Cationic surfactant/bile salt interaction studied by fluorescence quenching

Area of Science:

• Colloid and Surface Science
• Physical Chemistry
• Supramolecular Chemistry

Background:

• Understanding surfactant aggregation is crucial for various applications.
• Bile salts are biologically relevant surfactants influencing micelle formation.
• Cetyltrimethylammonium halide (CTAX) is a common cationic surfactant.

Purpose of the Study:

• To investigate the aggregation behavior of cetyltrimethylammonium halide (CTAX) in aqueous mixtures with two bile salts: sodium cholate (NaC) and sodium deoxycholate (NaDC).
• To characterize the structural transitions of mixed micelles using fluorescence quenching techniques.
• To determine the impact of bile salt structure on the resulting aggregate morphology.

Main Methods:

• Fluorescence quenching measurements using pyrene as a probe and dimethylbenzophenone (DMBP) as a quencher.
• Analysis of time-resolved fluorescence decay data.
• Application of kinetic models, including a generalized model for diffusion-controlled quenching in rod-like micelles.

Main Results:

• CTAX/NaC mixtures formed small, spherical mixed micelles across all compositions.
• CTAX/NaDC mixtures exhibited a transition from spherical micelles to larger, rod-like aggregates at equimolar and near-equimolar concentrations.
• Complex kinetics were observed for CTAX/NaDC rod-like aggregates, necessitating advanced modeling.
• The mutual diffusion coefficient for the pyrene-DMBP pair was successfully determined.

Conclusions:

• Bile salt structure significantly influences the aggregation behavior of CTAX.
• Sodium cholate promotes the formation of small, stable spherical micelles with CTAX.
• Sodium deoxycholate induces the formation of larger, anisotropic rod-like aggregates with CTAX, altering the system's dynamics.