Cooperative effect of carbamoylmethylene phosphine oxide on the extraction of lanthanides(III) to water-in-oil microemulsion from concentrated nitric acid medium

The extraction of metals to a water-in-oil microemulsion by using the combination of an anionic surfactant and an electrically neutral organic ligand, which causes a very strong cooperative effect, has been introduced as a novel technique for effective metal extraction and separation from a highly acidic medium. Aerosol OT (AOT) was used as an anionic surfactant to form a microemulsion in hexane, and a carbamoylmethylene phosphine oxide (CMPO), which has two aliphatic chains, was employed as a typical neutral organic ligand. The microemulsion system containing only AOT (0.002 M) showed poor extractability and selectivity for lanthanides(III) in their extraction from 0.5 M aqueous nitric acid solution; furthermore, the microemulsion itself was not very stable. However, by adding only a small amount of the CMPO (0.003 M), the AOT-based microemulsion system was greatly stabilized and exhibited extremely enhanced extractability, which has not been seen in the other microemulsion systems studied before. Also, the separation of the lanthanide series was also improved considerably, compared with simple solvent extraction using only the CMPO. The experimental results show clearly that metal ions complexed with the CMPO have much stronger affinities for the AOT microemulsion than uncomplexed ones. The organometallic complexes, which are amphiphilic, can be retained (solubilized) very strongly in the AOT microemulsion by electrostatic and hydrophobic interactions. The hydrophobic interaction, in particular, should be the most important driving force in microemulsion extraction; the electrostatic interaction is common to microemulsion systems containing and not containing the CMPO, since the ligand does not neutralize the charge of metal ions. The success of the experiment seems to be brought about by the use of an electrically neutral organic ligand, CMPO, whose metal complex is always cationic; a cationic species has the great advantage of a strong electrostatic interaction in such an anionic surfactant-based microemulsion. Furthermore, it also seems to be very important that the CMPO itself is not surface-active, and thus does not play the role of a cosurfactant to form mixed reversed microemulsions; a surface-active ligand, acting as a cosurfactant in such mixed microemulsions, has its chelating activity more or less spoiled.