Solvation Dynamics of HEHEHP Ligand at the Liquid-Liquid Interface

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Solvation Dynamics of HEHEHP Ligand at the Liquid-Liquid Interface An T. Ta, Govind A. Hegde, Brian D. Etz, Anna G. Baldwin, Yuan Yang, Jenifer C. Shafer, Mark Peter Jensen, C. Mark Maupin, and Shubham Vyas J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b03165 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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The Journal of Physical Chemistry

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Solvation Dynamics of HEHEHP Ligand at the

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Liquid-Liquid Interface

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An T. Ta, Govind A. Hegde, Brian D. Etz, Anna G. Baldwin, Yuan Yang, Jenifer C. Shafer, Mark

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P. Jensen, C. Mark Maupin, and Shubham Vyas*

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Department of Chemistry, Colorado School of Mines, Golden, Colorado 80401, United States

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*Corresponding author: phone +1-303-273-3632, E-mail: [email protected]

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ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract. Actinide-Lanthanide Separation (ALSEP) has been a topic of interest in recent years as

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it has been shown to selectively extract problematic metals from spent nuclear fuel. However, the

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process suffers from slow kinetics prohibiting it from being applied to nuclear facilities. In effort

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to improve the process, many fundamental studies have been performed, but the majority have

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only focused on the thermodynamics of separation. Therefore, to understand the mechanism

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behind the ALSEP process, molecular dynamics (MD) simulations were utilized to obtain the

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dynamics and solvation characteristics for an organic extractant, 2-ethylhexylphosphonic acid

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mono-2-ethylhexyl ester (HEHEHP). Simulations were conducted with both pure and biphasic

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solvent systems to evaluate the complex solvent interactions within the ALSEP extraction method.

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The MD simulations revealed solvation and dynamical behaviors that are consistent with

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experimentally observed chemical properties of HEHEHP for the pure solvent systems (e.g.

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hydrophobic/hydrophilic behaviors of the polar head group and alkyl chains and dimer formation

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between the ligands within an organic solvent). When present in a biphasic solvent system,

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interfacial behaviors of the ligand revealed that, at low concentrations, the alkyl side chains of

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HEHEHP were parallel to the interfacial plane. Upon increasing the concentration to 0.75 M,

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tendency for the parallel orientation decreased and a more perpendicular-like orientation was

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observed. Analysis of ligand solvation energies in different solvents through the thermodynamic

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integration method demonstrated favorability towards n-dodecane and biphasic solvents, which is

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in agreement with the previous experimental findings.

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I. Introduction

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One of the major challenges faced by nuclear facilities is the management of spent nuclear

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fuel. Approximately 2,500 tons of spent fuel is generated in the United States each year and, if left

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untreated, the long-term radiotoxicity will pose a threat to the environment.1–4 Of the many isotopes

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present within nuclear waste, transuranic actinides foster major concern due to their large

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radiotoxicities. Such elements are primary contributors to long-term toxicity and, contribute major

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strain on waste repositories.1–3 To simplify repository design and siting, recent efforts have been

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dedicated to minimizing the transuranic content within spent nuclear fuel.4,5

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Although separation processes such as PUREX have been successfully implemented at the

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industrial6,7 scale for the extraction of plutonium and uranium, recovery of other prominent

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actinides, such as americium (Am) and curium (Cm), remains difficult due to the presence of

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fission product lanthanides. The separation of Am and Cm is crucial because the lanthanides

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possess high neutron capture cross sections, which may inhibit the transmutation of the actinides

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in advanced nuclear reactors.8 Different separation processes have been developed to address this

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issue but solvent (liquid-liquid) extractions are most commonly used.4 This method utilizes two

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immiscible solvents to partition the actinides from the lanthanides in solution. Some solvent

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extraction separations that have shown to be capable of selectively separating Am and Cm are

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SANEX, GANEX, advanced TALSPEAK, and ALSEP.4,5,9 However, implementation of these

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processes often suffers from slow kinetics resulting in unsatisfactory throughput when

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implemented at the industrial scale.

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The ALSEP process displays a fast solvent loading step where trivalent lanthanides and

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actinides are extracted from molar concentrations of nitric acid but the selective stripping step

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where actinides and lanthanides are actually separated from each other is too slow to implement



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in conventional centrifugal contractors.5,10 The separation step in the ALSEP process is very

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similar to that of the reverse TALSPEAK and advanced TALSPEAK processes, in which an

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organophosphorus extractant such as di-(2-ethylhexyl)phosphoric acid (HDEHP) or 2-

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ethylhexylphosphonic acid mono-2ethylhexyl ester (HEHEHP) is used to extract both trivalent

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actinide and lanthanide cations into the organic phase. Then the actinides are selectively stripped

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from the organic phase with aqueous diethylenetriaminepentaacetic acid (DTPA) or N-(2-

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[bis(carboxymethyl)amino]ethyl)-N-(2-hydroxyethyl)glycine

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conceptually simple, a detailed and quantitative understanding of the chemical process behind

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ALSEP have yet to be uncovered.

(HEDTA).5,11

Although

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Currently, most fundamental research of the ALSEP process has only considered

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thermodynamics and little attention has been directed towards understanding mechanisms that

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drive the kinetics of separation. Nevertheless, previous studies of extraction kinetics on

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TALSPEAK have demonstrated that the rate of partitioning is dependent on chemical reactions at

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or near the interface of the system.12–14 Due to the chemical similarities between the processes, it

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is believed that such rate dependence will also be the same for the ALSEP method. Therefore,

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understanding the various phenomena occurring at the interface and identification of important

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interfacial reactions are essential to determining the origin of the kinetic barriers in ALSEP. Once

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the critical interfacial reactions are identified, and their kinetics barriers are evaluated, the rate

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limiting steps can be identified and possible improvements can be proposed. In an effort to deepen

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our understanding of the events occurring at the interface, computational modeling through ab

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initio calculations and molecular dynamics (MD) simulations were used to analyze dynamics and

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solvation behaviors of HEHEHP (Figure 1) in n-dodecane, water, and a biphasic n-dodecane +

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water solvents.


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Figure 1. Structure of HEHEHP. Oxygen atoms are depicted in red, phosphorous in orange, carbon

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in grey, and hydrogen in white colored spheres. The labeling scheme is as follows: ester oxygen –

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(O1), hydroxy oxygen – (O2), phosphonyl oxygen – (O3), ethyl carbon – (E1/2), and hexyl carbon

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(H1/2).

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In this study, new atomic charge force field parameters were developed for HEHEHP

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through ab initio calculations and were used within MD simulations to model the ligand dynamics

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within different solvents. These simulations were analyzed to determine solvation spheres, cluster

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formations, interfacial orientations, and solvation energies in two scenarios (single and multi-

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ligand systems). Investigations of these scenarios probed the impact of extractant concentration on

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HEHEHP behavior within different solvents and provided critical insights that are useful to

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developing a mechanistic understanding of the ALSEP process.

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II. Computational Methods

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A. Ab initio Calculations

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Ab initio calculations were performed using the Gaussian09 software package.15 Geometry

100

optimizations of HEHEHP and n-dodecane were carried out using the Minnesota global hybrid

101

functional, M06-2X,16 with the CBSB7 basis sets (i.e. 6-311g(2d,d,p)).17 The nature of the

102

stationary state was confirmed to be a minimum by the absence of imaginary frequencies. Single



103

point calculation on the previously obtained geometries using Møller-Plesset second order

104

perturbation theory (MP2)18 and the cc-PVTZ basis sets19 were then utilized with the restrained

105

electrostatic potential (RESP) method to obtain the classical point charges.20 These charges and

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the optimized geometry coordinates are reported in Table S1 and S2 of the supporting information.

107

Similar to previously published work21, charge calculations were employed in conjunction with

108

the General Amber Force Field (GAFF)22 using the antechamber program23 within the Amber14

109

software package24 to generate charge modified GAFF parameter sets. These modified GAFF

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parameter sets were then used in subsequent MD simulations within n-dodecane, water, and a

111

biphasic (n-dodecane + water) solvent system.

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B. Molecular Dynamics Simulations

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All MD simulations were performed using the Amber14 package while analyses were

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conducted with AmberTools14, Visual Molecular Dynamics (VMD)25, and in-house programs,

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which are detailed in the respective results sections. Simulations within three different solvents

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were conducted at two different scenarios, as shown in Table S3. Single-ligand systems represent

117

one molecule of HEHEHP in a box of solvent and multi-ligand systems contain a HEHEHP

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concentration of 0.75 M, which was chosen to study the behavior of HEHEHP at typical ALSEP

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concentrations.5 Water was modeled using the SPC/FW26 flexible water model whereas HEHEHP

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and n-dodecane were described by the charge modified GAFF parameters obtained through

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calculations done in Section II.A. The Packmol program27 was utilized to produce initial

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coordinates for all systems, which consisted of a random distribution of the molecules across the

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simulation box unless otherwise noted.

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All systems were initially minimized for 2500 steps using the steepest descent algorithm,

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followed by 2500 steps using the conjugate gradient algorithm. After minimization, an isobaric-


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isothermal ensemble (NPT) simulation was performed at 298 K and 1 atm for 1 ns to ensure that

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the densities of the systems had converged. To ascertain the accuracy of the force field and

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sufficiency of the NPT simulation lengths, the density of a pure HEHEHP system (500 molecules)

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was calculated to be of 0.941 ± 0.001 g/cm3, which is in good agreement with the experimental

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value, 0.958 g/cm3. NPT simulations were followed by a production run using the canonical

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ensemble (NVT) at 298 K for 20 ns and a microcanonical ensemble (NVE) simulation for 50 ns.

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The NVE simulations were then used to calculate diffusion coefficients through an in-house

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program. All MD simulations utilized a 1 fs time step.

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C. Solvation Energy Calculations

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Solvation energies of HEHEHP in different solvents were calculated using

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Thermodynamic Integration (TI) as implemented in the AMBER14 software package.28 The TI

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procedure involves defining a thermodynamic cycle to probe the free energy difference between

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two states and running a separate simulation to represent each discrete segment in the

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thermodynamic cycle. In this case, the cycle involves a pathway with an initial state corresponding

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to the ligand having no interactions with the solvent (𝜆 = 0) and a final state corresponding to the

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ligand having complete interactions with the solvent (𝜆 = 1), which represents the fully solvated

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state of the ligand. The free energy is calculated using:

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1

Δ𝐸 = ∫0 𝑑𝜆

(1)

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where Δ𝐸 is the free energy difference of solvation, U is the potential energy of the system, and 𝜆

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is a parameter that varies the potential from the initial state where 𝜆 = 0 to the final state where 𝜆

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= 1. Solvation free energy was obtained with simulations ran at discrete values of 𝜆 from the initial

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to final state at intervals of 0.1 and numerical integration of Equation 1 by an in-house program.



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The procedure used in this study utilizes the new and more efficient implementation of TI in

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Amber28, wherein the transformation of both the charges and the van der Waals parameters from

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the initial state to the final state are done in the same step. Earlier implementations of TI required

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the electrostatic and van der Waals transformations to be done separately in two separate

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simulations. Furthermore, the new implementation allows for the calculation of free energies at

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both the initial and final states, which reduces error compared to previous implementations

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wherein the potential energy calculation became unstable at values of 𝜆 close to the initial and

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final states.

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D. NMR Measurements

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The average diffusion coefficient of a 0.2 M sample of HDEHP in n-dodecane was

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measured with a 500 MHz JEOL NMR spectrometer at 21ºC by implementing a pulsed field

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gradient stimulated echo experiment as described in previous works of Tanner and Baldwin et

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al.29,30 To achieve 90% attenuation (or greater) of the HDEHP peak at 4 ppm, 16 gradients of 16

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scans at a magnetic field gradient strength of 270 mT/m was utilized. A gradient pulse width of 2

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ms and a diffusion time of 200 ms was used in conjunction with a 5 s relaxation delay. The self

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diffusion coefficient was then calculated from the Stejskal-Tanner equation that relates decay of

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NMR signal intensity with increasing applied magnetic field gradient strength to the displacement

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of NMR-active nuclei. These results were used to compare with and validate the accuracy of the

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computational protocol used to find diffusion coefficients of HEHEHP.

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III. Results and Discussion

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A. Solvation of HEHEHP

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Interactions of HEHEHP in solution were investigated by computing radial distribution

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functions (RDFs) for seven atoms (labelled in Figure 1) with respect to solvent molecules and each

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other (when applicable) from the NVT simulation. To examine the immediate solution

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environment around the ligand (i.e. first solvation sphere), coordination numbers (CNs) were

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calculated by integrating the first peak of the respective atom’s RDF. Analyses in which a distinct

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peak was absent were interpreted as the atom having an asymptotic relationship with the solvent

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and the coordination number was deemed to be zero. Specific values of the cutoff distances for all

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RDFs can be found in Table S4 in the supporting information.

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RDFs of HEHEHP within pure n-dodecane and water solvents are reported within the

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supporting information as Figure S1 and S2 respectively. Both single- and multi-ligand systems

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displayed expected hydrophobic/hydrophilic behavior towards solvent molecules as the alkyl

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chains coordinated with n-dodecane within the pure organic system while the oxygen atoms

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exhibited interactions with water within the pure aqueous system. In n-dodecane, analysis of the

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ethyl carbons revealed a coordination number of 0.0 for both single- and multi-ligand systems.

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Although the carbon is hydrophobic and should favor interaction with n-dodecane, this can be

187

attributed to the relatively large size of the solvent molecules and the solute’s polar head group,

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which restricts interaction between the chains and solvent. Analyses of ligand-ligand interactions

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also showed expected behaviors with coordination primarily occurring between the head group in

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n-dodecane and interactions largely found to be between alkyl chains in water. In depth analyses

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of these expected behaviors within pure solvent systems can be found within the supporting

192

information.



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Figure 2. Cluster distribution of HEHEHP (0.75M) in water (blue), n-dodecane (red), and biphasic

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water + n-dodecane (green) solvent.

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To quantify ligand-ligand aggregation in the MD simulations, cluster analysis was

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performed on HEHEHP within the different solvents. It is well known that HEHEHP, when

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coordinating with a metal cation, forms dimer clusters between the head groups in an organic

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phase.31–36 Therefore, to investigate whether the MD simulations can reproduce the same behavior,

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cluster analysis was conducted in the absence of metal cations. A cluster was considered to be

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formed if phosphorous atoms of different ligand molecules came within a cut-off distance of 4.9

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Å. This distance was selected based on the ligand-ligand RDF from Figure S1C of the supporting

203

information. The presence of clusters was determined throughout the entire simulation using an

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in-house hierarchical clustering algorithm. A distribution of various multimeric complexes is

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shown in Figure 2 and a table of these values are reported in Table S5 of the supporting

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information. Multiple clusters were observed within the pure n-dodecane solvent with dimers

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primarily present throughout the simulation at ~54%. This illustrates HEHEHP’s tendency to form

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dimers, however, other multimeric complexes (e.g. trimers, tetramers, pentamers, and even

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hexamers), were also present, which demonstrates a more aggregative behavior in the absence of


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cations. Such behavior was suggested previously for HDEHP based on isopiestic measurements37

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and hints at the impact of the metal cation acting as a driver for dimer formation. Based off the

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criteria used, extractant clustering within the water solvent was observed to result primarily in

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monomers (~96%) with little dimer formation. This is in agreement with the RDF analyses of the

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aqueous solution, which showed that the polar P(=O)OH group of HEHEHP primarily hydrogen

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bonds to water molecules with ligand-ligand interactions mainly occurring between alkyl chains. (A)

(C)

(B)

(D)

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Figure 3. Radial distribution functions for (A) single-ligand HEHEHP to n-dodecane molecules,

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(B) multi-ligand HEHEHP to n-dodecane molecules, (C) single-ligand HEHEHP to water

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molecules, and (D) multi-ligand HEHEHP to water molecules in n-dodecane + water solvent.

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RDF assessment of HEHEHP in the biphasic solvent (Figure 3) revealed that the ester

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oxygen (O1) had an unexpectedly larger coordination number with water than the other oxygen

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atoms at the interface. This larger coordination number suggests that the orientation of the alkyl



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chains is parallel to the interface. Such orientation may be due to the hydrophobic n-dodecane

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molecules being parallel at the interface (vide infra, Section III.C) and, in turn, drive parallelization

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of the alkyl chains, which has been previously hypothesized.38 In addition, the single-ligand system

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revealed that the ethyl carbon (E1/2) had little interaction with n-dodecane molecules while they

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had a coordination number of ~2 in the multi-ligand system. This difference in interaction between

228

ethyl carbon and n-dodecane implies that the paralleled orientation of HEHEHP may be more

229

prominent at low concentrations. Under ALSEP concentrations, coordination between ligands was

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observed to be primarily between alkyl chains (Figure 4), which may help facilitate a more

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perpendicular conformation to the interfacial plane as conventional thought would suggest.31,32

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This was also seen within the cluster analysis (Figure 2) which showed monomers being primarily

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present from the perspective of the polar head groups.

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Figure 4. Radial distribution function of HEHEHP to HEHEHP in n-dodecane + water system.

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Note: "H" represents the head group and "T" represents the alkyl chains of the ligand.

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B. Diffusivity of HEHEHP

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To investigate the dynamical behavior of HEHEHP in various solvation environments,

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diffusion coefficients were calculated from the NVE simulation using Einstein’s relationship

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between the mean square displacement and diffusion coefficient as shown in equation 2,

243

𝐷=

𝜕 6(𝜕𝑡)

(2)

244

where D is the diffusion coefficient, is the mean squared displacement, and t is time. The

245

accuracy of the protocol used to determine diffusivity for HEHEHP was evaluated by comparing

246

experimental values available for the 0.2 M HDEHP in n-dodecane to values found from the MD

247

simulations. The MD simulations on HDEHP were performed with the same procedure as

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described in the section II.A and II.B. The diffusion coefficient for the steric bulk of HDEHP (alkyl

249

side chain) was 0.22 x 10-5 ± 0.07 cm2/s as determined from NMR measurements, while the MD

250

simulations yielded 0.09 x 10-5 ± 0.02 cm2/s. Considering experimental and theoretical error, these

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values are of acceptable agreement and demonstrate that the protocol used to capture the diffusion

252

of HDEHP in solution is of reasonable accuracy. HDEHP is quite similar to HEHEHP with the

253

only structural difference being the head group is a phosphoric and not a phosphonic acid (i.e.

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HEHEHP contains one less ester oxygen than HDEHP). Separately calculated diffusivities of the

255

alkyl chains in HEHEHP revealed that the ester oxygen has little effect on the diffusion of the

256

steric bulk with a value of 0.04 ± 0.04 x 10-5 cm2/s and 0.04 ± 0.02 x 10-5 cm2/s for ester containing

257

and non-ester containing chains, respectively. Keeping in mind structural similarity and the little

258

effect from the ester oxygen, it is concluded that the diffusion of HEHEHP is captured accurately

259

within this study.



260

To evaluate the dynamics of various parts of HEHEHP in solution, diffusion coefficients

261

for the polar head group (represented by the phosphorous atom), alkyl chains (represented by the

262

average of the two hexyl-terminal carbons), and the entire molecule on average were calculated at

263

25 °C as shown in Table 1. Within the single-ligand system, it was observed that the diffusion of

264

the head group was lower in a water system than in n-dodecane while the opposite was observed

265

for the alkyl chain. This trend may be attributed to favorable interactions (aqueous solvent

266

molecules interacting with the polar head group, etc.) causing the first solvation sphere to be tightly

267

bound. When such spheres are formed, the effective size will become larger and a slower

268

diffusivity is observed. A mixture of both n-dodecane and water solvents allows for both the polar

269

head group and alkyl chains to achieve favorable interactions at the interface and, thus, all diffusion

270

coefficients found within the biphasic solvent was observed to be smaller than that in pure solvents.

271

However, this was not observed within the multi-ligand system.

272

Table 1. Diffusion coefficients of different molecular subunits of HEHEHP and average diffusion

273

coefficients of HEHEHP for single- and multi-ligand systems in different solvents.

System

Group

Single-Ligand System

Head Group Alkyl Chains Average Head Group Alkyl Chains Average

Multi-Ligand System

Diffusion Coefficient (x 10-5 cm2/s) n-dodecane water n-dodecane + water 0.20 ± 0.08 0.11 ± 0.09 0.10 ± 0.03 0.15 ± 0.01 0.16 ± 0.09 0.13 ± 0.05 0.14 ± 0.07 0.17 ± 0.09 0.10 ± 0.03 0.02 ± 0.01 0.02 ± 0.02 0.10 ± 0.01 0.02 ± 0.01 0.07 ± 0.02 0.09 ± 0.02 0.02 ± 0.01 0.03 ± 0.02 0.10 ± 0.01

274 275

Dynamics of HEHEHP in the multi-ligand system showed diffusion coefficients being smaller

276

within the pure solvents. This may be due to the increased concentration of HEHEHP resulting in

277

the introduction of ligand-ligand interactions and the formation of clusters. These interactions and


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278

clusters help accentuate added favorable contacts that were not possible with a single ligand. As

279

mentioned previously, HEHEHP formed clusters between hydrogen bonds of the hydrophilic head

280

groups in n-dodecane while clusters were formed between dispersion of the hydrophobic alkyl

281

chains in water. Ligands of a biphasic system reside at the interface, which prevents the formations

282

of such clusters and, thus, a larger diffusion constant is observed. Comparison of the single- and

283

multi-ligand systems also support this as the diffusion of HEHEHP did not change significantly in

284

a biphasic solvent. In addition, ligand clusters may explain why the diffusivity of HEHEHP is

285

similar in both n-dodecane and water for the head group and overall average. Conversely, alkyl

286

chains were observed to diffuse much faster in water (0.07x10-5cm2/s) than in n-dodecane

287

(0.02x10-5cm2/s). This exception is presumably a result of unfavorable interactions occurring

288

between the alkyl chains and water molecules. From the structure of HEHEHP, it was believed

289

that the ligand would form micelle-like structures in the aqueous phase as this is observed with n-

290

alkane phosphonic acids.39 However, unlike n-alkane phosphonic acids, HEHEHP contains a

291

mono-ester group, which may hinder the ligand’s ability to form complete micellar structures and,

292

instead, only aggregate towards one another incapable of fully avoiding unfavorable interactions.

293

C. Interfacial Analysis of HEHEHP

294

To fully understand the interfacial orientation of HEHEHP, a separate set of NVT

295

simulations was performed, in which the ligand was initially placed at the interface. By setting the

296

original coordinates of n-dodecane and water to be separated from each other, these systems

297

contained a well-defined interfacial plane. The conformation of n-dodecane molecules and

298

HEHEHP alkyl chains were analyzed with an in-house code, in which vectors were defined as the

299

starting atom to the terminal carbon of the relevant chain. For example, with the ester containing

300

ligand chain, the vector was defined from the starting oxygen atom (O1, Figure 1) to the terminal



301

hexyl-carbon (H2, Figure 1). Vectors with an angle of 0-20o with respect to the interface were

302

considered to be parallel while those with an angle of 70-90o were deemed to be perpendicular.

303 304

Figure 5. Average angle of n-dodecane molecules with respect to the distance of the MD box in

305

(A) single-ligand and (B) multi-ligand systems throughout the simulation. Blue region indicates

306

the aqueous phase while orange region represents the organic phase. The dashed line represents

307

the interface.

308

As mentioned in section III.A., it has been hypothesized that immiscible solvent molecules

309

are parallel to the interface near the interfacial plane and, in turn, orientation of any solute near or

310

at the interface is driven to also be parallel.38 Testing this hypothesis, orientation angles of n-

311

dodecane molecules were analyzed as a function of the simulation box distance (Figure 5). Both

312

single- and multi-ligand systems supported the hypothesis made by Vandegrift et al.38 with the

313

angles of the organic solvent being < 15o at the interface. In addition, it was observed that the n-

314

dodecane molecules exhibited much larger angles (~30o to ~48o) at increased distances from the

315

interface, which represents a random orientation and, hence, a bulk organic phase. This indicates

316

a distinct connection between the parallel configuration and proximity to the interfacial plane.


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317

Noticeably, the highest angle observed within the multi-ligand system was not of the expected

318

bulk distribution with an angle ~30° and not 45°. This can be attributed to the smaller box size

319

preventing the solvent molecules from attaining bulk configuration. Analysis of the HEHEHP

320

alkyl chains revealed that parallelization was present regardless of concentration (Figure 6A) with

321

the steric bulk primarily possessing an orientation that is 10o with respect to the interface.

322

Moreover, such configuration was observed to be more prominent within the alkyl chain

323

containing the ester oxygen. These results support the findings from RDF analyses shown earlier

324

in Figure 2 with the ester oxygen of HEHEHP coordinating more with the aqueous phase than the

325

other oxygen atoms.



(A)

(B)

(C)

326


Page 18 of 26

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327

Figure 6. Occurrence of different interfacial angles analyzed (A) statistically for alkyl chains of

328

single-/multi-ligand systems and with respect to time for (B) the O-chain of the multi-ligand

329

system and (C) the C-chain of the multi-ligand system within n-dodecane + water solvent. Note:

330

O-chain and C-chain represent the alkyl chain with and without the ester oxygen, respectively.

331 332

Notably, due to steric hindrance, not all HEHEHP ligands were capable of being initially

333

placed at the interface within the multi-ligand simulation. Visual tracking by VMD revealed that

334

ligands began to partition to the interface and did not completely reside at the binary boundary till

335

later timeframes in the NVT simulation (Figure S3, supporting information).

336

quantifiably monitor the effects of interfacial ligand concentration, time resolved analyses were

337

performed on the orientation angles of alkyl side chains with respect to the interface. From Figure

338

6B and 6C, it was observed that ligand-ligand interactions cause the orientation to shift towards

339

larger angles. Most notably, results from the 15-20 ns timeframe revealed that angles ≥ 50o were

340

significantly more present than at earlier times. This escalation in the occurrence of higher angles

341

suggests that increasing the concentration instigates ligand configuration to shift towards a more

342

perpendicular-like orientation presumably due to the increased dispersion interaction of the alkyl

343

chains, which is supported by the previous RDF analyses (Figure 3). When present in the single-

344

ligand system, majority of the interfacial plane is occupied by n-dodecane molecules, which helps

345

facilitate the parallel alignment of HEHEHP’s steric bulk. On the contrary, in the multi-ligand

346

system, dispersion interactions between the alkyl chains of HEHEHP facilitate an increase in

347

perpendicular arrangement, despite the parallel alignment of the solvent molecules. Moreover, this

348

relationship indicates the critical role of ligand concentration in the extraction mechanism. It is

349

known that the extraction of metal cations by HEHEHP occurs between a dimer formation of the


Hence, to


350

head group coordinating with metal cation.31,32 Such an extraction would be greatly hindered if the

351

ligand was parallel to the interface, limiting aqueous participation of the hydroxyl and phosphoryl

352

oxygen.

353

D. Solvation Energy of HEHEHP in Different Solvents

354

To understand and devise a mechanism for the ALSEP process, it is critical to be able to

355

quantitatively analyze the solvation of HEHEHP. TI method as implemented in AMBER14

356

software package was used to obtain the free energies of HEHEHP in multi-ligand systems

357

containing n-dodecane, water and biphasic n-dodecane + water solvent. The TI method was

358

applied in this study for reasons explained within the supporting information. In these calculations,

359

the presence of the ligand is gradually removed in solution, and the energetics of the nonphysical

360

disappearance is monitored. Therefore, to account for ligand-ligand interactions in solution, the

361

method was implemented such that only one ligand was disappearing within the solvent.

362

Normalizing to the water solution, the relative free energies of n-dodecane and biphasic solvents

363

were found to be favored with a value of -9 ±5 kcal/mol and -8.6 ±0.6 kcal/mol. In addition, it was

364

observed that the free energies between n-dodecane and biphasic solvent were similar (difference

365

of 0.4 kcal/mol) which was expected as it is well known that HEHEHP is soluble in both n-

366

dodecane and biphasic solvent.31–33,38–40 As the experimental observations of favorable and

367

unfavorable interactions of HEHEHP with the organic and aqueous phase respectively are

368

reflected, it is believed that the energies calculated through TI are quantitatively representative of

369

the HEHEHP ligand.

370

IV. Conclusion


Page 20 of 26

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371

Within this study, force field parameters were developed for the organic HEHEHP

372

extractant and were used to investigate the solvation and dynamical behavior of the ligand in n-

373

dodecane, water, and biphasic n-dodecane + water solvents. Solvation behavior of HEHEHP

374

demonstrated expected hydrophilic and hydrophobic behavior of the ligand within the pure organic

375

and aqueous phases respectively. Within n-dodecane, ligand-ligand interactions were found to be

376

primarily between the head groups while such interactions were primarily between alkyl chains

377

within the water and biphasic n-dodecane + water solvents. Contrary to what is observed when

378

HEHEHP coordinates with metals, cluster analyses showed that the ligand exhibited a more

379

aggregative behavior than dimer formation in the absence of metal cations. The calculated thermal

380

diffusivities were in good agreement with the experimental data. Calculated diffusion coefficients

381

of HEHEHP within different solvents for a single- and multi-ligand system showed that favored

382

non-covalent interactions were found to primarily dictate the diffusivity within the single-ligand

383

system while cluster formation was observed to be the major influence within the multi-ligand

384

system. It is worth noting that the alkyl chains of the multi-ligand system exhibited a larger

385

diffusion coefficient in water than n-dodecane presumably due to the mono-ester functional group

386

prohibiting a complete micellar structure.

387

Conformational analysis of the orientation of the molecules revealed that n-dodecane and

388

HEHEHP possessed a parallel orientation at the interface, which confirms the hypothesis of

389

Vandegrift et al.38 Moreover, it was observed that increased concentration of the ligand instigated

390

a more perpendicular orientation at the interface, which suggests that concentration plays a critical

391

role in the extraction mechanism of HEHEHP since the ligand is known to extract metals by

392

coordinating with metal cations through their polar head groups. Solvation energies obtained by

393

the TI method were observed to describe expected solution behavior of HEHEHP by



394

demonstrating relative favorability towards n-dodecane and biphasic solvent when compared to

395

water.

396

Solvation and dynamical analyses carried out in this study shows that the protocol used to

397

generate charge modified GAFF parameters is of reasonable accuracy and, thus, can be used for

398

future parameterization of extractant ligands. Additionally, interfacial analyses suggest that

399

concentration is key for HEHEHP to obtain optimal configuration for the extraction of metals.

400

These effects will be studied in future work along with systems that fully mimic an ALSEP process

401

such as the inclusion of TEHDGA, nitric acid, and ultimately heavy metals.

402

V. Acknowledgements

403

Authors gratefully acknowledge the allocated computational resources from the High

404

Performance Computing Facility by the Computing, Communications, and Information

405

Technologies (CCIT) center at the Colorado School of Mines. This research was financially

406

supported by the U.S. Department of Energy, Office of Nuclear Energy, and Nuclear Energy

407

University Program (NEUP).

408

VI. Supporting Information

409

Values of calculated relativistic charges and coordinates of optimized structures used within this

410

investigation, RDF cut-off distances, cluster distribution of HEHEHP (0.75M) within three

411

solvents, RDF plots for pure solvent (n-dodecane/water) systems, in depth analysis of expected

412

HEHEHP behavior within pure solvent systems, time stamps of the canonical simulation used for

413

interfacial analyses of HEHEHP within a multi-ligand system, and reasons for using the TI method

414

are provided within the supporting information along with this manuscript.

415

VII. References


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Solvation Dynamics of HEHEHP Ligand at the Liquid-Liquid Interface

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