Elsevier

Journal of Molecular Structure

Volume 1148, 15 November 2017, Pages 381-387
Journal of Molecular Structure

Structural and energetic properties of La3+ in water/DMSO mixtures

https://doi.org/10.1016/j.molstruc.2017.07.068Get rights and content

Highlights

  • The structure of a mixed solvent (water:DMSO) around the Lanthanum (III) ion is analyzed by MD simulations.

  • Specific force field parameters for the La(III) ion are tested and validated.

  • DMSO appears to be the dominant specie in the first shell.

Abstract

By using molecular dynamics based on a custom polarizable force field, we have studied the solvation of La3+ in an equimolar mixture of dimethylsulfoxide (DMSO) with water. An extended structural analysis has been performed to provide a complete picture of the physical properties at the basis of the interaction of La3+ with both solvents. Through our simulations we found that, very likely, the first solvation shell in the mixture is not unlike the one found in pure water or pure DMSO and contains 9 solvent molecules. We have also found that the solvation is preferentially due to DMSO molecules with the water initially present in first shell quickly leaving to the bulk. The dehydration process of the first shell has been analyzed by both plain MD simulations and a constrained dynamics approach; the free energy profiles for the extraction of water from first shell have also been computed.

Introduction

Lanthanoid (III) (Ln3+) together with Actinoid (III) ions represent a unique opportunity in chemistry because of the uniformity of their physico-chemical properties along the series and of their oxidation states [1]. The Lanthanoid ions typical charge in solution is +3 and, given their relatively small ionic radii, when their salts are dissolved in a polar solvent, a well structured solvation shell is formed whose shape and size depend on the ion size and the polarity and bulkiness of the solvent [2], [3]. The Ln3+ ions in solution have been the subject of much experimental and theoretical investigation and researchers have engaged in the study of how the solvation properties vary when moving from light to heavy elements in series [1], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14].

In this respect, the molecular simulations of their behavior in bulk solvents represent an extremely useful tool to understand the ion-solvent interaction from a nanoscopic point of view and they provide many details toward which experiments are blind. This has certainly been the case, for example, for the hydration of Ln3+ ions where the complementarity of the information provided by theory and experiments has allowed a very detailed understanding of the structure and of the dynamics of their hydration shells [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Other spectroscopic properties such as UV/Vis absorption or NMR chemical shift can be obtained from the MD trajectories by cleverly selecting an appropriate ensemble of conformation using suitable algorithms such as those reported in Refs. [29], [30], [31].

While hydration of Ln3+ ions has been explored by many experimental and theoretical studies [1], the solvation in organic solvents remains much less investigated despite the fact that these studies are crucial for the evaluation of possible technological routes to the separation and recovery from waste of these (sometimes very) expensive elements. Dimethylsulfoxide (DMSO) as a solvent for Ln3+ ions has been investigated experimentally in Refs. [13], [14] and in the presence of additional ligand molecules as, for example, the complexation of Ln3+ ions by different polyamines [32], [33], [34], [35].

A recent experimental determination based on EXAFS measurements [13], [36] has suggested that the DMSO forms a regular shell of eight oxygen atoms surrounding the central ion for all the lanthanoid (III) series. This eight-fold coordination motif recurs in the solids as well in the solutions and it is reminiscent of what had been found in water [9], [14], [32]. Anyway, the coordination state of Ln3+ ions in DMSO solution is still controversial, since EXAFS has a ±1 accuracy in determining the coordination numbers and, in addition, other studies have reported evidence for changes in the average coordination number along series [33], [37].

Recently we engaged in the theoretical description of the solvation features of lantanoids ion in pure DMSO using both a gas phase ab initio approach [38] and a molecular dynamics (MD) one [39]. We have shown that, very likely, the first coordination shell is made by a complex environment where a structure of eight first shell ligands coexists with a structure of nine, the latter being substantially iso-energetic with the former. By using a polarizable potential model, obtained by matching accurate ab-initio data, we have shown that the coordination number does actually vary along the series although in a much more limited way with respect to water, passing from 8.5 to 8 when going from La3+ to Lu3+. MD simulations, despite the fact that their reliability is bound to the goodness of the model potentials, are one of the most efficient approaches to provide details that experimental measurements are not able to grasp as, for example, the nanoscopic picture of ion solvation in which thermal coexistence between two (or more) coordination numbers takes place [40], [41], [21].

The great majority of the cited studies above have dealt with pure solvents. The necessary step in order to study the processes related to separation and recovery of these ions necessarily requires the study of their solvation in mixtures of solvents. This is the task that we have undertaken in this work where we have studied, as far as we know for the first time, the solvation of La3+ ions in an equimolecular mixture of water and DMSO. DMSO-water mixtures have been a very active field of research because of their unconventional properties [42], [43], [44] due to the local nanoscopic structuring of water and DMSO into sub-domains whose formation is, very likely, driven by hydrogen bonding formation and dynamics.

In recent works [45], [38] we have shown how an approach based on the accurate DFT study of small clusters of organic solvent containing the heavy ion not only is able to obtain information on the structural properties of first hydration shell, but it is also a natural testing playground for the development of force field parameters. A variant of the Amoeba [46] force field which includes Van der Waals (VdW) parameters for the Ln3+ ions has been recently developed by us [38], [39]. In particular the ab-initio geometries and energies of the [La (DMSO)n]3+ clusters have been used to tailor a specific value for the VdW coefficients of the ion. The resulting force field has been successfully used to characterize the solvation shell of the ions in bulk DMSO. Here we extend those studies to the water/DMSO mixture.

Section snippets

Methods

The force field we have used here is the Amoeba [46] one with the modifications reported in our work on the solvation of La3+ in DMSO [39]. The force field has been validated again in this work by using it to compute the interaction energies and the geometries of the La[H2O]8[DMSO]83+ cluster which is representative of the solvation environment we expect to find in the 1:1 solution. The results and details of this validation are reported in the Supporting Information. Due to the nature of the

Results and discussion

In this section we present the results that we have obtained in the liquid phase. Various molecular dynamics simulation of La3+ ion in an equimolecular mixture of DMSO and H2O molecules have been performed and trajectories of different time lengths have been collected. The longest simulation has a production time of about 20 ns and was prepared by equilibrating a box where 50 water molecules and 50 DMSO molecules were placed randomly. In Fig. 1 we report the radial distribution function (rdf)

Conclusions

In this work we have analyzed the solvation process of La3+ ion in a mixture of DMSO and water. We have found that the first solvation shell dehydrates and that the preferred solvent in the first shell is almost exclusively DMSO even of the two solvents have similar interaction energies. By performing several simulations using different initial conditions we have concluded that, even though we start from a hydrated first shell of solvation, a dehydration process ultimately takes place leaving

Acknowledgement

EB gratefully acknowledges grants C26H15FBBC and C26A142SCB from “La Sapienza” and grants AAOX (IsC44) and POLIL (IsC36) from Cineca. RS thanks the French National Research Agency (ANR) on project ACLASOLV (ANR-10-JCJC- 0807-01) for partial support.

References (51)

  • C. Cossy et al.

    A change in coordination number from nine to eight along the lanthanide(III) aqua ion series in solution: a neutron diffraction study

    New J. Chem.

    (1995)
  • D. Lundberg et al.

    Structural study of the n,n′-dimethylpropyleneurea solvated lanthanoid(III) ions in solution and solid state with an analysis of the ionic radii of lanthanoid(III) ions

    Inorg. Chem.

    (2010)
  • A. Abbasi et al.

    Crystallographic and vibrational spectroscopic studies of octakis(dmso)lanthanoid(III) iodides

    Inorg. Chem.

    (2007)
  • P. D'Angelo et al.

    Analysis of the detailed configuration of hydrated lanthanoid(III) ions in aqueous solution and crystalline salts by using K- and l-3-edge XANES spectroscopy

    Chem. Eur. J.

    (2010)
  • I. Persson et al.

    Hydration of lanthanoid(III) ions in aqueous solution and crystalline hydrates studied by exafs spectroscopy and crystallography: the myth of the gadolinium break

    Chem. Eur. J.

    (2008)
  • P. D'Angelo et al.

    Revised ionic radii of lanthanoid(III) ions in aqueous solution

    Inorg. Chem.

    (2011)
  • P. D'Angelo et al.

    K-edge xanes investigation of octakis(dmso)lanthanoid(III) complexes in dmso solution and solid iodides

    Phys. Chem. Chem. Phys.

    (2013)
  • I. Persson et al.

    X-ray absorption fine structure spectroscopic studies of octakis(dmso)lanthanoid(III) complexes in solution and in the solid iodides

    Inorg. Chem.

    (2007)
  • A. Villa et al.

    Dynamics and structure of Ln(III)-aqua ions: a comparative molecular dynamics study using ab initio based flexible and polarizable model potentials

    J. Phys. Chem. B

    (2009)
  • T. Kowall et al.

    Molecular dynamics simulations study of lanthanides ions Ln3+ in aqueous solutions. Analysis of the structure of the first hydration shell and of the origin of symmetry fluctuations

    J. Phys. Chem.

    (1995)
  • T. Kowall et al.

    Molecular dynamics simulations study of lanthanides ions Ln3+ in aqueous solutions including water polarization. Change in coordination number from 9 to 8 along the series

    J. Am. Chem. Soc.

    (1995)
  • F.M. Floris et al.

    A study of aqueous solutions of lanthanide ions by molecular dynamics simulation with ab initio effective pair potentials

    J. Chem. Phys.

    (2001)
  • C. Clavaguéra et al.

    Molecular dynamics study of the hydration of lanthanum(III) and europium(III) including many-body effects

    J. Phys. Chem. B

    (2005)
  • C. Clavaguera et al.

    Theoretical study of the hydrated Gd3+ ion: structure, dynamics, and charge transfer

    J. Chem. Phys.

    (2006)
  • M. Duvail et al.

    A dynamical model to explain hydration behaviour trough lanthanide series

    ChemPhysChem

    (2008)
  • 1

    Present Address: Leibniz-Institut für Polymerforschung Dresden e.V. (Leibniz Institute of Polymer Research Dresden), Dresden.

    View full text