PNNL

Abstract

Surfactant-laden liquid/liquid interfaces mediate numerous chemical processes, from commercial applications of microemulsions to chemical separations. Classical molecular dynamics simulation is a prevalent method for studying microscopic and thermodynamic properties of such interfaces, yet the extent to which these features can be reliably predicted depends upon the force field parameters employed. At present, the impact of force fields upon simulated properties is relatively understudied, however recent advances to sampling and analysis algorithms are increasing the interpretation of simulation data and thus the force field dependence is becoming highly relevant. In this study, the impact of the surfactant tri-n-butyl phosphate (TBP), as well as water force field, is investigated at a water/(n-hexane + surfactant) interface. Empirical charge scaling (used to modulate polar interactions) has been employed to represent a range of effective hydrophilicities (and hydrogen bonding interaction energies) of the surfactant. As anticipated, the adsorbed oncentrations of TBP at the interface are highly dependent upon charge scaling. Macroscopic properties that result from ensemble hydrogen bonding interactions, such as as interfacial tension and width are also highly dependent - even when measured at similar interfacial TBP concentrations. Interestingly, competitive interactions can “cancel” the effects of changing TBP charge on interfacial properties. Once example is the orientation and hydrogen bonding structure of interfacial water, where the direct TBP-water hydrogen bonding competes against the indirect TBP-induced interfacial roughness. Thus these features are relatively insensitive to TBP force field. The dynamic properties of solvents at the interface are strongly modulated by the variation in hydrogen bond strength caused by different charges of TBP. This includes the residence times of water at the interface, where stronger water-TBP hydrogen bonding causes long-lived residences. In combination, the observations from this study may assist future simulation studies in understanding the impact of force field on the above range of dynamic, structural and thermodynamic properties.