Exploring the Depth-Dependent Microviscosity inside a Micelle Using Butterfly-Motion-Based Fluorescent Probes.

Area of Science:

• Supramolecular Chemistry
• Physical Chemistry
• Materials Science

Background:

• Micellar interiors exhibit complex viscosity gradients, challenging to measure experimentally.
• Understanding these gradients is crucial for applications in drug delivery and catalysis.

Purpose of the Study:

• To develop and utilize novel fluorescent surfactants for quantifying the depth-dependent microviscosity within micelles.
• To investigate the relationship between surfactant alkyl chain length and micellar interior viscosity.

Main Methods:

• Synthesis of fluorophore-substituted surfactants (DPAC-Fn) by functionalizing alkyl-trimethylammonium bromides with a viscosity sensor (DPAC).
• Utilizing DPAC-Fn's viscosity-sensitive multicolor emissions to probe micellar environments.
• Employing external standards for quantitative viscosity measurements within cetrimonium bromide (CTAB) micelles.

Main Results:

• DPAC-Fn's immersion depth in CTAB micelles varied with alkyl chain length (n).
• Efficient, viscosity-sensitive multicolor emissions were observed, correlating with immersion depth.
• A significant viscosity gradient was quantified within a ~4 nm CTAB micelle, ranging from ~190 Pa·s at the core to ~1 Pa·s at the surface.

Conclusions:

• Developed DPAC-Fn surfactants serve as powerful tools for exploring micellar interior microviscosity.
• The study experimentally confirms and quantifies the dramatic viscosity variation from the center to the surface of micelles.
• This provides a tailored approach for in-depth micellar characterization.