Ionic Liquids in external electric and electromagnetic fields: a molecular dynamics study Niall J English, Damian Anthony Mooney, Stephen O’Brien To cite this version: Niall J English, Damian Anthony Mooney, Stephen O’Brien. Ionic Liquids in external electric and electromagnetic fields: a molecular dynamics study. Molecular Physics, Taylor & Francis, 2011,109 (04), pp.625-638. 10.1080/00268976.2010.544263. hal-00670248 HAL Id: hal-00670248 https://hal.archives-ouvertes.fr/hal-00670248 Submitted on 15 Feb 2012 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Molecular Physics For Peer Review Only Ionic Liquids in external electric and electromagnetic fields: a molecular dynamics study Journal: Molecular Physics Manuscript ID: TMPH-2010-0292.R1 Manuscript Type: Full Paper Date Submitted by the 27-Oct-2010 Author: Complete List of Authors: English, Niall; University College Dublin, School of Chemical & Bioprocess Engineering Mooney, Damian; University College Dublin, School of Chemical & Bioprocess Engineering O'Brien, Stephen; University College Dublin, School of Chemical & Bioprocess Engineering Ionic Liquids, Electric Fields, Electromagnetic Fields, Conductivity, Keywords: Diffusion Note: The following files were submitted by the author for peer review, but cannot be converted to PDF. You must view these files (e.g. movies) online. SOURCE FILES.zip URL: http://mc.manuscriptcentral.com/tandf/tmph Page 1 of 37 Molecular Physics 1 2 3 4 5 6 1 7 8 9 Ionic Liquids in external electric and electromagnetic fields: a molecular 10 11 dynamics study 12 * * 13 Niall J. English , Damian A. Mooney and Stephen O’Brien 14 15 16 For Peer Review Only 17 The SEC Strategic Research Cluster and the Centre for Synthesis and Chemical 18 Biology, School of Chemical and Bioprocess Engineering, University College Dublin, 19 20 Belfield, Dublin 4, Ireland. 21 22 23 24 Abstract 25 26 The non-thermal effects of external electric and electromagnetic fields in the 27 28 microwave to far-infrared frequency range and at (r.m.s.) electric field intensities of 29 -3 30 10 to 0.25 V/År.m.s. on neat salts of 1-3-dimethyl-imidazolium hexafluorophosphate 31 32 ([dmim][PF6]) and 1-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim][PF6]) 33 34 have been investigated by means of non-equilibrium molecular dynamics simulation. 35 36 Significant alterations in molecular mobility were found vis-à-vis zero-field 37 38 conditions. Using Green-Kubo and transient time correlation function (TTCF) 39 40 analysis, the electrical conductivity of these ionic liquids has been estimated. Results 41 42 indicate that ionic liquids respond most significantly to frequencies much lower than 43 44 those of smaller polar solvents such as water, although the mechanism of field 45 response is almost exclusively translational. 46 47 48 49 50 51 52 53 54 55 56 57 * Corresponding authors: [email protected], [email protected] 58 59 60 URL: http://mc.manuscriptcentral.com/tandf/tmph Molecular Physics Page 2 of 37 1 2 3 4 5 6 2 7 8 9 Introduction 10 11 Ionic liquids (here referred to as ionic liquids, or ILs) are organic salts in the liquid 12 13 state, which offer considerable possibilities in the chemical process and allied 14 15 industries due to their unique physical and chemical properties. They are excellent 16 For Peer Review Only 17 solvents for a wide variety of organic and inorganic substances, often bringing 18 together otherwise immiscible reagents and, in many cases, maintaining or enhancing 19 20 reaction rates, extents of reaction and selectivities. In addition, ILs are essentially 21 22 non-volatile and provide an opportunity for replacing traditional organic solvents, 23 24 thereby potentially reducing the emissions and energy requirements of unit operations 25 26 used for separations. Those ILs which are liquids at room temperature (‘room 27 28 temperature’ ILs), are of particular interest since they provide for more cost-effective 29 30 handling and processing. Therefore, ILs are regarded as very promising ‘green’ 31 32 solvents and, as such, the impetus for their introduction into existing processes is 33 1, 2 34 significant. Field Code Changed 35 3, 4 36 From the earliest studies showing the utility of microwave fields in synthesis, Field Code Changed 37 38 there has been significant growth in their application to a wide variety of reactions 39 5 40 and processes, accelerated by advances in microwave technology. For the case of Field Code Changed 41 42 ILs, their ionic nature, and, to a lesser extent, dipolar polarization, make them ideal 43 44 candidates for use with microwave technology. One of the earliest investigations on 45 46 microwaves in IL synthesis was in the solvent-free production of a series of 1-alkyl-3- 47 48 methyl-imidazolium (AMIM) halides, by the reaction of 1-methylimidazole with alkyl 49 6 50 halides/terminal dihalides using a household microwave oven. In this work, Field Code Changed 51 improvements in reaction rates were observed. To date, the utility of microwave 52 53 fields has been exploited in three areas of synthetic chemistry, namely in the synthesis 54 55 of ILs themselves, as already mentioned, as an aid for heating dipolar substances and 56 57 58 59 60 URL: http://mc.manuscriptcentral.com/tandf/tmph Page 3 of 37 Molecular Physics 1 2 3 4 5 6 3 7 8 9 when ILs are used as solvents and reactants and as an aid in the processing of 10 7 11 reactions carried out in the presence of ILs. Field Code Changed 12 13 Given the increasing importance of ILs, the use of electric and electromagnetic (e/m) 14 15 fields to exploit their physical and catalytic properties further constitutes a valuable 16 For Peer Review Only 17 avenue of research. ILs possess important properties making them amenable to 18 19 electric and e/m field processing, in particular their ionic nature, which offers many 20 21 intriguing questions and potential applications. Enhancing the mass and heat transfer 22 23 characteristics of ILs, and of other solvents in ILs, by e/m field exposure would result 24 in smaller reactor volumes and improved, less expensive distillation and liquid-liquid 25 26 extraction systems. Similarly, the use of microwave irradiation to improve the 27 28 synthesis efficiency of ILs themselves, and for organic reactions therein, could 29 1 30 potentially offer a means of process intensification in the chemical industry. This Field Code Changed 31 32 would serve to maximize the use of resources by their efficient use, necessary for the 33 34 long-term future of chemical and allied sectors. 35 36 This work builds on our earlier non-equilibrium molecular dynamics (MD) studies 37 8, 9 38 of the behavior of binary mixtures of water and ILs in external e/m fields. In those Field Code Changed 39 40 cases, the presence of water molecules led to the consideration of field-induced 41 42 dipolar alignment as an important consideration in characterizing the response to the 43 44 fields. In this work, we have chosen to investigate similar, imidazolium-based ionic 45 - 46 liquids, specifically hexafluorophosphate ([PF6] ) salts of 1-3-dimethyl-imidazolium 47 + + 48 and 1-butyl-3-methyl-imidazolium - [dmim] and [bmim] , respectively. This study 49 50 seeks to study in more detail the molecular mechanisms of pure IL species’ behavior 51 in electric and e/m field conditions, which is dominated by induced translational 52 53 motion for neat IL systems, given the absence of strongly dipolar species such as 54 55 water. This work examines non-thermal field effects on IL properties, especially those 56 57 58 59 60 URL: http://mc.manuscriptcentral.com/tandf/tmph Molecular Physics Page 4 of 37 1 2 3 4 5 6 4 7 8 9 determined by translational response, such as electrical conductivity and the variation 10 11 of the de facto self-diffusivity with frequency, as well as the comparative field 12 13 response of anions and cations, with simulations conducted over wide range of 14 -3 15 frequencies (2.45 GHz to 200 GHz) and e/m field intensities (10 to 0.25 V/År.m.s.). 16 For Peer Review Only 17 Using Green-Kubo and transient time correlation function (TTCF) analysis, the 18 electrical conductivity of these ionic liquids has been estimated, at lower applied 19 20 -3 electric field intensities than possible in previous molecular simulation studies (10 ). 21 22 A goal of this study is to provide insights into the optimization of the use of electric 23 24 and e/m fields in processes involving ILs. 25 26 27 28 Methodology 29 30 There exist a large number of force-fields developed especially for ILs, where the 31 32 trend has been toward a generic force-field model adaptable to a large number of 33 10-12 34 possible IL variants. In this work, we have employed the well documented and Field Code Changed 35 10 36 tested force-field of Lopes et al. for imidazolium salts, which itself is based on the 37 13, 14 38 AMBER/OPLS force-fields, with the general form given by Field Code Changed 39 12 6 40 N −1 N σ σ q q ij ij i j 41 Etot = ∑∑4εij − + + i=1 j >i r r r 42 ij ij ij 43 2 2 ∑ kr (r − ro ) + ∑ kθ (θ − θo ) + (1) 44 bonds angles 45 + φ − δ + ϕ − ϕ 2 46 ∑ k 1 cos(n ) ∑ kϕ ( o ) dihedrals improper 47 48 15 10 More recent work has attempted to refine the force-field of Lopes et al., Field Code Changed 49 Field Code Changed 50 specifically the non-bonded contributions, in order to replicate more closely hydrogen 51 52 bonding between cations and anions, as well as transport, including self-diffusion, 53 54 coefficients.