View PDF VersionPrevious ArticleNext Article

Open Access Article
This Open Access Article is licensed under a
Creative Commons Attribution 3.0 Unported Licence
DOI: 10.1039/C7CC00201G (Communication) Chem. Commun., 2017, 53, 2375-2377

Mixing cations with different alkyl chain lengths markedly depresses the melting point in deep eutectic solvents formed from alkylammonium bromide salts and urea†

Zhengfei Chen a, Tamar L. Greaves b, Gregory G. Warr c and Rob Atkin *a
aPriority Research Centre for Advanced Fluids and Interfaces, Newcastle Institute for Energy and Resources, The University of Newcastle, Callaghan, NSW 2308, Australia. E-mail: rob.atkin@newcastle.edu.au
bSchool of Science, College of Science, Engineering and Health, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia
cSchool of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia

Received 10th January 2017 , Accepted 1st February 2017

First published on 1st February 2017

Abstract

The melting point of a deep eutectic solvent formed from a ternary mixture of ethylammonium bromide (EABr), butylammonium bromide (BABr) and urea is 10 °C, which is almost 40 °C lower than the melting points of binary DESs formed from either EABr:urea or BABr:urea mixtures. This reveals a new route to prepare room temperature DESs via mixing different cations.

Deep eutectic solvents (DES) are formed by mixing two or more components at their eutectic point, which is the composition with the lowest freezing point.1 DESs are often formed from mixtures of quaternary ammonium halides and a molecular hydrogen bond donor (HBD), such as urea.2

The archetypal DES is formed from choline chloride (ChCl, mp = 301 °C) and urea (mp = 133 °C) at a molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]2 salt[thin space (1/6-em)]:[thin space (1/6-em)]HBD.1 In an effort to understand the marked reduction in freezing point in ChCl[thin space (1/6-em)]:[thin space (1/6-em)]urea (1[thin space (1/6-em)]:[thin space (1/6-em)]2) compared to the parent materials, interactions between the ions with each other and the HBD has been extensively studied computationally.3 In addition to the expected Cl⋯HN hydrogen bonds, urea also hydrogen bonds with the choline cation (e.g. OH⋯O[double bond, length as m-dash]C and OH⋯NH), resulting in both Ch–urea and Cl–urea clusters.3 This may explain why the eutectic point occurs at 1[thin space (1/6-em)]:[thin space (1/6-em)]2 ChCl[thin space (1/6-em)]:[thin space (1/6-em)]urea.

The properties of DESs are often compared with ionic liquids,4,5 which are pure salts that have been molecularly designed to have low freezing points.6 Many DESs have properties commonly associated with ionic liquids such as non-flammability, low volatility, high thermal stability and high ionic conductivity.7 Unlike most ionic liquids, DESs are easily prepared from relatively low cost starting materials, and can also be biocompatible and/or of low toxicity.1,8,9 The properties and current applications of DESs (mostly based on ChCl) have been comprehensively reviewed.2,10

Studies of DESs' applications are frequently related to their use as electrolytes.11–15 However, the ionic conductivities of DESs are relatively low, especially those with urea as the HBD. For example, the conductivity of 1[thin space (1/6-em)]:[thin space (1/6-em)]2 ChCl[thin space (1/6-em)]:[thin space (1/6-em)]urea at 40 °C is only 0.2 mS cm−1.16 Low conductivity is chiefly due to the intrinsically high viscosities of DESs, and these factors limit their practical application and wider uptake. Here we demonstrate a simple way to reduce DES melting points and viscosity using an equimolar mixture of ammonium cations with different alkyl chain lengths.

Differential scanning calorimetry (DSC) was used to assess the effect of adding the urea HBD to the salts EABr, BABr, and EABr:BABr mixtures.‡Fig. 1A presents DSC traces without added urea. The endotherms at 205 °C for BABr and 162 °C for EABr show the melting points of the pure salts. EABr has an additional endothermic peak at 95 °C, but the sample remained solid, revealing a solid–solid phase transition and indicating a protic organic ionic plastic crystal.17 The melting point of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr is 80 °C, much lower than either pure salt, and the endotherm is broader. Similar effects have been noted for mixtures of ionic liquids,18 and attributed to the different sized cations frustrating ion packing.19Fig. 1B shows the effect of adding an equal number of moles of urea to each salt. The melting points of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea are depressed to 46 °C and 52 °C, respectively, which are much lower than the pure salts, but still well above room temperature. On the other hand, the melting point of the 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea ternary mixture is lowered to approximately room temperature (23 °C).


image file: c7cc00201g-f1.tif
Fig. 1 DSC curves of (A) EABr, BABr and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr molar ratio and (B) 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea and 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea. A magnified region (indicated by the arrow) shows the melting point of the ternary mixture.

Fig. 2 presents melting points for EAB–urea, BABr–urea and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr–urea mixtures as a function of urea mole fraction. These were determined from DSC traces for each system, which are presented in the ESI† (Fig. S1). Melting points for BABr:urea are higher than those of EABr:urea at all compositions, which is attributed to stronger interactions between alkyl chains. For urea mole fractions above 0.55 the freezing point of EABr:BABr:urea is approximately the same as BABr:urea, but below 0.55 the freezing point of EABr:BABr:urea is significantly lower. The eutectic points are at 1[thin space (1/6-em)]:[thin space (1/6-em)]1.2 for BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 for EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea and 0.6[thin space (1/6-em)]:[thin space (1/6-em)]0.6[thin space (1/6-em)]:[thin space (1/6-em)]1 for EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea.


image file: c7cc00201g-f2.tif
Fig. 2 Melting points of EAB–urea, BABr–urea and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr–urea mixtures as a function of urea mole fraction. The inset shows the melting point of EABr:BABr:urea mixtures versus cation composition where (EABr + BABr)[thin space (1/6-em)]:[thin space (1/6-em)]urea = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.

Fig. 2 (inset) shows the effect of cation composition on the melting point for the ternary EABr:BABr:urea mixtures, where the ratio of the total number of moles of cation (EA+ + BA+) to urea is fixed at 1[thin space (1/6-em)]:[thin space (1/6-em)]1. The melting point minimum occurs when equal numbers of EA+ and BA+ ions are present; when the ion ratios are slightly unbalanced the melting point is ∼20 °C higher. The reason for this marked depression in melting temperature when equal numbers of cations and anions are present, and the nature of their interaction with urea, is currently the topic of further study using neutron diffraction experiments with empirical potential structure refinement fitting, and will be reported in a forthcoming article.

For simplicity of comparison, subsequent experiments use 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea and 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea mixtures, where the total number of moles of salt and urea are equal.

The 1H NMR spectra of the alkylammonium bromide:urea DESs are presented in the ESI† (Fig. S2). Chemical shifts for alkyl chain protons are the same for the three mixtures. However, protons of the cation ammonium group and the urea amine groups shift towards low field in the order 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, to 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, to 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea. As low field shifts are a consequence of stronger H-bonds,20,21 this reveals the strength of hydrogen bonding between –NH3 and –NH2 with bromide increases in the order 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea < 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea < 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea. The fact that the strongest hydrogen bonds are found for the ternary mixture, which has the lowest melting point, is at first glance surprising. However, the melting point is determined by the sum of the cohesive interactions in the liquid, which means increased hydrogen bond strength in the ternary mixture must be more offset by decreased strength of other cohesive interactions (e.g. electrostatic, van der Waals, etc.) where mixed cations are used. In ionic liquids, weaker electrostatic interactions enable ions to adopt orientations that are more favourable for hydrogen bonding.22 A similar effect could account for stronger hydrogen bonding yet lower melting points in the ternary system compared to the binary system.

The viscosities, η, of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea and 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea between 25 °C and 80 °C are shown in Fig. 3. The experiments were performed from high temperature to low, and the data represents average values from three repeats. Data is not presented for 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea below 40 °C because it solidified. Results are shown down to room temperature for 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea which remains in a supercooled liquid state below its freezing point (for a variable time period). The viscosities of all three liquids are high, similar to glycerol,23 over the temperature range probed. The viscosity of 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea lies between that of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, in accordance with results for mixtures of molecular liquids24 and ionic liquids, but critically it is a thermodynamically stable liquid at 25 °C whereas the binary mixtures are not.25


image file: c7cc00201g-f3.tif
Fig. 3 Viscosities of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea and 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea mixtures as a function of temperature. The open triangles for 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea indicate temperatures where the liquid was supercooled.

Fig. 4 shows that the ionic conductivity (σ) of 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea is between that of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea over the entire liquid range. In accordance with these viscosity results, the conductivity of the ternary mixture lies between that of the two binary mixtures. At 25 °C the conductivity of 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea is 0.88 mS cm−1, which is more than three time higher than the supercooled 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea DES, and at this temperature 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea is solid.


image file: c7cc00201g-f4.tif
Fig. 4 Ionic conductivities of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea and 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea mixtures as a function of temperature. The open triangles for 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea indicate temperatures where the liquid was supercooled.

At 0.88 mS cm−1, the conductivity of the ternary mixture is more than 4 times higher than 1[thin space (1/6-em)]:[thin space (1/6-em)]2 choline chloride[thin space (1/6-em)]:[thin space (1/6-em)]urea and other urea based DESs at the same temperature,16 and also significantly higher than for other choline-based DESs formed from less viscous HBDs.10 Most DESs are formed from 1[thin space (1/6-em)]:[thin space (1/6-em)]2 salt[thin space (1/6-em)]:[thin space (1/6-em)]HBD mixtures, whereas in the ternary mixture the (EABr + BABr)[thin space (1/6-em)]:[thin space (1/6-em)]urea is 1[thin space (1/6-em)]:[thin space (1/6-em)]1, i.e. the effective salt concentration is about 2 times higher, which leads to higher conductivity.

Like similar DESs, Walden plots (see ESI,† Fig. S3) for 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]urea, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea and 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1 EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea fall well below the ideal 0.01 M KCl line. (The densities as a function of temperature for the three liquids are also shown in the ESI,† Fig. S4.)

In conclusion, the eutectic temperature of EABr:BABr:urea is 10 °C, which is more than 40 °C lower than those of EABr:urea and BABr:urea. The EABr[thin space (1/6-em)]:[thin space (1/6-em)]BABr[thin space (1/6-em)]:[thin space (1/6-em)]urea eutectic composition is 0.6[thin space (1/6-em)]:[thin space (1/6-em)]0.6[thin space (1/6-em)]:[thin space (1/6-em)]1, which means the salt[thin space (1/6-em)]:[thin space (1/6-em)]urea is 1.2[thin space (1/6-em)]:[thin space (1/6-em)]1. This means the salt concentration is EABr:BABr:urea is much higher than common DESs like 1[thin space (1/6-em)]:[thin space (1/6-em)]2 choline chloride[thin space (1/6-em)]:[thin space (1/6-em)]urea, leading to significantly higher ionic conductivity. While the primary alkyl ammonium bromide salts examined here are less electrochemically stable than quaternary cations like choline, the approach of mixing cations could also be applied to electrochemically robust ions, thereby increasing the range of parent salts that can be liquefied. This is the topic of continued investigation.

GGW and RA acknowledge support from Australian Research Council Discovery Projects, and RA acknowledges the ARC for a Future Fellowship. The authors would like to thank Dr Leigh Aldous from The University of New South Wales for providing the density/microviscometer.

Notes and references

  1. A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed and V. Tambyrajah, Chem. Commun., 2003, 70 RSC.
  2. E. L. Smith, A. P. Abbott and K. S. Ryder, Chem. Rev., 2014, 114, 11060 CrossRef CAS PubMed.
  3. C. R. Ashworth, R. P. Matthews, T. Welton and P. A. Hunt, Phys. Chem. Chem. Phys., 2016, 18, 18145 RSC.
  4. S. L. Perkins, P. Painter and C. M. Colina, J. Phys. Chem. B, 2013, 117, 10250 CrossRef CAS PubMed.
  5. D. Y. Yue, Y. Z. Jia, Y. Yao, J. H. Sun and Y. Jing, Electrochim. Acta, 2012, 65, 30 CrossRef CAS.
  6. T. Welton, Coord. Chem. Rev., 2004, 248, 2459 CrossRef CAS.
  7. J. Luo, O. Conrad and I. F. J. Vankelecom, J. Mater. Chem., 2012, 22, 20574 RSC.
  8. K. D. Weaver, H. J. Kim, J. Sun, D. R. MacFarlane and G. D. Elliott, Green Chem., 2010, 12, 507 RSC.
  9. F. Ilgen, D. Ott, D. Kralisch, C. Reil, A. Palmberger and B. Konig, Green Chem., 2009, 11, 1948 RSC.
  10. Q. H. Zhang, K. D. Vigier, S. Royer and F. Jerome, Chem. Soc. Rev., 2012, 41, 7108 RSC.
  11. Q. W. Chu, J. Liang and J. C. Hao, Electrochim. Acta, 2014, 115, 499 CrossRef CAS.
  12. C. A. Nkuku and R. J. LeSuer, J. Phys. Chem. B, 2007, 111, 13271 CrossRef CAS PubMed.
  13. Y. You, C. Gu, X. Wang and J. Tu, J. Electrochem. Soc., 2012, 159, D642 CrossRef CAS.
  14. Q. B. Zhang, A. P. Abbott and C. Yang, Phys. Chem. Chem. Phys., 2015, 17, 14702 RSC.
  15. J. Luo, T. V. Tan, O. Conrad and I. F. J. Vankelecom, Phys. Chem. Chem. Phys., 2012, 14, 11441 RSC.
  16. A. P. Abbott, G. Capper and S. Gray, ChemPhysChem, 2006, 7, 803 CrossRef CAS PubMed.
  17. J. Luo, A. H. Jensen, N. R. Brooks, J. Sniekers, M. Knipper, D. Aili, Q. Li, B. Vanroy, M. Wubbenhorst, F. Yan, L. Van Meervelt, Z. Shao, J. Fang, Z.-H. Luo, D. E. De Vos, K. Binnemans and J. Fransaer, Energy Environ. Sci., 2015, 8, 1276 CAS.
  18. T. D. J. Dunstan and J. Caja, ECS Trans., 2007, 3, 21 CAS.
  19. H. Niedermeyer, J. P. Hallett, I. J. Villar-Garcia, P. A. Hunt and T. Welton, Chem. Soc. Rev., 2012, 41, 7780 RSC.
  20. J. E. Del Bene, S. A. Perera and R. J. Bartlett, J. Phys. Chem. A, 1999, 103, 8121 CrossRef CAS.
  21. G. Wagner, A. Pardi and K. Wuethrich, J. Am. Chem. Soc., 1983, 105, 5948 CrossRef CAS.
  22. R. Hayes, S. Imberti, G. G. Warr and R. Atkin, Angew. Chem., Int. Ed., 2013, 52, 4623 CrossRef CAS PubMed.
  23. R. W. Gallant, Hydrocarbon Process., 1967, 46, 201 CAS.
  24. A. Daprano, A. Capalbi, M. Iammarino, V. Mauro, A. Princi and B. Sesta, J. Solution Chem., 1995, 24, 227 CrossRef CAS.
  25. M. T. Clough, C. R. Crick, J. Grasvik, P. A. Hunt, H. Niedermeyer, T. Welton and O. P. Whitaker, Chem. Sci., 2015, 6, 1101 RSC.

Footnotes

† Electronic supplementary information (ESI) available: Experimental details. DSC curves, NMR spectra Walden plot and density values. See DOI: 10.1039/c7cc00201g
‡ The melting points of the salt–urea mixtures were determined by DSC at 10 °C min−1. It should be noted there may be some discrepancy in values if other techniques are used due to the slow kinetics in these mixtures.
This journal is © The Royal Society of Chemistry 2017
Click here to see how this site uses Cookies. View our privacy policy here.