Calixarene-based surfactants: conformational-dependent solvation shells for the alkyl chains

Area of Science:

• Supramolecular Chemistry
• Physical Chemistry
• Materials Science

Background:

• Amphiphilic p-sulfonatocalix[n]arenes are macrocyclic compounds with surfactant properties.
• Understanding their micellization behavior is crucial for designing novel self-assembling systems.

Purpose of the Study:

• To correlate thermodynamic parameters of micellization with alkyl chain length and the number of monomeric units (n) in p-sulfonatocalix[n]arenes.
• To elucidate the influence of molecular structure and conformation on surfactant behavior.

Main Methods:

• Thermodynamic analysis of micellization.
• Correlation of Gibbs free energy of micellization (ΔG(M)(o)) and critical micelle concentration (cmc) with structural parameters.
• Investigation of molecular conformation in solution and aggregates.

Main Results:

• Increasing alkyl chain length in p-sulfonatocalix[4]arene made micellization Gibbs free energy (ΔG(M)(o)) more negative.
• The transfer free energy per CH(2) group was lower than typical surfactants, suggesting intramolecular interactions.
• Critical micelle concentration (cmc) increased with the number of monomeric units (n).
• Calix[4]arenes in cone conformation showed efficient micellization, while calix[6] and calix[8]arenes incurred energetic costs due to conformational changes.

Conclusions:

• The cone conformation of p-sulfonatocalix[4]arenes is favorable for globular aggregate formation and efficient micellization.
• Conformational flexibility in larger calixarenes (calix[6] and calix[8]) leads to less efficient micellization due to enthalpic and/or entropic costs.
• Solvation shells differ between calix[4] and calix[8] derivatives, indicating distinct hydration behaviors.