A Novel Approach for Quantifying Magnetic Susceptibility of Aqueous and Organic Solutions

A new method for measuring the magnetic properties of aqueous and organic solutions is presented. This approach is based on quantifying the force resulting from the sample's interaction with a magnetic field. The experimental setup utilizes neodymium magnets attached to a stepper motor to adjust the distance between the magnets and the test sample, while an analytical balance serves as a strain gauge. Magnetic susceptibility measurements were performed on selected inorganic and organic solutions. A series of finite element simulations allowed us to convert experimental results to physical quantities describing magnetic susceptibilities of substances. The limit of detection (LoD) and limit of quantification (LoQ) values for the developed method of determining magnetic susceptibility were equal to 6.67·10-3 M and 2.02·10-2 M, respectively.

Controlling Extraction of Rare Earth Elements Using Functionalized Aryl-vinyl Phosphonic Acid Esters

Ligands that can discriminate between individual rare earth elements are important for production of these critical elements. A set of aryl-vinyl phosphonic acid ligands for extracting rare earth elements were designed and synthesized under the hypothesis that the strength of the rare earth-ligand interactions could be tuned by changing the dipole moment of the ligand. The ligands were synthesized via a two-step reaction procedure using a Heck coupling reaction to functionalize vinyl phosphonic acid, followed by Steglich esterification to obtain high-purity styryl phosphonic acid monoesters with varying dipole moments along the P-C bond. The metal binding strength and composition of the rare earth complexes formed with these styryl phosphonic acid monoesters were experimentally studied by liquid-liquid extraction techniques, while DFT calculations were performed to determine the dipole moments of the free and complexed ligands and the electronic structure of the complexes formed. All three prepared ligands were much stronger extracting agents for europium(III) than the dialkylphosphonic acids usually used for this separation. However, the order of increasing extraction strength was found to match the order of the decreasing calculated dipole moment along the P-C bond of the three styryl-based ligands, rather than correlating with increasing ligand basicity, as reflected by the pKa of the ligands. These findings suggest that this approach can be used to systematically alter the extraction strength of aromatic phosphonic monoesters for rare earth element purification.

Magnetic forces in paramagnetic fluids

An overview of the effect of a magnetic field gradient on fluids with linear magnetic susceptibilities is given. It is shown that two commonly encountered expressions, the magnetic field gradient force and the concentration gradient force for paramagnetic species in solution are equivalent for incompressible fluids. The magnetic field gradient and concentration gradient forces are approximations of the Kelvin force and Korteweg-Helmholtz force densities, respectively. The criterion for the appearance of magnetically induced convection is derived. Experimental work in which magnetically induced convection plays a role is reviewed.

Coordination Chemistry-Driven Approaches to Rare Earth Element Separations

Current projections for global mining indicate that unsustainable practices will cause supply problems for many elements, called critical raw materials, in the next 20 years. These include elements necessary for renewable technologies as well as artisanal sources. Energy critical elements (ECEs) comprise a group used for clean, renewable energy applications that are in low abundance in the Earth's crust or require an economic premium to extract from ores. Sustainable practices of acquiring ECEs is an important problem to address through fundamental research to provide alternative energy technologies such as wind turbines and electric vehicles at cheaper costs for our global energy generation and usage. Some of these green technologies incorporate rare-earth (RE) metals (Sc, Y and the lanthanides), which are challenging to separate from mineral sources because of their similar sizes (i.e., ionic radii) and chemical properties. The current process used to provide REs at requisite purities for these applications is counter-current solvent-solvent extraction, which is scalable and works efficiently for any ore composition. However, this method produces large amounts of caustic waste that is environmentally damaging, especially to areas in China that house major separation facilities. Advancement of the selectivity of this process is challenging since exact molecular speciation that affords separations is still relatively unknown. In this context, we developed a program to investigate new RE separations systems that were aimed at minimizing solvent use, controlled by molecular speciation, and could be targeted at problems in recycling these critical metals.The first ligand system that was developed to impart solubility differences between light and heavy rare-earth ions was [{(2-^tBuNO)C₆H₄CH₂}₃N]^3- (TriNOx^3-) (graphic below). A differential solubility allowed for a separation of Nd and Dy of SF_Nd:Dy = ∼300 in a single step. In other words, a 50:50 Nd/Dy sample was enriched to give 95% pure Nd and Dy through a simple filtration, which is potentially impactful to recycling magnetic materials found in wind turbines. This separations system compares favorably to other state-of-the-art molecular extractants that are based on energetic differences of the thermodynamic parameter to affect separations for neighboring elements. This straightforward, thermodynamically driven method to separate REs primed our future research for new coordination chemistry approaches to separations.Another separations system was accomplished through the variable rate of a redox event from one arm of the TriNOx^3- ligand. It was determined that the rate of this one electron oxidation, which operated through an electrochemical-chemical-electrochemical mechanism, was dependent on the identity of the RE ion. This kinetically driven separation afforded a separation factor (SF) of SF_Eu:Y = 75. We have also described other transformations such as ligand exchange, substituent dependent, and redox-driven chelation processes with well-defined speciation to afford purified RE materials. Recently, we determined that magnetic properties can be used to enhance both thermodynamic and kinetic RE separations processes to give an approximately 100% boost for pairs of paramagnetic/diamagnetic REs. These results have shown that both thermodynamic and kinetic RE separations were efficient for different selected RE binary pairs through coordination chemistry. The focus of this Account will detail the differences that are observed for RE separations when promoted by thermodynamic or kinetic factors. Overall, the development of rationally adjusted speciation of REs provides a basis for future industrial separations processes for technologies applied to ECEs derived from wind turbines, batteries for electric vehicles, and LEDs.

Harnessing magnetic fields for rare-earth complex crystallization–separations in aqueous solutions

Magnetic field-directed crystallization separation of rare-earth (RE) metals is emerging as a new direction in the field of separation science, due to its simplicity, low energy input, and low cost of operation, as compared to traditional separation methods such as solvent extraction. Here, we report the use of Fe₁₄Nd₂B magnets for selective crystallization of paramagnetic Nd, Dy, Er, and Tm rare earth compounds from a mixture with diamagnetic La ones using the RE–DOTA complex system. All the separations were performed at milder temperatures of 3 °C to provide a thermal gradient, and the crystallizations were set up in aqueous solutions using the benign solvents water and acetone. A four-fold increase in the separation factor (41.4 ± 0.6) was observed for the Dy/La pair in the presence of a magnetic field as compared to the separation factor (10.5 ± 0.9) obtained without the application of the field. These results indicate that the use of the magnetic crystallization method for RE separations is effective in aqueous systems and can be a useful strategy for energy-efficient molecular separations of RE metals.

Magnetic crystallization was used as an energy-efficient technique for selective separation of paramagnetic rare-earth ions from lanthanum ions. An air-stable and simple RE-DOTA complex system was used to achieve separation in aqueous conditions.

Neutron Imaging of Paramagnetic Ions: Electrosorption by Carbon Aerogels and Macroscopic Magnetic Forces

The electrosorption of Gd³⁺ ions from an aqueous 70 mM Gd(NO₃)₃ solution in monolithic carbon aerogel electrodes was recorded by dynamic neutron imaging. The aerogels have a bimodal pore size distribution consisting of macropores and mesopores centered at 115 and 15 nm, respectively. After the uptake of Gd³⁺ ions by the negatively charged surface of the porous structure, an inhomogeneous magnetic field was applied to the system of discharging electrodes. This led to a convective flow and confinement of Gd(NO₃)₃ solution in the magnetic field gradient. Thus, a way to desalt and capture paramagnetic ions from an initially homogeneous solution is established.

A Trimetallic Cobalt/Iron/Nickel Phytate Catalyst for Overall Water Splitting: Fabrication by Magnetic-Field-Assisted Bipolar Electrodeposition

Electrodeposition is an effective method to prepare various materials. We have established a bipolar electrodeposition system assisted by a constant magnetic field to fabricate a Co/Fe/Ni phytate catalyst with good electrocatalytic activity for overall water splitting. The effects of magnetic and electric fields on the catalytic properties of the material were studied. The catalyst prepared with an N-pole magnetic field (NPMF) exhibited good overall water splitting performance. Benefiting from the synergistic effect of the Co/Fe/Ni-phytate and the advantages of the N-pole magnetic field the NPMF electrode has a continuous 25 hours high-efficiency hydrogen evolution and oxygen evolution reaction at a current density of 100 mA cm^-2 in1.0 M KOH compared with commercial RuO₂ and Pt/C. Bipolar electrodeposition with a constant magnetic field is thus an efficient means to fabricate electrocatalytic water splitting catalysts.

Magnetic Levitation in Chemistry, Materials Science, and Biochemistry

All matter has density. The recorded uses of density to characterize matter date back to as early as ca. 250 BC, when Archimedes was believed to have solved "The Puzzle of The King's Crown" using density.^[1] Today, measurements of density are used to separate and characterize a range of materials (including cells and organisms), and their chemical and/or physical changes in time and space. This Review describes a density-based technique-magnetic levitation (which we call "MagLev" for simplicity)-developed and used to solve problems in the fields of chemistry, materials science, and biochemistry. MagLev has two principal characteristics-simplicity, and applicability to a wide range of materials-that make it useful for a number of applications (for example, characterization of materials, quality control of manufactured plastic parts, self-assembly of objects in 3D, separation of different types of biological cells, and bioanalyses). Its simplicity and breadth of applications also enable its use in low-resource settings (for example-in economically developing regions-in evaluating water/food quality, and in diagnosing disease).

Oscillatory surface deformation of paramagnetic rare-earth solutions driven by an inhomogeneous magnetic field

The deformation of the free surface of a paramagnetic liquid subjected to a nonuniform magnetic field is studied. A transient deformation of the surface caused by the interplay of gravity, magnetic field, and surface tension is observed when a permanent magnet is moved vertically downward to the free surface of the liquid. Different concentrations of rare-earth-metal salt (DyCl_{3}) are used and different magnet velocities are studied. The deformation of the interface is followed optically by means of a microscope and recorded with a high-speed camera. The experimental results are compared and discussed with complementary numerical simulations. Detailed results are given for the static shape of the deformed surface and the temporal evolution of the surface deformation below the center of the magnet. The frequency of the surface oscillations is found to depend on the concentration of the salt and is compared with analytical findings. Finally, a potential application of the effects observed is presented.

Magnetic-Field-Induced Electrochemical Performance of a Porous Magnetoplasmonic Ag@Fe3O4 Nanoassembly

The Lorentz or Kelvin force generated by an externally applied magnetic field may introduce additional convection of the electrolyte near the working electrode and consequently produces magnetocurrent (MC), which can be attributed to the magnetohydrodynamic (MHD) flow and an extra electrochemical reaction. A magnetoplasmonic (MagPlas) composite of metallic and superparamagnetic nanoparticles (NPs) with a permanent dipole or magnetic moment have additional degree or order, which corresponds to directional correlation to electric and magnetic dipoles. In particular, an ordered self-assembly may boost up the MHD flow on a collectively reactive surface, leading to remarkable electrochemical performance. In this article, a proof-of-concept work explores the effect of the magnetic field on the electrocatalytic activity of the oxygen reduction reaction (ORR) as well as [Fe(CN)₆]^3-/4- redox probes using a precisely controlled three-dimensional (3D) nanostructure of a silver core and a porous magnetic shell (Ag@Fe₃O₄) assembly. Then, the reduction current was carefully monitored in the presence of a magnetic field (B, up to 380 mT), resulting in an extraordinary increment of reduction current (I_R) of [Fe(CN)₆]^3- by 23% and a 1.13-fold high ORR efficiency owing to the additional magnetic field (B_in) from the 3D magnetoplasmonic nanoassembly. The computational simulation explained the plausible mechanism of current enhancement from the MagPlas nanoassembly. From our experimental and computational studies, it is probable that the 3D MagPlas nanoassembly is a unique and efficient catalyst under a low external magnetic field, which would be useful for further biomedical and energy-related applications.

Magnetomigration of Rare-Earth Ions Triggered by Concentration Gradients

Mach-Zehnder interferometry was applied to explore the effects of inhomogeneous magnetic fields on the mobility of rare-earth ions in aqueous solutions. No migration of ions was observed in a thermodynamically closed system when a homogeneous solution was subjected to a magnetic field gradient alone. However, magnetomigration could be triggered by a concentration gradient of the rare-earth ions in the solution. When a concentration gradient was introduced in the sample by solvent evaporation, consistent migration of paramagnetic Dy³⁺ ions from the bulk solution to regions with stronger magnetic fields was observed. By contrast, no movement was detected for diamagnetic Y³⁺ ions in the presence of a concentration gradient.

Tunable photoluminescence and room temperature ferromagnetism of In2S3:Dy3+,Tb3+ nanoparticles

In₂S₃:6%Dy³⁺,yTb³⁺ nanoparticles exhibit tunable optical properties and room temperature ferromagnetism, which have been further investigated using VASP calculations.