Fluorination Increases Hydrophobicity at the Macroscopic Level but not at the Microscopic Level

TL;DR

This study compares macroscopic and microscopic hydrophobicity in fluorinated polystyrene, revealing increased macroscopic hydrophobicity due to fluorination, but unchanged microscopic hydrophobic forces, suggesting different underlying mechanisms.

Contribution

It demonstrates that fluorination enhances macroscopic hydrophobicity without affecting microscopic hydrophobic forces, highlighting different mechanisms at play.

Findings

Fluorinated PS shows higher contact angles than PS.

Microscopic force spectroscopy reveals similar unfolding forces for PS and FPS.

Hydrophobicity mechanisms differ between microscopic and macroscopic levels.

Abstract

Hydrophobic interactions have been studied in detail in the past based on hydrophobic polymers, such as polystyrene (PS). Because fluorinated materials have relatively low surface energy, they often show both oleophobicity and hydrophobicity at the macroscopic level. However, it remains unknown how fluorination of hydrophobic polymer influences hydrophobicity at the microscopic level. In this work, we synthesized PS and fluorine-substituted PS (FPS) by reversible addition-fragmentation chain transfer polymerization method. Contact angle measurements confirmed that FPS is more hydrophobic than PS at the macroscopic level due to the introduction of fluorine. However, single molecule force spectroscopy experiments showed that the forces required to unfold the PS and FPS nanoparticles in water are indistinguishable, indicating that the strength of the hydrophobic ffect that drives the self-assembly of PS and FPS nanoparticles is the same at the microscopic level. The divergence of hydrophobic effect at the macroscopic and microscopic level may hint different underlying mechanisms: the hydrophobicity is dominated by the solvent hydration at the microscopic level and the surface-associated interaction at the macroscopic level.