ATR-FTIR studies on the effect of strong salting-out salts on the phase separation scenario in aqueous solutions of poly(N-isopropylacrylamide) [PNIPA]

Attenuated total reflectance (ATR) FTIR spectroscopy was used to investigate the effect of two strong salting-out salts (Na2SO4 and K2SO4) on the temperature-induced phase separation process in aqueous solutions of poly(N-isopropylacrylamide) [PNIPA]. It was found that the addition of these salts not only depresses the phase transition temperature (as was pointed out by other groups in the past) but, in fact, changes the whole scenario below, at, and above the aggregation temperature. Deconvolution of the amide II peak into three sub-bands reflecting three types of populations that differ by their hydrogen bonding types revealed that the sulfate ions decreased the amount of amide-water hydrogen bonds, while increasing the amount of amide-amide intramolecular H-bond population. This led to a relatively compact structure even at low temperatures. Based on the behavior of the hydrophobic IR peaks it is suggested that addition of sulfate ions increases the importance of hydrophobic interactions below the phase separation temperature. This study demonstrates the potential of ATR-FTIR in providing information at a molecular level on phase separation phenomena in aqueous solutions of hydrophobic polymers and, in particular, on the effects of co-solutes in such systems. Copyright (C) 2003 John Wiley Sons, Ltd.