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Thermochimica Acta

Volume 686, April 2020, 178547
Thermochimica Acta

Heat capacities of ionic liquids based on tetrahydrothiophenium cation and NTf2 anion

https://doi.org/10.1016/j.tca.2020.178547Get rights and content

Highlights

ILs [S-alkyl-tetrahydrothiophenium][NTF2] prepared and purified.

Heat capacities of seven ILs were measured by DSC.

Linear heat capacity chain-length dependence was observed.

General regularities in heat capacities of NTf2 ILs established.

Group-additivity method used for validation of experimental data.

Abstract

The isobaric heat capacities of seven ionic liquids (ILs) based on the S-alkyl-tetrahydrothiophenium cation [CnTHT] with alkyl chain lengths of Cn = 4,5,6,7,8,9,10 and the bis[(trifluoromethyl)sulfonyl]imide, [NTf2] anion were measured in a differential scanning calorimeter in the temperature range 310−376 K. Measurements have been performed for the first time. Experimental results were extrapolated to the reference temperature of 298 K. A linear trend of the Cp,mo(liq,298K) -values for [CnTHT][NTf2] with the growing alkyl chain length was observed and compared with trends available for the other ILs with 5-membered (imidazolium and pyrrolidinium) and 6-membered (pyridinium) ring cations as [NTf2] salts. The new Cp,mo(liq,298K) -values for for [CnTHT][NTf2] were used for mutual validation of experimental and theoretical results by a group-additivity method available in the literature. The agreement observed between experimental and additive calculations, together with the reasonable chain-length dependence demonstrated for the [CnTHT][NTf2] series was considered as evidence of internal consistency of the heat capacities measured in this work.

Introduction

Ionic liquids (ILs) are considered as candidates for replacements of volatile solvents and heat-transfer liquids used in industry. For these applications, it is important to know their thermophysical and thermochemical properties. The ionic liquids bearing the bis[(trifluoromethyl)sulfonyl]imide (or NTf2) anion combined with imidazolium [1], pyridinium [2], pyrrolidinium [3], and phosphonium [4] cations were studied in our laboratory, recently. ILs with the tetrahydrothiophenium cation (see Fig. 1) have attracted our attention due to their hydrophobic nature and their wide electrochemical window resulting in possible applications like dye-sensitized solar cells [5] or electric double-layer capacitors [6].

This paper extends our previous studies on the ILs containing fluorinated anions [[1], [2], [3], [4],[7], [8], [9]] and deals with heat capacity measurements on tetrahydrothiophenium based ILs with the [NTf2] anion (see Fig. 1).

Section snippets

Materials

The samples of [CnTHT][NTf2] were prepared according to literature procedures based on the synthesis of Paulsson et al. [10] Purities of ILs are given in Table S1. They were measured by using ion chromatography. Detailed description of synthesis, purification, and analytics will be published in a forthcoming paper. Before beginning the heat capacity measurements, the IL samples were kept at 333 K and 10−5 Pa for 1 h to remove possible traces of moisture and residual solvents. The negligible

Heat capacities as a function of temperature

Primary experimental data on heat capacities of ILs measured in the range 310 K–376 K are given in Table S2. For all ILs under study, the relationship between isobaric heat capacity and temperature was found to be approximately linear (see Fig. 2), with only about 55−65 J·K−1·mol−1 increase observed over the temperature range used.

The experimental heat capacities temperature dependences were approximated with help of the linear equation:Cp,mo(liq)/J·K-1·mol-1=A0+A1·(T/K)

Coefficients of

CRediT authorship contribution statement

Dzmitry H. Zaitsau: Investigation, Validation, Formal analysis, Writing - review & editing. Alexa Schmitz: Investigation, Writing - review & editing. Christoph Janiak: Conceptualization, Methodology, Writing - original draft, Supervision. Sergey P. Verevkin: Conceptualization, Methodology, Writing - original draft, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work has been supported by the German Science Foundation (DFG) in the frame of the priority program SPP 1807 “Control of London Dispersion Interactions in Molecular Chemistry”, as well as of the priority program SPP 1708 “Material Synthesis Near Room Temperature” (grants VE 265-14/1 to SPV and JA 466/31-2 to CJ). DHZ acknowledges the financial support from DFG, grant ZA 872/3-1, 407078203. This work has been also partly supported by the Russian Government Program of Competitive Growth of

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