Short-Range Order and Transport Properties in Mixtures of the Protic Ionic Liquid [C₂HIm][TFSI] with Water or Imidazole

Abstract

We investigate the effect of adding different molecular cosolvents, water or imidazole, to the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide, i.e., [C 2 HIm][TFSI]. We explore how the added cosolvent distributes within the ionic liquid by means of molecular dynamics simulations and X-ray scattering. We also analyze the degree of short-range heterogeneity in the resulting mixtures, finding that while imidazole easily mixes with the protic ionic liquid, water tends to form small clusters in its own water-rich domains. These differences are rationalized by invoking the nature of intermolecular interactions. In aqueous mixtures water−water hydrogen bonds are more likely to form than water−ion hydrogen bonds (water−TFSI bonds being particularly weak), while imidazole can interact with both cations and anions. Hence, the cation−anion association is negligibly influenced by the presence of water, whereas the addition of imidazole creates solvent-separated ion pairs and is thus able to also increase the ionicity. As a consequence of these structural and interactional features, transport properties like self-diffusion and ionic conductivity also show different composition dependencies. While the mobility of both ions and solvent is increased considerably by the addition of water, upon adding imidazole this property changes significantly only for molar fractions of imidazole above 0.6. At these molar fractions, which correspond to a base-excess composition, the imidazole/[C 2 HIm][TFSI] mixture behaves as a glass-forming liquid with suppressed phase transitions, while homomixtures such as imidazole/[HIm][TFSI] can display a eutectic point.

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“…Simultaneously, a signature of the ring vibration observed at 1452 cm –1 broadens in the spectra of both [HC 6 Im]­[TfO]/Im and CNC/Im/[HC 6 Im]­[TfO]­(38). These observations indicate that the PIL and imidazole are well mixed and are at molecular proximity within the film and establish hydrogen bonds. − The weak signatures of imidazole in the Raman spectra of [HC 6 Im]­[TFSI]/Im and CNC/Im/[HC 6 Im]­[TFSI]­(29) (Figure S3b) are explained similarly to the case of the composites with [HC 6 Im]­[TfO]. However, the lesser effects on the modes at 742 cm –1 (δ s CF 3 ) and 1243 cm –1 (ν s CF 3 ) characteristic for the [TFSI] anion indicate a weaker interaction between imidazole and [TFSI] − , which is consistent with previous findings in our group that imidazole added to an [HC n Im]­[TFSI] ionic liquid preferably interacts with the alkylimidazolium cation .…”

“…These observations indicate that the PIL and imidazole are well mixed and are at molecular proximity within the film and establish hydrogen bonds. − The weak signatures of imidazole in the Raman spectra of [HC 6 Im]­[TFSI]/Im and CNC/Im/[HC 6 Im]­[TFSI]­(29) (Figure S3b) are explained similarly to the case of the composites with [HC 6 Im]­[TfO]. However, the lesser effects on the modes at 742 cm –1 (δ s CF 3 ) and 1243 cm –1 (ν s CF 3 ) characteristic for the [TFSI] anion indicate a weaker interaction between imidazole and [TFSI] − , which is consistent with previous findings in our group that imidazole added to an [HC n Im]­[TFSI] ionic liquid preferably interacts with the alkylimidazolium cation . The only distinguishable change is a slight asymmetric broadening (toward higher wavenumbers) of the peak at 742 cm –1 in CNC/Im/[HC 6 Im]­[TFSI] compared to the pure PIL or its mixture with imidazole.…”

“…Unfortunately, the 1 H NMR spectra of the CNC/Im/PIL composite membranes do not provide a resolution high enough to perform diffusion experiments and hence study the contribution from exchangeable mobile protons. Nevertheless, our previous works have already shown that the copresence of imidazole and imidazolium cations promotes the self-diffusion of protons, , which is a desired phenomenon for fuel cell applications.…”

“…However, the lesser effects on the modes at 742 cm −1 (δ s CF 3 ) and 1243 cm −1 (ν s CF 3 ) characteristic for the [TFSI] anion indicate a weaker interaction between imidazole and [TFSI] − , which is consistent with previous findings in our group that imidazole added to an [HC n Im]-[TFSI] ionic liquid preferably interacts with the alkylimidazolium cation. 37 The only distinguishable change is a slight asymmetric broadening (toward higher wavenumbers) of the peak at 742 cm −1 in CNC/Im/[HC 6 Im][TFSI] compared to the pure PIL or its mixture with imidazole. This may arise from confinement and/or interactions established between the PIL and CNCs.…”

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“…Ionic liquids (ILs) are a novel group of salt-like materials that are liquid under ambient conditions [8] and have great potential for use in different industries. Although their transport properties have been widely investigated in recent years, investigations of the thermal properties are very limited in the literature [9][10][11][12]. Because of their excellent thermal properties, such as high thermal conductivity (TC) and high heat capacity (C p ), they are recognized as an appealing candidate for heat transfer applications [13,14].…”