TBP at the water-oil interface: The effect of TBP concentration and water acidity investigated by molecular dynamics simulations

Molecular dynamics simulations provide microscopic pictures of the behavior of TBP (tri-n-butyl phosphate) at the water-" oil " interface, and in wate r-" oil " mixtures where "oil" is modeled by chloroform. It is shown that, depending on the TBP concentration and water acidity, TBP behaves as a surf actant, an interface modifier, or a solute in oil. At low concentrations, T BP is surface active and forms an unsaturated monolayer at the " planar " i nterface between the pure water and oil phases, adopting an " amphiphilic o rientation ". Increasing the TBP concentration induces water-oil mixing at the interface which becomes very rough while TBP orientations at the phase boundary are more random and TBP molecules solubilize in oil. The effect of water acidity is investigated with three nitric acid models: neutral NO3H, ionic NO3- H3O+ and TBPH+ NO3-. The role of these species on the propertie s of the water-oil boundaries and on the outcome of water-oil demixing expe riments is presented. The neutral NO3H form is highly surface active. Hydro gen bonding between TBP and NO3H, TBPH+, or H3O+ disrupts the first TBP lay er and leads, at high TBP concentrations, to a mixed third phase or to a mi croemulsion. These results are important for our understanding of the micro scopic solution state of liquid-liquid extraction systems.