The complexation of solid uranyl nitrate with tri-n-butyl phosphate (TBP) in supercritical CO2 is quite different from that of a liquid–liquid extraction system because fewer water molecules are involved. Here, the complexation mechanism was investigated by molecular dynamics simulation, emphasising on speciation distribution analysis. In the anhydrous uranyl nitrate system, poly-core uranyl-TBP species [UO2(NO3)2]2·3TBP and [UO2(NO3)2]3·3TBP were formed in addition to the predominant [UO2(NO3)2]·1TBP and [UO2(NO3)2]·2TBP species. The poly-core species was mainly constructed via the linkage of U![[double bond, length as m-dash]](http://www.rsc.org/images/entities/char_e001.gif)
O⋯U contributed by pre-developed [UO2(NO3)2]·1TBP species. However, in the hydrated uranyl nitrate system, TBP·[UO2(NO3)2]·H2O species form, preventing the formation of the poly-core species. The complexation developed differently depending on the TBP to the uranyl nitrate ratio, the solute densities and the participation of water. It suggested that the kinetically favoring species would gradually convert into the thermodynamically stable species [UO2(NO3)2]·2TBP by ligand exchange.
