(36f) Molecular-Dynamics-Assisted Molecular-Thermodynamic Framework to Predict the Interfacial Tensions of Non-Ionic Surfactants

The reduction in interfacial tension by surfactants underlies several natural phenomena in multi-phase systems, including emulsions and foams. This effect is also responsible for many industrial processes such as spray painting, emulsion polymerization, distillation in packed bed columns, and froth flotation. The mechanisms underlying these phenomena have been described by several mathematical models in the colloid science literature. Since the interfacial tension is an essential input for the implementation of all these models, their accuracy depends on the accuracy of the estimate (s) of the interfacial tension (s) used. Here, we propose a method to evaluate the surface tensions of non-ionic surfactants using a combination of molecular dynamics (MD) simulations and molecular-thermodynamic (MT) theory.
We have developed an automated, computational workflow to carry out a series of simulations of surfactant molecules at fluid-fluid interfaces. One set of simulations studies the interactions between surfactant molecules, and attempts to characterize the intermolecular interactions in a surfactant monolayer in terms of two molecular parameters – a hard-disk radius (representative of strong intermolecular repulsions) and a second-virial coefficient (representative of weak, intermolecular, van der Waals attractions). These parameters are obtained by first calculating the potential of mean force between the two surfactant molecules at the interface, and then analyzing this potential using principles of statistical mechanics. A second set of simulations studies the change in the free energy of a surfactant molecule as it is brought from bulk solution to an interface. This free energy of adsorption is a measure of the affinity of the surfactant of interest for the interface of a given system, and provides a metric capable of ranking families of surfactants. Together with the intermolecular interaction parameters, the free energy of adsorption enables prediction of the interfacial tension as a function of the surfactant bulk concentration through the use of a suitable equation of state for the interface.

In this talk, we will present the details of our proposed computational methodology, including: (1) the results obtained by its application to a series of nonionic alkyl poly (ethylene oxide) (CiEj) surfactants at fluid-fluid interfaces, and (2) our perspective on applying this methodology to high-pressure/high-temperature systems.