The Chaotropic Effect as an Assembly Motif in Chemistry

Area of Science:

• Supramolecular Chemistry
• Physical Chemistry
• Biophysical Chemistry

Background:

• Scattered reports indicate interactions between chaotropic ions (e.g., iodide, thiocyanate, perchlorate) and various molecules/surfaces in water.
• The hydrophobic effect is a primary driver of self-assembly in aqueous solutions.

Purpose of the Study:

• To establish the chaotropic effect as a generic driving force for supramolecular assembly, distinct from the hydrophobic effect.
• To introduce superchaotropic ions as highly effective agents in chaotropic assembly.
• To present a unified scale of water-solute interactions, including solvation and cavitation.

Main Methods:

• Review of existing literature on chaotropic ion interactions.
• Discussion of the Hofmeister series and its extension to superchaotropic ions.
• Analysis of solvation, cavitation, and the association of chaotropic anions with hydrophobic sites.

Main Results:

• The chaotropic effect is identified as a generic driving force for supramolecular assembly, operating orthogonally to the hydrophobic effect.
• Superchaotropic ions, such as borate clusters and polyoxometalates, demonstrate enhanced chaotropic effects.
• A continuous scale encompassing kosmotropic, chaotropic, and hydrophobic solute interactions, including cavitation, is proposed.

Conclusions:

• The chaotropic effect provides a fundamental principle for understanding water-solute interactions and supramolecular assembly.
• Superchaotropic ions represent a new frontier in manipulating self-assembly processes.
• This framework unifies diverse solute interactions within aqueous systems.