J. Phys. Chem. C 2009, 113, 18233–18243
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Polyoxometalate Keggin Anions at Aqueous Interfaces with Organic Solvents, Ionic Liquids, and Graphite: a Molecular Dynamics Study Alain Chaumont and Georges Wipff* Laboratoire MSM, UMR CNRS 7551, Institut de Chimie, 4 rue B. Pascal, 67 000 Strasbourg, France ReceiVed: June 12, 2009; ReVised Manuscript ReceiVed: July 21, 2009
A recent computational study (Chaumont, A; Wipff, G. Phys. Chem. Chem. Phys. 2008, 10, 6940) showed that, in spite of their high charge and spherical shape, polyoxometalate R-PW12O403- Keggin anions PW3can “attract each other” and form oligomers in water. This led us to speculate that these anions should also display surface-active properties, a feature supported by molecular dynamics results presented here at aqueous interfaces with an organic solvent (chloroform), with three ionic liquids (ILs) and a solid (graphite). Simulations have been performed with 20 PW3- per box and, in some cases, with different Mn+ counterions (Cs+, H3O+, H5O2+, NBu4+, Eu3+). At the chloroform interface, the highest PW3- activity (90-100%) is found with NBu4+ counterions, while with all other counterions, it is ca. 50%. The results obtained in “standard conditions” (juxtaposed solvent boxes simulated with additive potentials and TIP3P water) are confirmed with tests using either bigger boxes, SPC/E water, or a polarizable force field and with mixing/demixing experiments. PWs are also attracted at aqueous interfaces with [XMI][Y] ionic liquids, with marked effects of their constitutive XMI+ (butylmethylimidazolium BMI+ or octylmethylimidazolium OMI+) and Y- (PF6- or Tf2N-) ions. The most spectacular result is found with the [OMI][PF6] IL where nearly all PWs adsorb at the interface, attracted by interfacial OMI+ cations, while PF6- anions solubilize in the bulk water (“anion exchange mechanism”). PWs also adsorb onto the neutral graphite surface, without forming a saturated monolayer, though. As expected, when the graphite surface gets positively charged, PW adsorption is enhanced. These results have bearing on the supramolecular organization and reactivity of polyoxometalate anions at surfaces, as well as on the mechanism of ion exchange between water and ILs. Introduction Polyoxometalate anions (POMs) represent an important class of polynuclear metal-oxygen clusters, with remarkable molecular tunability that has made them useful probes of fundamental structural issues in chemistry, such as catalysis, electron transfer in solution and at metal oxide interfaces, self-assembly, magnetism in multicomponent systems, and association with biological systems.1 Many of their applications (catalysis,2 electrochemistry,3 corrosion protection,4 metallic patterning,5 and lithography6) involve at some stage solutions in contact with surfaces. An in-depth understanding of such complex multistep processes requires an analysis of the composition and structure of POMs, their solvation properties, and supramolecular organization. Computer simulations contributed to a detailed analysis of the structure of POMs and of their solvation. Features like electronic structure, intrinsic oxygen basicity have been studied by quantum mechanical calculations on the isolated ion,7 whereas solvation features have been recently investigated by molecular dynamics (MD) simulations with explicit representation of the solvent.8 Hydration, dynamic properties and ion pairing of [XW12O40]n- anions (X ) P/Si/W) with different alkali counterions “at infinite dilution” have been recently studied by MD.8,9 Other MD simulations revealed that in more concentrated conditions, Keggin R-PW12O403- anions (hereafter noted PW3- or PW; see Figure 1) can display solvent-induced specific supramolecular arrangements, with marked Mn+ coun* To whom correspondence should be addressed. E-mail: wipff@chimie. u-strasbg.fr.
Figure 1. Simulated PW3- anion (PW12O403-; W atoms in blue; O atoms in red).
terion effects.10 In spite of their mutual repulsions, they have been shown to display short contacts, forming dimers or even higher oligomers in water. Marked aggregation was observed with hydrophobic ammonium NBu4+ counterions or in “acidic conditions”, i.e., when neutralized by H3O+ or H5O2+ counterions. The specific effect of water was highlighted by comparing the Mn+ · · · PW3- and PW3- · · · PW3- relationships in water versus methanol solutions.10 Furthermore, it was speculated that PWs, in spite of the high charge and nearly spherical shape should display some propensity to adsorb at aqueous interfaces.10 In this paper, we investigate this issue by simulating solutions of PW3- (20 PW3- per box, corresponding to aqueous concentrations of ca. 0.02 mol L-1) neutralized by different types of Mn+ counterions as in ref 10 (Cs+, NBu4+, H3O+, H5O2+, Eu3+)
10.1021/jp905518p CCC: $40.75 2009 American Chemical Society Published on Web 09/25/2009
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with the main aim to analyze their distributions at interfaces. In fact, the nature of counterions can be critical with respect to POMs features such as acidity, solubility, porosity, and thermal stability and plays an important role in processes like polymerization, assembling,11 catalytic properties.12 We thus want to investigate to what extent counterions affect the PWs distribution near aqueous interfaces. As apolar phase in contact with water we successively consider an organic solvent (chloroform, noted CLF), hydrophobic ionic liquids (ILs), and a solid (graphite). While there have been many experiments and simulations involving CLF/ water interfaces, studies on interfaces with ionic liquids ILs are still in their infancy.13 We thus decided to select three ILs, based on common cations and anions. The ILs will be noted [XMI][Y] to specify the nature of their constitutive XMI+ cations (butylmethylimidazolium (BMI+) or octylmethylimidazolium (OMI+)) and Y- anions (PF6- or Tf2N- ) (CF3SO2)2N-)). Insights into the effect of the IL’s anion will be obtained by comparing [OMI][Tf2N] to [OMI][PF6], whereas the effect of IL’s cation will be studied by comparing [BMI][Tf2N] to [OMI][Tf2N]. Because of computer time limitations, PWs were simulated at the water/ILs interfaces only with hydrophilic monovalent counterions (Cs+, H3O+, and H5O2+) and with Eu3+ in the case of [OMI][PF6] (see the list of simulated systems in Table 1). Finally, given the analogy between apolar “oil” surfaces and the surface of graphite, and because POMs are known to condense at the graphite surface,14 we decided to also investigate PWs (neutralized by H3O+ counterions) at the water/ graphite interface, comparing three different charge models for the surface carbon atoms of graphite. Methods The systems were simulated by classical MD using the AMBER 7.0 software15 in which the potential energy, U, is described by a sum of bond, angle, and dihedral deformation energies and pairwise additive 1-6-12 (electrostatic + van der Waals) interactions between non bonded atoms:
Chaumont and Wipff
U)
∑ kb(r - r0)2 + ∑ kθ(θ - θ0)2 +
bonds
angles
∑ ∑ Vn(1 + cos(nφ - γ))
+
∑ i
