The effect of the Hofmeister series on the deswelling isotherms of poly(N-isopropylacrylamide) and poly(N,N-diethylacrylamide)

Abstract

The influence of Na₂SO₄, NaCl, NaBr, and NaI was studied on the deswelling isotherms of thin films of photo-crosslinked poly(N-isopropylacrylamide) and poly(N,N-diethylacrylamide) to help elucidate the mechanisms by which ions of the Hofmeister series affect the solubility of neutral, amide-based polymers. The films were characterized with both ellipsometry and FTIR to determine both water content and the frequency of the CO, N–H, and CH₃ vibrations associated with the polymers. When compared at the same water content, the frequency of the N–H bend in poly(NIPAAm) red-shifts in the order I⁻ > Cl⁻ > Br⁻ > SO₄²⁻. The red-shift is consistent with disrupted hydrogen bonding of the N–H moiety due to an ion–dipole attraction. The CO stretch on the other hand is insensitive to the ion, suggesting that the ion pairs primarily with the partially positive end of the HN–CO dipole. For poly(DEAAm), which lacks an N–H moiety, the CO stretch is now sensitive to the ion, with the trend following the order of the ion in the Hofmeister series. Both of these findings signify that I⁻ > Cl⁻ > Br⁻ > SO₄²⁻ with respect to the strength of the ion–dipole interaction. Moreover, the ion–dipole interaction is stronger in poly(NIPAAm) than poly(DEAAm), suggesting that the specificity of the Hofmeister ions arises from the effect of vicinal groups on the amide dipole.