Co-nonsolvency of PNiPAM at the transition between solvation mechanisms

TL;DR

This paper explores the co-nonsolvency of PNiPAM in water-alcohol mixtures, revealing two distinct solvation mechanisms that govern its phase behavior in different solvent regimes, and explaining the phenomenon through changes in solvation contributions.

Contribution

It identifies and differentiates the solvation mechanisms responsible for PNiPAM's phase behavior in water-rich and alcohol-rich regimes, providing a detailed understanding of co-nonsolvency.

Findings

Hydrophobic hydration dominates in water-rich regime.

Mixing contributions dominate in alcohol-rich regime.

Co-nonsolvency occurs where neither mechanism prevails.

Abstract

We investigate the co-nonsolvency of poly-N-isopropyl acrylamide (PNiPAM) in different water-alcohol mixtures and show that this phenomenon is due to two distinct solvation contributions governing the phase behavior of PNiPAM in the water-rich and alcohol-rich regime respectively. While hydrophobic hydration is the predominant contribution governing the phase behavior of PNiPAM in the water-rich regime, the mixing contributions governing the phase behavior of classical polymer solutions determine the phase behavior of PNiPAM in the alcohol-rich regime. This is evidenced by distinct scaling relations denoting the energetic state of the aqueous medium as a key parameter for the phase behavior of PNiPAM in the water-rich regime, while the volume fractions of respectively water, alcohol and PNiPAM become relevant parameters in the alcohol-rich regime. Adding alcohol to water decreases the energetics of the aqueous medium, which gradually suppresses hydrophobic hydration, while adding water to alcohol decreases the solvent quality. Consequently, PNiPAM is insoluble in the intermediate range of solvent composition, where neither hydrophobic hydration nor the mixing contributions prevail. This accounts for the co-nonsolvency phenomenon observed for PNiPAM in water-alcohol mixtures.