(256c) The Formation of a Micelle Free Zone in the Transport of Surfactant From An Aqueous Micellar Solution to An Oil Phase
This paper presents theoretical and experimental studies of the surfactant transport from an aqueous micellar solution through an oil/water interface to an initially surfactant-free oil (hexadecane) phase. The nonionic polyethoxylated surfactant used (C14E6) is soluble in both phases; adsorption from the micellar solution to the oil/water interface is followed by transfer of surfactant molecules across the interface into the oil phase. The transport of surfactant across the interface involves transport of monomer and micelles. Surfactant monomers adsorb onto the interface depleting the aqueous sublayer concentration, and creating a diffusive flux of monomer to the interface. Surfactant adsorbed onto the interface then desorbs into the oil phase. The decrease in the monomer concentration near the interface causes micelles to break-up and provide monomer to supplement the monomer diffusion process. The depletion of micelles by break-up creates a diffusive flux of micelles to the surface. In the low micelle concentration regime the diffusion flux of micelles cannot compete with the adsorption flux at the interface. In this limit, assuming the micelle break-up rate to be very large, micelles become completely depleted near the interface creating a micelle free zone. As monomer adsorption proceeds, this zone emerges from the interface and extends into the bulk. For an aqueous phase with a finite thickness, the entire phase can become depleted of micelles if the oil phase is large enough.
To visualize this micelle free zone, we have used a small hydrophobic, environment-sensitive dye molecule (Nile Red), which partitions itself into the hydrophobic core of the micelles. When sequestered in the hydrophobic core, the dye fluoresces upon excitation. When micelles break-up and the dye is released into the aqueous phase, the fluorescence is quenched; hence the dye can be used to demarcate the presence of micelles and detail a micelle free zone. The experimental arrangement situates a lens of hexadecane on top of a low concentration micellar solution of C14E6 in a chamber with a water layer thickness of the order of a millimeter. The spatial evolution of the micelle free zone from the lens-aqueous interface is tracked with the fluorescence signal from the dye molecules within the micelles by using Confocal Laser Scanning Microscope (CLSM) to image the fluorescence in planes extending from the interface into the aqueous phase. A diffusion limited model is also developed to predict the micelle concentration distribution and the location of the micelle zone as a function of time. The simulation results for the movement of the micelle free zone boundary compares favorably with the experimental results obtained from CLSM.
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