Structure and dynamics of acetate anion-based ionic liquids from molecular dynamics study Aneesh Chandran, Karthigeyan Prakash, Sanjib Senapati * Department of Biotechnology, Indian Institute of Technology Madras, Chennai 600 036, India article info Article history: Received 19 April 2010 In final form 4 June 2010 Available online 13 June 2010 Keywords: Ionic liquids Molecular dynamics simulations Physicochemical properties Dynamics abstract Acetate anion-based ionic liquids (ILs) have found wide range of applications. The microstructure and dynamics of this IL family have not been clearly understood yet. We report molecular dynamics simula- tion results of three acetate anion-based ionic liquids that encompass the most common IL cations. Sim- ulations are performed based on a set of proposed force field parameters for IL acetate anion which can be combined with existing parameters for IL cations to simulate large variety of ILs. The computed liquid density and IR spectral data for [BMIM][Ac] are found to match very well with available experimental results. The strong amino-group-associated interactions in [TMG][Ac] are seen to bring about higher cohesive energy density, stronger ion packing, and more restricted translational and rotational mobilities of the constituent ions. The IL anions are found to track the cation movements in all systems, implying that ions in ILs travel in pairs or clusters. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Ionic liquids (ILs) are organic salts with low melting points, fre- quently below room temperature. They have been regarded as green solvents because of their greater stability and lower volatil- ity than the conventional organic solvents in engineering applica- tions [1–3]. The areas of application include media for synthesis and separation, matrices in mass spectroscopy, agents in gas absorption, electrolyte for batteries, etc. [4–7]. This in conjunction with their ease of preparation at different cation–anion combina- tions provides a great opportunity to obtain task-specific ILs for a multitude of applications [8–10]. The most common ILs are based on imidazolium, pyridinium, and guanidinium cations. These cations can combine with a range of anions such as halide, tetrachloroaluminate, nitrate, tetrafluoro- borate, hexafluorophosphate, dicyanamide, trifluoromethanesul- phonyl, bis-(trifluoromethylsulphonyl)imide, methide, etc. to produce a large variety of room temperature ionic liquids [1–10]. Different analytical techniques, such as X-ray diffraction, IR and Raman spectroscopy, NMR spectroscopy, neutron diffraction, and microcalorimetry have been employed to characterize the physico- chemical properties of these ILs [11–19]. Molecular simulation has emerged as an effective tool to understand the structure and dynamics of liquids and macromolecules in detail. Many past com- puter simulation studies have provided insight into the thermody- namic and transport properties of various ionic liquids. Lynden- Bell and coauthors [20,21] have reported the intermolecular poten- tials for the simulations of liquid imidazolium chloride and hexa- fluorophosphate salts which predicted the correct structures and diffusion rates. Maginn and coworkers [12,16,22,23] have carried out extensive simulation studies of various ILs which provided deeper understanding of this emerging class of solvents. Andrade et al. [24] presented a complete force field for condensed phase simulations of 1-ethyl-3-methylimidazolium and 1-n-butyl-3- methylimidazolium tetrachloroaluminate and tetrafluoroborate that successfully reproduces the experimental results. Lopes et al. [25–27] proposed a systematic all-atom force field for the molecu- lar modeling of the imidazolium, pyridinium, and phosphonium salts which was validated against crystal structures and liquid- state densities. The structure and dynamics of 1-ethyl-3-methyl imidazolium nitrate was studied by Popolo and Voth [28]. The principal contribution to the configurational energy was found to come from the long-range Coulombic interactions. Liu et al. [29] proposed a refined force-field in the framework of AMBER param- eters that is expected to describe the PVT relationship and other properties of the ILs correctly. Though most of the past simulation studies have concentrated on ionic liquids with imidazolium cat- ions, the study of other IL cations such as guanidinium [30], pyrid- inium [16,27], and pyrrolidinium [31] is not rare. The studies of gas-IL and water-IL interfaces have also shown promise for this class of liquid to become a suitable replacement of conventional organic solvents [32–35]. Very recently, the ionic liquids composed of acetate anions have received attention due to their growing applications. It is reported that the IL domain of N,N,N 0 ,N 0 -tetramethylguanidium acetate 0301-0104/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2010.06.011 * Corresponding author. Tel.: +91 4422574122; fax: +91 4422574102. E-mail address: sanjibs@iitm.ac.in (S. Senapati). Chemical Physics 374 (2010) 46–54 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys