Aggregation of dialkyl-substituted diphosphonic acids and its effect on metal ion extraction. Page: 4 of 23
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Aqueous soluble diphosphonic acids are powerful complexing agents for a wide
variety of metal ions, especially actinides and lanthanides (1,2). The chelating ion
exchange resin Diphonix contains a geminally substituted diphosphonic acid ligand
chemically bonded to a styrene-based polymeric matrix. Because of its extraordinarily
strong affinity for actinide ions and for iron(III), the resin has found application in
procedures for actinide separations and in hydrometallurgical processes where efficient
separation of Fe(III) from other transition metals is required (3).
P,P'-di(alkyl) alkyldiphosphonic acids are the solvent extraction equivalent of the
Diphonix resin. A general formula for some recently developed dialkyl-substituted
diphosphonic acid solvent extraction reagents is shown in Structure I:
HO I\ (CH2)n OH
where R is the 2-ethylhexyl group and n is one, two or four, for P,P'-di(2-ethylhexyl)
methane- (H2DEH[MDP]), ethane- (H2DEH[EDP]), and butane- (H2DEH[BuDP])
diphosphonic acids, respectively. The recently developed extraction chromatographic resin
Dipex is a noteworthy example of a practical application of H2DEH[MDP] in actinide
separation procedures (4).
It is well established that acidic organophosphorus extractants strongly aggregate
in non-polar diluents (5,6). Monoprotic acids usually dimerize to form an R2(8) ring,
analogous to that formed in the familiar dimerization of carboxylic acids. R2(8), in the
Etter hydrogen bond assembly classification (7), denotes an 8-membered ring structure
containing two hydrogen bond donors and two hydrogen bond acceptors. Hydrogen
bonding in organophosphorus acid dimers, however, is known to be stronger than in
carboxylic acid dimers (8,9). The aggregation behavior of diprotic acids is more
complicated. When two monomers hydrogen bond to form a dimeric species containing
one R2(8) ring, the resulting aggregate has additional -OH groups that can serve as sites
for further aggregation (10). This explains why di(2-ethylhexyl) phosphoric acid, HDEHP,
is dimeric in benzene (6,11), while the diprotic acid mono(2-ethylhexyl) phosphoric acid,
H2MEHP, can have an aggregation number as high as 12 in the same diluent (11,12).
Since HDEHP and H2MEHP can be regarded as monofunctional analogues of the
diphosphonic acids shown in Structure I, the question arises whether the aggregation
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Chiarizia, R.; Barrans, R. E., Jr.; Ferraro, J. R. Herlinger, A. W. & McAlister, D. R. Aggregation of dialkyl-substituted diphosphonic acids and its effect on metal ion extraction., article, October 22, 1999; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc708730/m1/4/: accessed March 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.