Structure, Interfacial Properties, and Dynamics of the Sodium Alkyl Sulfate Type Surfactant Monolayer at the Water/Trichloroethylene Interface: A Molecular Dynamics Simulation Study

Abstract

In this work, we perform a series of molecular dynamics (MD) simulations on the category of sodium alkyl sulfate (SDS-type) surfactant monolayers at the water/trichloroethylene (TCE) interface. Three separate tail-length SDS-type molecules are used. We investigate the conformation of surfactant chain (i.e., packing, orientation, and order), interfacial properties (i.e., interfacial thickness, interfacial tension, area compressibility, and bending modulus), their dependence on the chain length, and the average area per surfactant chain. We also examine the behavior of the surfactant monolayer in the metastable regime of negative surface tension with reference to collapse. The simulation has clearly shown that the very dilute monolayer is well described as a two-dimensional gas. With the increase of interfacial surfactant coverage, the monolayer is in the liquid-expanded (LE) phase. The surfactant tails at the interface become straighter, more ordered, and thicker at higher surfactant coverage. At the same time, interfacial tension of long-tail systems is always lower than that of short-tail systems. In the LE phase, the area compressibility modulus and the bending modulus increase with an increase in tail length. With a further decrease in molecular areas, the monolayer with large negative surface tension becomes unstable. Our simulations show that buckling of the monolayers is of dynamic nature as a response to mechanical instability. The further transformation pathway from buckling to bud can be controlled by the bending modulus, which depends crucially on the tail length and interfacial surfactant coverage. At a given area per molecule, the short tail chain makes the monolayer softer, and the budding process becomes more probable. For the supersaturated softer SDS monolayer, the collapse transition is initiated by the buckling of monolayers, followed primarily by budding and detachment of the nanoscale swollen micelle from the monolayer. Despite a number of extensive studies of monolayer collapse at the air/water interface, to our knowledge the conversion of surfactants from the liquid-liquid interface to swollen micellar aggregates as described here has not been reported in the literature.