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Separation Science and Technology

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Insight into nitric acid extraction and aggregation of N, N, N’, N’-Tetraoctyl diglycolamide (TODGA) in organic solutions by molecular dynamics simulation Meena B Singh, Suneha R Patil, Aishwarya A Lohi & Vilas G. Gaikar To cite this article: Meena B Singh, Suneha R Patil, Aishwarya A Lohi & Vilas G. Gaikar (2018): Insight into nitric acid extraction and aggregation of N, N, N’, N’-Tetraoctyl diglycolamide (TODGA) in organic solutions by molecular dynamics simulation, Separation Science and Technology, DOI: 10.1080/01496395.2018.1445107 To link to this article: https://doi.org/10.1080/01496395.2018.1445107

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SEPARATION SCIENCE AND TECHNOLOGY https://doi.org/10.1080/01496395.2018.1445107

Insight into nitric acid extraction and aggregation of N, N, N’, N’-Tetraoctyl diglycolamide (TODGA) in organic solutions by molecular dynamics simulation Meena B Singha, Suneha R Patilb, Aishwarya A Lohic, and Vilas G. Gaikara a

Department of Chemical Engineering, Institute of Chemical Technology, Mumbai, India; bDepartment of Petrochemical Engineering, Dr. Babasaheb Ambedkar Technological University, Raigad, India; cDepartment of Chemical Engineering, Dr. Babasaheb Ambedkar Technological University, Raigad, India ABSTRACT

ARTICLE HISTORY

The molecular dynamics (MD) simulation of TODGA in n-dodecane shows formation of nanostructures of TODGA aggregates with nitric acid and water. These aggregates are dispersed in dodecane phase or form well defined reverse micelles grown sufficient in size depending on the acid concentration. With increasing nitric acid concentration, aggregation number of TODGA in reverse micelles also increases which, however, is independent of TODGA concentration. Aggregation number rises from 2 to 8 in presence of 0–3.5 M nitric acid in corresponding aqueous phase. The formation of the aggregates explains remarkable acid co-extraction from aqueous phase to organic dodecane phase by TODGA.

Received 5 April 2016 Accepted 21 February 2018

Introduction High-level liquid waste (HLLW) generation and isolation of actinides and lanthanides from highly acidic HLLW is a major nuclear waste management concern.[1] Solvent extraction is routinely used in nuclear industry for isolation of long life species, such as neptunium, americium, and curium, before their transmutation to short life species.[2,3] Apart from the conventional phosphorous based extractants, the recent DIAMEX[4] and TRUEX[5] processes advocate use of completely incinerable CHON amides, such as N,N’-dimethyl-N,N’-dioctyl ethylethoxymalonamide (DMDOHEMA) and octyl-(phenyl)-N,N’-diisobutyl-carbamoylmethyl phosphine oxide (CMPO) as ligands for the extraction of radioactive metal ions.[4–7] These bidentate amide ligands leave no solid waste for disposal on incineration.[8] The natural progress from a bidentate diamide to tridentate diglycolamide ligands, gave the same extraction performance at ligand concentrations lower by an order of magnitude, and thus made a new extractant chemistry possible.[9,10] Stephan et al.[11,12] were the first to report extraction of lanthanides, actinides and other heavy metal ions by diglycolamides, followed by Sasaki et al.[13,14] for extraction of Th, Eu, Am, U and Np from aqueous acidic solutions. N,N’Dimethyl-N,N’-diphenyl-3-oxapentanediamide has been reported for solvent extraction of trivalent lanthanides

KEYWORDS

Aggregates; molecular dynamics (MD) simulation; reverse micelles; acid extraction; nitric acid; TODGA

from aq. HNO3 and HCl solutions into chloroform.[15,16] The tridendate diglycolamide ligands have shown potential particularly for selective extraction of f-block elements. The hydrophilic and lipophilic balance of diglycolamide depends on the nature and length of alkyl side chains attached to the N-atom of the amide. The extractability rate and solubility of diglycolamide ligands in aqueous nitric acid-dodecane system depend on the side chain length. Diglycolamides with shorter chain length of 1–2 carbons are water soluble and cannot be used for extraction, but those with longer alkyl side chains, i.e., octyl, decyl, and dodycyl, readily dissolve in non-polar solvents and exhibit poor solubility in water. N,N,N’,N’-Tetraoctyl-diclycolamide (TODGA) (Fig. 1) is a promising extractant for selective extraction of lanthanides and actinides from highly acidic solutions because higher extractability ratios.[17–20] The organic solvent’s polarity and aqueous acidity seem to affect interaction between metal ions and TODGA while determining stoichiometry of stable speciation in organic solutions. TODGA has shown an excellent extraction ability for di-, tri- and tetra-valent cations with ionic radii of 100 pm, 87 pm and 113 pm, respectively.[10] TODGA reportedly encounters a problem of third phase formation at higher acid and metal concentrations, which decreases its efficiency.[21–25] Tachimori

CONTACT Vilas G. Gaikar [email protected] Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400019, India. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsst. Supplemental data for this article can be accessed here. © 2018 Taylor & Francis

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Hydrophobic

Hydrophilic

Figure 1. Hydrophilic and hydrophobic groups in TODGA.

et al.[26] have reported formation of TODGA aggregates at higher metal ion concentrations in liquid–liquid extraction of Nd(III). The molecular size of the organic solvent, temperature, acid concentration in corresponding aqueous phase and nature of counter-ions show considerable influence on loading capacity of ligand and formation of aggregates which in some cases can be avoided by addition of a phase modifier, such as monoamide, dodecane.[27] The research continues for better understanding of stoichiometry and chemistry involved in the diglycolamide based extraction systems and to develop full-scale operational process. The hyperstoichiometric acid dependency is the most interesting feature of diglycolamide systems. Although, the aciditydriven aggregation of diglycolamide is the most commonly attributed reason with supporting experimental data,[28] association of nitric acid molecules in the metal–diglycolamide complex also has been considered by Shimada et al.[29] and Sasaki et al.[30] Similarly, experimental studies on acid extraction in absence of coordinating metal ions have been reported by Nave et al.[31] and Bell et al.[32] Surprisingly, there are no molecular modeling simulation studies to understand these interesting features of the system. In this paper, we have investigated aggregation behavior of TODGA as a function of increasing acidity and subsequent formation of reverse micellar structures in dodecane in the absence of metal ions by a systematic molecular dynamic (MD) simulation. The objective was to investigate the reverse micellar structures, if any, and particularly increase in size of micelles during extraction at higher acidity in organic phase. This simulation was expected to provide a basis for explaining experimental observations and to predict the behavior of the system under practical conditions.

The MD simulation of 400C12H26·10TODGA·yH2O·z (NO3− and H3O+) system determines aggregation behavior of TODGA in an organic system in the presence, as well as absence of extracted nitric acid and water molecules. As no attempt was made in the past to study the aggregation properties of TODGA through the molecular modeling, these results are first of its kind indicating clearly formation of TODGA reverse micelles at molecular level and the effect of nitric acid and TODGA concentrations on microstructures present in dodecane phase.

Methods A cubic unit cell of 5 nm dimension was constructed with predefined number of dodecane, water, TODGA and nitric acid molecules to represent different concentrations of nitric acid and 3D periodic boundary conditions were applied to the unit cell.[33] In organic phase, nitric acid was considered in dissociated as well as molecular forms. Initially, the unit cell contained different numbers of nitrate and hydronium ions, water and TODGA molecules distributed randomly in 400 dodecane molecules. The number of dodecane molecules was estimated on the basis on the density of dodecane and size of the unit cell. For each system, different subsystems depending on concentration of nitric acid were considered for the MD simulation (Table 1). Three cases were considered with respect to nitric acid molecules, (i) in completely dissociated form, (ii) in undissociated molecular form and (iii) partially dissociated form in the organic phase to get clear information of interaction of TODGA with nitric acid. The number of nitric acid molecules in the system was estimated by considering experimental data on transfer of nitric acid from aqueous phase to organic

SEPARATION SCIENCE AND TECHNOLOGY

3

Table 1. Details of the periodic box used for MD calculations. Sr. No. System 1 2 3 4 System 5 6 7 System 8 9 10

Dodecane (No.)

TODGA (No.)

NO3− ions (No.)

H3O+ ions (No.)

HNO3 (No.)

[HNO3]aq (M)

[HNO3]org, (M)

Water (No.)

5.26 5.28 5.27 5.16

400 400 400 400

10 10 10 10

3 5 12 –

3 5 12 –

– – – –

1.12 2.0 3.5 0.0

0.033 0.055 0.132 0. 0

12 20 48 40

5.48 X 5.48 X 5.48 5.46 X 5.46 X 5.46 5.48 X 5.48 X 5.48

400 400 400

10 10 10

– – –

– – –

3 5 12

1.12 2.0 3.5

0.033 0.055 0.132

12 20 48

5.27 X 5.27 X 5.27 5.26 X 5.26 X 5.26 5.28 X 5.28 X 5.28

400 400 400

10 10 10

1 2 6

1 2 6

2 3 6

1.12 2.0 3.5

0.033 0.055 0.132

12 20 48

Size of periodic box in nm I

II

III

5.26 5.28 5.27 5.16

X X X X

5.26 5.28 5.27 5.16

X X X X

phase. It has been reported that 0.02–0.13 M nitric acid exists in the organic phase, at 0.1 M concentration of TODGA, when corresponding equilibrium aqueous phase acid concentration is in the range of 1–3.5 M.[32] For every nitric acid molecule, four water molecules also get transferred into the organic phase because of hydrogen bonding between nitric acid or nitrate ions and water in the aqueous phase.[34] Therefore, in all the cases the number of water molecules in the organic phase was taken in the ratio of 4:1 with respect to nitric acid. The extracted water molecule in the organic phase system should not be confused with the bulk aqueous phase, there is no aqueous phase considered in the simulation. An open source code for MD, GROMACS, was used to perform MD simulation. The force field parameters for n-dodecane,[35] nitric acid,[36] nitrate and hydronium ions[36] (Table S1 from supporting information), which had been tested previously were selected from literature.[35,36] For TODGA, the charges were obtained from Quantum Mechanics calculations by performing geometry optimization with DFT calculations and using B3LYP exchangecorrelation energy density functional and 6–311 + G (d,p) basis set in GAUSSIAN 09 program.[37] (The details are provided in Table S2 of supporting information.) The potential parameters were taken from methyl, methylene carbonyl ester and tertiary amine groups in OPLS-AA library as they predict the physiochemical properties for the organic liquid systems quite accurately.[38,39] The SPC/E model was chosen for water molecules due its higher efficiency for liquid water simulation.[40] The OPLS force field uses a combination of electrostatic interactions and Lennard–Jones potentials.[41] A cut off at 1.0 nm was used for the interaction potential as MD simulation predicts accurately properties of the system for larger cut-off distance.[42] The

immediate neighbor list was updated at every fifth time step. The particle mesh Ewald method was used to treat long range electrostatic interaction beyond the cut-off distance.[41,43]An integration time step of 2.0 fs, a temperature coupling of 0.1 ps and a pressure coupling of 2.0 ps were used. Both, steepest descent and conjugated gradient, algorithms were employed to minimize energy gradient of the system below 1000 kJ mol−1 nm−1. After the energy minimization, a series of simulated annealing runs were performed as follows: (i) In the first simulation, equilibration of the system under an NVT ensemble was conducted for 20 ns to stabilize temperature of the system at 300 K with position constraints; (ii) Next step of equilibration of pressure was conducted under an NPT ensemble for 20 ns with position restraints; (iii) Upon completion of two equilibration phases, the system was well-equilibrated at desired temperature and pressure conditions, so the position restraints were released and production MD run was performed for 100 ns. Five independent simulations of 20 ns each were performed to avoid uncertainty in calculated diffusion coefficients and this time was sufficient enough to get steady state structure. For MD calculations, a Verlet type multiple time step algorithm was used, as it is simple and effective and stable, specially with a large number of interacting particles.[41,43] The diffusion coefficients of various species were estimated from mean square displacement (MSD) function.[33,44]

Results and discussion TODGA has an affinity for nitric acid and, therefore, the acid gets co-extracted into organic phase along with other metal ions.[32] It thus becomes important to know the way nitric acid interacts with TODGA and the form

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(associated or dissociated) in which it exists in the organic phase along with water molecules. The MD simulation results, presented below, show effect of nitric acid concentration and temperature on aggregation of TODGA and corresponding co-extraction of the acid and water. Effect of acid concentration on aggregation of TODGA In the absence of nitric acid, TODGA forms hydrogen bonding with water (Fig. 2a). Its hydrophilic part is aligned towards water while hydrophobic part remains in contact with dodecane. In the absence of nitric acid, only hydrogen bonding interaction is seen between water and TODGA. The nitric acid concentration in organic phase increases with increase in corresponding aqueous phase acid concentration upto 3.0 M.[32] All nitric acid molecules were initially considered in their dissociated form. Some groups[31,45,46] have reported a third phase formation in the TODGA–Nitric acid–dodecane mixtures in presence of metal ions, but only reverse micellar aggregates, different from the third phase formation, are observed in current simulation. The third phase is usually different from both, the aqueous acidic phase and the organic dodecane phase[21] and essentially contains TODGA with extracted nitrate and

hydronium ions and water molecules with little of dodecane. Reverse micelle is a cluster of specific number of TODGA molecules surrounding nitric acid and water molecules with a definite shape in dodecane phase. However, reverse micelle formation can be the initiation of the third phase formation, by merger of several reverse micelles. During the simulation the total number of clusters and their stability with respect to simulation time is shown in Fig. 3. Presence of cluster is indicated by small square dot at given time, which if present continuously, appears to be line. Dots when are more connected indicates more stable cluster therefore appearing as a line while scattered dots indicates unstable cluster therefore doesn’t appear as line. TODGA molecules which are not forming clusters and remain as monomers are also counted as individual cluster in cluster number analysis plot. At 1.12 M aqueous nitric acid concentration, up to three TODGA molecules are seen to form a reverse micelle in dodecane, trapping 1–2 nitrate and hydronium ions and five to eight water molecules in a poorly defined structure (Fig. 2b). Partial density profile shows the presence of TODGA, nitrate and water throughout the simulation box. In the system, three types of clusters are seen, i.e. 60% TODGA is in trimer or dimer form while the rest remains as monomer but interacting with either 1 or 2 water molecules or not at all. Cluster number analysis shows the presence of total 10

a. Aggregation of 2 TODGA molecules in organic phase in the absence

b. Aggregate of 2 TODGA molecules in 0.03M organic phase HNO3

of nitric acid

concentration

c. Aggregate of 5 TODGA molecules in 0.05M organic phase

d. Aggregate of 8 TODGA molecules in 0.132M organic phase

HNO3concentration

HNO3concentration

Figure 2. Aggregation of TODGA molecules in organic phase with different concentrations of nitric acid at 300K.

SEPARATION SCIENCE AND TECHNOLOGY

a. Cluster number analysis in 0.03M organic phase HNO3concentration

5

b. Cluster number analysis in 0.05M organic phase HNO3concentration

c. Cluster number analysis in 0.132 M organic phase HNO3concentration

Figure 3. Cluster number analysis with respect to simulation time in organic phase with different concentrations of nitric acid at 300K.

clusters (including TODGA monomers) during the simulation out of which four clusters are stable shown by longer horizontal lines (Fig. 3a). At 0.05 M organic phase acid concentration, which corresponds to 2.0 M aqueous phase nitric acid concentration, the biggest cluster consists of 5 TODGA molecules with 2–3 nitrate and hydronium ions in equal numbers and 12–14 water molecules (Fig. 2c). Of the remaining TODGA molecules, 20% formed a dimer while 30% of TODGA remain as monomers interacting with 1–2 water molecules each or no water molecules. No ionic species are present in the organic phase without water. The acid gets coextracted only with water in the reverse micelles. Partial density profile (Fig. 2c) shows that major percentages of TODGA are getting aggregated at one position while few are still present at other sites. Cluster number analysis shows the presence of total four clusters (including TODGA monomers) during the simulation out of which two clusters are stable shown by longer horizontal lines (Fig. 3b). For 3.5 M nitric acid in aqueous phase, the corresponding organic phase acid concentration was 0.13 M. The organic phase shows 8 TODGA molecules forming the biggest cluster in the box with all 12 or 11 nitrate and corresponding hydronium ions and 46–48 water molecules (Fig. 2d) consisting of 80% of initial TODGA molecules, while remaining 20% of TODGA

remains as monomers or dimers. Partial density profile shows accumulation of TODGA, nitrate and water at the specific position indicating the presence of big cluster (Fig. 2d). Cluster number analysis shows the presence of total two clusters during the simulation out of which one cluster is prominent and stable shown by longer horizontal line (Fig. 3c). The number of TODGA molecules participating in a biggest (single) cluster increased from thre to eight as the nitric acid concentration increases in the aqueous phase from 1.12 M to 3.5 M. These results are in good agreement with the dimensions of the aggregates reported in literature by small angle neutron scattering (SANS) studies.[28] Yaita et al.[28] had reported that atHNO3concentration less than 0.7 M in aqueous phase, TODGA existed as a monomer or a dimer in the organic phase, but as concentration of the acid was increased, TODGA formed bigger aggregates containing 6.9 ± 0.9, i.e., on an average seven molecules in the cluster. This experimental value is close to the current simulated aggregation number of eight of TODGA for 3.5 M HNO3 in the aqueous phase. The numbers of TODGA, nitric acid and water molecules each in the aggregates as a function of nitric acid concentration are specified in Table 2. Nitric acid brings stronger electrostatic forces into the system. The hydrogen bonding between molecular form of nitric acid and TODGA or electrostatic

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Table 2. Composition of reverse micelle (largest) formed by TODGA in bulk organic phase with respect to various forms of nitric acid. Organic phase conc. of HNO3

No. of TODGA

No. of HNO3

No. of NO3−

No. of H3O+

No. of water

– – –

3 3 12

3 3 12

1 13 48

2 3 10

– – –

– – –

1 14 40

1 3 4

1 2 5

1 2 5

5 12 42

System 1 (All Nitric acid in dissociate form) 0.033 3 0.055 4 0.132 8 System 2 (All Nitric acid in molecular form) 0.033 3 0.055 3 0.132 8 System 3 (Both in dissociate and molecular form) 0.033 3 0.055 4 0.132 7

interactions between nitrate ion via hydronium ion and TODGA are stronger than the hydrogen bonding between water and TODGA.

Effect of different forms of acid (dissociated form, molecular form and mixture of dissociated and molecular forms) on aggregation of TODGA Due to presence of local aqueous pools inside the reverse micellar structures of TODGA, nitric acid is considered to be present in dissociated form. The effective concentration of the acid in the aqueous microdomains of reverse micelles can be very high, increasing the possibility of presence of a mixture of both dissociated and molecular forms of nitric acid. So, MD simulations were also carried out for additional systems. In all the three systems, the number of TODGA molecules in an aggregate remains almost the same (Table 2). All hydronium ions remain in contact with nitrate ions throughout the simulation giving an impression of undissociated nitric acid. In partially dissociated nitric acid system too, similar observation has been made. Due to strong electrostatic forces of attraction, the two oppositely charged ions always remain in contact with each other. As there are only few water molecules (

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