Rodrigo M. A. SILVA1, Hádrian MONTES-CAMPOS1, Ana I. M. C. LOBO FERREIRA1, Eduards BAKIS2, Luís M. N. B. F. SANTOS1
1CIQUP, Institute of Molecular Sciences (IMS) – Department of Chemistry and Biochemistry, Faculty of Sciences, University of Porto, Porto, Portugal
2Faculty of Chemistry, University of Latvia, Riga, Latvia
The viscosity and density of ionic liquids can be reduced by replacing the alkyl side chain of the cation by an alkylsilane or an alkylsiloxane chain [1-3]. In this work we present the study of the thermophysical properties of ILs with alkylsilane and alkylsiloxane chains, as well as some of their analogs with carbon-based chains. With this work, we aim to understand the impact that alkylsilane and alkylsiloxane chains have on the thermophysical properties of ionic liquids.
The phase behavior of these compounds was studied by differential scanning calorimetry (DSC). Their heat capacity was measured, as a function of temperature (from T = 283 K to T = 333 K) by means of high-precision differential scanning microcalorimetry (iSenseDSC) and, at T = 298.15 K, via high-precision drop calorimetry [4]. Thermal stability was studied by thermogravimetric analysis (TGA), and their volatility was studied by means of Knudsen effusion method coupled with a quartz crystal microbalance (KNQ) [5].
Although no first-order transitions were detected for the ILs with carbon-based chains, DSC experiments revealed that some of the ILs with alkylsilane and alkylsiloxane chains were obtained in the crystalline state. The IL with an alkylsiloxane chain has a lower melting temperature than its alkylsilane analog. The replacement of carbon with silicon in the cation chain causes an increase in the molar heat capacity and in thermal stability. The vaporization studies shows that the ILs with alkylsilane chains are slightly more volatile and have similar cohesive energy to their carbon-based chain analogs. The effect on volatility is more significant for the IL with an alkylsiloxane chain and seems to arise due to a lowering of the cohesive energy.
Acknowlegdements
This work was supported by the Fundação para a Ciência e Tecnologia (FCT) (funded by national funds through the FCT/MCTES (PIDDAC)) to CIQUP, Faculty of Science, University of Porto (Project UIDB/00081/2020), IMS-Institute of Molecular Sciences (LA/P/0056/2020). RMAS is grateful to FCT for the award of his PhD grant (U1/BD/153093/2022). AIMCLF is also financed by national funds through the FCT-I.P., in the framework of the execution of the program contract provided in paragraphs 4, 5 and 6 of art. 23 of Law no. 57/2016 of 29 August, as amended by Law no. 57/2017 of 19 July. EB acknowledges PostDoc Latvia for financial support (Grant No. 1.1.1.2/VIAA/3/19/549).
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