(699f) Interfacial Properties of Water-Surfactant Systems Using Classical Density Functional Theory

Surfactants play an important role in nature and industry. However, the amphiphilic character of surfactant molecules presents a challenge when it comes to modelling interfacial properties in the presence of surfactants.

Classical density functional theory (DFT) extends classical thermodynamics to describe microscopically inhomogeneous systems. Typical applications are the study of all varieties of interfaces including adsorption phenomena and porous media. The computational effort is significantly reduced compared to molecular simulation, but it comes at the cost of increased modelling necessities. Instead of a force field, the Helmholtz energy functional is required as input. For simple model fluids like hard spheres, precise models exist. For real fluids, the development of Helmholtz energy functionals based on equations of state has proven successful. [1]

To be able to model surfactants with DFT, the amphiphilic character of the molecules must be considered. We therefore combine the iSAFT approach by Jain et al. [2], that resolves the densities not only of molecules, but also of individual segments on the molecules, with the PCP-SAFT equation of state [3]. We use the model to efficiently calculate the influence the surfactant has on the pure vapor-liquid interface and the water-alkane liquid-liquid interface. Preliminary results also show that the model can describe structures such as bilayers and micelles. Since the approach is consistent with the bulk equation of state, the stability of different phases and structures can be determined from one underlying model.

For many problems, DFT provides a fast and quantitative method, that still captures the thermodynamics at a molecular level. We aim to show, that the behavior of surfactants at interfaces and in solution is another application for it.

References
[1] E. Sauer, J. Gross: “Classical Density Functional Theory for Liquid–Fluid Interfaces and Confined Systems: A Functional for the Perturbed-Chain Polar Statistical Associating Fluid Theory Equation of State” Industrial & Engineering Chemistry Research, 2017, 56, 4119-4135
[2] S. Jain, A. Dominik, W. Chapman: “Modified interfacial statistical associating fluid theory: A perturbation density functional theory for inhomogeneous complex fluids.” Journal of Chemical Physics, 2017, 127, 244904
[3] J. Gross, G. Sadowski: “Perturbed-chain SAFT: An equation of state based on a perturbation theory for chain molecules." Industrial & Engineering Chemistry Research, 2001, 40.4: 1244-1260.