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Synthesis and characterization of new unsymmetrical diglycolamide extractants for lanthanide ion partitioning: part two – branched alkyl derivatives

This study reports the synthesis and characterization of six new unsymmetrical diglycolamide ligands with branched alkyl derivatives, evaluating their effectiveness and complexation stoichiometry for partitioning trivalent lanthanide ions from acidic nitrate media through radiometric distribution measurements and luminescence spectroscopy.

Original authors: Benjamin G. Tokheim, Shane S. Kelly, Robert C. Ronald, Kenneth L. Nash

Published 2026-09-23

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Benjamin G. Tokheim, Shane S. Kelly, Robert C. Ronald, Kenneth L. Nash

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the complex world of nuclear energy, one of the most persistent challenges is managing the waste left behind after fuel is used. When nuclear reactors generate power, they leave behind a mixture of elements, including valuable materials that could be reused and hazardous radioactive isotopes that must be isolated. Among these are the lanthanides, a group of metals that often get in the way of recovering more useful elements like plutonium. To separate these metals from the liquid waste, scientists use a technique called solvent extraction. Imagine two liquids that do not mix, like oil and water. If you add a special chemical to the oil, it can grab onto specific metal ions floating in the water and pull them across into the oil layer. The goal is to find a chemical that is strong enough to grab the right metals, gentle enough to let them go when needed, and stable enough to survive in the harsh, acidic environment of nuclear waste without falling apart or creating a messy third layer of sludge.

A team of researchers at Washington State University, Southwest Minnesota State University, and Pacific Northwest National Laboratory set out to design a new generation of these separating chemicals. They focused on a specific family of molecules known as diglycolamides, which are already known for their ability to grab onto metal ions. Previous versions of these chemicals worked well but had a flaw: under certain conditions, they would clump together and form a thick, unwanted third phase that made the separation process difficult and inefficient. The researchers hypothesized that by changing the shape of the molecule—specifically by adding branched, tree-like carbon chains instead of straight ones—they could prevent this clumping while keeping the chemical's ability to grab metals intact. They synthesized six new variations of these molecules, each with a slightly different shape and size, to see which one offered the best balance of performance and stability.

The researchers began by creating these six new molecules in the laboratory. Each one was built from a central core that acts as the metal-grabbing hand, with two different types of carbon chains attached to it. One side of the molecule featured a compact, ring-shaped structure, while the other side carried a long, branched chain designed to keep the molecule dissolved in the oil-like solvent. They tested these new chemicals by mixing them with a solution containing a radioactive form of europium, a metal that behaves similarly to the lanthanides found in nuclear waste. By measuring how much of the europium moved from the water into the oil, they could determine how effectively each new molecule worked. They also varied the acidity of the water to see how the chemicals performed under different conditions, mimicking the real-world environment of a nuclear processing plant.

The results revealed a clear pattern in how the shape of the molecule influenced its performance. The chemicals with the smallest, most compact ring structures proved to be the most effective at grabbing the metal ions, especially when the water was highly acidic. One specific molecule, featuring a five-membered ring and a long branched chain, stood out as the most promising candidate. It was able to pull nearly all of the europium out of the water in just one minute of mixing, a speed that is crucial for industrial processes. In contrast, the molecules with larger, six-membered rings were less effective, and those with a specific type of oxygen-containing ring struggled the most, often failing to grab the metal at all in low-acid conditions. The researchers also discovered that the branched chains helped prevent the formation of that troublesome third phase, keeping the mixture clear and easy to separate.

To understand exactly how these molecules held onto the metal, the team used a technique called luminescence spectroscopy. This method involves shining light on the mixture and measuring the glow emitted by the metal ions, which changes depending on how many chemical molecules are attached to them. This analysis confirmed that the most effective molecules formed stable complexes with the metal, holding onto them tightly. Interestingly, the study showed that the molecules with the compact rings were significantly stronger at holding the metal than the older, straight-chain versions currently used in the industry. However, the researchers noted that the molecules with the oxygen-containing rings, while they did not clump as easily, were too weak to be practical for separating metals in a real-world setting.

The study concludes that the best path forward lies in the molecules with the compact ring structures and long, branched chains. These new designs offer a rare combination of high speed, strong metal grabbing, and resistance to forming messy sludge. While the researchers found that the branched chains slightly reduced the overall strength of the metal grab compared to straight chains, this trade-off was necessary to keep the chemical soluble and stable in the organic solvent. The most successful molecule identified in this work is now a leading candidate for further development, potentially offering a cleaner, more efficient way to recycle nuclear fuel and manage radioactive waste. The work demonstrates that by carefully tweaking the shape of a molecule, scientists can solve long-standing problems in chemical separation without introducing new complications.

Technical Summary: Synthesis and Characterization of New Unsymmetrical Diglycolamide Extractants for Lanthanide Ion Partitioning (Part Two – Branched Alkyl Derivatives)

Problem Statement
Diglycolamide (DGA) extractants, particularly the symmetrical $N,N,N',N'$ -tetraoctyldiglycolamide (TODGA), are effective for partitioning trivalent lanthanides and actinides from acidic nitrate media. However, TODGA is prone to forming a third (heavy organic) phase at high acidities or high metal loadings due to self-aggregation into reverse micelles. While phase modifiers like 1-octanol can mitigate this, they introduce phosphorus-free alternatives that adhere to the CHON principle (Carbon, Hydrogen, Oxygen, Nitrogen) are desirable. Previous work on unsymmetrical diglycolamides (UDGAs) with straight alkyl chains (e.g., $n$ -hexyl) showed promise but suffered from aggregation issues in highly acidic media. This study addresses the need for extractants that balance high metal extraction efficiency with improved solubility and resistance to third-phase formation by introducing branched alkyl chains ( $C_8$ and $C_{10}$ ) and cyclic amide moieties.

Methodology
The research involved the synthesis and characterization of six new unsymmetrical diglycolamide ligands (designated 7–12). These ligands feature a "Type 1" architecture: one side of the diglycolamide core is substituted with a cyclic secondary amine (pyrrolidine, piperidine, or morpholine), while the other side bears two branched alkyl chains. The alkyl groups investigated were bis(2-ethylhexyl) (DEH, $C_8$ ) and bis(3,7-dimethyloctyl) (DMO, $C_{10}$ ).

Synthesis: The ligands were synthesized via a three-step process: reductive amination/hydrogenation of citronellal to create bis(3,7-dimethyloctyl)amine; amidation of diglycolic anhydride with the branched amines; and subsequent methylation and amidation with the cyclic amines.
Characterization: Ligands were purified and characterized using $^1$ H and $^{13}$ C NMR, IR spectroscopy, and high-resolution mass spectrometry (HRMS).
Solvent Extraction: Liquid-liquid extraction experiments were conducted using radiotracer $^{152,154}$ Eu(III) in varying concentrations of nitric acid ($0.01$ to $3.0$ M). The organic phase consisted of the ligand dissolved in $n$ -dodecane with $5.0\%$ v/v 1-octanol as a phase modifier. Distribution ratios ( $D_{Eu}$ ) were measured to determine extraction efficiency, equilibration times, and nitric acid dependence.
Stoichiometry and Stability: Ligand dependence studies (varying ligand concentration at constant $1 $M$ HNO_3$) were used to determine metal-ligand stoichiometry and conditional extraction constants ( $K_{ex}$ ). Additionally, luminescence spectroscopy titrations in acetonitrile were performed to determine conditional stability constants ( $\beta$ ) for Eu(III)-ligand complexes.

Key Results

Solubility and Third-Phase Formation: The introduction of branched alkyl chains improved solubility compared to shorter straight chains, though the morpholino derivatives (9 and 12) still exhibited limited solubility and a tendency to form third phases at higher ligand concentrations. The pyrrolidinyl and piperidinyl derivatives generally maintained clear biphasic systems.
Extraction Efficiency:
- Acidity Dependence: All ligands showed low extraction at low acidity ($0.01 $M$ HNO_3$) and high extraction at higher acidity ( $\ge 1$ M $HNO_3$ ). The pyrrolidinyl derivatives (7 and 10) were the strongest extractants, achieving quantitative extraction ( $D_{Eu} > 100$ ) at $1 $M$ HNO_3$.
- Structure-Activity Relationship: Extraction strength followed the trend: Pyrrolidinyl > Piperidinyl > Morpholino. The DMO derivatives (10–12) generally exhibited slightly higher distribution ratios than their DEH counterparts (7–9), suggesting that the shorter methyl branches in the $C_{10}$ chain may be less detrimental to extraction than the ethyl branches in the $C_8$ chain.
- Comparison to TODGA: The pyrrolidinyl ligands (specifically DMOpyrDGA, 10) and the DMO piperidinyl ligand (11) demonstrated extraction strengths significantly higher than TODGA (up to 100-fold for ligand 10). The morpholino ligands were weaker than TODGA.
Stoichiometry: Slope analysis of ligand dependence data indicated a metal-to-ligand stoichiometry of approximately 1:3 for the Eu(III)-UDGA complexes in the organic phase.
Stability Constants: Luminescence spectroscopy in acetonitrile confirmed the formation of 1:1 through 1:4 complexes. The conditional stability constants ( $\log \beta$ ) generally followed the extraction strength trend (Pyrrolidinyl > Piperidinyl > Morpholino). The 1:4 complex of DMOpyrDGA (10) showed the highest stability ( $\log \beta_4 = 24.8 \pm 0.3$ ).
Kinetics: Equilibration was rapid, with distribution ratios reaching steady state within one minute of mixing, significantly faster than typical TODGA systems.

Significance and Claims
The authors claim that this study successfully identifies new unsymmetrical diglycolamide candidates that offer a favorable trade-off between metal extraction efficiency and phase stability. Specifically:

DMOpyrDGA (Ligand 10) is identified as the most promising candidate due to its combination of high extraction efficiency at high acidity, effective metal stripping at low acidity, and improved solubility in aliphatic diluents compared to previous straight-chain analogs.
The study demonstrates that branched alkyl chains can mitigate the third-phase formation issues associated with straight-chain DGAs without sacrificing extraction performance, provided the cyclic amide moiety is appropriately selected (favoring pyrrolidine or piperidine over morpholine).
The work provides a comprehensive dataset comparing the effects of alkyl branching (2-ethylhexyl vs. 3,7-dimethyloctyl) and ring size (5-membered vs. 6-membered) on the partitioning behavior of lanthanides, offering design principles for future ligand development in nuclear fuel reprocessing.

The paper concludes that while TODGA remains a benchmark for solubility and high-acid extraction, the new unsymmetrical derivatives, particularly the branched alkyl pyrrolidinyl variants, offer superior extraction strengths and faster kinetics, making them viable candidates for further development in large-scale solvent extraction processes.

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