Mechanistic insights into amplification of specific ion effect in water-nonaqueous solvent mixtures

Ethylene glycol (EG) and hydrogen peroxide (H2O2) can act as both hydrogen-bond donors and acceptors in the formation of solvent complexes with water molecules. In the present work, we have systematically investigated the ion-specific lower critical solution temperature (LCST) behavior of poly(N-isopropylacrylamide) (PNIPAM) in H2O–EG and H2O–H2O2 mixtures. The results obtained from turbidity measurements show that the specific anion effect is amplified with the increasing molar fraction of EG (x(EG)) but is independent of the molar fraction of H2O2 (x(H2O2)). The studies of Raman spectra and differential scanning calorimetry indicate that the discrepancy in amplification of specific anion effect between H2O–EG and H2O–H2O2 mixtures is due to the difference in the anion–solvent complex interactions rather than the anion–polymer or solvent–polymer interactions. On the other hand, the specific cation effect can also be amplified with the increasing x(EG) but changes only slightly with the x(H2O2). The discrepancy in amplification of specific cation effect between the two types of solvent mixtures is attributed to the difference in the solvent–polymer interactions.