ISBN 978-1-84626-081-0 Proceedings of the 2010 International Conference on Application of Mathematics and Physics Volume 1: Advances on Space Weather, Meteorology and Applied Physics Nanjing, P. R. China, May 7-9, 2010, pp339-344 The micro-properties of [hmpy+] [Tf 2 N-] Ionic liquid: a simulation study Guo Y.C. 1, Chen J.W. 1, Liu H.L. 2, Liu W.W. 1, Lu H.Y. 1, Zhu X.B. 1 and Gu B. 1 1 College of Math and Physics, Nanjing University of Information Science and Technology, Nanjing 210044; 2 School of Atmospheric Physics, Nanjing University of Information Science and Technology, Nanjing 210044; Abstract. The micro-properties of [hmpy+][tf2n-] ionic liquid at room temperature (300K) and 1atm pressure are studied with molecular dynamic simulations. The radial distribution functions and three-dimensional distributions between different particle pairs are shown for the view of structural properties, while the mean square displacements of different ions, which are proofs of its liquid state, are also studied in detail. Our results are consistent with some experimental data. This work provides the basis for our theoretical study on the capability of ionic liquid as promising catcher and remover of pollutions in atmosphere. Keywords: [hmpy+][tf2n-] ionic liquids, molecular dynamic simulation, microscopic properties, atmospheric contaminants remover and catcher 1. Introduction The pollution of atmosphere has become a global environment topic. Petroleum, coal and natural gas have been used as the primary global fuel and will continue to be widely used in the following years. [1,2] Regarded as the cleanest one of these fuels, the natural gas will be consumed at an accelerating pace, but the problem of the undesirable gases created by fuels will be extremely serious, especially greenhouse gases such as CO2 and atmospheric contaminants (CO 2, NOx, SO2, H2S, etc.). In recent years, ionic liquids (ILs) have been used to capture these undesirable gases in the atmosphere. Room-temperature ionic liquids are useful because a variety of organic and inorganic molecules are able to be dissolved in them. [3-5] The environmental friendly properties of the liquids also make them useful as a substitute for other more damaging organic solvents. As the structures of ILs are designable, there are many factors which are relevant for the capability of ILs with particular cation or anion. Computer simulation is a prefer method for the study of the properties of ILs, as some detail views of the micromechanics of their functions can be derived form computer simulations before they been designed in lab. In this paper, we study the micro properties of one typical IL which consists the catio n of 1-n-hexyl-3-methylpyridinium ([hmpy+]) and anion of bis(trifluoromethanesulfonyl)imide ([Tf2N-]) at room temperature (300K) and 1atm pressures with molecular dynamic simulations. Corresponding author. Tel.: + 86-25 5873 1160. E-mail address: gubin@nuist.edu.cn 339
2. Computational methods 2.1 System and Model Fig. 1 present a pair of [hmpy+][tf2n-] (left) and a snap of the system which is studied in this work and contains 216 ion pairs with a cubic periodic boundary condition (right). Fig. 1 The structure of one cation and anion of [hmpy+][tf2n-] (left) and one snap of the system study in this work(rignt). In our study, the intra-molecular potential energy is calculated with the amber potential in the following form: 2 2 b eq a eq 0.5 t1 cos( - ) V K rr K K mt t bonds angles torsional 12 6 4 / r / r qq / r i j ij ij ij ij ij i j ij (2.1) It contains not only bonded, angle, torsion terms but also Lennard-Jonse(L-J) interaction and electrostatic Coulomb interactions between atom pairs separated more than two bonds. The inter-molecule interactions include L-J and Coulomb interaction. The potential parameters developed by Cesar Cadena [6] are adopted in this work. 2.2 Simulation Details The following procedures were used to in our simulations. Before the bulk IL was constructed, a standard steepest descent and conjugate gradient energy minimization procedure were applied to relax the [hmpy+][tf2n-] pair. Then, 216 optimized ion pairs were randomly placed in a big cubic box (low-density) with periodic boundary conditions. Anti-bumping constraints were used to prevent ions from overlapping. Next, the density of the system was equilibrated by carrying out a 500ps isobaric-isothermal (NPT) simulation. Following this, a 1ns canonical (NVT) simulation was conducted. Finally, microcanonical ensemble runs were performed for 2ns. All of these runs were made at temperatures ranging from 297 to 305K. The last 1ns NVE simulations were taken as production data. The properties of the system are analyzed from 200ps to 800ps of the trajectory. In all simulations, electrostatics was handled by means of a Particle-Mesh Ewald (PME) method. Nose-Hoover temperature coupling was used for the temperature control and 340
Parrinello-Rahman pressure coupling was used for the pressure control. Long-range electrostatic interactions were treated in the same way. The barostat oscillation time and damping factors were 1ps. The thermostat damping factor was 0.1ps. All our simulations were performed with the MD software tools of GROMACS. [7-8] 3. Results and Discussion 3.1 Structural Properties The density of simulated [hmpy+][tf2n-] IL is 1.26g/cm³, which is close to the experimental value at the temperature of 300K and under the pressure of 1atm. [6] Various site-site radial distribution functions (RDFs) were computed to obtain insight into the local organization of the ionic liquid. Fig. 2 shows the center-of-mass RDFs for cation-cation, anion-anion, and cation-anion interactions. Fig. 2 Radial distribution functions between the centers of mass of different kinds of ionic pairs. The first peak of cation-anion correlation g(r)+- locates at approximately 0.5 nm, while the first peaks between ions with same charges [both g(r)++ and g(r)--] appear at approximately 0.8 nm. This can be explained that ions attract those with opposite charge and banish ions with the same charges. In this way ions with opposite-charge keep closer, resulting in a short correlation distance. Ions which appear around one center ion with opposite charge will attract ions which have the same charge as the center one. The result is that ions with the same charge can stay around the reference ion over one ion with opposite charge and makes the first peaks in the RDF of cation-cation or anion-anion. It can be seen that the peaks and bottoms of the g(r)+- and that of both g(r)++ and g(r)-- appear alternately with complementation along the radial direction. The cation-anion peaks appear at 0.5nm, 1.2nm and 1.9nm where the bottom of RDF between ions with same charge locates. Because the volumes of [hmpy+] are bigger than [Tf2N-], the first shell of g(r)++ is broader and lower than that of that of g(r)--. As the long range coulomb interaction are stronger than normal liquids, the RDFs in the [hmpy+][tf2n-] IL present a long range correlations extended beyond 2.0nm where the relative density reaches 1.0nm. 341
Fig. 3 The 3D SDF of atom N in [Tf2N-](a) and [hmpy+](b) around [hmpy+]. (c) is the mixture of both (a) and (b) For better understanding of the microcosmic structure of the [hmpy+][tf2n-] IL, the relative angular orientation distribution between different particles more than radial distributions should be taken into account. Spatial distribution functions (SDFs) present more information about the orientational distribution of particles around certain interesting particles. We analyze SDFs in [hmpy+][tf2n-] and generate the average 3-D isosurfaces of both cation and anion around one cation. Fig.3 shows some 3D probability distributions of nitrogen(n) atoms around [hmpy+]. It is known that, in [hmpy+], N atom locating at the sub-tip part of the hexyl with positive partial charge. Fig.3 (a) and (b) show that the possibility for each anion to arrange around the positive-charge area of one cation is obvious, while for cations, the areas with negative charge are prefer. As shown in Fig.3 part (c), there is an evident gapping filling effect for the orientational distribution of cations and anions. 3.2 Dynamic Properties The mean-square displacement (MSD), which contains information on the atomic diffusivity, is defined: [9] MSD r t r 2 ( ) (0) (3.1) where rt () is the position of the interesting particals at time t. For solid state, MSD tend to a finite value (a line with little slope). For fluid, MSD increases linearly with time. [9] Fig. 4 shows the MSD curves of particles in the [hmpy+][tf2n-] IL studied here. During the beginning 200ps of the simulation, the curves are non-linear, because the box is not stable yet. During the last 200ps, there is evident fluctuations for the insufficient sampling. In this way, the diffusion coefficients ( D ) are analyzed based on the data from 200 to 800ps by Einstein equation: 1 D r t r 6t lim 2 ( ) (0) (3.2) t The results show that at 300K and 1 atm, the average diffusion coefficient of the particles in the system is (0.69±0.06) 10-11 m 2 /s, that of [hpmy+] and [Tf2N-] are (0.74±0.03) 10-11 m 2 /s and (0.66±0.12) 10-11 m 2 /s individually. 342
Fig. 4 Mean-squared displacement of ion pairs, anions and cations from left to right. Generally speaking, the diffusion coefficients are between 10-9 m 2 /s and 10-14 m 2 /s in solids-state, bewteen10-10 m 2 /s and 10-9 m 2 /s in liquid-state, and larger than 10-5 m 2 /s in gas-state. [10] At 300K and 1atm, the fluidity of [hmpy+][tf2n-] ionic liquid is so small as a kind of solid. One possible reason to this phenomenon is that cation-anion interactions are much stronger than particle interactions in molecular liquids. This result indicates that at room temperature and atmospheric pressure, the ions [hmpy+][tf2n-] of does not move vigorously. It may be difficult for undesirable gases to be dissolved in [hmpy+][tf2n-]. On the other hand, the captured gas molecules will be immovable in it. For [hmpy+][tf2n-] the process of absorbing gases is quite slow at room temperature and atmospheric pressure. High temperature is needed to expedite the gas molecules dissolved in it, and high vapor pressure may be needed to increase the solubility of gases. For the sake of easy-to-use, the temperature and pressure should not deviate much away from the room temperature and atmospheric pressure. The further study are need to be carried out. 4. Conclusion and Expectations The micro structural and dynamical properties of [hmpy+][tf2n-] liquid at 300K and 1atm pressure are studied with molecular dynamic simulations in this work. As the result of the strong coulomb interactions originated from partial charges, there exists long-range spatial correlations with complementarities between cations and anions in orientational distributions in the system. The diffusion coefficient of the particles in the ionic liquid is small at room temperature and normal pressure. So the gas absorbing velocity of [hmpy+][tf2n-] may be small. The best choice of the circumstance conditions should be studied further more. The simulations about the absorbing process of CO 2 into [hmpy+][tf2n-] liquid at various conditions are in progress. In addition, as complementation to these molecular dynamics studies, the chemical reactions during the dissolving of CO 2 into [hmpy+][tf2n-] may be studied with ab initial time dependent density functional calculations. [11] Our ultimate goal is to construct different kinds of room temperature ILs and study their properties as promising catcher and remover of pollutions [11] such as CO 2, SO 2, NO x, etc, in atmosphere with simulation experiments. 5. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grants No.10847147) and the research foundation of Nanjing University of Information Science and Technology (Grants No.20080279). 343
6. References [1] D. B. Eleanor, et al. CO2 Capture by a Task-Specific Ionic Liquid. J.Am.Chem.Soc. 2002, 124(6): 926-927 [2] M. P. Mills. Energy Policy in the Electron Age. Mills-McCarthy & Associates, Inc. http://www.fossilfuels.org/electric/electron.htm. [3] X.H. Huang, et al. Why is the partial molar volume of CO2 so small when dissolved in a room temperature Ionic Liquid? J.Am.Chem.Soc. 2005, 127, 17842-17851. [4] Anthony J. L, et al. Anion Effects on Gas Solubility in ionic Liquids. Phys.Chem. B 2005, 109, 6366-6374. [5] R. Lynden-Bell, et al. Simulation of interfaces between room temperature ionic liquids and other liquids. Faraday Discuss. 2005, 129, 57-67. [6] Cesar Cadena. Molecular modeling of the thermophysical and transport properties of ionic liquids. (dissertation). University Of Notre Dame. 2006 [7] H. Stern, et al. SIM molecular dynamics simulation program, 2000. [8] Berendsen, H. J. C, et al. GROMACS: A message-passing parallel molecular dynamics implementation. Comput.Phys.Comm. 1995, 91, 43-56. [9] F. Ercolessi, et al. A molecular dynamics primer. International School for Advanced Studies. Spring College in Computational Physics. Trieste, Italy. 1997. pp,24. [10] C. Cadena, et al. Why is CO2 so soluble in Imidazolium-Based Ionic Liquids? J.Am.Chem.Soc. 2004,126(16): 5300 5308. [11] L. Cui. B. Gu, et al. Effect of different laser polarization direction on high order harmonic generation of N2 and H2 Appl. Phys. Lett. 2006, 89, 211103-3. [12] Thomas Welton. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99 (8), pp 2071 2084. 344