To simulate the evolution of an oil-in-water emulsion toward flocculation and coalescence, a modification of a standard Brownian dynamics algorithm was made. The resulting program takes into account the effects of surfactant diffusion and interfacial adsorption on the drop-drop interaction potential. Different realizations of the possible surfactant distributions are considered. In this work, the evolution of a small 64-particle system in the presence of a surfactant concentration gradient is studied. These results are compared with the predictions of well-known analytical formulas, which do not account for non-homogeneous surfactant distributions or a time-dependent surfactant adsorption. The particles are assumed to interact through a DLVO potential, which changed with surfactant concentration. The variation of the total number of particles with time follows the analytical predictions of Borwankar et al. for initial and intermediate steps of the flocculation/coalescence process. However, significant differences in the drop size distribution were found for longer times.