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

The influence of Na(2)SO(4), 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 C=O, N-H, and CH(3) 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(-) > SO(4)(2-). The red-shift is consistent with disrupted hydrogen bonding of the N-H moiety due to an ion-dipole attraction. The C=O stretch on the other hand is insensitive to the ion, suggesting that the ion pairs primarily with the partially positive end of the HN-C=O dipole. For poly (DEAAm), which lacks an N-H moiety, the C=O 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(-) > SO(4)(2-) 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.