Specific Anion Effect in Water–Nonaqueous Solvent Mixtures: Interplay of the Interactions between Anion, Solvent, and Polymer

Ethanol (EtOH) and dimethylsulfoxide (DMSO) are polar protic and aprotic organic solvents, respectively. In the present work, we have investigated the anion-specific lower critical solution temperature (LCST) and upper critical solution temperature (UCST) behaviors of poly­(N-isopropylacrylamide) (PNIPAM) in the H₂O–EtOH and H₂O–DMSO mixtures. The turbidity and differential scanning calorimetry studies show that the LCST for the anions follows the Hofmeister series at the molar fraction of EtOH (x_E) or DMSO (x_D) of 6%. At x_E of 26%, the UCST for the anions also follows the Hofmeister series because the dominating interactions for the UCST behavior are similar to that for the LCST behavior in the H₂O–EtOH mixtures. In the H₂O–DMSO mixture at x_D of 70%, an inverted V-shaped anion series is observed for the UCST behavior of PNIPAM. Our studies demonstrate that the specific anion effect on the phase transition behaviors of PNIPAM is influenced not only by the anionic polarization of hydrogen bonding between solvent molecules and PNIPAM but also by the anion adsorption on the PNIPAM chain surface.