Role of Micelles in Facilitating Molecular Diffusion

Several recent studies have focused on the role that micelles play in mediating the mass transfer between emulsion droplets, the hypothesis being that micelles may facilitate mass transfer by acting as carriers of oil molecules. Karabomi and co- 

The lack of a strong dependence of the mass transfer on micelle concentration for ionic surfactants may result from electrostatic repulsions between the emulsion droplets and micelles, which would provide a high energy barrier preventing droplet/micelle collisions. Taisne et al. estimated that the disjoining pressure between a micelle and emulsion droplet would be of the order of 10 Pa, indicating little probability for a direct collisional mechanism. Karabomi et al. also noted in their molecular dynamics simulations that the collision mechanism is more likely with nonionic surfrctants and less likely with ionic surfactants.  

In mechanism (ii), a micelle in the vicinity of an emulsion droplet rapidly takes up dissolving oil from the continuous phase. The swollen micelle then diffuses away to another droplet, where the oil is redeposited. Such a mechanism would be expected to result in an increase in the rate of mass transfer over and above LSW theory by a factor given below  

A variation in the molecular diffusion theme was proposed by Miller. He suggested that a prohibited zone exists around the emulsion droplets due to electrostatic repulsions with the micelles. Mass transport within this zone can be accomplished only by molecular diffusion. Such a process would be expected to have a ripening rate lower than that predicted by eqn. (9.8) however, it would be expected that the average particle size would show a quadratic scaling with time. As discussed, experimental studies have shown a cubic scaling, and Miller s mechanism, although important, is not likely to be rate determining. 

To reconcile the apparent mismatch between theory and experiment it is important to consider the kinetics of the micellar solubilization process. Kabalnov thought it likely that the micelles are not in equilibrium with the droplets. He proposed that the rate of oil monomer exchange between oil droplets and micelles is slow, and rate-determining. Thus, at low micelle concentrations, only a small proportion of the micelles are able to rapidly solubilize the oil. This leads to a small but measurable increase in the ripening rate with micellar concentration. 

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