Specific ion effects have been known for more than a century, yet attaining a molecular-level understanding of these effects, which constitute the Hofmeister series, has been and continues to be a challenging task. Anionic Hofmeister series have received more significant focus in research, whereas the cationic side has been relatively neglected. However, a significant interest in cations has built up in recent years. This work presents a filling of a gap present in the cation Hofmeister effects, complementing the body of work on metallic cations with the hitherto unexamined behavior of greasy/hydrophobic cations. Aqueous salt solutions of tetraalkylammonium cations with variable chain lengths have been investigated for their molecular interaction mechanisms with a neutral model macromolecule (poly(N-isopropyl acrylamide); PNIPAM). Systematic investigation of lower critical solution temperatures (LCST) of the macromolecule is combined with spectroscopic methods (FTIR and NMR) over increasing salt concentration. This reveals a new specific cation binding mechanism, and indicate that different Hofmeister effects become dominant with increasingly greasy cations. The ion - macromolecule interactions become more favorable with increasing degree of hydrophobicity. The greasiest cations apparently interact with the polymer in a similar manner to weakly-hydrated anions, e.g., SCN-. These results furnish us with a more complete understanding of the Hofmeister series and demonstrate an expanded scope and diversity in cationic Hofmeister effects.

Specific ion effects have been known for more than a century, yet attaining a molecular-level understanding of these effects, which constitute the Hofmeister series, has been and continues to be a challenging task. Anionic Hofmeister series have received more significant focus in research, whereas the cationic side has been relatively neglected. However, a significant interest in cations has built up in recent years. This work presents a filling of a gap present in the cation Hofmeister effects, complementing the body of work on metallic cations with the hitherto unexamined behavior of greasy/hydrophobic cations. Aqueous salt solutions of tetraalkylammonium cations with variable chain lengths have been investigated for their molecular interaction mechanisms with a neutral model macromolecule (poly(_N_-isopropyl acrylamide); PNIPAM). Systematic investigation of lower critical solution temperatures (LCST) of the macromolecule is combined with spectroscopic methods (FTIR and NMR) over increasing salt concentration. This reveals a new specific cation binding mechanism, and indicate that different Hofmeister effects become dominant with increasingly greasy cations. The ion - macromolecule interactions become more favorable with increasing degree of hydrophobicity. The greasiest cations apparently interact with the polymer in a similar manner to weakly-hydrated anions, e.g., SCN^-^. These results furnish us with a more complete understanding of the Hofmeister series and demonstrate an expanded scope and diversity in cationic Hofmeister effects.

Molecular interactions of greasy ions with macromolecules in aqueous solution

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.