High-Accuracy Vapor Pressure Data of the Extended [C_nC₁im][Ntf₂] Ionic Liquid Series: Trend Changes and Structural Shifts

Abstract

For the first time, two distinct trends are clearly evidenced for the enthalpies and entropies of vaporization along the [Cnmim][Ntf2] ILs series. The trend shifts observed for Δ(l)(g)H(m)(o) and Δ(l)(g)S(m)(o), which occur at [C6mim][Ntf2], are related to structural modifications. The thermodynamic results reported in the present article constitute the first quantitative experimental evidence of the structural percolation phenomenon and make a significant contribution to better understanding of the relationship among cohesive energies, volatilities, and liquid structures of ionic liquids. A new Knudsen effusion apparatus, combined with a quartz crystal microbalance, was used for the high-accuracy volatility study of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide series ([Cnmim][Ntf2], where n = 2, 3, 4, 5, 6, 7, 8, 10, 12). Vapor pressures in the (450–500) K temperature range were measured, and the molar standard enthalpies, entropies, and Gibbs energies of vaporization were derived. The thermodynamic parameters of vaporization were reported, along with molecular dynamic simulations of the liquid phase structure, allowing the establishment of a link between the thermodynamic properties and the percolation phenomenon in ILs.

Novel 2-alkyl-1-ethylpyridinium ionic liquids: synthesis, dissociation energies and volatility

“…At the reference temperature, T = 460 K, the 2-alkyl-1-ethylpyridinium based ionic liquids presents significant lower volatility than the imidazolium ionic liquids series. 50 Analogous to imidazolium based ionic liquids, 49 decreases along the alkyl side chain length of the cation, similarly as it was observed for the imidazolium series. 50 The non-electrostatic potential is expected to increase nearly linearly along the alkyl chain length.…”

“…[80][81][82] The graphic representations of the standard molar enthalpies and entropies of vaporization at reference temperature, T = 460 K, as a function of the total number of carbon atoms in the two alkyl side chains of the cation, N, are presented in Fig. 3 50,76 ionic liquid families decreases, being however in all the cases higher, indicating that the electrostatic interactions contribution should be slightly higher in the pyridinium based ILs than in the imidazolium ILs.…”

“…19 Our group has been focused on the vapor pressure measurements of several families of ionic liquids, [50][51][52][53][54][55] using the quartz microbalance Knudsen effusion apparatus. 56 In these studies, the effect of the alkyl chain length of the cation was explored in order to get more insights about the nanostructuration of ionic liquids, which is related to the transition from a dispersed to a continuous non-polar phase, as the alkyl-side chain of the cation becomes longer.…”

“…5 It was possible to give a thermodynamic view of the nanostructuration of the imidazolium bistriflamide ionic liquids, for which a limit for the beginning of the nanostructuration/nano segregation at C 6 was observed. 50,55,57 Recently, the first volatility study of the 1-alkylpyridinium bis 50,51 It was found that the volatility of pyridinium ionic liquids is five times lower than that of imidazolium and that it is driven by their higher enthalpy of vaporization. 53 Imidazolium based ionic liquids are the most investigated family in several research areas.…”

“…Electrospray ionization mass spectrometry data were further used to gather additional insights into the interpretation of the thermodynamic properties of vaporization in terms of the IL's interaction energies. In this work, by means of a comparative analysis with previous studies, 50 the effect of nanostructuration on the pyridinium bistriflamide ionic liquids was explored.…”