Theoretical studies on tri-n-butyl phosphate: MD simulations in vacuo, in water, in chloroform, and at a water/chloroform interface.

We report Molecular Dynamics studies on TBP, representative of neutral organophosphorous ligands used in metal ion extraction experiments. First, the conformation of the TBP monomer is analyzed as a function of its environment (in the gas phase, in pure water or pure chloroform phases, and at a water/chloroform interface). Then the structure and stability of (TBP)(n) clusters are studied (n = 2 and 10), and shown to be markedly dependent on the environment. For instance, the dimer (n=2), unstable in a dry organic phase, remains bound in the gas phase or in chloroform via two bridging water molecules. In an aqueous phase, another type of the dimer is found, stabilized by hydrophobic interactions between butyl chains. The (TBP)(10) aggregate behaves differently in pure water and in pure chloroform solutions. The simulations of (TBP)(l) and (TBP)(10) at the water/chloroform interface demonstrate the strong adsorption and orientational preference of TBP at the interface, related to its amphiphilic nature. With different starting configurations, a monolayer forms at the interface during the simulation, with the phosphoryl dipoles pointing towards the aqueous phase and the alkyl chains more or less in the organic phase. These results are discussed in relation with experimental data.