LAUSR

Abstract Indeed, the relative strength of specific ion effect on macromolecule/biomacromolecules increases with increasing concentration. However, the utilization of high ion concentration may lead to aggregation or precipitation of macromolecule/biomacromolecule. Therefore, in the present study, we provided a potential alternative method to tune the strengths of “salting-in” and “salting-out” effects of Hofmeister series anions on the coil-globule transition of poly(N-isopropylacrylamide) (PNIPAM) by using a crown ether, 18-crown-6 (18-C-6) as a crowding agent. The ionic liquids (ILs) having the same cation, 1-butyl-3-methylimidazolium (Bmim+) and commonly used anions such as CH3COO−, Cl− and I− were selected to explore the strengths of “salting-in” and “salting-out” effects of Hofmeister series anions on a coil-globule transition of PNIPAM in the crowded environment of 18-C-6. Our experimental results (differential scanning calorimetry, hydrodynamic radius, fluoresce intensity, Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy) demonstrated that the salting-out effect exerted by a kosmotropic anion, CH3COO−, is strengthened, while the salting-in effect exerted by a chaotropic anion, I−, is weakened in the presence of 18-C-6. The results from the present study may certainly provide an alternative method to tune the relative strength of Hofmeister effect without increasing salt concentration.