“Ion Specificity from Small Molecules to Oligomers and Beyond Amide-Based Macromolecules”
By Sobia Farooq
Advisor: Emrah Özensoy,
Co-advisor: Halil İ. Okur
Bilkent University, Department of Chemistry
Place: Chemistry Meeting Room (SB Building)
All are cordially invited to attend.
Presence of ions in aqueous solution regulate the properties of molecules in the same aqueous environment. Such alteration processes are mainly dependent on the concentration and the identity of ions. In this thesis, two parts of ion specific effects were aimed to be explored.
First the synthesis and characterization of PNIPAM oligomers by using both reversible addition-fragmentation chain transfer (RAFT) and radical polymerization methods will be shown. Both of these methods give the control over molecular size of the polymer. Oligomers with charged and neutral end group were synthesized to comparatively investigate ion specific effect. These oligomers were also systematically characterized by using various analytical techniques such as phase transition temperature measurement, 1H-NMR and Gel permeation chromatography (GPC).
Such oligomers were employed to investigate the specific ion effects via the salt influence on the phase transition temperature (LCST). By employing two chloride salts, NaCl (strongly hydrated) and NaSCN (weakly hydrated), it was found that strongly hydrated anions salt-out both charged and neutral oligomers, whereas weakly hydrated anions increase the phase transition temperature with a salting in mechanism. By empirical modeling with a Langmuir-type binding isotherm, a weak binding with a dissociation constant KD = 0.57 M for charged and KD = 1.13 M for neutral oligomers were demonstrated.
The second part of this thesis focused on the specific ion effects beyond amide-based macromolecules i.e. hydroxypropyl cellulose (HPC) as a model for sugar-based macromolecules. Eight sodium salts were employed to demonstrate the entire Hofmeister series. Namely; NaSCN, NaI, NaNO3, NaClO4 NaCl, Na2SO4, Na2CO3, NaH2PO4 were measured on the phase transition temperature and 1H-NMR measurements. Salts of weakly hydrated anions; NaSCN, NaI, NaClO4 and NaNO3 showed a salting in mechanism and demonstrate a non-monotonic phase transition behavior. In contrast, salts of strongly hydrated anions; NaCl, Na2SO4, NaH2PO4 and Na2CO3 showed salting out mechanism with a monotonic decrease in the phase transition temperature. Additionally, the site-specific ion-macromolecule interaction was studied by 1H-NMR, and 2D-COSY NMR measurements. Although, the exact binding site cannot be ruled-out, it was concluded that the ion binding site is at the side-chain hydroxypropyl groups and that yields the salting-in effect that was observed for the weakly-hydrated anions.
Keywords: Macromolecules, Oligomers, Specific Ion Effects, Hydroxypropyl Cellulose, PNIPAM, RAFT Polymerization