Thermophysical properties of the 1-alkyl-3-methylimidazolium triflate series

Ana Imc LOBO FERREIRA¹, Rodrigo Ma SILVA¹, Luís Mnbf SANTOS^1
1CIQUP, Institute of Molecular Sciences (IMS) – Department of Chemistry and Biochemistry, Faculty of Sciences, Porto, Portugal

One of the most interesting properties of ionic liquids is the existence of nanostructuration in the liquid phase. This happen when the alkyl side chains of the cations are sufficiently long, leading to their aggregation, which gives rise to two distinct nanodomains: a polar nanodomain (where the polar parts of the cation and the anions are located) and a non-polar domain (where the end parts of the alkyl tails are located) [1]. This work presents the study of the phase behavior, heat capacity and volatility of the 1-alkyl-3-methylimidazolium triflate ionic liquid series, [C_nC₁im][OTf] (with n = 2, 4, 6, 8, 10, 12). 
 
The phase behavior of this IL series was studied by differential scanning calorimetry (DSC). The heat capacity for these ILs was determined, as a function of temperature (between 283 K and 333 K), using a customized differential scanning microcalorimeter (iSenseDSC) and, at T = 298.15 K, using a high precision drop microcalorimeter [2]. The volatility was measured by means of a Knudsen effusion method coupled with a quartz crystal microbalance (KNQ) [3].
 
While evaluating the different studied properties as a function of the number of carbon atoms in the alkyl side chain of the cation, n, a trend shift near n = 6 is highlighted (critical alkyl size, CAS). For the volumetric heat capacity, a minimum is reached at n = 6, evidencing the intensification of the nanostructuration [4,5].

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.
 
References
[1]    J. Canongia Lopes, A. Pádua, J. Phys. Chem. B, 110 (2006) 3330-3335.
[2]    L. Santos, M. Rocha, M. Bastos, J. Chem. Thermodyn., 43 (2011) 1818-1823.
[3]    L. Santos, A. Lobo Ferreira, M. Rocha, J. Chem. Thermodyn., 126 (2018) 171-186
[4]    M. Rocha, L. Santos, M. Bastos, J. Chem. Thermodyn.,53 (2012) 140-143.
[5]    P. Serra, F. Ribeiro, L. Santos, J. Mol. Liq., 248 (2017) 678-687.