Structure and Aggregation in the 1,3-Dialkyl-imidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Family: 2. From Single to Double Long Alkyl Side Chains

Abstract

A systematic molecular dynamics study using large simulation boxes has been performed in order to extend the analysis of the mesoscopic segregation behavior observed in ionic liquids of the 1,3-dialkyl-imidazolium bis(trifluoromethylsulfonyl)imide homologous series, [C(n)C(mim)][Ntf2] (2 ≤ n ≤ 10, 2 ≤ m ≤ n). The analyses include the discussion of the structure factors, S(q), in the low-q range (1.6 ≤ q/nm(-1) ≤ 20); the confirmation of the periodicity of the polar network of the ionic liquid and its relation to the so-called intermediate peaks; and the characterization of the polar network and the nonpolar regions that are formed along the series using aggregate analyses by means of five different statistical tools. The analyses confirmed that the percolation of the nonpolar regions into a continuous domain occurs when the total number of carbon atoms in the alkyl chains exceeds six but that this is not a sufficient condition for the emergence of a distinct and intense prepeak. The existence of such a peak also requires that the longer alkyl chain contains more than a critical alkyl length (CAL) of five carbon atoms.

Vapor pressures of 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids with long alkyl chains

“…The symmetry of the ionic liquid cation, changes dramatically the thermophysical properties, as it was already shown in the literature. 30,33,34,43,64 In fact, only recently the extended series of [C N/2 C N/2 im][NTf 2 ] ionic liquids was fully studied concerning the heat capacities, viscosities, densities and the liquid structure using SWAX 33,34,64 and more recently using molecular dynamics simulations, 9 and two regions were identified, with a shift around [C 6 , is in agreement with the expected effect arising from the nanostructuration in ionic liquids. 9 It is well established that in the liquid phase of ionic liquids, the polar regions tend to organize themselves in order to enhance the electrostatic interactions between the cation and anion and the alkyl chains of the cation for a non-polar segregated network.…”

“…30,33,34,43,64 In fact, only recently the extended series of [C N/2 C N/2 im][NTf 2 ] ionic liquids was fully studied concerning the heat capacities, viscosities, densities and the liquid structure using SWAX 33,34,64 and more recently using molecular dynamics simulations, 9 and two regions were identified, with a shift around [C 6 , is in agreement with the expected effect arising from the nanostructuration in ionic liquids. 9 It is well established that in the liquid phase of ionic liquids, the polar regions tend to organize themselves in order to enhance the electrostatic interactions between the cation and anion and the alkyl chains of the cation for a non-polar segregated network. 5,8,9,31,32 The segregation of the non-polar nanostructured regions leads to an increase of the contribution of the methylene group, to the absolute entropy of the liquid as well as to the cohesive energy, due to the overlapping of the decreasing electrostatic interaction potential by the increasing van der Waals potential.…”

“…29 The effect of short alkyl chain length as well as the symmetry of the cation on the vaporization thermodynamics was evaluated based on the thermodynamic properties of vaporization of the [C 1 2 ] ionic liquids using molecular dynamics simulations. 9 The analysis confirmed that the beginning of the nanostructuration/ alkyl chain segregation occurs when the total number of carbon atoms in the alkyl chain exceeds six. This findings support the previous interpretation of the experimental thermophysical results published on the literature.…”

“…9 It is well established that in the liquid phase of ionic liquids, the polar regions tend to organize themselves in order to enhance the electrostatic interactions between the cation and anion and the alkyl chains of the cation for a non-polar segregated network. 5,8,9,31,32 The segregation of the non-polar nanostructured regions leads to an increase of the contribution of the methylene group, to the absolute entropy of the liquid as well as to the cohesive energy, due to the overlapping of the decreasing electrostatic interaction potential by the increasing van der Waals potential. The results and discussion presented in this work is a strong support and indication that for the symmetric series, [C N/2 C N/2 im][NTf 2 ], the limit for the beginning of the nanostructuration/alkyl chain segregation is [C 6 C 6 im][NTf 2 ], presenting the same size for the longer alkyl chain than the asymmetric series where the limit was found to be [C 6 …”

“…ionic liquids in different molecular simulation techniques. [5][6][7][8][9] Despite the importance of the vapor pressures of the ionic liquids and the related thermodynamic properties of vaporization (enthalpies, entropies, and Gibbs energies of vaporization), most of the works performed until know were focused in estimate/determine the enthalpies of vaporization. 7,[10][11][12][13][14][15][16][17][18][19][20][21] The nature of the gas phase of ionic liquids is still in debate; however, the theory that the vaporization of ionic liquids occurs as a direct liquid to gas transfer of the intact ionic pair has been supported by several studies using photoionization, 22 line of sight mass spectrometry, 11,12,23 Fourier transform ion cyclotron resonance, 24,25 field ionization method, 26 tunable vacuum ultraviolet photoionization, 15 and more recently by molecular dynamics simulations.…”