Molecular simulation of water extraction into a tri-n-butylphosphate/n-dodecane solution

Molecular dynamics simulations were performed to investigate water extraction into a solution of 30 vol % tri-n-butylphosphate (TBP) in n-dodecane. Our computational results indicate that the TBP electric dipole moment has a significant effect on the predicted water solubility. A larger TBP dipole moment decreases the aqueous-organic interfacial tension, leading to increased roughness of the aqueous-organic interface. Interfacial roughness disrupts the interfacial water hydrogen bonding structure, resulting in a presence of dangling water molecules at the interface. The increased interfacial roughness enhances the probability of water molecules breaking away from the aqueous phase and migrating into the organic bulk phase. By varying the atomic partial charges of the TBP molecules to reproduce a dipole moment close to the experimentally measured value, we were able to predict water solubility in close agreement with experimental measurements. In addition, our simulation results reveal the detailed molecular mechanism of the water extraction process, and the various structural forms of water molecules both at the interface and in the bulk organic phase.