Ion Specificity Of Macromolecules In Complicated Environments

In this dissertation, we have systematically investigated the specific ion effects on the phase transition behavior of poly(N-isopropylacrylamide) (PNIPAM) in the water-organic solvent mixtures and in aqueous solutions in the presence of small-molecule osomolytes. The main results are as follows:(1) We have systematically investigated the ion-specific lower critical solution temperature (LCST) behavior of PNIPAM in the 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 (XEG), but it is independent on the molar fraction of H2O2 (xh2o2). The studies of Raman spectra and differential scanning calorimetry (DSC) 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 xEG, but it only slightly changes with the xH2o2. 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.(2) We have investigated the anion-specific LCST and upper critical solution temperature (UCST) behaviors of PNIPAM in the H2O-EtOH and H2O-DMSO mixtures. In these two types of solvent mixtures, the turbidity studies show that the anion sequence in terms of the specific anion effect on the LCST behavior follows the Hofmeister series. The ordering of anions according to the UCST of PNIPAM in the H2O-EtOH mixture also follows the Hofmeister series. However, an inverted V-shaped anion series is observed for the anion-specific UCST behavior of PNIPAM in the H2O-DMSO mixture. The studies of Raman spectra and solution conductivities suggest that the anions to influence the LCST and UCST behaviors are in the similar manner in the H2O-EtOH mixtures, leading to the similar anion specificity between the LCST and UCST behaviors. In contrast, the combined effect of the polarization of hydrogen bonding by anions and the adsorption of chaotropic anions onto the PNIPAM chain surface results in the occurrence of the inverted V-shaped anion series for the UCST behavior in the H2O-DMSO mixture.(3) We have investigated specific anion effect on the LCST behavior of PNIPAM in the presence of methylated urea or sugars. The DSC studies reveal that tetramethylurea can adsorb on the PNIPAM surface but glucose will be excluded from the PNIPAM surface. The specific anion effect on the LCST behavior of PNIPAM is amplified by methylated urea but not by sugars. The amplification of anion specificity by methylated urea is attributed to the increasing difference in the anion-specific polarization of hydrogen bonds induced by the formation of PNIPAM/methylated urea complexes via the hydrophobic interactions. As the number of methyl group on the methylated urea increases, the extent of amplification of the anion specificity increases due to the increasing hydrophobic interactions between PNIPAM and methylated urea. Additionally, no amplification of the anion specificity is observed in the presence of urea because no PNIPAM/urea complex can be formed via the hydrophobic interactions.