This poster presents theoretical and experimental studies of transport of surfactant from an aqueous micellar solution to a nonpolar oil phase in which surfactant is soluble. When an initially clean (surfactant free) oil phase is contacted with an aqueous solution consisting of surfactant aggregates (micelles) as well as monomers in equilibrium with each other, surfactant monomers from the aqueous sublayer kinetically adsorb onto the newly formed oil-water interface. As the nonionic polyethoxylated surfactant used (C14E6) is soluble in both phases, adsorbed surfactant molecules desorb into the oil phase and diffuse away from the interface. This adsorption from an aqueous sublayer depletes the monomer concentration in the sublayer and disturbs the monomer-micelle equilibrium causing the micelles to break-down. Depletion of monomers (due to adsorption) and micelles (due to break-down) drives diffusive flux of these species towards the interface. However, micelle break-down takes place at very fast time-scale as compared to the diffusion. In this quasi-equilibrium limit, micelles always maintain monomer concentration around them at critical micelle concentration (CMC). At low micelle concentration, micelle diffusion flux cannot keep up with the adsorption flux at the interface. In such a case, micelle break-down takes place away from the interface and micelles deplete completely in a region extending from the interface, which we call a micelle-free zone.
To visualize the micellar transport, we have used a small hydrophobic dye molecule (Nile Red), which is sensitive to its surrounding environment. Being a small hydrophobic molecule, it partitions itself into the hydrophobic core of the micelles where it exhibits a strong fluorescence upon excitation. However when the micelles break-open, Nile Red is released into the aqueous solution and its fluorescence is quenched. This property of the Nile Red we use to demarcate the presence of micelles and locating the micelle-free zone. In the experimental arrangement we gently place a lens of hexadecane on the top of a low micelle concentrated aqueous solution of C14E6. As soon as the oil lens is placed, micelle-free zone forms at the interface and starts to penetrate into the aqueous bulk phase. Spatial evolution of the micelle-free zone from the oil-water is tracked with the fluorescence signal from the dye molecules trapped within the micellar core, with the help of Confocal Laser Scanning Microscope (CLSM), in horizontal planes extending from the oil-water interface into the aqueous phase. A diffusion limited model has also been developed to study the micellar transport from aqueous to oil phase, with which micelle concentration profiles are predicted and correspondingly the location of the micelle-free zone boundary as a function of time. Results of this numerical simulation for the micelle-free zone boundary movement compared favorably with the experimental results obtained from CLSM.