On the Extraction of HCl and H2PtCl6 by Tributyl Phosphate: A Mode of Action Study

Combining computational modeling with experimental measurements has revealed the self-assembly of nano-aggregate structures in the transfer of HCl and PtCl62– from an aqueous phase into toluene by the common industrial extractant tributyl phosphate (TBP). Molecular dynamics simulations have been coupled to analytical measurements to provide an atomistic interpretation of the mode of action of TBP under 6 M and 10 M HCl conditions. The structures conform to reverse micelles, where the Cl– or PtCl62– core is encapsulated by a hydration shell that acts as a mediating bridge to the electronegative oxygen atom in the TBP phosphate groups. For the 6 M HCl extraction model, the data support stable aggregates forming from 2–3 TBP molecules around one chloride anion if the number of water molecules encapsulating the chloride anion is no more than five; increasing the water content to 10 molecules allows a fourth TBP molecule to coordinate. For the 10 M HCl extraction model, stable structures are obtained that conform to the empirical formula (TBP.HCl.H2O)3–5. At 6 M HCl, extraction of PtCl62– is achieved by encapsulation by four TBP molecules; the data for extraction at 10 M HCl indicate larger aggregates containing multiple PtCl62– anions are likely to be forming. In all cases, the hydrated core regions of the reverse micelles are considerably exposed. The diameters of the self-assembled structures around chloride ions agree well with available literature data from small-angle neutron-scattering experiments.