Visualization of an adsorption model for surfactant transport from micelle solutions to a clean air/water interface using fluorescence microscopy

This work pertains to visualizing a transport model for adsorption of surfactants from micelle solutions onto a clean air/water interface. Under the condition of surfactant adsorption from very dilute solutions, the time scale for diffusion of a surfactant monomer is much slower than the time scale for kinetic breakdown of the aggregates. A theoretical model predicts two regimes for the adsorption dynamics. We visualize these two regimes under the mechanism of solubilization using fluorescence microscopy, in which an insoluble fluorescent probe, NBD-HAD (4-(hexadecylamino)-7-nitrobenz-2-oxa-1,3-diazole), is used to illuminate the micelles. The dye fluoresces in the microenvironment of micelles but is quenched in the aqueous solution on laser excitation. The region containing micelles is illuminated, but the region which does not contain micelles appears dark. For surfactant solution of C14E6 at concentration just above the critical micelle concentration (CCMC), CCMC = 4.4 mg/L, a dark region between the bright image of the air/water interface and the micelle-containing zone is observed. This dark region becomes smaller with time and finally disappears once equilibrium is reached. For a surfactant solution of C14E6 at the concentration of 4.74CCMC  , which is higher than a critical total surfactant concentration (CTc) of 4.25CCMC, we observe bright images through surfactant solutions during the adsorption process. Fluorescence images validate the theoretical model.

This work shows how to visualize a transport model for adsorption of surfactant from micellar solutions onto clean air/water interfaces.Figure optionsDownload high-quality image (80 K)Download as PowerPoint slideResearch highlights
► There are two regimes for adsorption dynamics when surfactants absorb from very dilute micellar solution.
► Regime I is the case at lower aggregate concentration, in which micelle diffusion cannot keep up with the kinetic adsorption and micelle-free zone emerges from the surface and move back into the bulk.
► Regime II is the case at higher aggregate concentration, in which the micelle diffuse flux is commensurate with the kinetic adsorption and a micelle-free zone does not form.
► Fluorescence microscopy visualized these two regimes.