Study of the molecular array behaviours and interfacial activities of green surfactant alkyl polyglycoside and the mixed systems with other surfactants on oil–water interface

Determining interfacial tension and critical micelle concentrations of surfactants from atomistic molecular simulations

Hypothesis Atomistically-detailed models of surfactants provide quantitative information on the molecular interactions and spatial distributions at fluid interfaces. Hence, it should be possible to extract from this information, macroscopical thermophysical properties such as interfacial tension, critical micelle concentrations and the relationship between these properties and the bulk fluid surfactant concentrations. Simulations and Experiments Molecular-scale interfacial of systems containing n-dodecyl β-glucoside (APG12) are simulated using classical molecular dynamics. The bulk phases and the corresponding interfacial regions are all explicitly detailed using an all-atom force field (PCFF+). During the simulation, the behaviour of the interface is analyzed geometrically to obtain an approximated value of the critical micelle concentration (CMC) in terms of the surfactant area number density and the interfacial tension is assessed through the analysis of the forces amongst molecules. New experimental determinations are reported for the surface tension of APG12 at the water/air and at the water/n-decane interfaces. Findings We showcase the application of a thermodynamic framework that inter-relates interfacial tensions, surface densities, CMCs and bulk surfactant concentrations, which allows the in silico quantitative prediction of interfacial tension isotherms.

Molecular Dynamics Simulation of the Effects of Complex Surfactants on Oil-Water Interaction and Aggregation Characteristics at the Interface

In response to the problem of complex interaction between oil and water in the oil-water interface, especially heavy oil and water, this study investigated the effects of complex surfactants on the interaction of two phases and their aggregation characteristics by molecular dynamics simulation. The results showed that increasing the content of sodium lauryl polyether carboxylate (AEC-9Na) was beneficial to the coordination between it and alkyl glycoside (APG-10), improved the interfacial activity, and enhanced the interfacial stability of the composite system, and the best effect was achieved when AEC-9Na:APG-10 = 8:2. The thickness of the oil and water film on the oil-water interface was irregular. When the concentration of AEC-9Na was lower than that of APG-10, the total thickness of the interfacial film (ttotal) first increased. When the content of AEC-9Na is higher, a large number of sodium ions were adsorbed near the -COO- group of AEC-9Na, which will polarize out of the hydration layer structure and attract water molecules from the second hydration layer on the heavy oil surface to the first hydration layer through electrostatic interaction. Then, the thickness of the interface film was compressed, and the interface film was reduced. When the ratio increased to 10:0, the oil and water phase competed to adsorb surfactant molecules, and the headgroup tended to lay on the interface. Moreover, the hydrophilicity of the surfactant layer was weakened, and the thickness of the water film decreased. The distribution of surfactant was looser than 8:2, the light components of heavy oil molecules (saturated and aromatic hydrocarbons) entered the gap between surfactants in large quantities, and the hydrophobic tail chain tended to be laid on the oil-water interface. The oleophilicity of the surfactant layer increased, and the thickness of the oil film remarkably increased, so the total thickness of the interface film increased again.