temperature (Tmin) decreases and the corresponding volume fraction of methanol (XM) shifts to a larger value. Our results demonstrate that anion specificity is due to the anionic structure making/breaking effect on water/methanol complexes. Cations are found to have a lesser but still significant effect on the reentrant transition, and as Tmin decreases the corresponding XM also shifts to larger values as with the anions. Our studies show that cation specificity is induced by specific interactions between cations and PNIPAM chains. Furthermore, both anion and cation specificities are amplified as XM is increased due to the formation of additional water/methanol complexes. Calorimetry measurements demonstrate that the ion specificity is dominated by changes in entropy.