Influence of surfactant distribution on the stability of oil/water emulsions towards flocculation and coalescence

A modification of a standard Brownian Dynamics algorithm is used to simulate the evolution of a 64-particle system interacting through DLVO forces. The algorithm accounts for drop coalescence and surfactant adsorption effects. For this purpose, the electrostatic component of the potential is expressed as a function of the surface excess. Surfactant adsorption changes with the total surfactant concentration of the system, the initial spatial distribution of amphiphilic molecules, and time. The explicit diffusion of surfactant molecules towards the interface is avoided, but its effects on the total stability of the system are accounted for. Thus, depending on the surroundings and adsorption times, drops of equal sizes show dissimilar repulsive barriers towards coalescence. Whenever coalescence occurs, the interfacial area diminishes and the exceeding surfactant is re-distributed. Here, the effects of a time-dependent surfactant adsorption and the consequences of inhomogeneous surfactant distributions upon the Drop Size Distribution (DSD) of the emulsion are studied. For the bitumen in water emulsions simulated, the calculations confirm the stabilizing effect of homogeneous surfactant distributions and short surfactant adsorption times.