MSc in Chemistry Thesis Defense
by Ertan İşsever
Title: Molecular Interaction Mechanisms of Bulky Ions on Aqueous Neutral Macromolecules
Advisor: Asst. Prof. Halil Ibrahim Okur
Date: September 9, 2025
Time: 13.00
Place: Chemistry Department Meeting Room (SB Building, 1. Floor)
Abstract:
The ion specific effects on the solubility of macromolecules have been demonstrated for the first time more than a century ago yet the molecular level detailed picture of ion – macromolecule interactions has not been fully drawn. When considering the effects of salt on macromolecules, it was generally considered that the overall effect is dominated by anions, in which the counter cations acting mainly for the charge balance. Recently, the chloride salts of tetra alkyl ammonium (NR4+) ions refuted such assumptions via demonstrating that weakly hydrated cations can interact with macromolecules as effectively as weakly hydrated anions (I-, NO3-, SCN-). In this thesis, we asked how tetra alkyl ammonium cations (NR4+) behave in conjunction with surface-active anions (I-, NO3-) and whether their effects on macromolecules are additive. When we examined the phase transition temperature changes of tetra alkyl ammonium iodide (NMe4I, NEt4I, NPr4I, NBut4I) salts on PNIPAM, a thermo-responsive polymer that allows us to construct a thermodynamic model, we observed an unexpected salting-out behavior, particularly for tetra butyl ammonium iodide (NBut4I). while a strong salting-in behavior is expected due to the presence of two surface-active ions. To elucidate the mechanism of this non-additive behavior, we investigated the solubilized and collapsed states of the macromolecule using 1H-NMR and ATR-FTIR experiments, respectively. Our experimental results indicate that when the cation and anion are bound simultaneously, the surface charge remains close to neutral, and therefore, a salting-out behavior can occur. On the other hand, when the hydrophobicity of the cation decreases, this cooperative binding is disrupted, and the weakly hydrated anion becomes dominant. In the second part, tetraphenyl borate (TPB-) and phosphonium (TPP+) ions are also investigated, which are bulky molecular ions with charge asymmetry.
Hydration spectroscopy (MCR-Raman) demonstrated the influence of charge asymmetry on pi-water interactions via dangling O-H shifts. In light of this, the effects of these ions on three different model neutral macromolecules (PNIPAM, PDEA and ELP) were investigated. The ion with the highest water interaction (TPB-) exhibited a salting-out behavior, while the one with the weakest water interaction (TPP+) exhibited salting-in action. NMR and ATR-FIR experiments demonstrated that the tetraphenyl phosphonium ion (TPP+), in particular, exhibited an apparent non-site-specific interaction with the macromolecule. Consequently, the different hydration fingerprints of hydrophobic ions directly correlate with the well-known strongly hydrated, and weakly hydrated ion concepts, and clearly rationalize their effect on neutral macromolecule.