Mark R. Antonio

Direct observation of contact ion-pair formation in aqueous solution.

The formation of ion pairs in aqueous solution orchestrates vital processes in nature and engineered systems, including electron transfer and other chemical reactions, 5] and crystallization/precipitation of synthetic and natural matter. Polyoxometalates (POMs) have received special attention with regard to ion pairing. POMs are highly charged anions with ideal topologies for binding and associating metal cations, as their assembly, redox, and catalytic properties in aqueous solution are influenced by ion pairing. Methods for determining ion pairing are largely considered to be indirect in that they observe physical changes in a solution that results from ion association. Spectroscopic methods are the exception, but these are prone to insufficient resolution, and are unable to detect solvent-separated ion pairing. Herein we show that smallangle X-ray scattering (SAXS) of the Lindqvist [Nb6O19] 8

Aggregation in Organic Solutions of Malonamides: Consequences for Water Extraction

The molecular organization of N,N′‐dimethyl‐N,N′‐dioctylhexylethoxymalonamide (DMDOHEMA), the current reference extractant for the DIAMEX (DIAMide EXtraction) process, is correlated with its water extraction properties from neutral media. The aggregation of DMDOHEMA in n‐heptane was investigated by vapor pressure osmometry (VPO) and the aggregate speciation characterized by combined small‐angle neutron and X‐ray scattering (SANS and SAXS, respectively). Two approaches were taken to model the aggregation of the diamide and the water extraction as a function of the diamide concentration by taking into account a single aggregation equilibrium with an average aggregation number N equal to 4.28 ± 0.05; and a competition between two types of aggregates in the organic phase, namely, aggregates of the reverse micelle type with 4 diamides per aggregate, and an oligomeric structure composed of about 10 diamide molecules which appears at high extractant concentration (>1 mol/L). In both cases, the supramolecular speciation representing the monomers/aggregates distribution was determined, and for each supramolecular organization, a solubilization parameter was calculated using the Sergievskii‐Dannus relationship. Thus, the correlation between the two types of micellization of the diamide and the extraction of water into the organic phase was demonstrated. The larger aggregates can extract about five times more water than monomers.

Extraction of Lanthanides(III) and Am(III) by Mixtures of Malonamide and Dialkylphosphoric Acid

Abstract N,N′‐dimethyl‐N,N′‐dioctylhexylethoxymalonamide, DMDOHEMA, and di‐n‐hexylphosphoric acid, HDHP, are the extractants of reference for the French DIAMEX–SANEX process for the separation of trivalent actinide ions from the lanthanide ions. In this work, the extraction of Eu3+ and Am3+ by the two extractants, alone or in mixtures, has been investigated under a variety of experimental conditions. The two cations are extracted by HDHP as the M(DHP · HDHP)3 complexes with an Eu/Am separation factor of ∼10. With DMDOHEMA, Eu3+ and Am3+ are extracted as the M(NO3)3(DMDOHEMA)2 disolvate species with an Am/Eu separation factor of ∼2. The metal distribution ratios measured with a mixture of the two reagents indicated that almost all lanthanides are extracted equally well. The extraction of Eu3+ and Am3+ by HDHP‐DMDOHEMA mixtures exhibits a change of extraction mechanism and a reversal of selectivity taking place at ∼1 M HNO3 in the aqueous phase. Below this aqueous acidity, HDHP dominates the metal extraction by the mixture, whereas DMDOHEMA is the predominant extractant at higher aqueous acidities. Some measurements indicated apparent modest antagonism between the two extractants in the extraction of Eu3+ and synergism in the extraction of Am3+. These data were interpreted as resulting from the formation in the organic phase of mixed HDHP‐DMDOHEMA species containing two HDHP and five DMDOHEMA molecules.

Aggregation in Solvent Extraction Systems Containing a Malonamide, a Dialkylphosphoric Acid and their Mixtures

Abstract Aggregation phenomena in n-alkane solutions of di-n-hexylphosphoric acid (HDHP), N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (DMDOHEMA), and their mixtures, were investigated by electrospray ionization–mass spectrometry (ESI-MS), vapor pressure osmometry (VPO), and small-angle X-ray and neutron scattering (SAXS and SANS). The objective of the study was to probe the formation of mixed HDHP-DMDOHEMA species before and after extraction of trivalent lanthanide and actinide (M3+) nitrates. The most important species formed by HDHP upon metal extraction has the composition M(DHP)3(HDHP)3(H2O). These species exist as spherical aggregates of the reverse micelle type with a polar core diameter of ∼ 7 Å and total diameter of ∼ 11 to ∼ 15 Å. The aggregation of DMDOHEMA is a progressive phenomenon, with an average aggregation number of ∼ 2 in the 0.2 to 0.6 M range and larger aggregates forming at higher concentrations. The metal loaded DMDOHEMA aggregates can be considered as interacting spheres with a polar core diameter between ∼ 11 and ∼ 16 Å, depending on composition, a total diameter of up to ∼ 25 Å, and a weight-average aggregation number of ∼ 9. The results obtained in this work provide strong evidence for the formation of mixed aggregates when mixtures of HDHP and DMDOHEMA are used for the extraction of trivalent Ln and An cations. These mixed reverse micelles have a diameter between 19 and 24 Å with a polar core diameter of 10 and to 14 Å. The most recurrent micellar composition is 2 HDHP and either 4 or 5 DMDOHEMA molecules.

X-ray excited optical luminescence (XEOL) detection of x-ray absorption fine structure (XAFS)

The x-ray excited optical luminescence (XEOL) from a variety of rare-earth ions was used as a detection mode for the collection of L-edge x-ray absorption fine-structure (XAFS) data. In order to understand the source of the observed optical signal, advantage is taken of the known luminescent response of f ions in a variety of transparent host materials. Whereas some samples exhibit an optical response that is indistinguishable from the transmission XAFS data, other samples show marked differences between the data obtained with the two different detection schemes. The unexpected optical luminescence of a Gd2O3 sample is traced to a Eu impurity. An optical spectrum of 0.4% Tb in Gd2O2S, excited by x-ray photons at the Gd edge, is used to demonstrate that the optical signal may arise from an ion different from the absorbing ion. The implications of this energy transfer are discussed in terms of the suitability of XEOL as a detection scheme for XAFS spectroscopy.

Design of spectroelectrochemical cell for in situ X-ray absorption fine structure measurements of bulk solution species

A purpose-built spectroelectrochemical cell for in situ fluorescence XAFS (X-ray absorption fine structure) measurements of bulk solution species is described. The cell performance was demonstrated by the collection of europium L3-edge XANES (X-ray absorption near edge structure) during constant-potential electrolysis of 14.2mmEuCl3.6H2Oin1mH2SO4 aqueous electrolyte. Additionally discussed in this report are the probabilities of 2p3/2→5d electronic transitions pertaining to Euiii and Euii ions. Implications for the use of XANES in studying intermediate-valence materials, and some potential applications of the presently reported spectroelectrochemical cell are described.

A comparison of neptunyl(V) and neptunyl(VI) solution coordination: the stability of cation-cation interactions.

The solution coordination environments of pentavalent and hexavalent Np are studied by high-energy X-ray scattering. Np5+ and Np6+ both exist as the neptunyl moiety coordinated with five equatorial waters at Np-O distances of 2.46(2) and 2.37(2) A, respectively. NpO2(2+) also has a second coordination sphere of 6-10 waters at 4.37(3) A. The NpO2+ scattering is complicated by the presence of scattering at about 4.2 A that is attributed to Np-Np cation-cation interactions. The analysis of changing intensity of this peak as a function of Np concentration is used to determine a stability constant of Keq=0.74(9) M(-1) for the dimeric complex.

New materials synthesis: Characterization of some metal-doped antimony oxides

Abstract In order to understand the chemistry of altermetal dopants in antimony oxide, the detailed structural characterization of two β-Sb 2 O 4 compounds is reported, Mo-doped β-Sb 2 O 4 (1.5 metal%) and V-doped β-Sb 2 O 4 (5 metal%). The methods used to character...

A SAXS Study of Aggregation in the Synergistic TBP–HDBP Solvent Extraction System

The macroscopic phase behaviors of a solvent system containing two extractants, tri-n-butyl phosphate (TBP) and di-n-butyl phosphoric acid (HDBP) in n-dodecane, were investigated through use of liquid-liquid extraction and small-angle X-ray scattering (SAXS) experiments. Five organic solutions, each containing a total extractant concentration (TBP + HDBP) of 1 M in varying molar ratios (0, 0.25, 0.5, 0.75, and 1.0 [TBP]:[TBP + HDBP]), were contacted with 0.2 M HNO3 aqueous solutions without and with dysprosium(III) at a concentration of 10(-4) M. An enhancement of the extraction of Dy(3+)--due to effects of synergism arising from the binary combination of extractants--was observed. SAXS data were collected for all solution compositions from 0 to 1 mol-fraction end ratios of TBP after contact with the acidic aqueous solutions both in the absence and presence of Dy as well as for the organic phases before aqueous contact. In the precontacted solutions, no notable changes in the SAXS data were observed upon combining the extractants so that the scattering intensity (I) measured at zero angle (Q = 0 Å(-1))--parameter I(0)--the experimental radius of gyration (R(g)), and the maximum linear extent (MLE) of the extractant aggregates were arithmetic averages of the two end members, 1 M HDBP, on the one hand, and 1 M TBP, on the other. In contrast, after contact with the aqueous phases with and without Dy(3+), a significant reorganization occurs with larger aggregates apparent in the extractant mixtures and smaller in the two end member solutions. In particular, the maximum values of the metrical parameters (I(0), R(g), and MLE) correlate with the apparent optimal synergistic extraction mole ratio of 0.25. The SAXS data were further analyzed using the recently developed generalized indirect Fourier transformation (GIFT) method to provide pair-distance distribution functions with real-space information on aggregate morphology. Before aqueous contact, the organic phases show a systematically varying response from globular-like reverse micelles in the case of 1 M TBP to rod-shaped architectures in the case of 1 M HDBP. After aqueous contact, the aggregate morphologies of the mixed extractant systems are not simple linear combinations of those for the two end members. Rather, they have larger and more elongated structures, showing sharp discontinuities in the metrics of the aggregate entities that are coincident with the synergistic extraction mixture for Dy(3+). The results in this initial study suggest a supramolecular, micellization aspect to synergism that remains underexplored and warrants further investigation, especially as it concerns the contemporary relevance to decades-old process chemistry and practices for high throughput separations systems.

Berkelium redox speciation

Summary The inner-sphere hydration environments of the Bk3+·nH2O and Bk4+·n′H2O aquo ions in 1 M HClO4 were determined and are viewed in the context of other 5f (actinide) and 4f (lanthanide) aquo ions. The formal potential of the Bk4+/Bk3+ redox couple and the reorganization of the hydration sphere, in terms of the average Bk–OH2 interatomic distances and number of coordinated water molecules (n and n′), were determined through in situ X–ray absorption spectroelectrochemistry. The participation of H2O in the electrochemical conversion and the rearrangement of the hydration environment upon reduction–oxidation are summarized in the equation: [Bk(OH2)8]4+ + H2O + e− ⇆ [Bk(OH2)9]3+. The change of hydration about 249Bk between the trivalent and tetravalent states is believed to reflect predominantly steric factors related to the difference of ionic radii, rather than electronic or bonding influences.