Modelling of mass-transfer induced instabilities at liquid–liquid interfaces based on molecular simulations

Abstract

Interfaces and especially mass transfer across interfaces are of great importance in many fields of chemical engineering. Interfacial convection, which is generally called the Marangoni effect, may improve mass transfer across interfaces quite drastically and has not been investigated adequately in detail. In order to investigate the influence of mass transfer on a liquid–liquid interface molecular computer simulations have been performed. Since many molecules have to be considered for a significant modelling of the interface, cubic lattice systems have been chosen for the simulation which proceeds according to the Monte-Carlo scheme. The parameters that describe the thermodynamic and transport properties resemble those of realistic standard EFCE test systems for extraction. Results of various Monte-Carlo simulations show that under certain conditions mass transfer across interfaces induces the formation of nano droplets in the close vicinity of the interface. The different combinations of the nano droplet behaviour due to attractive or repulsive long-range forces together with the characteristics of coalescence may lead to different macroscopic interfacial instabilities such as spontaneous emulsification or eruptions. Based on diffusive and thermodynamic properties of the chosen lattice system a first stability criterion which allows the prediction of the onset of nano droplet formation is developed. The theoretical results compare well with experimental observations at a single drop and in a two-phase cell where the instabilities are investigated optically via Schlieren optics.