Extractive insights in the cesium ion partitioning with bis(2-propyloxy)-calix [4]crown-6 and dicyclohexano-18-crown-6 in ionic liquid-water biphasic systems

Accepted Manuscript Extractive insights in the cesium ion partitioning with bis(2-propyloxy)-calix [4]crown-6 and dicyclohexano-18-crown-6 in ionic liquid-water biphasic systems

Rima Biswas, Pallab Ghosh, Tamal Banerjee, Sk. Musharaf Ali, K.T. Shenoy PII: DOI: Reference:

S0167-7322(16)34159-9 doi: 10.1016/j.molliq.2017.06.015 MOLLIQ 7461

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

21 December 2016 26 April 2017 2 June 2017

Please cite this article as: Rima Biswas, Pallab Ghosh, Tamal Banerjee, Sk. Musharaf Ali, K.T. Shenoy , Extractive insights in the cesium ion partitioning with bis(2-propyloxy)calix [4]crown-6 and dicyclohexano-18-crown-6 in ionic liquid-water biphasic systems, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.06.015

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ACCEPTED MANUSCRIPT Extractive Insights in the Cesium Ion Partitioning with Bis(2-propyloxy)calix[4]crown-6 and Dicyclohexano-18-crown-6 in Ionic LiquidWater Biphasic Systems

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Rima Biswasa, Pallab Ghosha, Tamal Banerjeea,*, Sk. Musharaf Alib, K.T. Shenoyb

Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati

Chemical Engineering Division, Bhabha Atomic Research Center, Mumbai 400085, India

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781039, Assam, India

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* Corresponding author; E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract

We report the molecular dynamics studies on the interfacial behavior of cesium (Cs+) extraction by bis(2-propyloxy)calix[4]crown-6 (BPC6) and dicyclohexano-18-crown-6 (DCH18C6). For the benchmarking study, the phase separation for [BMIM][Tf2N]water was validated. Thereafter, to understand the mechanism of complexation and the behavior of the crown ether

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ligand, crown ether (CE) molecules and Cs+NO3 ions were inserted randomly in the ionic liquid (IL)–water biphasic system. It was observed that the 2:1 Cs+-BPC6 complex formed during the

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simulation was more stable at the interface and was found to diffuse slowly to the IL side of the interface. On the contrary the 1:1 Cs+-DCH18C6 complex was found to be fully solvated in the

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bulk IL phase. The [BMIM]+ cation was found to partition to the aqueous phase and exchange with Cs+ ion in presence or absence of CE. The solubility, density plots and the snapshots of the

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[BMIM]+ cation at the IL‒ water interface indicates the dual cationic exchange. A comparison of the interfacial behavior of Cs+-BPC6 complex and Cs+-DCH18C6 complex in IL‒ water

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preformed interface system, depicted that the Cs+-BPC6 complex was stable at the interface till the end of the simulation. One the other hand, the uncomplexed DCH18C6 resided at the

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interface, while the Cs+ ion diffused to the bulk aqueous phase. The calculated interaction energy of Cs+-BPC6 was found to higher as compared to Cs+-DCH18C6. These results display the

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extraction of metal ions.

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importance of the dual cationic exchange properties of IL and the extraction efficiency of CE for

effect

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Keywords: BPC6, DCH18C6, Interface, Cs+ ion extraction, Molecular dynamics, Solvation

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ACCEPTED MANUSCRIPT 1. Introduction Ionic Liquids (IL) are based on organic cations such as 1-butyl-3-methylimidazolium (BMI+) and inorganic anions such as bis(trifluoromethylsulfonyl)imide (Tf2N) or hexafluorophosphate (PF6). They make biphasic systems with water and are widely used in liquid-liquid extraction [1, 2]. Crown ethers, phosphoryl derivatives, diketonates or calixarenes, are used in classical

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liquidliquid extraction to extract metallic ions. In these processes, interfacial phenomena play a

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major role where the metal ions (aqueous phase) and the extractants (IL phase) meet and form a hydrophobic complex at the interface. Cesium is an extensive fission product in spent nuclear

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fuels [3, 4]. It’s removal from the nuclear wastes forms an integral part of waste remediation startegy [3]. The extraction of cesium ion with crown ethers from the aqueous medium has been

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studied by McDowell [4] and Dietz et al. [5] Further, macrocyclic crown ether [6] and calix[n]arene compounds [7] were also reported to extract Cs+ ion from aqueous solution. It was

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established that the phenyl or cyclohexyl derivatives of 18-crown-6 (18C6) and alkyl substituted have very low separation factors (βCs/Na= 0.3–4.57) [8] as compared to ditertiary butyl-dibenzo-

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18C6 (DTBDB18C6) (βCs/Na = 210). It has been concluded that calix is less attracted with metal

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ions when it is not functionalized with some substituent [9].

Recently it was observed that the ILs are efficiently solvated by polar ligands and

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their complexes. This eventually reduces the interfacial activity of IL and water thereby enabling efficient separation of metal ions such as cesium [10]. Luo et al. [2] used BOBCalixC6

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(calix[4]arene-bis(tert-octylbenzocrown-6)) as an adequate extractant for the extraction of Cs+ ion when dissolved in imidazolium-based ILs. Furthermore, the extraction mechanism for metal

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ion transfer from the bulk water phase to the IL phase was found to be different as compared to organic solvents. Xu et al. [11] reported the mechanism and radiation effect for the remediation of cesium ions from aqueous phase using a crown ether. This was confirmed by Levitskaia et al. [12] where calix-crown were found to be a more efficient extractant than the other crown ether ligands. The important difference between IL and organic solvent is the cohesive forces, which are more vigorous with the IL than with the organic solvent. This leads to several orders of magnitude higher relaxation times (which is in the order of nanosecond) and faster diffusion of molecules as compared to the organic solvents. Among the theoretical studies, molecular 3

ACCEPTED MANUSCRIPT dynamics (MD) simulation has been studied by Sieffert et al. [10] on the removal of Cs+ by a calix[4]arene-crown-6 host (L). They compared their predictions with an IL and an organic solvent i.e. chloroform. The complexation and solvation properties of L were then examined by MD in pure [BMI][Tf2N] as well at water-oil interfaces. Theoretical studies concerning the thermo-physical properties of ILs [13] and calix-crown ligands [14, 15 ] has been recently studied by our group. Our initial studies have explored the dual mode of metal ion extraction by

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calculation in conjunction with COSMO solvation technique [16].

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dicyclohexano-18-crown-6 and bis(2-propyloxy)-calix[4]crown-6 in ILs by employing quantum

In continuation with the research concerning biphasic systems involving ILwater with

bis(2-propyloxy) calix[4]crown-6 (BPC6)

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metal ions, we have attempted to study cesium ion extraction by two crown ethers : namely and dicyclohexano-18-crown-6 (DCH18C6). The

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crown ethers were specially chosen, since the experimental data for the distribution constant for both the ethers were very encouraging. For BPC6, it was greater than 1000, whereas for

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DCH18C6, it was 91 [16]. In their computations, the authors had calculated the binding energy from DFT calculations and explained the reason for this striking difference in the extractive

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ability between BPC6 and DCH18C6. Hence a need for finite temperature classical MD simulations was felt so as to explore the trajectory of the systems. The outcome of such an

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exercise will be the prediction of both thermodynamic and dynamics properties such as density, demixing index, the radial distribution functions (RDF), diffusivity, surface and interfacial

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tensions. Demixing results will also allow us sample the phase equilibria configuration along with the partitioning of the solutes. Both these phenomena tend to depend on the partial charge

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of the components and the nature of the crown ether ligands. For the Cs+ complexes with the ionophore, the distribution of the solvent molecules around the complex can be quantified by RDF. On a similar note, the interfacial behavior of the Cs+-CE complex has also been studied by both surface tension and diffusivity computation. Finally, the binding free energies obtained from the MD and quantum chemical (QC) calculations were also compared to shed some light on the nature of the capture.

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ACCEPTED MANUSCRIPT 2. Computational Details The systems were simulated by classical (MD) using the NAMD (version 2.9) software [17]. Classical (MD) simulations were carried out by the OPLS force field parameters [18, 19]. In such a system the gemoetry optimization was first performed using B3LYP along with SVP basis set without prohibiting any symmetry correction.The equilibrium structure for checked with the

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absence of imaginary freqencies using the freq command. Thereafter the optimized coordinates

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were further used for the single point energy calculation with MP2 level and SDD basis set. This

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was performed to check the accuracy of B3LYP functional. The Stuttgart/Dresden (SDD) ECP basis set with relativistic effect corrections is usually recommended for heavier and lanthanide atoms such as cesium. The free macrocycle, crown ether and complex was optimized separately.

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Thereafter the binding energies were obtained from the following expression in which “complex” represents the “crown ether and the cesium ion.

(1)

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EBinding  EComplex  ECrown Ether  ECesium

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The partial atomic charges were calculated by CHELPG procedure [20] using electron densities generated at the same level. Additional force field parameters for IL were obtained from Canonga-Lopes et al. [21, 22 ]. Water was defined by the TIP3P model [23]. The force field

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parameters for nitrate ion was taken from Lopes et al. [21] while parameters for DCH18C6 and

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BPC6 were taken from literature [24, 25]. The systems were defined within a 3D periodic boundary condition, and for each computation, three random configurations were generated. The nonbonded interactions were determined using a 12 Å atom-based cutoff while long-range

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electrostatics interactions were distinguished using the Ewald summation method (PME approximation) [26, 27]. Initially, to verify the accuracy of the OPLS force field and their respective parameters for the IL, [22] the force field was validated with the surface tension of IL and water. The solutes (219 [BMIM][Tf2N] molecules and 2657 H2O molecules) were initially immersed in cubic boxes of 45 Å length. The molecules were chosen such that both the phases had an equal number of atoms. The systems were equilibrated after 10000 steps of energy minimization, followed by a slow and steady heating to reach 300 K using temperature reassigning parameters of NAMD (version 2.9). Thereafter NPT simulations of 10 ns were run to

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ACCEPTED MANUSCRIPT set the system to its experimental density. In such a case, a low density configuration was started for the NPT run. Here the temperature was monitored using a Langevin thermostat, while the pressure was monitored using Langevin piston with a period of 100 fs and decay of 50 fs. The Verlet algorithm was implemented with a time step of 1 fs to integrate the equations of motion. Once the density or the cell volume was optimized, we took the final configuration for the production run in an NVT ensemble. This was required so as to obtain a stable interface. For the

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production stage, a 2 ns equilibration was initially performed. Starting with random velocities,

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the production run in NVT ensemble was then started for a length of 40 ns. The different

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simulated systems are presented in Table 1. Typical snapshots were taken by the VMD software [28]. The length of the simulation box in the z-direction was divided into sections of 0.5 Å in

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thickness. Thereafter, the atomic weight densities of water   w  and IL   IL  were computed separately for each section. These densities were plotted with respect to the position of the

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corresponding section in the z-direction. The position of the interface was dynamically estimated by the intersection of the IL and water density curves (Gibbs dividing surface). Three regions

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were defined as (a) the interfacial region (within ±12 Å from the interface) and (b) bulk IL and (c) bulk water phases (beyond 12 Å from the interface, respectively). The width of the interface

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region was determined as the distance between z positions where the densities of the solvents reach 90% of their bulk density. The degree of phase separation was observed with time by the

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demixing index (  ). This ranges from 1 (fully mixed system) to 0 (completely separated phase).

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



1 1 1     N   w  IL 

(2)

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Here N is the normalization factor. Potential of mean force (PMF) is calculated in order to indicate the stability of Cs+CE complex in the solvents using the following formula:

W ( r )  kBT ln  g (r ) 

(3)

Here the pair correlation function g  r  , corresponds to the interaction between the metal ion (Cs+) and the centre of mass (COM) of the crown ether or solvent molecules. T is the temperature (300 K) and k B refers to the Boltzmann constant. The diffusion coefficients were calculated using the Einstein equation [25] over the last 1 ns of dynamics by selecting the solute molecules or the complex present in the bulk solvent phase. 6

ACCEPTED MANUSCRIPT 6 Dt  ri  t   ri  0 

(4)

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Here ri  0  and ri  t  are the positions of the ith atom at time instants 0 and t , respectively. The ensemble block average of the squared displacements on the right hand side of eq. (4) indicates the mean square displacement (MSD). The diffusivity was obtained by computing the slope from

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the MSD versus time variation.

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Surface tension  p  has been calculated according to the pressure tensor method. This method

Pxx  Pyy Lz   Pzz  2 2 

   

(5)

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p 

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is depend on the difference between pressure tensor components [29].

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Where Pxx , Pyy and Pzz are the diagonal components within the pressure tensor while Lz denotes the length along the z direction for the PBC box. The interfacial tensions were calculated based

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on the capillary wave technique where  is the interfacial width. Thereafter, the simulation results were adapted with an error function [30]. The profile accomplished for two 40 ns

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z h  1 1   A   B     A   B  erf   2 2  2 

(6)

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 ( z) 

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simulations of different system size in the xy plane, i.e. Lx ,i and Lx , j takes the form:

Where  A and  B are the bulk densities of each phase and h is the average z of the interface.

c 

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The interfacial tension is then calculated by the following formula:

k BT

   

2   2   2 i 

L ln  x ,i   Lx , j j 

  

(7)

Here T is the temperature (300 K) and k B is the Boltzmann constant.

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ACCEPTED MANUSCRIPT 3. Results and Discussion In the initial part, we have investigated the distribution of only Cs+ ions at the preformed ILwater interface, while the subsequent part presents the simulations involving Cs+NO3– along with the crown ethers in IL-water interface.

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3.1. Cesium Nitrate at IL-Water Interface

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In this section, we simulated 6 Cs+NO3- ion pairs at the preformed [BMIM][Tf2N]-water interface. All the ion pairs were initially immersed in the bulk aqueous phase (Fig. 1). During the

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simulation, Cs+ ions were found to diffuse from the bulk water to the interface without the presence of crown ethers. This can be distinguished from the density profile of Cs+ and NO3-

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ions where both displays a broad peak near the interface at 20 ns (Fig. 1). It is interesting to observe than the concentration of [BMIM]+ at the interface is more oriented towards bulk water

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phase when compared to [Tf2N]-. The driving force for this movement can be seen with the effect of Cs+ ion which display a sharp peak towards the IL side of the interface. Moreover, the

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zoomed view of Fig. 2 depicts the [BMIM]+ starting to get enclosed towards the aqueous side of the interface.

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On the other hand, in Fig. 2, Cs+ is found to solvate in a greater extent with the anion i.e. Tf2N- towards the IL side of interface. As expected the counter ion namely NO3- is seen to

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approach the [BMIM]+ cation so as to attain charge neutralization in aqueous side of the interface. We hence can qualitatively assume a dual mode of extraction where [BMIM] + cation is

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exchanged with Cs+ ion in aqueous side of the interface. This is evident as the solubility of

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[BMIM]+ (0.0304) was found to be higher at the interface when compared to [Tf2N]- (0.0267). 3.2. Phase Separation of Immiscible Liquids The demixing simulation of [BMIM][Tf2N] with water was earlier performed by Sieffert et al. [10]. However, in their study, the preformed complex with the cesium ion was directly inserted into the two phases in an orderly fashion. This does not tell us the mechanism of the insertion for the metal ion. A useful way which has been explored in this work is to randomly place these ions and crown ether within the entire periodic box. The complexation part has been explained in the ensuing section. Fig. 3(a) and 3(b) represent such a phase separation of IL and water in the presence of BPC6 and DCH18C6, respectively. For a clearer visualization, the coordinates of the 8

ACCEPTED MANUSCRIPT Cs+NO3- and crown ethers are not depicted in the figures. This provides the snapshots of the [BMIM][Tf2N]–water mixed configuration system with coordinates of IL and water displayed independently side-by-side, taken with the evolution of simulation time The position of the interface was dynamically estimated by the intersection of the IL and water density curves (Gibbs dividing surface) as shown in Fig. 3 (a) and 3(b). Starting from the dimixing simulation, it was observed that at time t  10 ns the phases became strongly separated with time. The

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component species formed adjacent slabs of bulk IL–water phases by the end of 40 ns of

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simulation. The degree of phase separation was determined along the length of simulation run

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(Fig. 4(a) and 4(b)) by the demixing index,  , which would range from 1 (fully mixed system) to 0 (perfectly separated phase). The rate of phase separation was quantified using the initial

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slope of demixing index (Fig. 4(a) and 4(b)). For the [BMIM][Tf2N]–water binary mixed system, the rate of phase separation was rapid (0.0749) for BPC6 extractants (Fig. 4(a)) as compared to

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(0.0509) DCH18C6 ( Fig. 4(b)). In both cases,  fell to a value of 0.4 at 10 ns and then reached an equilibrium value of 0.2, corresponding to well-separated phases at 40 ns. For BPC6 based

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system the rate of phase separation is faster since  reached a plateau of 0.2 in only 24 ns. On the other hand when DCH18C6 is represented as extractant, the phases gets separated at a lower rate

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as the  is found to reach an equilibrium value of 0.2 at 37 ns. The value of  =0.2 is recommended by Wipff et al. [31] as the metal ion based system tends to reach equilibrium and

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is stable at that value. The probable reason for the formation of a quick interface is that scaled

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partial charges were used for IL, which turned it more hydrophobic. The intersolvent mixing, i.e. the number of IL molecules in the “bulk” water or the water

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molecules in the “bulk” IL was apparently very poor, as solubility of water in the bulk IL phase is 0.0673 mole fraction. On the contrary, there was an absence of IL in the bulk water phase. Here we note that a higher solubility of [BMIM]+ (0.03174) over [Tf2N] (0.0138) at the interface (within ±12 Å from the interface) makes the [BMIM] cation extraction viable to a cation exchange mechanism. It is observed that the solubility of BPC6 (0.0319) in bulk IL phase is higher than that of DCH18C6 (0.0228). These predictions agree qualitatively from a previous work [16], where distribution constant of Cs+ (DCs ~1000) was found to be much higher in BPC6 system when compared to DCH18C6 (DCs ~91). It should be noted that an exact order or value

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ACCEPTED MANUSCRIPT could not be reciprocated due to the constraint of system size and simulation time. Nevertheless, it serves a qualitative tool in assessing the ionophores. 3.3. Cs+–BPC6 Complex in [BMIM][Tf2N]-Water Binary Systems To understand the mechanism of complexation and the behavior of BPC6, Cs+NO3– ions and

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BPC6/Cs+ were inserted in the IL–water biphasic system. Twelve molecules of BPC6 crown

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ether and six molecules of Cs+NO3– were randomly introduced into the biphasic systems. The

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system conditions were taken closer to the experimental study performed by our earlier work [16]. The insertion was performed using PACKMOL [32] by randomly inserting the 18

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molecules in the system, which was same as that used in the phase separation simulation. The subsequent dynamics have been carried out for 40 ns (see Fig. 5). During the simulation, liquids were found to be separated into two clean bulk phases after 20 ns (Fig. 5), with the free crown

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ether moieties migrating to the IL phase and the Cs+NO3– ions to the water phase. This can be comprehended from the density profiles of BPC6 and Cs+ ions, which portrays density curves

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displaying a broad peak near the interface at 40 ns (Fig. 5). However, the complexation of Cs+

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with BPC6 was observed only after 5 ns of simulation time at the interface. The formation of 2:1 Cs+–BPC6 complex was also confirmed from the RDF of the Cs atom and the nearest oxygen

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atom in the crown ether computed over the final 5 ns (i.e. from 35 ns to 40 ns) (Fig. 6a). The 2:1 complex is provided in Fig. 6(b) where both the Cs+ ion are seen to complex with BPC6 at a

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distance of 3.44 Å and 5.20 Å respectively. Both the distances can be seen in RDF of Fig. 6(a). The RDF peak position at 3.05 Å and 5.69 Å at a distance close to the optimal value suggests

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two Cs+ are surrounded by six oxygen atoms of BPC6. This suggests that the 2:1 complexation indeed took place and the entity remained physically stable till the end of the simulation (40 ns). However, a 1:1 stoichiometry is preferred since the distance of 5.20 Å is fairly large enough to remain stable for longer time scale. The large magnitude of the RDF peak between Cs–O is similar to that obtained by Sahu et al. [33] for the Li+‒ DB18C6 complex at the aqueousorganic interface. To understand the interfacial behaviour of the Cs+-BPC6 complex in IL-water binary system, we simulated three complexes at the preformed [BMIM][Tf2N]water interface (Fig. 7). The complexes were inserted one each at the bulk water phase, interface and bulk IL phase. Here we 10

ACCEPTED MANUSCRIPT observe that the complex at the bulk water phase moved slowly to the bulk IL phase and was stable. However, the Cs+ ion was found to decomplex after 1 ns run and remained in the bulk aqueous phase till the end of the simulation. The complex at the interface slowly moved towards the IL side of the interface. The complex at the bulk IL phase remained intact till the end of simulation. This indicates that the complex is hydrophobic where it either prefers to diffuse to

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the interface or the bulk IL phase.

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3.4. Cs+–DCH18C6 Complex in [BMIM][Tf2N]-Water Binary Systems

We repeated the simulation with the other crown ether namely DCH18C6 in order to

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study the effect of crown size on the efficiency of Cs+ extraction. The subsequent dynamics have been pursued for 40 ns and snapshots were taken at regular intervals (Fig. 8). The density curves

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of DCH18C6 and Cs+ also depicts broad oscillations at the interface (Fig. 8). In order to obtain a detailed insight, the binding energies were also computed at the MP2/TZVP level of theory. The binding energy with BPC6 for Cs+ ion was found to be higher by 9.1–9.8 kcal mol1 when

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compared with DCH18C6 [16]. Comparing Fig. 5 and 8 at the 40th ns run, the crown ether

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DCH18C6 could only capture a single Cs+ ion during the simulation run of 40 ns. The complex was then found to remain in the IL phase. This is lesser than what we had observed for BPC6

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(Fig. 5), where it was able to attract two Cs+ ions towards its crown cavity. Moreover, both the captured Cs+ ions were found to lie towards the IL part of the interface. In order to understand

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the Cs+-BPC6 (2:1) complex, we extended the simulation up to 60 ns. We observe that the 2:1 complex is more stable at the interface and diffuses slowly to the IL side of the interface. It

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suggests that the calix crown (BPC6) has more extraction efficiency as compared to DCH18C6.

In the simulation results for preformed interface, complex is directly inserted in the interface, bulk IL and bulk water phase respectively. The preformed simulation has been depicted for BPC6 (Fig.7) and DCH18C6 (Fig. 9). For both BPC6 and DCH18C6 the complexes are seen to decomplex in the water phase. This is contrary to the IL phase where respective complexes are seen to be stable till the end of the simulation.

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ACCEPTED MANUSCRIPT 3.5. Structure of the Solvent Molecules The RDFs of free Cs+ with uncomplexed BPC6 and free Cs+ with Oxygen atom of water were further calculated over a trajectory from 3540 ns of the simulation as shown in Fig. 10(a). The first solvation shell of Cs+-BPC6 is observed at ~ 6.49 Å which is at the same distance for the second solvation shell (~ 6.12 Å) of Cs+-O(Water). The peak height of the RDF for all free Cs+

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cations with uncomplexed BPC6 is thus found to be higher than that of Free Cs + in water. Fig.

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10(b) represents the RDF plot for free Cs+ with uncomplexed DCH18C6 and Cs+ in water

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calculated over a trajectory from 3540 ns of the simulation. The peak height of the RDF for Cs+ with DCH18C6 was found to be higher than that of Cs+ in water. The free energy profile (PMF),

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which is contrary to RDF, are shown in Fig. 11(a). The PMF profile depicts one single local minimum. The ‘black line’ indicates the minimum corresponding to the direct contact of Cs+

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with crown ether (BPC6) and the ‘red line’ indicates the minimum corresponding to the direct contact of Cs+ with water. The PMF of Cs+–BPC6 complex shows a well-defined minimum of 4

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kcal/mol near 3 Å. This is a significantly a larger minimum than that corresponding to Cs+ with DCH18C6 (i.e. –1 kcal/mol) as shown in Fig. 11(b). The large minimum for the Cs+‒ BPC6

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complex is indicative of an actively bound Cs+ ion to BPC6 which corresponds to the formation of the complex at the interface (Fig. 5).

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3.6. Interaction Energies of Cs+–CE Complex

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In order to explore the behavior at the interface, the interaction energies of Cs+–BPC6 complex, and Cs+–DCH18C6 complex have also been plotted for 0–40 ns. Fig. 12(a) and 12(b)

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depict the interaction energies for the crown ether namely BPC6 and DCH18C6 with Cs+ respectively. The interaction energies during the initial stages of the simulation have values close to zero suggesting that the Cs+ is not present in the sphere of influence of the crown ether. In order to confirm the interaction energies, all (QC) energies were computed by SDD basis set at the same DFT level of theory (B3LYP) [16]. From the values presented in Table 2, it is clear that the magnitudes of the binding energies of Cs+ with BPC6 obtained from the QC and MD approaches are reasonably close. The binding energies of Cs+ with BPC6 and DCH18C6 obtained from the QC calculations are larger than those computed from the MD simulations. The values of non- bonded energies are traced out by the ‘green’ line in Fig. 12(a), 12(b). The

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ACCEPTED MANUSCRIPT extraction of metal ions is governed by the solvation process, which it inherits within the aqueous phase. A higher charge on the metal ion makes it tougher to be removed. These crown ether molecules actually reduced the charge on the metal ion further so as to reduce the solvation and made it diffuse easily towards the interface. The capture of the cesium ion decreased the magnitude of the interaction energy indicating a movement towards the crown ether cavity. It is observed that BPC6 can capture two out of six Cs+, whereas DCH18C6 can capture only a single

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Cs+ during the simulation run of 40 ns. However, to reconfirm the predictions further, we have

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performed simulations by comparing the interaction energy of Cs+ with both the crown ethers in

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vacuo (i.e. in the absence of solvents). We simulated six Cs+NO3‒ with twelve BPC6 and DCH18C6 respectively in two different systems. This has been depicted in Fig. 13. Here BPC6

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could capture five Cs+ during a simulation run of 5 ns (Fig. 13a) whereas the other crown ether, i.e. DCH18C6 could capture a single Cs+ at 5 ns. We have observed that the maximum number

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of Cs+NO3‒ molecules are surrounded by DCH18C6 but the capture within the crown ether cavity did not occur even after a 5ns run (Fig. 13b).

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It should be noted that the difference between QM and MD(Table 2) is obvious as in the former the interaction energy is calculated from ab initio and in the later it is force field

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based. The QM calculations were performed in vacuum. In order to confirm our methodology the interaction energies from QC were computed by SDD basis set at the same DFT level of theory

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(B3LYP) as per our earlier scheme[16]. The binding energies between DCH18C6 and Cs+ in vacuum for both QC and MD strategy are presented in Table 2. The MD simulations in vacuo

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tends to converge on Cs+ ion getting entrapped further away from the DCH18C6 core (-29.73 kJ/mole) as opposed to MD(solvents) (-45.85 kJ/mole). This may be due to the solvation effect

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of the individual ion(s) when in solvents which makes the bonding stronger. However this is in contrary to what was observed with BPC6, where the Cs+ ions is captured entirely within the cavity (figure 5) for QC, MD(solvents) and MD(vacuo). 3.7. Mean Square Diffusivity (MSD) The section discusses the effect of diffusivities of the individual components. This is essential in order to understand the motion of the captured species i.e. the cesium complex near or at the interface. The diffusion coefficient of each species was computed by the MSD profiles as given

13

ACCEPTED MANUSCRIPT in Table 3. Cs+NO3– was soluble in water and was found to dissociate easily into cesium and nitrate ions. The self-diffusion coefficient of each species was computed by the MSD profiles as given in Table 3. Cs+NO3– was soluble in water and was found to dissociate easily into cesium and nitrate ions. Thus, these species have large diffusivities of 0.798  109 and 0.570  109 m2/s, respectively. The MSD profiles of Cs+NO3– and BPC6 in the binary [BMIM][Tf2N]–water system are depicted in Fig. 14(a). Fig 14(b) depicts the other crown ether namely DCH18C6. The

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diffusion coefficient of Cs+-BPC6 complex at the interface is found to be much lower (0.036

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109 m2/s) as compared to other species. This indicates that the complex is more stable at the

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interface and is insoluble in water. On the other hand the diffusion coefficient of Cs+–DCH18C6 complex was 0.250  109 m2/s, which is lower than for the same free DCH18C6 (0.309  109

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m2/s) in bulk IL. This contrary nature of Cs+-DCH18C6 complex may be attributed due to its presence inside the IL bulk phase where it is solvated by the IL molecules (Fig. 8). The diffusion

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coefficient of BPC6 (0.826109 m2/s) is also higher in bulk IL phase as compared to DCH18C6 (0.309109 m2/s). In a similar manner, the diffusion coefficient of Cs+-BPC6 and Cs+-

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DCH18C6 complexes at the interface were found to be 0.113 and 0.107  109 m2/s

in

preformed planar interface system (Fig. 14(c)). The lower diffusivity of complex at the interface

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indeed confirm the complexes stay near at the interface and is insoluble in water.

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3.8. Surface and Interfacial Tensions

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The predicted surface tensions together with its interfacial tension for all of the systems are listed in Table 4. When Cs+NO3– and crown ether (BPC6, DCH18C6) were inserted into the binary

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[BMIM][Tf2N]–water system, the surface tension of the bulk IL (29.4 mN m1 and 31.09 mN m1) was found to decrease as compared to the surface tension of the bulk IL (31.38 mN m1) of the [BMIM][Tf2N]–water system. This is expected because the molecules of BPC6 and DCH18C6 enter the matrix of the IL layer. Thus, the forces acting on the individual IL molecule reduces. On the contrary, it is worthwhile to mention that the enhancement of the surface tension is governed by the addition of a strong electrolyte to water [34, 35]. Hence, as compared to a surface tension of 71.61 mN m1 for pure water, this increases to 74.30 mN m1 and 73.02 mN m1 for BPC6 and DCH18C6 respectively. Finally, to further validate and confirm the force field, the interfacial tension was also computed and compared. The interfacial tension of ILwater in 14

ACCEPTED MANUSCRIPT the presence of both the crown ethers is higher when compared to pure IL-water interface (Table 4). The calculated interfacial tension (8.72 mN m1) and the surface tension agree well with the experimental data [34-36].

4. Conclusions

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MD simulations were carried out for the extraction of cesium ions using BPC6 and DCH18C6

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with ionic liquids. The rate of phase separation with respect to demixing index was higher for BPC6 as compared to DCH18C6 in the binary systems. The broad peak height of density file and

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high solubility of [BMIM]+ at the interface may attributed to dual cationic exchange. Further the binding energy of Cs+ ion with BPC6 was higher as compared to DCH18C6. For BPC6, the

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complexation took place at around 5 ns, well before the complete phase separation, on the other hand, for DCH18C6, the complexation took place at 13 ns and the complexing entities were

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found to be stable till 40 ns to simulation run. Further, comparing the two crown ethers in vacuo, BPC6 could capture 5 Cs+ during the simulation run of 5 ns whereas DCH18C6 could only

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capture two Cs+ at 15 ns. This is fully dependable with the higher extraction efficiency to BPC6 compared to DCH18C6. The potential mean force and the corresponding binding free energies

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for both the crown ethers were also obtained from the QC and MD predictions. The selfdiffusivity of Cs+-BPC6 complex was lower at the interface. These results suggest that the

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complex is more stable at the interface and insoluble in water. The presence of crown ethers in the [BMIM][Tf2N]–water system was found to reduce the surface tension of bulk IL. On the

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contrary an enhancement of the surface tension was observed with the addition of the metal salt. Overall, the simulation study reported in this work provides a molecular picture of the

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solutesolvent, crown ether–cesium and complexsolvent interactions. Acknowledgement

This work was funded by the Board of Research in Nuclear Sciences (BRNS, Government of India) vide scheme no. 2013/36/30-BRNS/2352, dated 26.11.2013.

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ACCEPTED MANUSCRIPT References [1] A.E. Visser, M.P. Jensen, I. Laszak, K.L. Nash, G.R. Choppin, R.D. Rogers, Uranyl coordination environment in hydrophobic ionic liquids: an in situ investigation, Inorg. Chem. 42 (2003) 2197-2199. [2] H. Luo, S. Dai, P.V. Bonnesen, A. Buchanan, J.D. Holbrey, N.J. Bridges, R.D. Rogers,

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ACCEPTED MANUSCRIPT [36] M. Tariq, A.P. Serro, J.L. Mata, B. Saramago, J.M. Esperança, J.N.C. Lopes, L.P.N. Rebelo, High-temperature surface tension and density measurements of 1-alkyl-3-methylimidazolium bistriflamide ionic liquids, Fluid Phase Equilib. 294 (2010) 131-138. [37] R.L. Gardas, R. Ge, N. Ab Manan, D.W. Rooney, C. Hardacre, Interfacial tensions of imidazolium-based ionic liquids with water and n-alkanes, Fluid Phase Equilib. 294 (2010) 139-

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. Initial and final snapshots of the distribution of 6 Cs+NO3 ion pairs in IL-water system. Plots for density are affixed to the right side. Fig. 2. Solvation of Cs+, NO3- ions at the IL side of the interface and [BMIM]+ ion at the aqueous side of the interface.

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Fig. 3. Snapshots of configurations at 0, 10 and 40 ns, for the crown ether containing binary mixture of [BMIM][Tf2N]–water: (a) BPC6 as crown ether and (b) DCH18C6 as crown ether. Plots for density are affixed to the right side. BPC6 and DCH18C6 are not shown in the figure.

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Fig. 4. Variation of demixing index () with time in [BMIM][Tf2N]–water system with DCH18C6 and BPC6 as extractants: (a) demixing index of BPC6 and (b) demixing index of DCH18C6

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Fig. 5. Snapshots of configurations at 0, 10, 20 and 40 ns, for cesium nitrate as Cs+NO3 and BPC6, inserted in the [BMIM][Tf2N]–water system. Plots for density are affixed to the right side.

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Fig. 6. Optimized geometry of 2:1 Cs+–BPC6 complex. (a) RDF for 2:1 Cs+-O complex over the trajectory run from 5 to 40 ns. (b) Equilibrium distance between Cs+ (purple) and O of BPC6 (red) is shown by red dotted line.

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Fig. 7. Initial and final snapshots of the distribution of 3 Cs+-BPC6 complex in IL-water system. Plots for density are affixed to the right side.

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Fig. 8. Snapshots of configurations at 0, 10, 20 and 40 ns for cesium nitrate as Cs+NO3 and DCH18C6 inserted in the [BMIM][Tf2N]–water system. Plots for density are affixed to the right side. Fig. 9. Initial and final snapshots of the distribution of 3 Cs+-DCH18C6 complex in IL-water system. Plots for density are affixed to the right side.

Fig. 10. Radial distribution functions for (a) free Cs+– uncomplexed BPC6 (green) and free Cs+water (red), and (b) free Cs+– uncomplexed DCH18C6 (green) and free Cs+–water (red) at 300 K. Fig. 11. Potential mean force of (a) Cs+-BPC6 complex and water, and (b) Cs+ with DCH18C6 crown ether and water.

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ACCEPTED MANUSCRIPT Fig. 12. Interaction energies of (a) Cs+–BPC6 and (b) Cs+–DCH18C6 Fig. 13. Snapshots of configuration at 5 ns for (a) cesium nitrate as Cs+NO3 and BPC6 and (b) cesium nitrate as Cs+NO3 and DCH18C6 in vacuo.

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Fig. 14. MSD of CsNO3 and crown ethers in the binary [BMIM][Tf2N]water system: (a) BPC6 and Cs+–BPC6 complex at the interface, (b) free Cs+ and NO3– in the aqueous phase, and (c) free Cs+ and NO3– in the aqueous phase, and DCH18C6 at the interface.

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ACCEPTED MANUSCRIPT Table Captions Table 1 Size of the simulation boxes and number of molecules at 300K. Table 2 Calculated values of binding energies of Cs+ with crown ether as obtained from QC and MD calculations. Table 3 Diffusion coefficients of the species in IL and water.

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Table 4 Surface tensions of ionic liquid and water, and interfacial tension for IL and water

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calculated at 300 K.

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[BMIM]+ [Tf2N]-

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500

(a) BPC6 Cs+ NO3-

400

IP

T

300

200

CR

MSD(Å2)

Cs+-BPC6

0 0

200

400

US

100

600

800

1000

800

1000

M

AN

Time(ps)

200

ED

(b)

DCH18C6 Cs+-DCH18C6

PT

100

CE

MSD (Å2)

150

AC

50

0

0

200

400

600 Time (ps)

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ACCEPTED MANUSCRIPT

70

(c)

60

Cs+-BPC6 Cs+-DCH18C6

T IP

40 30

CR

MSD (Å2)

50

20

0 0

200

400

US

10

600

800

1000

Fig. 14

AC

CE

PT

ED

M

AN

Time (ps)

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Table 1. Size of the simulation boxes and number of molecules at 300K.

T

Box Size x,y,z (Å3)

Preformed planar interface

6 Cs+NO3‒ ; 12 BPC6

Random mixed

6 Cs+NO3‒ ; 12 BPC6

Preformed planar interface

6 Cs+NO3‒ ; 12 DCH18C6

Random mixed

6 Cs+NO3‒ ; 12 DCH18C6

Preformed planar interface

6 Cs+NO3‒ ; 12 BPC6

vacuum

6 Cs+NO3‒ ; 12 DCH18C6

vacuum

Time

44.63 44.63 89.26

20 ns

45.59 45.59 91.19

40 ns

44.87 44.87 89.75

20 ns

45.05 45.05 90.08

40 ns

44.68 44.68 89.37

20 ns

34.04 34.04 68.08

10 ns

31.76 31.76 63.52

10 ns

AC

CE

PT

ED

M

AN

US

6 (Cs+; NO3‒ )

IP

Systems

CR

Solutes

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ACCEPTED MANUSCRIPT Table 2 Calculated values of binding energies of Cs+ with crown ethers as obtained from QC and MD calculations

Binding Energy (kcal/mol)

Crown Ether

MD (with solvents)

BPC6

–58.85

–52.55

DCH18C6

–55.83

–45.85

MD (in vacuo)

T

QC

IP

-52.87

AC

CE

PT

ED

M

AN

US

CR

-29.73

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Table 3 Diffusion coefficients of the species in IL and water

Solvent

Diffusion Coefficient  10-9 (m2/s)

Free Cs+

Water

0.798

NO3–

Water

0.570

Free BPC6

IL

Cs+–BPC6 Complex

Interface

Free DCH18C6

IL

Cs+–DCH18C6 Complex

IL

Cs+-BPC6 complex

Preformed interface

Cs+-DCH18C6 complex

Preformed interface

IP

T

Species

CR

0.826 0.036

0.250 0.113 0.107

AC

CE

PT

ED

M

AN

US

0.309

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ACCEPTED MANUSCRIPT Table 3 Surface tensions of ionic liquid and water, and interfacial tension for IL and water calculated at 300 K.

 IL ( mN m−1)

System

 W ( mN m−1)

Interfacial tension(  c )

IL–water+BPC6+Cs+NO3–

29.40±4

IL– water+DCH18C6+Cs+NO3–

31.09±6

71.61±7 (72.37) [34]

IP

31.38±4 (30.04) [36]

CR

IL–water

T

(mN m−1)

73.02±4

12.95±4

9.58±6

AC

CE

PT

ED

M

AN

US

74.30±7

8.77±2 (9.21) [37]

41

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AC

CE

PT

ED

M

AN

US

CR

IP

T

Graphical Abstract

42

ACCEPTED MANUSCRIPT Research Highlights

Ionophores BPC6 and DCH18C6 used for Cesium Extraction in IL‒ water system Complexes of Cs-BPC6 more stable than Cs-DCH18C6

AC

CE

PT

ED

M

AN

US

CR

IP

T

Indicative of a cation exchange mechanism at the IL-water interface

43