Molecular dynamics study on TOTO-based ionic liquids with different cations
Introduction
The term, ionic liquids (ILs), represents salts whose melting points are below 100 °C. In modern chemistry and synthesis-related technology, ILs are often applied to replace traditional organic solvents due to their low vapor pressure. This property can markedly reduce the emission of volatile organic compounds to atmosphere. Another remarkable advantage of ILs is their diversity. More specifically, changing cations or anions or both ions will produce new ILs. It is reported [1] that there is at least a million binary ILs possibly. The easy tunability makes ILs have excellent thermal and physiochemical properties, meeting some specific application requirements, such as the application of temperature-responsive ILs in catalysis reaction [2]. Thermal stability, high conductivity, and good electrochemical performance of these compounds are made good use in batteries, supercapacitors, cells, energy storage devices, electrochemical sensors, electrodeposition, electrosynthesis, and electrocatalysis [3], [4], [5]. In many other aspects, ILs show efficient and reversible ability to capture CO2 [6] and play important roles in biocatalysis, biofuel production, liquid chromatography, chemical catalysis, micellar electrophoresis, and so on [7], [8], [9]. Thus, ILs are also called as “designer solvent” or “task-specific compounds”. Understanding intrinsic structure and properties is the foundation or the key to develop variously target-applicable ILs.
ILs including different cation-anion components have fine tunability. Through combining various cations with anions, ILs with the need of specific applications can be designed and synthesized. The ether-functionalized ILs have attracted much attention [10,11], and 2,5,8,11-tetraoxatridecan-13-oate (TOTO) as an anion is one representative. Compared to most studied and applied ILs, which are based on cations with imidazolium or pyridinium ring, TOTO-originated ILs (TOTO-ILs) are superior for their low toxicity and good biodegradability [12]. These compounds can satisfy the requirement of green chemistry and eco-friendly chemistry to a large extent. Moreover, most TOTO-ILs are room-temperature ILs. The low melting point is attributed to the flexibility of ethylene oxide group of TOTO anion. Primitive studies on this class of ILs are focused on the combination of TOTO anions and alkali metal cations. Zech et al. [13] investigated and compared electrochemical and thermal stability of series of alkali metal-TOTO, including Li+, Na+, and K+ cations. Both Li-TOTO and Na-TOTO are room-temperature ILs. Strong ion pairs were found in Na-TOTO ionic liquid. Further, with polarity analysis and dielectric spectra [14], they found that Na+ ion strongly interacted with the carboxylate group of TOTO. Daschakraborty et al. [15] found that Na-TOTO exhibits many similar features with respect to already well-studied imidazolium ILs. Through molecular dynamics simulation, Eilmes et al. [16] gave a microscopic description on alkali cation-TOTO anion combinations. They find that the coordination interactions of different alkali cations with carboxylate oxygen atoms were stronger than those with ether oxygen atoms.
Besides ILs consisting of simple alkali metal ions and TOTO ions, Klein et al. [17] gave a study on organic positive ions with TOTO ions that have different properties with those of alkali metal-TOTO ILs from experiments. They proposed that strong interionic interactions between small alkali metal cations and TOTO anions were considerably weakened when introducing large organic cations. Using long-chain ammonium (C8-C18) as cation [18], only cations with ≤C12 chain length could form room-temperature ionic liquid with TOTO anion. In addition, such combination has both ILs and surfactant attributes, which are attractive and profound points. Nevertheless, compared to specific ion pairs of Na-TOTO ILs, coordination interaction between organic cation and TOTO anion is unclear. Detailed information of interaction in organic cation-TOTO ILs needs further studies, especially in the microscopic views.
With the improvement of computer performance and theoretical methods, molecular dynamics (MD) simulation is effective and reliable to investigate structural and dynamical information of ILs [19], [20], [21], [22], [23], [24]. In this paper, we aim to understand the structure and interionic interactions in organic cation-TOTO ILs to promote the design of application-oriented and green ILs from MD simulation. The combination of tetraalkylammonium (TAA) and choline (Ch) cations with TOTO anion is systematically studied by Klein et al. [17] from experiment. However, the microscopic binding patterns between cations and anions is unclear. We choose TAA, Ch and Na with TOTO to give a systemic study on comparing the binding patterns between organic cation-TOTO anion and inorganic cation-TOTO anion.
Section snippets
Computational section
All MD simulations were performed with Gromacs 2019 package [25]. The generalized Amber force field (GAFF) parameters [26] were used to describe bonded and nonbonded interactions for all studied molecules. The energy function used is given by equation S1. Restrained electrostatic potential (RESP) charges calculated with AM1-BCC model [27] were used to describe electrostatic properties of all molecules. All ions of Ch, TEA, TPA, TBA and TOTO are separately optimized with calculating charges.
Density
First, we calculate the densities of different systems. Densities of all systems with the time evolution are plotted in Fig. 2. It shows that all studied systems achieve equilibrium. Compared to organic cation-TOTO ILs, Na-TOTO has higher density. The organic cation-TOTO ILs decrease with order of Ch-TOTO>TEA-TOTO>TPA-TOTO>TBA-TOTO, which is in good agreement with experimental sequences [17]. All equilibrated densities of studied ILs systems are listed and compared in Table S7. Expect for the
Conclusions
A series of molecular dynamics simulations are performed to study how structure and interionic interactions of TOTO based-ILs are affected via changing different cations. For ILs systems with organic cations, the increase in cation size has relatively small effect on the distribution of TOTO-chain length and conformation, as well as radical function distribution between cations and anions. As alkyl chain of cation grows, coordination numbers for OC-cation and OE-cation coordination both
CRediT authorship contribution statement
Xia Leng: Conceptualization, Methodology, Software, Formal analysis, Writing - original draft, Writing - review & editing. Yunzhi Li: Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing. Yaoyao Wei: Methodology, Software, Writing - review & editing. Guangli Zhou: Supervision, Data curation. Guokui Liu: Supervision, Funding acquisition, Writing - review & editing. Qiying Xia: Funding acquisition, Writing - review & editing. Honglei Wang: Supervision,
Declaration of Competing Interest
None.
Acknowledgements
This work is supported by the Natural Science Foundation of Shandong Province, China (ZR2019BB072, ZR2017LB011).
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Both authors contribute equally to this work.