Influence of Counterions on the Hydration Structure of Lanthanide Ions

Article Cite This: J. Phys. Chem. B 2018, 122, 2779−2791

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Influence of Counterions on the Hydration Structure of Lanthanide Ions in Dilute Aqueous Solutions Valentina Migliorati,*,† Alessandra Serva,† Francesco Sessa,† Andrea Lapi,†,‡ and Paola D’Angelo*,† †

Department of Chemistry, University of Rome “La Sapienza”, P.le Aldo Moro 5, 00185 Rome, Italy Istituto CNR di Metodologie Chimiche-IMC, Sezione Meccanismi di Reazione c/o Department of Chemistry, University of Rome “La Sapienza”, P.le Aldo Moro 5, 00185 Rome, Italy

‡

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S Supporting Information *

ABSTRACT: A synergic approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) has been used to investigate diluted (0.1 M) aqueous solutions of two lanthanide ions (Ln3+), namely, La3+ and Dy3+, with triflate, nitrate, and bis(trifluoromethylsulfonyl)imide (Tf2N−) as counterions. The different complexing ability of the three anions has been highlighted by the analysis of the MD simulations: Tf2N− does not form innersphere complexes, while a small amount of triflate coordinates both the La3+ and Dy3+ cations in their first solvation shell. On the other hand, the nitrate ion is almost absent in the La3+ first coordination sphere, while forming contact ion pairs with Dy3+. Both lanthanide ions are found to preferentially interact with the water molecules, and the total number of oxygen atoms coordinating the Ln3+ cations in their first solvation sphere is the same in all of the solutions, regardless of whether they belong to water molecules or to the counterion. The presence of counterions in the cation first or second shell changes neither the first shell distance nor the symmetry of the hydration complex formed in solution. The MD results have been confirmed by comparison with the Ln K-edge XAS experimental data, and the quantitative analysis of the extended X-ray absorption fine structure (EXAFS) spectra of the three salt solutions has provided a definite proof of the accuracy of the force field employed in the simulations and of the MD structural result. The anion−water and water−water hydrogen bond lifetimes have been analyzed highlighting the slow down effect of the triflate, nitrate, and Tf2N− anions on the hydrogen bond dynamics in the Ln3+ first solvation shell, with the effect being stronger in the Dy3+ solutions, due to the higher charge density of the Dy3+ ion as compared to La3+.

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and it is well established that they do not enter the Ln3+ first hydration shell.15−22 Conversely, nitrate anions coordinate Ln3+ ions in their first coordination sphere, assuming either a bidentate or monodentate coordination geometry,17,19,23−25 while for chloride ions an inner-sphere complexation has been found to occur only at high salt concentrations.17,19,20,26−29 In this work, we have carried out a combined molecular dynamics (MD) and extended X-ray absorption fine structure (EXAFS) investigation on aqueous solutions of two Ln3+ ions (La 3 + and Dy 3 + ) with triflate, nitrate, and bis(trifluoromethylsulfonyl)imide (Tf2N−) as counterions in dilute conditions (0.1 M). The schematic molecular structure of the counterions investigated in this work is shown in Figure 1. Tf2N− and nitrate anions have been chosen, since they are the most common anions in imidazolium-based and protic ionic liquids that are very promising solvents for Ln3+ separation processes from aqueous media.30−36 The triflate anion has been studied, as it is a weakly complexing species with a structure

INTRODUCTION Understanding the hydration structure of metal cations in aqueous solution is very important to address their solvation properties and chemical reactions. Several experimental and theoretical studies on lanthanide (Ln) ions in water have been carried out over the past years, and the hydration structure of these ions is now well established.1−12 With the exception of Ce in aqueous solution, lanthanides exist exclusively in the highly charged trivalent state Ln3+ which makes them strongly hydrated. The average distance between the Ln3+ ions and the water molecules of the first hydration shell decreases across the series, and the coordination number goes from nine for the lighter elements to eight for the heavier ones.1,2,4,5,9−11,13 An important topic that is under debate in the literature is the influence of counterions on the coordination structure and dynamics of metal ions.14−20 In highly concentrated solutions (>1 M), contact ion pairs do form due to the low number of water molecules that are not sufficient to complete the first and second hydration shells of the ions. For more dilute solutions, the effect of the counterions strongly depends on their chemical nature. Perchlorate (ClO4−) and triflate (trifluoromethanesulfonate, TfO−) are considered weakly complex-forming anions, © 2018 American Chemical Society

Received: December 21, 2017 Revised: February 22, 2018 Published: February 26, 2018 2779

DOI: 10.1021/acs.jpcb.7b12571 J. Phys. Chem. B 2018, 122, 2779−2791

The Journal of Physical Chemistry B

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Article

METHODS

Molecular Dynamics. Classical MD simulations of 0.1 M aqueous solutions containing lanthanide salts (Ln(TfO)3, Ln(Tf2N)3, and Ln(NO3)3, with Ln3+ = La3+ and Dy3+) have been performed by means of the Gromacs package.55 The SPC/E model56 was adopted for water molecules, while the force field parameters for the TfO−, Tf2N−, and NO3− anions were taken from Lopes and Padua.57,58 As regards the La3+ and Dy3+ cations, we used standard point charge 3+ and LennardJones parameters (σLnLn and εLnLn) developed by us in order to obtain accurate structural properties of La3+ and Dy3+ in pure water.40 All of the Lennard-Jones cross-terms between different atom types have been calculated using the Lorentz−Berthelot combining rules, with the exception of the interaction between the Ln3+ cation and the O atoms of the NO3− anions. The Lennard-Jones parameters for this interaction have been obtained by combining σLnLn and εLnLn with the σOO and εOO values taken from the SPC/E water model.56 The systems were composed of 5 Ln3+ cations, 15 anions, and 2775 water molecules in a cubic box, created by randomly assigning initial positions to all ions and molecules and then equilibrated under NPT conditions in order to obtain the starting configuration. The simulations have been carried out in the NVT ensemble at 300 K for a total time of 20 ns, after a 3 ns equilibration run, using a 1 fs time step. The lengths of the simulation box edges are 44.14, 44.60, and 43.87 Å for the Ln(TfO)3, Ln(Tf2N)3, and Ln(NO3)3 salt solutions, respectively. The Nosé−Hoover thermostat,59,60 with a relaxation constant of 0.5 ps, was used to control the system temperature. Long-range interactions have been evaluated by the particle-mesh Ewald method,61 while the cutoff of the nonbonded interactions was set to 9 Å. Periodic boundary conditions have been applied in order to simulate bulk material. X-ray Absorption Measurements. La(TfO)3, Dy(TfO)3, La(NO3)3·6H2O, and Dy(NO3)3·xH2O were purchased from Sigma-Aldrich, all with a stated purity of >99%, and further purification was not carried out. La(Tf2N)3 and Dy(Tf2N)3 were synthesized by reaction of the corresponding oxides (La2O3 and Dy2O3, Sigma-Aldrich) with bis(trifluoromethanesulfonyl)imide following the procedure reported by Ferry et al. in ref 62. Aqueous solutions (0.1 M) of Ln(TfO)3, Ln(Tf2N)3, and Ln(NO3)3 were made by dissolving a weighed amount of salt in freshly distilled water. La and Dy K-edge spectra were collected at room temperature at ESRF, on the bending magnet X-ray absorption spectroscopy beamline BM23, in transmission geometry. The spectra were collected by using a Si(311) double-crystal monochromator with the second crystal detuned by 20% for harmonic rejection. The aqueous solutions were kept in cells with Kapton film windows and Teflon spacers of 2 cm. EXAFS Data Analysis. The analysis of EXAFS data has been carried out with the GNXAS program, and all details on the theoretical framework can be found in ref 63. Phase shifts have been calculated using muffin-tin potentials starting from the coordination structure obtained from the MD simulations. The exchange and correlation parts of the potential are determined on the basis of the local-density approximation of the self-energy of the excited photoelectron using an appropriate complex optical potential. The real part of selfenergy has been calculated by using the Hedin−Lundqvist energy-dependent potential. Inelastic losses of the photoelectron in the final state have been accounted for intrinsically

Figure 1. Molecular structure of the triflate (TfO−) (a), bis(trifluoromethylsulfonyl)imide (Tf2N−) (b), and nitrate (NO3−) (c) anions.

more similar to the bulky Tf2N− anion as compared to perchlorate. On the other hand, La3+ and Dy3+ have been chosen, as they are placed at the beginning and at the middle of the lanthanide series and the Ln3+ hydration properties are known to change across the series. In previous works, it was found that the first shell coordination number of lanthanide and actinide ions decreases or increases depending on whether the counterions are located in the primary or secondary shell of the cation, respectively.37 A different picture of the influence of counterions on the hydration properties of Cm3+ and Th4+ has been provided by recent ab initio MD simulations.21,22 In these works, it was shown that the presence of the counterions induces a strenghtening of the cation primary hydration shell as a consequence of an altered dynamics of the hydrogen bonds between the water−water and water−counterion pairs. MD simulations have been frequently used to shed light onto the structural and dynamical properties of Ln3+ aqueous solutions, and most of these studies have been carried out in the so-called infinite dilution approximation with a single ion immersed in bulk water and with neutrality achieved by a uniformly distributed charge of opposite sign.1−8 However, counterions can be inserted in the simulation to neutralize the net charge and this allows one to shed light onto the formation of inner- or outer-sphere complexes with the cation. Several MD calculations have been carried out using parametrized polarizable force fields to account for the polarization induced on the water molecules by the 3+ charge of the Ln cations.1,5−7,38,39 However, it can be quite challenging for polarizable force fields to reproduce the fine balance among the different interactions taking place in the presence of the counterion, especially considering the long simulation time that is required to accurately sample the phase space visited by these systems. On the other hand, in a recent MD simulation study of the whole lanthanide series, a force field without explicit polarization has been developed and it has been found to properly reproduce the structural properties of the Ln3+ hydration complexes in water40 and in ethylammonium nitrate.41 Here, this force field has been used to provide a microscopic description of the Ln3+ ions in aqueous solutions including counterions having different complexing properties. From an experimental point of view, Ln3+ complexation with perchlorate, halide, nitrate, and triflate has been studied with different techniques such as EXAFS,15,16,26 17O and 19F NMR,18,42 Raman,19,20 UV−visible,17 and time-resolved laserinduced fluorescence spectroscopies.27 However, although these techniques provide reliable structural or dynamic information, a thorough microscopic description of the shortand medium-range structure of Ln3+ salts in water is still lacking. Here, MD simulations have been used in combination with the EXAFS spectroscopy that provides accurate shortrange structural information around a selected ion,43−54 to clarify the Ln3+ complexation with anions in dilute aqueous solutions and to obtain a complete picture of the hydration structure of both the cations and anions in water. 2780


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The Journal of Physical Chemistry B by the complex potential. Previous investigations on the solvation structure of lanthanide ions,13 metal ions,64−66 and halides67−69 have shown that in the case of solutions a correct approach to compute the χ(k) signal uses radial distribution functions g(r) according to the equation χ (k ) =

∫0

∞

dr 4πρr 2g (r )A(k , r ) sin[2kr + ϕ(k , r )]

(1)

where A(k, r) and ϕ(k, r) are the amplitude and phase functions, respectively, and ρ is the density of the scattering atoms. Given the short-range sensitivity of the EXAFS technique, the total theoretical χ(k) signal has been calculated starting from the MD Ln−O and Ln−H g(r)’s and only those associated with the water molecules have been found to provide a detectable contribution. Least-squares fits of the EXAFS raw spectra have been carried out keeping fixed the structural parameters, while two nonstructural parameters have been optimized, namely, E0 (core ionization threshold) and S02 (many-body amplitude reduction function).

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RESULTS AND DISCUSSION Lanthanide Salts: Inner- versus Outer-Sphere Complexation. The first question that we address here is whether lanthanide (La3+ and Dy3+) triflate, Tf2N−, and nitrate salts form inner-sphere complexes in water. To this end, we have carried out MD simulations of aqueous solutions containing the lanthanide salts, as described in detail in the Methods section. When dealing with salt association processes in solution, it is very important to properly sample the phase space visited by the systems, as a poor statistics could result in completely erroneous results. In particular, if the salt dissociation/ association process is slow and the simulation time is too short, the initial conditions can bias the results that are obtained from the calculations. For this reason, we have simulated large boxes containing 5 Ln3+ cations (together with 15 anions and 2775 water molecules) starting from randomly assigned initial positions of the ions and water molecules; after an NPT equilibration, the systems were additionally equilibrated for 3 ns in the NVT ensemble and production runs were then collected for a simulation time of 20 ns. In order to investigate the complexation properties of lanthanide salts in water, we have calculated the g(r)’s between the La3+ and Dy3+ cations and the anions (Ln−X g(r)’s), that are shown in Figure 2. In particular, we have chosen the sulfur atom, S, as the observed atom for the TfO− anion and the nitrogen atom, N, for the Tf2N− and NO3− anions. The g(r) peak positions r and the coordination numbers CN are listed in Table 1 for all of the solutions. Lanthanide Triflates. The triflate anions are considered as weakly complex-forming species and are often used as counterions for structural studies of the hydration properties of lanthanide ions in water.9,10,12 The g(r)’s between the La3+ and Dy3+ cations and the triflate anions calculated from our MD simulations of the lanthanide triflate aqueous solutions show a first shell peak with maxima at 3.90 and 3.80 Å for the La3+ and Dy3+ ions, respectively, followed by a second shell peak and some less-defined oscillations beyond. However, the Ln−S first shell coordination numbers, obtained by integrating the g(r) first peak up to the first minimum, are very low for both ions (0.2), indicating that the lanthanide triflate salts are substantially dissociated in water. This result is in agreement with the common assumption that triflate does not form ion

Figure 2. Ln−X radial distribution functions, g(r)’s, calculated from the MD simulations of 0.1 M Ln(TfO)3, Ln(Tf2N)3, and Ln(NO3)3 aqueous solutions. Note that X = S for TfO− and X = N for Tf2N− and NO3− anions.

pairs with lanthanides in aqueous solution, and also with two NMR investigations, the former carried out on Dy(TfO)3 aqueous solutions18 and the latter on Ln(TfO)3 in water for the entire lanthanide series,42 where it was found that the triflate anion does not enter the Ln3+ first coordination sphere. The same result has also been obtained for the La3+ ion in a recent combined Raman scattering and DFT study.19 In order to get additional insights into the salt association process and to evaluate the sampling of the system phase space, it is useful to plot the evolution of the Ln−S distances for each single Ln3+ cation in the course of the simulations. The time evolutions of the Ln distances for a single Ln3+ ion taken as an example are shown in panels a and b of Figure 3, while the time evolutions for all of the Ln3+ ions are shown in Figures S1 and S2 of the Supporting Information for La3+ and Dy3+ ions, respectively. As concerns the La3+ ion, at the end of the NVT equilibration, there are two cations with one triflate anion in their first solvation shell and then during the simulation many different anions enter, for short simulation times, the La3+ first coordination sphere. Conversely, a lower number of triflate anions approaches the Dy3+ cation in the course of the simulation, but sometimes the anions remain in the Dy3+ first coordination shell for longer times as compared to the La3+ ion. Lanthanide Tf2N−. The analysis of our MD simulations shows that lanthanide Tf2N− salts are fully dissociated in water. Indeed, the g(r)’s between the La3+ and Dy3+ cations and the Tf2N− anions show only a broad low intensity peak at long Ln− N distances (Figure 2). Inspection of the Ln−N time profiles (panels c and d of Figure 3 and Figures S3 and S4 of the Supporting Information) reveals that the Tf2N− anions approach the La3+ and Dy3+ cations (Ln−N distances in the range 4.5−5.0 Å) many times in the course of the simulations, but they never enter the Ln3+ first solvation shell. Lanthanide Nitrates. The g(r)’s between the La3+ and Dy3+ cations and the nitrate anions calculated from our MD simulations show different trends: for the La3+ ion, a low 2781


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Table 1. Structural Properties Calculated from the MD Simulations of Ln3+ Salt Aqueous Solutions, Together with Those of Ln3+ Ions in Pure Water40 a La3+ system

CN(1) OW

r(1) La−OW

pure water TfO− Tf2N− NO3−

9.5 9.3 9.5 9.5

2.57 2.57 2.57 2.57

CN(1) O

r(1) La−O

0.2

2.53

0.04

2.65

CN(2) OW

r(2) La−OW

17.9 16.8 17.1 17.0

4.71 4.71 4.71 4.71

CN(1) X

r(1) La−X

CN(2) X

r(2) La−X

0.2

3.90

0.04

3.80

0.4 0.5 0.8

5.80 7.00 5.20

CN(1) X

r(1) Dy−X

CN(2) X

r(2) Dy−X

0.2

3.80

0.5

3.61

0.4 0.5 0.6

5.65 6.90 5.00

Dy3+ system

CN(1) OW

(1) rDy−OW


9.0 8.8 9.0 8.5

2.39 2.39 2.39 2.39

CN(1) O

r(1) Dy−O

0.2

2.35

0.5

2.43

CN(2) OW

(2) rDy−OW

16.6 15.6 15.9 15.3

4.54 4.53 4.52 4.54

(2) (1) Coordination numbers of the Ln3+ first (CN(1) OW) and second (CNOW) hydration shells and position (Å) of the Ln−OW g(r) first (rLn−OW) and (2) (1) 3+ second (rLn−OW) peaks. Coordination numbers of oxygen atoms belonging to the anions that are in the Ln first (CNO ) coordination shell and (2) 3+ first (CN(1) position (Å) of the Ln−O g(r) first (r(1) Ln−O) peak. Coordination numbers of anions that are in the Ln X ) and in the second (CNX ) (1) (2) − − coordination shells and position (Å) of the first (rLn−X) and second (rLn−X) Ln−X g(r) peaks (X = S for the TfO anion and X = N for the Tf2N and NO3− anions). a

sphere.19 On the other hand, in a combined MD and UV/vis spectroscopy study, nitrate anions were observed in the Dy3+ first coordination shell only for aqueous solutions with a salt concentration above 2.0 mol kg−1.17 An open question concerning nitrate coordination of lanthanide ions is whether the nitrate ions act as monodentate or bidentate ligands. In solid state structures, the dominant coordination of nitrate is the bidentate one,72 while quantum chemical calculations have suggested that an increase of the number of water molecules in the Ln3+ first coordination shell promotes the monodentate coordination mode of nitrate with respect to the Ln3+ ion.25 The presence of a single peak in our Ln−N g(r)’s indicates that a single nitrate coordination mode takes place in both La3+ and Dy3+ nitrate aqueous solutions, i.e., the monodentate one. This finding is in agreement with the results obtained by Duvail et al. from MD simulations of Nd3+ and Dy3+ nitrates in water.17 As mentioned above, La3+ and Dy3+ nitrate salts show different structural properties, and a different dynamic behavior of such salts can also be appreciated by looking at the time profile of the Ln−N distances (panels e and f of Figure 3 and Figures S5 and S6 of the Supporting Information): for La3+, the nitrate anions enter the first solvation shell only for short simulation times (the longest lifetime of the anions in the La3+ first shell is about 0.2 ns), while, in the case of Dy3+, the nitrate can durably coordinate the cation also for longer time intervals (longest lifetime of 7.7 ns). The slow dynamics of the Dy3+ nitrate salt has been previously pointed out by Duvail et al., who did not observe any salt dissociation in their 2 ns MD simulation of a dilute Dy3+ nitrate aqueous solution.17 Ln3+ First Coordination Shell. La3+ Salts. The results of our MD simulations of La3+ salts in water have highlighted a different behavior of the counterions also under dilute conditions. In particular, the Tf2N− ion does not form innersphere complexes, while a very small amount of triflate and nitrate anions coordinate the La3+ cation in its first coordination shell. In order to shed light into the first shell solvation properties of the La3+ ion, we have computed the La−OW and La−HW g(r)’s that are reported in Figure 4 multiplied by the numerical density of the observed atoms ρ (OW and HW refer to the water oxygen and hydrogen atoms, respectively). When treating systems with different densities, the simple comparison

Figure 3. Time evolution of the Ln−X distances of one Ln3+ cation calculated from the MD simulation of 0.1 M La(TfO)3 (a), Dy(TfO)3 (b), La(Tf2N)3 (c), Dy(Tf2N)3 (d), La(NO3)3 (e), and Dy(NO3)3 (f) aqueous solutions. Note that X = S for TfO− and X = N for Tf2N− and NO3− anions. The plotted distance ranges have been chosen to highlight possible exchange events between the first and second solvation shells of the Ln3+ ions. Each different color indicates that the X atom belongs to a different counterion present in the solution.

intensity first shell peak with a maximum at 3.80 Å is found, while, for Dy3+, the first shell peak with a maximum position of 3.61 Å is much more intense (see Figure 2). This result is at variance with our findings on lanthanide triflate and Tf2N− salts in aqueous solutions, where the two Ln3+ cations have shown very similar structural properties. In the nitrate salt solutions, the Ln−nitrate first shell coordination number obtained for La3+ is very low, while, in the case of the Dy3+ ion, a larger number of nitrate ions enter the cation first coordination sphere (first shell coordination number of 0.5). These findings are consistent with the low association constant values of Dy3+ and other Ln3+ nitrate salts determined by microcalorimetry and solvent extraction techniques.70,71 Moreover, the coordination number obtained for the La3+ ion is in line with the results of a recent Raman and DFT investigation, where it was shown that, in dilute La3+ nitrate aqueous solutions, a very small percentage of nitrate ions coordinates La3+ in its first coordination 2782


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Figure 4. Ln−OW and Ln−HW radial distribution functions, g(r)’s, multiplied by the numerical density of the observed atoms ρ and Ln−OW running coordination numbers obtained from the MD simulations of 0.1 M Ln(TfO)3 (green), Ln(Tf2N)3 (blue), and Ln(NO3)3 (red) aqueous solutions. OW and HW are the water oxygen and hydrogen atoms. The Ln−water g(r)ρ’s (black) calculated from the MD simulations of Ln3+ ions in pure water (see ref 40) are also shown.

identify the first shell symmetry of an ion in solution.76 For this analysis, we have extracted from the La3+ simulations configurations containing 9-fold coordinated La3+ ions only, and on these trajectories, we have calculated the CDFs combining the distribution functions of the La−O distances and O−La−O angles, where O represents oxygen atoms belonging either to water molecules or to anions in the La3+ first coordination shell. The CDFs obtained for the salt solutions are very similar to each other and to the function calculated in pure water (Figure 5): two peaks are found, and they are centered at a La−O distance of about 2.57 Å and O− La−O angles of 70 and 135°. In all of the simulations, the form of the distributions, with the characteristic L-shape of the low

of the g(r)’s can be misleading, as already pointed out in the literature.73,74 The La−OW running coordination numbers are also reported in Figure 4. All of the g(r)ρ’s show very sharp and structured first shell peaks, as strong cation−water interactions take place and the neat separation between the La−OW and La−HW g(r) first peaks indicates that the water molecules are strongly oriented due to the electrostatic field of the La3+ ion. Note that the Ln−water g(r)ρ’s computed for the three salts are very similar among each other and also to the Ln−water g(r)ρ’s calculated from our previous MD simulation of La3+ ion in pure water, carried out in the infinite dilution approximation with a single La3+ ion immersed in bulk water and using the same interaction potential parameters adopted here (for additional details, see ref 40). The structural parameters of the La−OW g(r)’s are listed in Table 1: the position of the La−OW g(r) first peak maximum is the same in all systems (2.57 Å), independent of the counterion. Since the cation−anion interactions take place between the lanthanide ion and the oxygen atoms of the anion (O), it is useful to calculate the La−O g(r)’s that are shown in Figures S7 of the Supporting Information, while the corresponding structural parameters are listed in Table 1. Inspection of this table shows that the La3+ ion coordinates the oxygen atoms of the nitrate and triflate anions at longer and shorter distances, respectively, as compared to the La−OW first shell distance. A longer distance of the nitrate oxygen atoms with respect to the water oxygen has been previously found in an EXAFS investigation of lanthanide nitrate salts in water.75 Note that the total number of oxygen atoms in the La3+ first coordination shell is always the same (9.5) regardless of if they belong to water molecules or the counterion. This finding suggests that the La3+ coordination is mainly driven by electrostatic forces and the first shell structure is preserved also when a small amount of ionic pair is formed, as in the case of the triflate solution. It is now interesting to check whether the geometry of the first hydration shell of oxygen atoms surrounding the La3+ ion is the same in the different solutions. To this end, we have used a new tool developed by us that is based on the calculation of combined distribution functions (CDFs) and allows one to

Figure 5. Combined distribution functions (CDFs) between the La− O distances and the O−La−O angles evaluated for the 9-fold configurations only extracted from the MD simulations of La(TfO)3, La(Tf2N)3, and La(NO3)3 aqueous solutions. The CDF calculated from the MD simulation of La3+ ion in pure water (see ref 40) is also shown. 2783


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Figure 6. MD snapshots showing the local environment of anions that are in the La3+ second coordination shell. The water molecules of the La3+ first and second coordination shells are colored blue and yellow, respectively, while those belonging to the anion first hydration shell are orange. In particular, the water molecules that are simultaneously in the first coordination shell of the cation and the anion are colored magenta, while those that belong to the second coordination shell of the cation and to the first shell of the anion are cyan.

the La3+ ion, by looking at the structural parameters (Table 1) of the Dy−O g(r)’s with O belonging to the triflate and nitrate anion (Figures S7 of the Supporting Information), a longer and shorter Dy−O distance is found, respectively, as compared to the distance between Dy3+ and the oxygen atoms of first shell water molecules. In order to shed light into the threedimensional arrangements of the oxygen atoms surrounding the Dy3+ ion, we have calculated the CDF combining the Dy− O g(r) and the O−Dy−O angle distribution, where O are oxygen atoms belonging either to water molecules or to anions in the Dy3+ first coordination shell (see Figure S8 of the Supporting Information). As observed for the La3+ salts, the form of the CDF, with two peaks centered at a distance of about 2.39 Å and O−Dy−O angles of 70 and 135°, is consistent with the existence of a TTP symmetry of the Dy3+ first coordination shell. Moreover, the strong similarity of the CDF obtained for the different systems shows that the geometry of the oxygen atoms surrounding the Dy3+ ion is the same in all of the salt solutions. Note that the higher charge density of the Dy3+ ion as compared with La3+ gives rise to a first shell complexation mainly with the nitrate anion that, in turn, is the one with the highest charge density. However, the presence of counterions in the first or second shell changes neither the Ln−O first shell distance nor the symmetry of the hydration complex formed in solution. Ln3+ Second Coordination Shell. La3+ Salts. The second hydration shell of the La3+ ion is very similar in the aqueous solutions of the three La3+ salts (Figure S9 of the Supporting Information), and resembles the La3+ second hydration shell found in pure water. In particular, the La−OW second shell distance is identical in all of the systems, while a slight decrease of the number of second shell water molecules is found with respect to the one obtained with no counterions in the simulation box (Table 1). This reduction is due to the presence of anions in the La3+ second coordination sphere, as revealed by the second peak of the Ln−X g(r)’s shown in Figure 2 and by the CN(2) X coordination numbers that increase from 0.4 to 0.8 going from triflate to nitrate salt solutions (see Table 1). The La−X g(r) second peak is due to the formation of solventseparated ion pairs between cation and anion, and it becomes wider and more unstructured going from the NO3− to the TfO− and finally to the Tf2N− anion. The peculiar forms of the Ln−X g(r) second peaks suggest that they are the superposition of at least two peaks, originating from different structural arrangements. In the triflate and nitrate solutions, two possible

angle peak, is consistent with a tricapped trigonal prism (TTP) geometry of the first shell cluster, in agreement with the results reported in the literature for light Ln3+ ions.2,5,77 The close similarity of the CDFs indicates that the three-dimensional arrangement of the oxygen atoms in the La3+ first coordination shell is the same in all of the salt solutions, regardless of if all oxygen atoms belong to water molecules (pure water, nitrate, and Tf2N− solutions) or a small fraction of oxygen atoms belong to the anion (triflate solution). Dy3+ Salts. It is well-known that the lanthanide ionic radius in water decreases along the series giving rise to an increase of the charge density of the Ln3+ ions with increasing atomic number.2 This gives rise to different coordination properties of the lanthanide ions and can cause a different behavior in the presence of counterions along the series. The diverse charge density between La3+ and Dy3+ has no effect on the interaction with the bulky Tf2N− and triflate species for which identical complexation properties have been found for the two Ln3+ cations. In particular, Tf2N− does not enter the Dy3+ first hydration shell, while a small percentage of the triflate anion forms an inner-shell complex with the Dy3+ cation (see Figure 2). A different result has been obtained in the case of the nitrate counterions, as the increase of the Dy3+ charge density determines the formation of contact pairs in the 0.1 M aqueous solution that are not formed in the case of the larger La3+ ion. The Dy−OW and Dy−HW g(r)ρ’s and Dy−OW running coordination numbers calculated for the three salt solutions are shown in Figure 4, together with the Dy−water g(r)ρ’s calculated in the infinite dilution approximation.40 Also, in this case, the Dy−OW distance (2.39 Å) is identical in all of the simulations (see Table 1), and it is shorter as compared to the La−OW one, as a consequence of the lanthanide contraction.2 The Dy−OW g(r)ρ of the Tf2N− solution is identical to that of pure water, while small differences can be observed in the case of the triflate and nitrate anions, as also evident from the Dy−OW running coordination numbers. The Dy−OW first shell hydration number calculated in the Tf2N− solution is 9.0 as in pure water, and as expected, it is slightly lower than that of La3+ ion, in agreement with the decrease of lanthanide hydration numbers going from the first to the last element of the series.2 As concerns the Dy3+ triflate and nitrate salts, we observe a decrease in the Dy−OW first shell hydration number (8.8 and 8.5 for triflate and nitrate, respectively), but also in this case, the sum of the coordination numbers of water molecules and anions is always 9.0 (Table 1). As observed for 2784


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Table 2. Structural and Dynamic Properties Calculated from the MD Simulations of Ln3+ Salt Aqueous Solutions, Together with Those of Ln3+ Ions in Pure Water40 a La3+ system

t(2) WAT


11 13 15 13

t(2) X

n1HB

25 23 41

3.5 3.4 3.4 3.4

n2HB 3.5 3.5 3.5

Dy3+ n3HB

τ1

5.7 5.0 6.9

1.0 1.9 1.9 1.9

τ2

t(2) WAT

1.2 0.7 1.5

14 15 15 16

t(2) X

n1HB

27 22 57

3.5 3.4 3.4 3.4

n2HB 3.5 3.5 3.5

n3HB

τ1

τ2

5.9 5.1 6.5

1.2 2.2 2.2 2.3

1.2 0.7 1.5

a (2) tWAT

1 3+ and t(2) X are the mean residence times (ps) of water molecules and anions in the Ln second coordination shell, respectively. nHB is the average number of water−water hydrogen bonds per water molecule, n2HB is the average number of hydrogen bonds per solvent molecule by including both water−water and anion−water hydrogen bonds, while n3HB is the average number of anion−water hydrogen bonds per anion. τ1 and τ2 are the lifetimes (ps) of the hydrogen bonds formed between the water molecules belonging to the first solvation shell of the Ln3+ ion and all of the other species and formed between the anions and all of the water molecules, respectively.

configurations of the anions in the second shell of the La3+ ion are found, the former with two oxygen atoms of the anion pointing toward the cation, the latter with only one oxygen atom pointing toward La3+. As far as the Tf2N− anion is concerned, the large conformational freedom of this bulky anion results in a broad La−X g(r) second peak that extends over a large La−X distance range. Note that La−X distances below 6 Å are due to configurations where the anions approach the cation via the N atom, and not by means of interactions involving oxygen atoms. To give an idea of some possible anion orientations in the cation second coordination shell, we report in Figure 6 some representative MD snapshots extracted from the La3+ MD simulations. In these examples, one oxygen atom of the anion points toward the La3+ ion and it is coordinated with (i) two water molecules (magenta) belonging to the La3+ first hydration shell (see the TfO− example) or (ii) one water molecule (magenta) being in the La3+ first hydration shell (see the Tf2N− example). Conversely, in the example reported for the nitrate ion, two anion oxygen atoms point toward the central cation and they interact with two distinct water molecules belonging to the La3+ first hydration shell. Note that only three examples have been shown, one for each anion, but we have found the same three possible configurations for all of the three anions. Moreover, the anions can also be coordinated by one or two water molecules (cyan) being in the La3+second hydration shell. From a dynamic point of view, in the course of the simulations, the water molecules, as well as the anions, frequently enter and leave the Ln3+ second coordination shell. The exchange process rate has been evaluated by computing the mean residence times of both water molecules (t(2) WAT) and 3+ ) in the Ln second coordination sphere (see Table anions (t(2) X 2), by means of the Impey method.78 Following previous studies,67,69,79−85 a t* value of 0.5 ps has been chosen to define a real exchange in the Impey procedure. All of the calculated residence times are of the order of magnitude of picoseconds, thus revealing the flexible nature of the La3+ second solvation shell. The residence times of water molecules are quite similar for all of the solutions and comparable to the value calculated for the second hydration shell of La3+ in pure water. These values are also of the same order of magnitude of those calculated in a previous MD investigation of Ln3+ ion in water, where a polarizable force field has been employed.5 The residence times of anions are longer than those of water molecules, and the calculated values for triflate and Tf2N− are very similar and shorter than the residence time of nitrate ions. Dy3+ Salts. As already observed for La3+ salts, the second hydration shell of Dy3+ in the salt solutions resembles the

second hydration sphere found in pure water (Figure S9 of the Supporting Information), with a decrease of the number of water molecules due to the presence of counterions (see Table 1). The Dy−X g(r) second peaks calculated for the different salt solutions are very similar to the ones obtained for the La3+ ion, apart from a shift toward shorter distances as a consequence of the lanthanide contraction (Figure 2).2 The broad peak shape can be ascribed to the presence of several anion orientations in the Dy3+ second coordination shell, and a visual inspection of the Dy3+ MD simulations has shown that the possible configurations of the anions are similar to those found in the La3+ systems. The flexible nature of the Dy3+ second coordination sphere is reflected in the residence times of water molecules and anions in such shells that are found in the picosecond time scale (Table 2). As observed for the La3+ ion, the exchange process of water molecules is faster than the anion one, and among different anions, the residence time of nitrate is the longest one (57 ps). This value is in agreement with the results obtained in a MD study of Dy(NO3)3 in water.17 EXAFS Results. The MD simulations have shown that in dilute aqueous solutions of Ln3+ TfO−, Tf2N−, and NO3− salts the cation preferentially interacts with water molecules, and only a very small amount of TfO− and NO3− enters the first hydration shell. However, the counterions have been found to have no influence on the number of oxygen atoms coordinating the Ln3+ ions in the first solvation shell. In particular, both the Ln−O first shell distance and the geometry of the hydration complex is the same in all of the investigated salt solutions, and they are equal to the ones found in pure water. To verify the reliability of these results, we have collected XAS data of all of the investigated solutions. EXAFS is a short-range technique, and the structural information achievable is limited to the first coordination shell around the photoabsorber in the case of disordered systems and aqueous solutions. The La3+ and Dy3+ raw K-edge XAS spectra are shown in the top panels of Figure 7. As expected, they are identical independently from the nature of the counterion, thus confirming that the Ln3+ first hydration shell structure is not influenced by the counterion. These results are confirmed by the EXAFS spectra extracted with a three-segmented cubic spline, shown in the bottom panels of Figure 7. A definitive proof of the accuracy of the force fields used in the MD simulations has been gained by carrying out a quantitative analysis of the EXAFS spectra starting from the structural results obtained from the simulations. In particular, starting from the MD Ln−OW and Ln−HW g(r)’s, we calculated for each solution a total χ(k) theoretical signal using 2785


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Figure 7. Top panels: Ln K-edge X-ray absorption spectra of the 0.1 M Ln(TfO)3 (green), Ln(Tf2N)3 (blue), and Ln(NO3)3 (red) aqueous solutions. Bottom panels: corresponding EXAFS signals extracted with a three-segmented cubic spline. Figure 8. Upper panels: comparison of the EXAFS experimental data (dotted red line) of the 0.1 M Ln(TfO)3 aqueous solutions with the theoretical spectrum (solid blue line) calculated from the MD Ln− OW and Ln−HW g(r)’s. The residual curves are plotted in green. Lower panels: non-phase-shift-corrected Fourier transforms of the EXAFS experimental data (dotted red line) and of the theoretical signals (solid blue line).

eq 2, and we compared it with the experimental EXAFS signal. Minimization procedures have been performed in the range k = 2.4−14.0 Å−1. Note that the structural parameters have been kept fixed during the fitting procedures and in such a way the first solvation shell structure obtained from the simulations can be directly compared with experimental data. The results of these analyses for the La3+ and Dy3+ triflate solutions are shown in Figure 8, as an example, and the same results have been obtained for the other systems. In particular, from the top, the Ln−OW and Ln−HW theoretical two-body signals are shown together with the total χ(k) curve compared with the experimental spectrum and the residual curves. It is important to stress that in this analysis the structural parameters have been kept fixed to those obtained from the MD simulations and only the nonstructural parameters have been refined. The agreement between the experimental and theoretical data is quite good in all cases, even if it can be noticed the presence of an oscillation at k values higher than 10 Å−1 in the residual curve that is known to be due to a wrong shape of the radial distribution around the photoabsorber atom.40 A perfect agreement is obtained when all of the structural parameters are refined, as previously shown in the literature.13 The lower panels of Figure 8 show the non-phase-shift-corrected Fourier transforms of the EXAFS experimental data compared with the theoretical signals, and a quite good agreement is observed also in the reciprocal space. Note that the nice agreement between the theoretical and experimental data proves that the LennardJones interaction potentials developed for the Ln3+ ions in pure water40 are able to properly reproduce the solvation properties of the La3+ and Dy3+ ions also in aqueous solutions containing different counterions. Hydration Properties of Anions. Our results have shown that in lanthanide triflate, Tf2N−, and nitrate solutions very little cation−anion first shell association occurs. On the other hand, the anions are solvated by water molecules, as evidenced by the anion−water g(r)’s calculated from our MD simulations and reported in Figure 9. In particular, the top and bottom panels of the figure show the O (anion)−HW and O (anion)−OW g(r)’s, respectively. The structural parameters of the anion−

Figure 9. Anion−water radial distribution functions, g(r)’s, obtained from the MD simulations of 0.1 M La(TfO)3 (green), La(Tf2N)3 (blue), and La(NO3)3 (red) aqueous solutions: O (anion)−HW g(r)’s (top panel) and O (anion)−OW g(r)’s (bottom panel), where OW and HW are the oxygen and hydrogen atoms of the water molecules, respectively.

water g(r)’s are listed in Table S1 of the Supporting Information. As evident from Figure 9, the interactions between the anions and the water molecules are formed via the water hydrogen atoms. In particular, in all cases, the O (anion)−HW (water) g(r)’s show a first distinct peak at shorter distances as compared to the O (anion)−OW g(r) first maximum, and a second and less defined double peak at longer distances, indicating that anion−water hydrogen bonds are found in all of the aqueous solutions. The strongest anion−water interactions are formed with the nitrate ions, as evidenced from higher and 2786


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The Journal of Physical Chemistry B more defined anion−water g(r) first peaks. Note that the structural parameters of the nitrate−water g(r)’s obtained from our MD simulation are comparable with calculated86,87 and experimental88 data of NO3− in water. Additional insight into anion−water interactions can be gained by looking at the spatial distribution functions (SDFs) of water molecules surrounding the anions (Figure 10). Note

containing either La3+ or Dy3+, while depending on the nature of the anion and increasing in the order Tf2N− < triflate < nitrate (Table 2). Note that in order to define an anion−water hydrogen bond we have used as cutoff distances the first minima of the O (anion)−HW (water) and O (anion)−OW (water) g(r)’s listed in Table S1 of the Supporting Information and the same angle value cutoff used for water (O (anion)− OW−HW angle lower than 30°).89 One of the fundamental dynamical processes occurring in aqueous solution is the formation and breaking of hydrogen bonds which generally occurs in the sub-picosecond time scale in pure water. The hydrogen bond dynamics can be investigated by constructing a hydrogen bond population function h(t) which is unity when a tagged pair of water molecules is hydrogen bonded at time t according to an adopted definition, and zero otherwise. To investigate the hydrogen bond breaking dynamics, we calculated the time correlation function89

Figure 10. Spatial distribution functions (SDFs) of hydrogen (iceblue) and oxygen (magenta) atoms of water molecules around the TfO−, Tf2N−, and NO3− anions. Note that in the case of TfO− and Tf2N− the atoms of one anion sulfonyl group have been used to build the internal reference system for the SDF calculation and the rest of the molecular structure has been shown for clarity, only.

SHB(t ) = ⟨h(0)H(t )⟩/⟨h⟩

(2)

where H(t) = 1 if the tagged pair of water molecules, for which h(0) is calculated, remains continuously hydrogen bonded until time t and zero otherwise and ⟨···⟩ denotes an average over all pairs. SHB describes the probability that an initially hydrogen bonded pair remains bonded at all times up to t. The associated relaxation time is obtained as the time integrals of SHB and can be interpreted as the average lifetime of a hydrogen bond. The average water−water hydrogen bond lifetime calculated from our MD simulations for bulk water molecules is 0.7 ps. Here, it is interesting to investigate the influence of the Ln3+ ion on the hydrogen bond dynamics by calculating the lifetimes of the hydrogen bonds formed between the water molecules belonging to the first solvation shell of the Ln3+ ion and all of the other species, i.e., water molecules and anions (τ1). The τ1 values calculated from our MD simulations are listed in Table 2. For a given Ln3+ ion, the τ1 value obtained for the system containing only the Ln3+ ion increases as compared to the value of bulk water, and it increases significantly in the aqueous solutions containing also the triflate, Tf2N−, and nitrate counterions. Therefore, the hydrogen bonded pairs in the ionic solutions are more strongly held as compared to bulk water molecules. This result is in agreement with previous findings of two ab initio MD studies of Atta-Fynn et al.,21,22 where counterions such as Cl− and Br− were found to slow down the hydrogen bond dynamics in the first solvation shell of Th4+ and Cm3+. Moreover, the τ1 values obtained for the triflate, Tf2N−, and nitrate solution of a given Ln3+ ion are almost the same, indicating that the different counterions have the same impact on the hydrogen bond lifetimes. On the other hand, if we compare the results of La3+ and Dy3+ systems, we see that the hydrogen bond lifetimes in the Dy3+ solutions are higher than those of the La3+ ones, and this stronger slow-down effect could be due to the higher charge density of the Dy3+ ion as compared to La3+. Finally, we have computed the lifetimes of the hydrogen bonds formed between the anions and all of the water molecules (τ2) that are listed in Table 2. The τ2 values show a strong dependence on the counterion nature, as expected, with the longest and shortest lifetimes found for nitrate and Tf2N−, respectively, reflecting the different strengths of the hydrogen bonds formed by the different anions. Note that the τ2 values are identical in the solutions containing either La3+ or Dy3+.

that these functions have been calculated in an internal reference system integral with the nitrate plane for the nitrate ion and integral with one sulfonyl group for the triflate and Tf2N− anions. The SDFs of water hydrogen and oxygen atoms are shown as iceblue and magenta surfaces, respectively. For all three anions, the water−anion association is mainly driven by interactions of water hydrogen atoms with the anion oxygen atoms, remarking the ability of the anions to form hydrogen bonds with water. Interestingly, the water molecules interacting with the Tf2N− anion are found along the anion S−O direction, while, in the case of the TfO− and NO3− ions, the hydrogen and oxygen atoms are found in torus-shaped distributions around the S−O and N−O vector directions, respectively. Water−Water and Anion−Water Hydrogen Bonds. In all of the aqueous solutions containing the Ln3+ ions and triflate, Tf2N−, and nitrate counterions, a network of hydrogen bonds is formed among the water molecules and also between the water molecules and the anions. To carry out the analysis of the water−water hydrogen bonds, we adopted a configurational criterion where two water molecules are hydrogen bonded only if their interoxygen distance is shorter than 3.5 Å and simultaneously the hydrogen−oxygen distance is shorter than 2.45 Å and the oxygen (acceptor)−oxygen (donor)−hydrogen (donor) angle is lower than 30°.89 The average number of water−water hydrogen bonds per solvent molecule (n1HB) calculated from our MD simulations is identical in all of the systems containing either La3+ or Dy3+ ions and the different counterions (see Table 2). The value is slightly lower than that calculated in the simulations of Ln3+ in pure water, but if we calculate the average number of hydrogen bonds per solvent molecule by including both water−water and anion−water hydrogen bonds (n2HB), we obtain exactly the same results in the solutions with and without counterions. This result indicates that the water molecules tend to form always the same number of hydrogen bonds with the surrounding species, namely, with other water molecules or triflate, Tf2N−, and nitrate anions. Conversely, the average number of anion−water hydrogen bonds per anion (n3HB) is rather similar in the solutions 2787



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DISCUSSION AND CONCLUSIONS

Article

ASSOCIATED CONTENT

S Supporting Information *

In this work, we have carried out a combined MD and EXAFS investigation of dilute aqueous solutions of Ln3+ (La3+ and Dy3+) salts with the triflate, nitrate, and Tf2N− anions. The results of our MD simulations highlighted the different complexing abilities of the three counterions: Tf2N− does not form inner-sphere complexes, while a small amount of triflate coordinates both the La3+ and Dy3+ cations in their first solvation shell. On the other hand, the nitrate ion has a different behavior in La3+ and Dy3+ salt solutions, as it is almost absent in the La3+ first solvation shell, while forming contact ion pairs with Dy3+. In all of the investigated systems, the La3+ and Dy3+ cations preferentially interact with the water molecules and the counterions have no influence on the geometry of the Ln3+ solvation complex formed in solution. The total number of oxygen atoms coordinating the Ln3+ cations in their first solvation shell is the same in all of the solutions, regardless of if they belong to water molecules or to the counterion, suggesting that the cation coordination is mainly driven by electrostatic forces. These theoretical findings have been confirmed by comparison with the Ln K-edge XAS experimental data. Moreover, a quantitative analysis of the EXAFS spectra of the systems has been carried out starting from the structural results obtained from the simulations. The very good agreement found between theory and experiment has allowed us to definitely assess the reliability of the force field employed in the MD simulations. It is important to stress that determining the complexing ability of different counterions in solution is in general a very difficult task. As concerns the triflate ion, our results are at variance with previous NMR or Raman scattering spectroscopy studies which concluded that triflate does not form ion pairs with the Ln3+ ions.18,19,42 Conversely, our results show that a very small amount of triflate enters the La3+ and Dy3+ first coordination sphere (Ln−triflate coordination number of 0.2). It is important to stress that the combined use of the EXAFS technique and MD simulations allowed us to provide direct structural information on the Ln3+ first solvation shell in solution. As concerns the choice of the simulation method, among different theoretical approaches, classical MD is the most proper choice to afford the long simulation time that is required to accurately sample the phase space visited by these systems. Indeed, the ion pair dynamics can be very slow, as shown, for example, in the case of the Dy3+ nitrate salt in which the anion exchange processes can be observed only in long simulation time length (tens of nanoseconds). The use of ab initio simulations to predict the complexing ability of anions could therefore result in erroneous outcomes due to the limited simulation times (tens of picoseconds) achievable using these computationally demanding methods. Besides the investigation of the Ln3+ solvation structure and dynamics, we have also studied the hydration properties of the anions and the water−water and water−anion hydrogen bond dynamics, showing that the hydrogen bonded pairs are more strongly held in the solutions as compared to pure water, with the hydrogen bond lifetimes being higher in the Dy3+ sytems as compared to the La3+ ones. In conclusion, the structural and dynamic properties of La3+ and Dy3+ triflate, Tf2N−, and nitrate aqueous solutions are dominated by electrostatic effects in which the differences found between La3+ and Dy3+ are mainly due to the higher charge density of the heavier ion.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b12571. Figures showing the time evolution of the distances, radial distribution functions, and CDFs and table showing structural parameters (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Valentina Migliorati: 0000-0003-4733-6188 Alessandra Serva: 0000-0002-7525-2494 Andrea Lapi: 0000-0001-9728-8132 Paola D’Angelo: 0000-0001-5015-8410 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the University of Rome “La Sapienza” (Progetto ateneo 2015 nos. C26H159F5B and C26N159PNB) and by the CINECA supercomputing center through the grant IscrC_DESTINIS (no. HP10CZTDIS).

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REFERENCES

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Influence of Counterions on the Hydration Structure of Lanthanide Ions

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