Meso- and molecular-scale modeling to provide new insights into interfacial and structural properties of hydrocarbon/water/surfactant systems

Water/oil emulsions become important in pharmaceutical, health, cosmetics, food, and oil and gas industries. The emulsion stability controls transport phenomena, transport in porous media, and surface science. Interfacial tension (IFT) is a vital property, which is related to emulsion stability. In this paper, dissipative particle dynamics (DPD) mesoscopic and molecular simulations are employed to study the impacts of temperature and oil type on the structural properties and IFT in order to obtain detailed insights into emulsion stability. Flory-Huggins chi parameter (χ) is determined as a function of temperature from molecular dynamics (MD) simulations; the DPD interaction parameter (aij) is then evaluated from the solubility parameter and energy of mixing, using Monte Carlo simulation framework. The predicted chi parameter is then compared to the experimental data. IFT is evaluated in the presence and absence of an emulsifier for different types of cyclic (cyclohexane and benzene) and aliphatic (octane and dodecane) oil molecules. The influence of temperature on the IFT is also investigated through the integration of DPD and MD approaches. The surfactant effect on the aggregation behaviour of the system is discussed by considering the structural characteristics of the surfactant molecules using the radius of gyration. Comparing the simulation snapshots taken at different simulation time steps, concertation profiles, and radius of gyration values, we find the surfactant to stretch more in the case of aliphatic hydrocarbon due to the linear structure of the oil molecules. For cyclic hydrocarbons, the radius of gyration is reported to be smaller since less space is available for the interactions of surfactant tail groups with oil molecules. It is also found that the IFT is a function of the molecular weight of hydrocarbon. Thus, the highest IFT is for dodecane (44.12 mN/m) and the smallest IFT is for benzene (28.68 mN/m), regardless of the structure and polarity of the molecules. By decreasing the IFT, the emulsion coalescence occurs faster and phases start to separate so that the mixture has no longer the characteristics of an emulsion. Constructing the force-field in the DPD simulation based on the molecular scale calculations of chi parameter improves the accuracy of simulation where the computational time and cost are reduced. The current study further highlights the importance of beading arrangement and the effect of oil molecules on the interfacial characteristic of hydrocarbon/surfactant/water systems.