A simple computational technique for the systematic study of adsorption effects in emulsified systems.: Influence of inhomogeneous surfactant distributions on the coalescence rate of a bitumen-in-water emulsion

In order to study the evolution of a bitumen-in-water (B/W) emulsion with time, a modification of a standard Brownian Dynamics (BD) algorithm was made. In these calculations. the 3-D movement of emulsion drops is mimicked by a combination of a steady DLVO (Derjaguin-Landau-Verwey-Oberbeek) inter-drop force and a random kick. The electrostatic component of the DLVO potential is allowed to depend on the surfactant population at the interface of each drop. Surfactant adsorption changes with the total surfactant concentration of the system, the available interfacial area, and the initial spatial distribution of amphiphilic molecules. Depending on these variables, the drops can flocculate and coalesce with either real drops or periodic-boundary images. As a consequence of coalescence. the interfacial area diminishes and surfactant is re-distributed among the surviving drops. The results are compared with well-known analytical equations developed for much simpler cases. With the present simulation technique, the exponential decrease of the number of drops with time reported for B/W systems can be reproduced. Evidence of a monotonic relationship between the total interfacial area of the surviving drops and the total surfactant concentration is also presented.