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Structural Determination of Ionic Liquids with Theoretical Methods: C8mimBr and C8mimCl. Strength and Weakness of Current Force Fields E. Bodo,*,† L. Gontrani,‡ A. Triolo,§ and R. Caminiti† †
Department of Chemistry and CNISM, University of Rome “Sapienza”, Italy, ‡Department of Chemistry, University of Rome “Sapienza”, Italy, and §CNR-Istituto di Struttura della Materia, Area della Ricerca di Roma 2, Tor Vergata, Rome, Italy
ABSTRACT In this work we report the first X-ray scattering study of imidazoliumbased ionic liquids containing the bromide anion. The system studied was 1-octyl3-methyl-imidazolium bromide ([C8mim]Br). The study was extended to the analogous salt, containing chloride as anion ([C8mim]Cl) which has been used for comparison. The measured diffraction patterns are compared with the theoretical spectra calculated from model geometries obtained with classical molecular dynamics simulations. The behavior and the performance of the available force fields in the description of bromide ion are discussed. SECTION Macromolecules, Soft Matter
onic liquids (ILs)1,2 with low volatilities and low melting points have represented in the past 10 years a rich field of research because of their many applications in diverse technological research areas. The large variety of applications includes their use as a replacement of organic solvents,3 lubricants,4 ingredients for pharmaceuticals,5,6 and reaction media.7-9 Their flexibility depends on the fact that their properties can be tuned by varying the substituents on the cationic partner and by a careful choice of the anion counterpart. In general, the ILs are very stable, have a good electrical conductivity, a high ionic mobility, and a negligible vapor pressure. Despite the very large number of experimental characterizations of ILs,10-14 a robust theoretical framework able to make reliable predictions of the physical properties of ILs is still missing. Recently, the sophistication of molecular dynamics (MD) simulation techniques along with the increase in computer power have reached the point of being able to describe IL from an atomistic point of view (see ref 2 and references therein). One of the most important advances in the theoretical description of ILs has been the construction of a general force field that would be applicable to a large number of ILs.15-24 These force fields have been able to account for several interesting properties of ILs such as densities, diffusion coefficients, melting points, and heat capacities. However, the agreement with experimental observables is often scattered, and, only in very few instances, a particular force field has been able to accurately describe more than one property at the same time such as density, heat of vaporization, transport, and viscous properties.25,26 Instead of focusing on bulk properties, which would allow only for an indirect validation of the force field, here we shall directly compare the theoretical results with the X-ray diffraction data, which somewhat allow a direct “observation” of the
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molecular structures in the liquid phase along the same lines of our previous work on other ILs (see, for example, ref 27). 1-Octyl-3-methyl-imidazolium bromide (C8H15BrN2 or [C8mim]Br, CAS number 61545-99-1) and 1-octyl-3-methylimidazolium chloride (C8H15ClN2 or [C8mim]Cl, CAS number 64697-40-1) were purchased from Iolitec.28 After a 24-h drying in vacuum at 60 °C, in order to reduce the moisture content, the samples were rapidly put into a cell sealed with Mylar windows. Sample and windows thicknesses were 3 mm and 6 μm, respectively. The large angle X-ray scattering (LAXS) experiments were conducted at room temperature using the noncommercial energy-scanning diffractometer built in the Department of Chemistry, Rome University “Sapienza” (Patent No. 01126484, 23 June 1993, Caminiti, R. et al.).29,30 White Bremsstrahlung radiation emitted by a tungsten tube (50 kV, 40 mA)was used. The expression for the scattering variable s (transferred momentum) is s ¼
4π sin θ =E 3 1:014 sin θ λ
ð1Þ
where E is expressed in kilo-electron volts, and s is in inverse angstroms. The measurement protocol, i.e., the number of angles and the measuring time needed to obtain spectra with a high signal-to-noise ratio, was different for the two samples, owing to the very different radiation absorption of the two halide anions in the energy range used. While chlorine is relatively transparent, in fact, bromine is heavily absorbent, and the energy interval usable is thus narrower. For [C8mim]Br, scattered intensities for the sample were measured at eleven Received Date: February 3, 2010 Accepted Date: March 5, 2010 Published on Web Date: March 15, 2010
1095
DOI: 10.1021/jz100146r |J. Phys. Chem. Lett. 2010, 1, 1095–1100
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different angles (24.0, 15.5, 10.4, 8.0, 5.0, 3.0, 2.0, 1.5, 1.0, 0.8, and 0.5 degrees), while for [C8mim]Cl only seven different angles (24.0, 15.5, 8.0, 3.0, 2.0, 1.0 and 0.5) were used. In both experiments, we were able to cover a wide range of s, namely between 0.12 and 19.56 Å-1, and to obtain a minimum of 300 000 counts per experimental point for s < 5 Å-1 and 600 000 counts for s g 5 Å-1. The primary beam intensity I0(E) was measured directly, by reducing the tube current to 10 mA without the sample. Transmission of the sample was measured under the same conditions. Both quantities are needed to carry out the necessary absorption corrections to experimental data. The ultrathin Mylar cell windows contribution to the diffraction intensity is less than 1/10000 of the total. After correction of experimental data for escape peak suppression,31 the various angular data were normalized to a stoichiometric unit of volume containing one ion pair and merged to yield the total static structure factor, I(s), in momentum-space. I(s) is equal to n X xi fi 2 ð2Þ IðsÞ ¼ Ie:u: -
fixed, and an additional mixing was provided by a short (10 ps) NVT run at 500 K where the electrostatic interactions were turned off. The resulting configurations were left to evolve in a constant volume NVT production runs of about 2 ns with T = 300 K after an equilibration period; • in the second kind of trajectory calculation, the initial configurations where allowed a full equilibration in a NPT simulation at 1 atm and 300 K for about 1 ns before again performing the mixing at 500 K and the NVT 2 ns production run at room temperature. The difference between the two approaches is that, in the former, we have the system constrained to the experimental density, while in the latter we allow the system to relax geometrically to its theoretical density. In the case of chlorine, we will see that the two densities are indeed very similar while they differ significantly for bromine. All the bonds involving the hydrogens have been kept constrained using the Shake algorithm, while no other degrees of freedom were restricted. The system was treated in a periodic box of around 50 Å containing 358 ionic couples, in order to try to reproduce the bulk behavior. Nonbonded interactions were computed up to 9 Å. In order to test the quality of this choice, for both anions, we run few hundreds of additional picoseconds with the cutoff value set to 12.0 Å. We found the results to be largely insensitive to this choice. The long-range electrostatic interactions were accounted for by means of the smoothed particle mesh Ewald (SPME) solver with an accuracy of 1 in 10-6. The integration time step for the leapfrog algorithm was of 2 fs in the production run (1 fs for the high temperature calculations in order to ensure convergence of the shake algorithm), and a snapshot of the configuration was saved every 20 ps in order to be used in the geometric analysis. Each production run produced few hundred snapshots that were then employed in the calculation of the structure function following the Debye's scattering equations:36 XX sinðsrij Þ IðsÞ ¼ fi fj xi xj ð5Þ srij i j
i ¼1
where fi are the atomic scattering factors, xi are the number concentrations of i-type atoms in the stoichiometric unit, and Ie.u. is the observed intensity in electron units. The function was multiplied by s and by an s-dependent sharpening factor, M(s), whose expression is fN 2 ð0Þ expð -0:01s2 Þ ð3Þ MðsÞ ¼ 2 fN ðsÞ having chosen nitrogen as the sharpening atom. Fourier transformation of I(s) led to radial distribution functions (RDFs) in distance-space, D(r): Z 2r smax sIðsÞMðsÞ sinðrsÞds ð4Þ DðrÞ ¼ 4πr2 F0 þ π 0 In this equation, F0 is the bulk number density of stoichiometric units. We used the value of 19.56 Å-1 (last experimental point) as the upper limit of integration. Subtraction of the 4πr2F0 term, corresponding to uniform distribution, from this formula, allows the isolation of the structural contribution to the distribution function (Diff(r)). This formulation of RDF is more suited to point out intermolecular (medium-long-range) interactions than the more commonly used g(r). Detailed description of procedures and formulas used can be found in refs 31-33. All-atom MD simulations were carried out on pure [C8mim]Br and [C8mim]Cl ILs using the DLPOLY234 package. The force field and the parameters for the ILs were taken from the papers of Canogia-Lopes and P adua.16,35 The initial configurations were generated by randomly distributing the 358 ionic couples in very large simulation cells. A short isobaricisothermal (NPT) run at 500 K and with 10 Katm pressure was carried out in order to provide dense mixed initial configurations. These initial configuration where then used in two different production runs: • the initial configurations were left to relax during a short NPT run at 1 atm and 300 K to the point ar which they reached the experimental density. The volume was then
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where fi are the atomic scattering factors and xi are the number concentrations of i-type atoms in the stoichiometric unit. From I(s) it is possible to extract the radial distribution function D(r) by means of eq 4. It is important to point out that the very low s range (s
