Radiolytic stability of N,N-dialkyl amide: effect on Pu(iv) complexes in solution.

The radiolytic degradation of three N,N-dialkyl amide ligands relevant to nuclear fuel reprocessing was studied. The degradation of these ligands: N,N di-2-ethyhexylbutyramide (DEHBA), N,N di-2-ethyhexylisobutyramide (DEHiBA) and N,N di-2-ethyhexyl-3-dimethylbutanamide (DEHDMBA) was examined to evaluate the effect of the structure on the formation of degradation products as well as to compare alpha induced degradation to gamma induced degradation. In situ alpha radiolysis by introduction of plutonium(iv) as the alpha source in the solution and ex situ gamma radiolysis with 60Co as the gamma source were compared. Upon identification of the main degradation products, a degradation scheme was proposed. The effects of radiation on the stability of Pu-monoamide complexes were discussed. Theoretical calculations were also performed to determine bond dissociation energy and estimate the relative strength of the bond in the molecule. The results show that neither the type of radiation (alpha vs. gamma) nor the structure modification (introduction of branching on the alkyl chain off the carbonyl carbon) of the molecule significantly impact the formation of degradation products under the conditions studied. Moreover, it was observed that the overall stability of the monoamide remains good and that Pu complexation is not greatly affected by either alpha or gamma irradiation.

Thermodynamic Description of Synergy in Solvent Extraction: I. Enthalpy of Mixing at the Origin of Synergistic Aggregation.

Revisiting aggregation of extractant molecules into water-poor mixed reverse micelles, we propose in this paper to identify the thermodynamic origins of synergy in solvent extraction. Considering that synergistic extraction properties of a mixture of extractants is related to synergistic aggregation of this mixture, we identify here the elements at the origin of synergy by independently investigating the effect of water, acid, and extracted cations. Thermodynamic equations are proposed to describe synergistic aggregation in the peculiar case of synergistic solvent extraction by evaluating critical aggregation concentration (CAC) as well as specific interactions between extractants due to the presence of water, acid and cations. Distribution of two extractant molecules in the free extractants and in reverse micelles was assessed, leading to an estimation of the in-plane interaction parameter between extractants in the aggregates as introduced by Bergström and Eriksson ( Bergström, M.; Eriksson, J. C. A Theoretical Analysis of Synergistic Effects in Mixed Surfactant Systems . Langmuir 2000 , 16 , 7173 - 7181 ). Based on this model, we study the N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide (DMDOHEMA) and di(2-ethylexyl) phosphoric acid (HDEHP) mixture and show that adding nitric acid enhances synergistic aggregation at the equimolar ratio of the two extractants and that this configuration can be related to a favored enthalpy of mixing.

Synergism in a HDEHP/TOPO Liquid-Liquid Extraction System: An Intrinsic Ligands Property?

Among the proposed mechanisms to predict and understand synergism in solvent extraction, the possibility of a preorganization of the mixture of extractant molecules has never been considered. Whether involving synergistic aggregation as for solubilization enhancement with reverse micelles or favored molecular interaction between the extractant molecules, evaluation of this hypothesis requires characterization of the aggregates formed by the extractant molecules at different scales. We investigate here the HDEHP/TOPO couple of extractant with methods ranging from vibrational spectroscopy and ESI-MS spectrometry to vapor pressure osmometry and neutron and X-ray scattering to cover both molecular and supramolecular scales. These experimental methods are subjected to DFT calculations and molecular dynamics calculations, allowing a rationalization of the results through the different scales. Performed in the absence of any cation, this original study allows a decorrelation of the mechanisms at the origin of synergy: it appears that no clear preorganization of the extractants can explain the synergy and therefore that the synergistic aggregation observed in the presence of cations is rather due to the chelation mechanisms than to intrinsic properties of the extractant molecules.

Synergy in Extraction System Chemistry: Combining Configurational Entropy, Film Bending, and Perturbation of Complexation.

Iron-uranium selectivity in liquid-liquid extraction depends not only on the mole fraction of extractants, but also on the nature of the diluent used, even if the diluent has no complexation interaction with the extracted ions. Modeling strong nonlinearity is difficult to parametrize without a large number of parameters, interpreted as "apparent constants". We determine in this paper the synergy curve versus mole fraction of HDEHP-TOPO (di(2-ethylexyl) phosphoric acid/tri-n-octyl phosphine oxide) and compare the free energy of aggregation to the free energy of extraction in various diluents. There is always a concomitant maximum of the two quantities, but with a gradual influence on intensity. The diluent is wetting the chains of the reverse aggregates responsible of the extraction. We show here that the intensity of the unexplained synergy peak is strongly dependent on the "penetrating" or "nonpenetrating" nature of the diluent. This experimental determination allows us to attribute the synergy to a combination of entropic effects favoring extraction, opposed to perturbation of the first coordination sphere by penetration as well as surfactant film bending energy.

Characterization of molecular associations involving L-ornithine and α-ketoglutaric acid: crystal structure of L-ornithinium α-ketoglutarate.

The crystal structure of L-ornithinium α-ketoglutarate (C5H13N2O2, C5H5O5) has been solved by direct methods using single crystal X-ray diffraction data. It crystallizes in the monoclinic system, space group P21, unit cell parameters a=15.4326(3), b=5.2015(1), c=16.2067(3) Å and ß=91.986(1)°, containing two independent pairs of molecular ions in the asymmetric unit. An extensive hydrogen-bond network and electrostatic charges due to proton transfer provide an important part of the cohesive energy of the crystal. The conformational versatility of L-ornithine and α-ketoglutaric acid is illustrated by the present results and crystal structures available from the Cambridge Structural Database.

Relation between the hydrophile/hydrophobe ratio of malonamide extractants and the stability of the organic phase: investigation at high extractant concentrations.

In the present paper, it is shown that in malonamide extractant/dodecane mixtures, change in the shape of the extractant aggregate may promote phase demixion. This phenomenon is known as the 3rd phase formation in extractant systems. The shape of the aggregates is dictated by the length of the alkyl chains stabilizing the reverse microemulsion by steric repulsion. Small angle scattering experiments are used to investigate the aggregation states of diamide extractant molecules, namely N,N'dimethyl-N,N'dibutyl-tetradecyl malonamide (DMDBTDMA) and N,N'dimethyl-N,N'dibutyl-pentyl malonamide (DMDBPMA), in dodecane. The results are described in a consistent way by the packing parameter concept and by expressing steric repulsion versus Van der Waals inter-aggregate interactions, both curvature dependants. Non-monotonous change in the extractant Critical Micellar Concentration (CMC) as a function of the extractant alkyl chain length is rationalized by comparing Van der Waals attraction between the alkyl chains of both the extractant and the solvent.

Influence of gamma irradiation on hydrophobic room-temperature ionic liquids [BuMeIm]PF6 and [BuMeIm](CF3SO2)2N.

Stability of neat hydrophobic Room-Temperature Ionic Liquids (RTIL) [BuMeIm]X, where [BuMeIm]+ is 1-butyl-3-methylimidazolium and X- is PF6-, and (CF3SO2)2N-, was studied under gamma radiolysis (137Cs) in an argon atmosphere and in air. It was found that the density, surface tension, and refraction index of RTILs are unchanged even by an absorbed dose of approximately 600 kGy. Studied RTILs exhibit considerable darkening when subjected to gamma irradiation. The light absorbance of ionic liquids increases linearly with the irradiation dose. Water has no influence on radiolytic darkening. A comparative study of [BuMeIm]X and [Bu4N][Tf2N] leads to the conclusion that the formation of colored products is related to gamma radiolysis of the [BuMeIm]+ cation. The radiolytic darkening kinetics of RTILs is influenced by the anions as follows: Cl- < (CF3SO2)2N- < PF6-. Electrospray ionization mass spectrometry and NMR analysis reveal the presence of nonvolatile radiolysis products at concentrations below 1 mol% for an absorbed dose exceeding 1200 kGy. Initial step of BuMeIm+ cation radiolysis is the loss of the Bu* group, the H* atom from the 2 position on the imidazolium ring, and the H* atom from the butyl chain. Radiolysis of ionic liquid anions yields F* and CF3* from PF6- and [Tf2N]-, respectively. Recombinations of these primary products of radiolysis lead to various polymeric and acidic species.