15 research outputs found
Coordination Structures and Supramolecular Architectures in a Cerium(III)–Malonamide Solvent Extraction System
The process chemistry and solution structures investigated
in the title system bridge the three ostensibly disparate fields of
separation sciences, soft matter research, and coordination chemistry.
We have explored this subject with synchrotron radiation research
and advanced analyses leading to original insights into aggregation
phenomena in solvent extraction. Herein we present findings showing
the coagulation of reverse micelles into wormlike aggregates in organic
phases (<i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-dibutyltetradecylmalonamideabbreviated
as DMDBTDMAin <i>n</i>-dodecane) obtained by liquid–liquid
extraction following contact with acidic and neutral aqueous media
containing trivalent cerium. The growth of solute architectures was
shown to prelude phase transition (i.e., the formation of a “third
phase”). The presence of acid was shown to promote the growth
of these micellar chains and, therefore, promoted third-phase formation.
Acid was also shown to hydrate and swell the reverse micelle units,
preorganizing them to allow for incorporation of cerium, leading to
different coordination structures and enhanced metal extraction. The
approach of linking both the coordination environment and supramolecular
structures to the process properties of a solvent extraction system
in a single study provides perspectives that are not available from
independent, uncorrelated experimentation. Moreover, the analysis
of small-angle X-ray scattering data from a solvent extraction system
using the generalized indirect Fourier transform method to gain real-space
information led to insights not otherwise available, showing that
micellar assemblies are larger and more ordered than previously thought.
This multipronged and multidisciplinary investigation opens new avenues
in the evolving understanding of solute architectures in organic phases
of practical relevance to solvent extraction and, simultaneously,
of fundamental relevance to structured fluids and, in particular,
phase transition phenomena
Molecular Scale Description of Anion Competition on Amine-Functionalized Surfaces
Many industrial and biological processes
involve the competitive
adsorption of ions with different valencies and sizes at charged surfaces;
heavy and precious metal ions are separated on the basis of their
propensity to adsorb onto interfaces, often as anionic ion clusters
(e.g., [MCl<sub><i>x</i></sub>]<sup><i>n</i>−</sup>). However, very little is known, both theoretically and experimentally,
about the competition of factors that drive preferential adsorption,
such as charge density or valence, at interfaces in technologically
relevant systems. There are even contradictory pictures described
by interfacial studies and real life applications, such as chlorometalate
extractions, in which charge diffuse chlorometalate ions are extracted
efficiently even though charge dense chloride ions present in the
background are expected to occupy the interface. We studied the competition
between divalent chlorometalate anions (PtCl<sub>6</sub><sup>2–</sup> and PdCl<sub>4</sub><sup>2–</sup>) and monovalent chloride
anions on positively charged amine-functionalized surfaces using <i>in situ</i> specular X-ray reflectivity. Chloride anions were
present in vast excess to simulate the conditions used in the commercial
separation of heavy and precious metal ions. Our results suggest that
divalent chlorometalate adsorption is a two-step process and that
the divalent anions preferentially adsorb at the interface despite
having a charge/volume ratio lower than that of chloride. These results
provide fundamental insight into the structural mechanisms that underpin
transport in phases that are relevant to heavy and precious metal
ion separations, explaining the high efficiency of low charge density
ion transport processes in the presence of charge dense anions
Ion Transport Mechanisms in Liquid–Liquid Interface
Interfacial
liquid–liquid ion transport is of crucial importance
to biotechnology and industrial separation processes including nuclear
elements and rare earths. A water-in-oil microemulsion is formulated
here with density and dimensions amenable to atomistic molecular dynamics
simulation, facilitating convergent theoretical and experimental approaches
to elucidate interfacial ion transport mechanisms. Lutetium(III) cations
are transported from the 5 nm diameter water pools into the surrounding
oil using an extractant (a lipophilic ligand). Changes in ion coordination
sphere and interactions between the interfacial components are studied
using a combination of synchrotron X-ray scattering, spectroscopy,
and atomistic molecular dynamics simulations. Contrary to existing
hypotheses, our model system shows no evidence of interfacial extractant
monolayers, but rather ions are exchanged through water channels that
penetrate the surfactant monolayer and connect to the oil-based extractant.
Our results highlight the dynamic nature of the oil–water interface
and show that lipophilic ion shuttles need not form flat monolayer
structures to facilitate ion transport across the liquid–liquid
interface
Molecular Origins of Mesoscale Ordering in a Metalloamphiphile Phase
Controlling the assembly of soft
and deformable molecular aggregates
into mesoscale structures is essential for understanding and developing
a broad range of processes including rare earth extraction and cleaning
of water, as well as for developing materials with unique properties.
By combined synchrotron small- and wide-angle X-ray scattering with
large-scale atomistic molecular dynamics simulations we analyze here
a metalloamphiphile–oil solution that organizes on multiple
length scales. The molecules associate into aggregates, and aggregates
flocculate into meso-ordered phases. Our study demonstrates that dipolar
interactions, centered on the amphiphile headgroup, bridge ionic aggregate
cores and drive aggregate flocculation. By identifying specific intermolecular
interactions that drive mesoscale ordering in solution, we bridge
two different length scales that are classically addressed separately.
Our results highlight the importance of individual intermolecular
interactions in driving mesoscale ordering
How Hydrogen Bonds Affect the Growth of Reverse Micelles around Coordinating Metal Ions
Extensive research on hydrogen bonds (H-bonds) have illustrated their critical role in various biological, chemical and physical processes. Given that existing studies are predominantly performed in aqueous conditions, how H-bonds affect both the structure and function of aggregates in organic phase is poorly understood. Herein, we investigate the role of H-bonds on the hierarchical structure of an aggregating amphiphile-oil solution containing a coordinating metal complex by means of atomistic molecular dynamics simulations and X-ray techniques. For the first time, we show that H-bonds not only stabilize the metal complex in the hydrophobic environment by coordinating between the Eu(NO<sub>3</sub>)<sub>3</sub> outer-sphere and aggregating amphiphiles, but also affect the growth of such reverse micellar aggregates. The formation of swollen, elongated reverse micelles elevates the extraction of metal ions with increased H-bonds under acidic condition. These new insights into H-bonds are of broad interest to nanosynthesis and biological applications, in addition to metal ion separations
How Hydrogen Bonds Affect the Growth of Reverse Micelles around Coordinating Metal Ions
Extensive research on hydrogen bonds (H-bonds) have illustrated their critical role in various biological, chemical and physical processes. Given that existing studies are predominantly performed in aqueous conditions, how H-bonds affect both the structure and function of aggregates in organic phase is poorly understood. Herein, we investigate the role of H-bonds on the hierarchical structure of an aggregating amphiphile-oil solution containing a coordinating metal complex by means of atomistic molecular dynamics simulations and X-ray techniques. For the first time, we show that H-bonds not only stabilize the metal complex in the hydrophobic environment by coordinating between the Eu(NO<sub>3</sub>)<sub>3</sub> outer-sphere and aggregating amphiphiles, but also affect the growth of such reverse micellar aggregates. The formation of swollen, elongated reverse micelles elevates the extraction of metal ions with increased H-bonds under acidic condition. These new insights into H-bonds are of broad interest to nanosynthesis and biological applications, in addition to metal ion separations
In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids
Coordination
polymers (CPs) of metal ions are central to a large
variety of applications, such as catalysis and separations. These
polymers frequently occur as amorphous solids that segregate from
solution. The structural aspects of this segregation remain elusive
due to the dearth of the spectroscopic techniques and computational
approaches suitable for probing such systems. Therefore, there is
a lacking of understanding of how the molecular building blocks give
rise to the mesoscale architectures that characterize CP materials.
In this study we revisit a CP phase formed in the extraction of trivalent
lanthanide ions by diesters of the phosphoric acid, such as the bis(2-ethylhexyl)phosphoric
acid (HDEHP). This is a well-known system with practical importance
in strategic metals refining and nuclear fuel reprocessing. A CP phase,
referred to as a “third phase”, has been known to form
in these systems for half a century, yet the structure of the amorphous
solid is still a point of contention, illustrating the difficulties
faced in characterizing such materials. In this study, we follow a
deductive approach to solving the molecular structure of amorphous
CP phases, using semiempirical calculations to set up an array of
physically plausible models and then deploying a suite of experimental
techniques, including optical, magnetic resonance, and X-ray spectroscopies,
to consecutively eliminate all but one model. We demonstrate that
the “third phase” consists of hexagonally packed linear
chains in which the lanthanide ions are connected by three O–P–O
bridges, with the modifying groups protruding outward, as in a bottlebrush.
The tendency to yield linear polynuclear oligomers that is apparent
in this system may also be present in other systems yielding the “third
phase”, demonstrating how molecular geometry directs polymeric
assembly in hybrid materials. We show that the packing of bridging
molecules is central to directing the structure of CP phases and that
by manipulating the steric requirements of ancillary groups one can
control the structure of the assembly
Subtle Effects of Aliphatic Alcohol Structure on Water Extraction and Solute Aggregation in Biphasic Water/<i>n</i>‑Dodecane
Organic
phase aggregation behavior of 1-octanol and its structural
isomer, 2-ethylhexanol, in a biphasic <i>n</i>-dodecane–water
system is studied with a combination of physical measurement, small-angle
X-ray scattering (SAXS), and atomistic molecular dynamic simulations.
Physical properties of the organic phases are probed following their
mixing and equilibration with immiscible water phases. Studies reveal
that the interfacial tension decreases as a function of increasing
alcohol concentration over the solubility range of the alcohol with
no evidence for a critical aggregate concentration (cac). An uptake
of water into the organic phases is quantified, as a function of alcohol
content, by Karl Fischer titrations. The extraction of water into
dodecane was further assessed as a function of alcohol concentration
via the slope-analysis method sometimes employed in chemical separations.
This method provides a qualitative understanding of solute (water/alcohol)
aggregation in the organic phase. The physical results are supported
by analyses of SAXS data that reveals an emergence of aggregates in <i>n</i>-dodecane at elevated alcohol concentrations. The observed
aggregate structure is dependent on the alcohol tail group geometry,
consistent with surfactant packing parameter. The formation of these
aggregates is discussed at a molecular level, where alcohol–alcohol
and alcohol–water H-bonding interactions likely dominate the
occurrence and morphology of the aggregates
On the Extraction of HCl and H<sub>2</sub>PtCl<sub>6</sub> by Tributyl Phosphate: A Mode of Action Study
<p>Combining computational modeling with experimental measurements has revealed the self-assembly of nano-aggregate structures in the transfer of HCl and PtCl<sub>6</sub><sup>2–</sup> from an aqueous phase into toluene by the common industrial extractant tributyl phosphate (TBP). Molecular dynamics simulations have been coupled to analytical measurements to provide an atomistic interpretation of the mode of action of TBP under 6 M and 10 M HCl conditions. The structures conform to reverse micelles, where the Cl<sup>–</sup> or PtCl<sub>6</sub><sup>2–</sup> core is encapsulated by a hydration shell that acts as a mediating bridge to the electronegative oxygen atom in the TBP phosphate groups. For the 6 M HCl extraction model, the data support stable aggregates forming from 2–3 TBP molecules around one chloride anion if the number of water molecules encapsulating the chloride anion is no more than five; increasing the water content to 10 molecules allows a fourth TBP molecule to coordinate. For the 10 M HCl extraction model, stable structures are obtained that conform to the empirical formula (TBP.HCl.H<sub>2</sub>O)<sub>3–5</sub>. At 6 M HCl, extraction of PtCl<sub>6</sub><sup>2–</sup> is achieved by encapsulation by four TBP molecules; the data for extraction at 10 M HCl indicate larger aggregates containing multiple PtCl<sub>6</sub><sup>2–</sup> anions are likely to be forming. In all cases, the hydrated core regions of the reverse micelles are considerably exposed. The diameters of the self-assembled structures around chloride ions agree well with available literature data from small-angle neutron-scattering experiments.</p
Outer-Sphere Water Clusters Tune the Lanthanide Selectivity of Diglycolamides
Fundamental
understanding of the selective recognition and separation of <i>f</i>-block metal ions by chelating agents is of crucial importance
for advancing sustainable energy systems. Current investigations in
this area are mostly focused on the study of inner-sphere interactions
between metal ions and donor groups of ligands, while the effects
on the selectivity resulting from molecular interactions in the outer-sphere
region have been largely overlooked. Herein, we explore the fundamental
origins of the selectivity of the solvating extractant <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetraoctyl diglycolamide (TODGA) for adjacent lanthanides
in a liquid–liquid extraction system, which is of relevance
to nuclear fuel reprocessing and rare-earth refining technologies.
Complementary investigations integrating distribution studies, quantum
mechanical calculations, and classical molecular dynamics simulations
establish a relationship between coextracted water and lanthanide
extraction by TODGA across the series, pointing to the importance
of the hydrogen-bonding interactions between outer-sphere nitrate
ions and water clusters in a nonpolar environment. Our findings have
significant implications for the design of novel efficient separation
systems and processes, emphasizing the importance of tuning both inner-
and outer-sphere interactions to obtain total control over selectivity
in the biphasic extraction of lanthanides