Maggel
Deetlefs
a,
Kenneth R.
Seddon
a and
Michael
Shara
b
aThe QUILL Centre, David Keir Building, Stranmillis Road, Belfast, Northern Ireland, UK BT9 5AG. E-mail: quill@qub.ac.uk
bAmerican Museum of Natural History, Division of Physical Sciences, Central Park West and 79th St., New York, NY 10024, USA. E-mail: mshara@amnh.org
First published on 31st January 2006
Refractive index determination of minerals and gems often requires their immersion in fluids with the same refractive index. However, these natural materials frequently have refractive indices above the ranges of common organic solvents. Most available high refractive index immersion materials are solid at room temperature, toxic, noxious, corrosive, carcinogenic, or any combination thereof. Since the physical properties of ionic liquids can be tuned by varying the cation and/or anion, we have developed immersion fluids for mineralogical studies which are relatively benign. We report here the syntheses of a range of ionic liquids (many novel) based on the 1-alkyl-3-methylimidazolium cation, which all have refractive indices greater than 1.4, and can be used as immersion fluids for optical mineralogy studies. We further show that for a series of ionic liquids with the same anion, the refractive indices can be adjusted by systematic changes in the cation.
Amongst gems, diamonds (which have a refractive index of 2.42) are of great interest to earth scientists, since they represent the deepest available samples3 of the earth’s interior, forming by crystallisation in the mantle at depths greater than 150 km. Since diamonds (as well as other minerals) form by crystallisation, they often contain inclusions of foreign material, such as gases, liquids or even other minerals that become trapped within the host mineral during crystallisation (Fig. 1). Often regarded as flaws by the jewellery industry, inclusions provide earth scientists with valuable clues regarding the pressure, temperature, and chemical conditions in which minerals grew, thus serving as hallmarks of the processes which gave rise to them.
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Fig. 1 A purple pyrope garnet inclusion hosted in a diamond octahedron.3 |
In order to non-destructively locate inclusions in unpolished diamonds and other minerals by optical means, it is desirable to immerse these minerals in a fluid with the same refractive index. Unfortunately, however, many available high refractive index immersion compounds, such as AsI3 (RI = 2.2) or SnI4 (RI = 2.1),4 are solid at room temperature, poisonous, unstable, and extremely unpleasant. Diiodomethane saturated with sulfur5 is another commercially available high refractive index (1.78) room temperature liquid, but this is also a harmful material.
In an attempt to assist in the optical examination of minerals, we were interested in developing ionic liquids (ILs) based on the most common cations viz. 1-alkyl-3-methylimidazolium [Cnmim]+, (Table 1).
|
|
---|---|
n | Y |
a Previously unreported ionic liquid. b Tetrafluoroborate. c Bis{(trifluoromethyl)sulfonyl}amide. d Trifluoromethanesulfonate. e Iodide. f Bromide. | |
2 | [IBr2], [BrI2], [ClI2], [I7],a [I9]a |
4 | [IBr2], [BrI2], [ClI2],a [Br3], [I3], [I5],a [I7],a [I9],a[BF4],b [NTf2],c [OTf]d |
6 | I,e [IBr2],a [BrI2],a [I3]a |
8 | Brf |
Room temperature ionic liquids are solvents composed entirely of ions with melting points below ca. 100 °C. The burgeoning interest in these non-volatile liquids has resulted in publication rates of (currently) 1000 papers per year, and many research books6,7 and reviews.8,9 Ionic liquids have also been described as ‘designer solvents’,10 since careful selection of the cation, anion, or both, allows the preparation of ionic liquids with specific physical properties. At present, however, physical property data for ionic liquids are still fairly limited, placing restrictions on effective interrogation of existing information to allow predictive ionic liquid preparation. Using the tuneability of ionic liquids, we report here a set of fluids which have been designed to have refractive indices >1.4, have low environmental impact and good shelf life. As high atomic number is associated with high refractive index, ionic liquids containing periodide anions became the cynosure of the investigation. The investigated ionic liquids thus contain various polyhalide anions, and exhibit higher refractive indices than common organic solvents. Although a number of ionic liquids consisting of 1-alkyl-3-methylimidazolium cations and triiodide anions,11 as well as other mixed trihalide anions,12 have previously been described, many of the ionic liquids prepared in this study (see Table 1) are, to the best of our knowledge, new materials.
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Fig. 2 Schematic representation of a Fabry–Perot interferometer. |
We employed a Fabry–Perot interferometric refractive index sensor manufactured by FISO Technologies of Quebec City, Canada to measure ionic liquid refractive indices. The sensor consists of two miniature fibre-optic refractive index transducers, with 0.8 mm outer diameter and 10 mm length Pyrex tubes at each tip. One transducer was optimised for refractive index measurements in the 1.3 to 1.8 range, and a second was optimised for refractive indices between 1.8 and 2.5. A model FTI-10 signal conditioner was coupled to the transducers. The signal conditioner fed an 800 nm light source through each fibre to the transducer tip. It is this near-infrared light that was multiply reflected between the partially silvered glass plates of Fig. 2 to produce the interferogram, which the sensor and detector electronics interpreted.
The sensor of an interferometer consists of two closely spaced, partially-silvered surfaces with spacing, d. The FISO interferometer we used has d = 10 microns, and operates at 800 nm wavelength. The combination of a very small light-absorbing pathlength and infrared wavelength allowed refractive index measurements of the intensely coloured ionic liquids.
When light reaches the silvered surfaces, part of it is transmitted each time the light reaches the second surface, resulting in multiple offset beams. The large number of interfering rays produces an interferogram with extremely high resolution. When the sensor of the interferometer is immersed in a liquid, the value of d becomes d/RI. For Bragg’s law,25 2dcos(α) = mλ (α = angle of incidence of light on the cavity, m = integer at successive interference maxima, and λ = wavelength of light used), to continue to hold true, the value of m must change. The new value of m is determined by the sensor and detector electronics. This is achieved by measuring the widths of the fringes and cross-correlating the fringe pattern with itself. In this way, the refractive index of any liquid, regardless of its appearance, can be determined.
The natural beryl and corundum samples are faceted, polished ovals, while the (artificial) cubic zirconia is a polished, faceted square shape (an “emerald cut”). The line of white dots near the middle of the beryl are small chips in the culet of that specimen, best seen in the [C8mim]Br image. Note that the corundum almost vanishes in the “best match” [C4mim][Br3], while its inclusions become very clear. The facets of the cubic zirconia become increasingly difficult to discern with fluids of increasing refractive index. The apparent “bending” of the edge of the cubic zirconia is due to surface tension of the water and ionic liquid near the side of the microbeaker which distorts the liquids’ surfaces.
Salts containing polyhalide anions have been known for many years and their preparation, solution-phase, and solid-state chemistry have been extensively investigated.28 The general method for the preparation of polyhalides involves direct combination of a halide salt, e.g. [C4mim]Br, and a halogen, e.g. I2, or interhalogen, e.g. IBr,28 the experimental conditions being dictated by the reactivity of the reactants and products. In the present study, an analogous approach was employed to synthesise the ionic liquids containing polyhalide anions.28,29 The more familiar ionic liquids, [C4mim][BF4], [C4mim][NTf2], [C4mim][OTf], [C4mim]Cl, [C4mim]Br, [C4mim]I, [C6mim]Br, [C6mim]I, and [C8mim]I were prepared using standard procedures (see Experimental section).
Microanalytical data (Table 2) confirmed the identity of the polyhalide anion ionic liquids, while the invariance of their refractive index data, which were regularly determined over a three year period (vide infra), demonstrated the stability of the fluids over time.
Microanalysesa | ||||
---|---|---|---|---|
Ionic liquid | % C | % H | % N | Mp (°C) |
a Required values given in parentheses. b Not determined for known samples. c Not observed. | ||||
[C2mim][ClI2] | 18.01 (17.99) | 2.72 (2.77) | 7.40 (7.00) | —c |
[C2mim][BrI2] | 16.25 (16.19) | 2.47 (2.49) | 6.50 (6.29) | 8.6 |
[C2mim][IBr2] | 18.20 (18.11) | 2.62 (2.79) | 6.85 (7.04) | 3.9 |
[C2mim][I7] | 7.34 (7.21) | 0.90 (1.11) | 2.86 (2.80) | —c |
[C2mim][I9] | 5.95 (5.75) | 0.47 (0.88) | 2.24 (2.24) | —c |
[C4mim][I3] | 18.83 (18.48) | 2.96 (2.90) | 5.58 (5.39) | —c |
[C4mim][ClI2] | 22.44 (22.42) | 3.17 (3.53) | 6.45 (6.53) | —c |
[C4mim][BrI2] | 20.17 (20.32) | 3.15 (3.19) | 6.04 (5.92) | —c |
[C4mim][IBr2] | 21.61 (22.56) | 3.23 (3.54) | 6.13 (6.58) | —c |
[C4mim][Br3] | 25.04 (25.34) | 3.88 (3.99) | 7.18 (7.39) | —c |
[C4mim][I5] | 12.48 (12.42) | 2.03 (1.95) | 4.00 (3.62) | —c |
[C4mim][I7] | 9.59 (9.35) | 1.27 (1.47) | 3.01 (2.73) | —c |
[C6mim][I3] | 21.80 (21.92) | 3.13 (3.49) | 5.05 (5.11) | −9.8 |
[C6mim]I | 39.78 (40.83) | 5.95 (6.51) | 9.23 (9.52) | —c |
[C6mim][BrI2] | 23.01 (23.98) | 3.51 (3.82) | 5.12 (5.59) | —c |
[C6mim][IBr2] | 25.64 (26.45) | 3.77 (4.22) | 5.79 (6.17) | —c |
[C8mim]Br | —b | —b | —b | —c |
[C4mim][BF4] | —b | —b | —b | —c |
[C4mim][NTf2] | —b | —b | —b | —c |
[C4mim][OTf] | —b | —b | —b | —c |
Differential scanning calorimetry (DSC) was employed to determine the melting points of the ionic liquids with polyhalide anions, but only [C2mim][IBr2], [C2mim][BrI2], and [C6mim][I3] displayed unambiguous melting transitions below 10 °C (Table 2). DSC scans of the remaining salts below 20 °C exhibited very broad transitions, if any, probably because the materials form glasses, a common feature of ionic liquids.13
The proton nuclear magnetic resonance (1H-NMR) spectra of the ionic liquids confirmed the structural assignments of the cations and also showed that halogenation of the imidazolium ring of the ionic liquids containing polyhalide anions had not occurred (Table 3), as is known to happen in reactions with dichlorine.30
Ionic liquid | CH2 (s) | CH4 (s) | CH5 (s) | NCH2α (t or q) | NCH3 (s) | N(CH2)n−1CH3 (t) | NCH2(CH2)n−2 (m) | NC2H4CH2 (m) |
---|---|---|---|---|---|---|---|---|
a Recorded in d4-methanol. b Recorded in d3-ethanenitrile. c Recorded in d6-propanone. | ||||||||
[C2mim][ClI2]c | 8.97 | 7.68 | 7.91 | 4.28 (q, J = 7.3) | 3.95 | 1.46 (J = 7.1) | — | — |
[C2mim][BrI2]c | 8.95 | 7.58 | 7.65 | 4.43 (q, J = 7.5) | 3.96 | 1.57 (J = 7.5) | — | — |
[C2mim][IBr2]c | 8.43 | 7.39 | 7.34 | 4.17 (q, J = 7.5) | 3.83 | 1.46 (J = 7.2) | — | — |
[C2mim][I7]a | 8.92 | 7.63 | 7.83 | 4.24 (q, J = 7.3) | 4.00 | 1.54 (J = 7.1) | — | — |
[C2mim][I9]a | 8.95 | 7.61 | 7.94 | 4.25 (q, J = 7.3) | 3.96 | 1.53 (J = 7.2) | — | — |
[C4mim][I3]c | 9.04 | 7.62 | 7.75 | 4.25 (t, J = 7.2) | 3.98 | 0.85 (J = 7.2) | 1.86 (2H) | 1.26 |
[C4mim][ClI2]c | 9.25 | 7.57 | 7.82 | 4.30 (t, J = 7.2) | 3.97 | 0.82 (J = 7.2) | 1.83 (2H) | 1.29 |
[C4mim][BrI2]a | 8.94 | 7.56 | 7.63 | 4.24 (t, J = 7.5) | 3.95 | 0.99 (J = 7.2) | 1.89 (2H) | 1.39 |
[C4mim][IBr2]b | 8.42 | 7.34 | 7.37 | 4.13 (t, J = 7.2) | 3.83 | 0.94 (J = 7.5) | 1.81 (2H) | 1.33 |
[C4mim][Br3]c | 8.91 | 7.54 | 7.61 | 4.22 (t, J = 7.2) | 3.93 | 0.99 (J = 7.5) | 1.89 (2H) | 1.46 |
[C4mim][I5]a | 8.94 | 7.58 | 7.64 | 4.24 (t, J = 7.2) | 3.96 | 1.00 (J = 7.2) | 1.91 (2H) | 1.40 |
[C4mim][I7]a | 8.95 | 7.58 | 7.65 | 4.24 (t, J = 7.2) | 3.98 | 1.01 (J = 7.5) | 1.91 (2H) | 1.41 |
[C6mim][I3]a | 8.94 | 7.65 | 7.58 | 4.24 (t, J = 7.2) | 3.96 | 0.92 (J = 6.9) | 1.38 (6H, Hγ,δ,ε); 1.91 (2H, Hβ) | — |
[C6mim][IBr2]b | 8.40 | 7.37 | 7.33 | 4.11 (t, J = 7.2) | 3.82 | 0.88 (J = 6.3) | 1.30 (6H, Hγ,δ,ε); 1.83 (2H, Hβ) | — |
[C6mim][BrI2]b | 8.59 | 7.62 | 7.41 | 4.17 (t, J = 7.2) | 3.90 | 0.95 (J = 7.1) | 1.30 (6H, m, Hγ,δ,ε); 1.96 (2H, m, Hβ) | — |
In line with previous observations, the colours of the polyiodide salts, [Cnmim][Ix] (x = 3, 5, 7, or 9), prepared in the current investigation become darker with increasing iodine content, with the [I3]− salts red and transparent, and the [I9]− salts exhibiting a characteristic metallic lustre.31 During the preparation of the [Ix]− (x = 3, 5, 7, or 9) salts, it was ensured that the corresponding precursor salts, [Cnmim]I, were extremely dry, since attempting to dry the polyiodide products in vacuo could result in their decomposition, evidenced by the visible liberation of iodine vapour. The observed iodine loss is in agreement with earlier studies, which have shown that polyhalide salts may thermally dissociate into free iodine and the precursor iodide salt.28
The solid state structures of many salts with polyiodide anions have shown that, with few exceptions, the structural entities can be divided into the fundamental and stable units I−, I2 and [I3]−.31 As illustrated by (a) and (b) in Fig. 3, the local structures of polyiodide salts can be rationalised in standard coordination chemistry terms where the ions, I− and [I3]− are ligated by neutral I2 molecules.
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Fig. 3 Rationalisation of [I5]− anion structures; (a) [I5]−, first modification32 and (b) [I5]−, second modification.32 |
The identity of the anions was confirmed by a combination of the liquid secondary ion (LSI) and electrospray ionisation (ESI) mass spectra (Table 4); the isotopic distributions of these ions containing chloride and bromide served as support for their identities. Both techniques required the use of the carrier solvents, 3-nitrobenzyl alcohol (noba) or ethanenitrile and methanol, respectively, which can affect the equilibria or nature of polyhalide anions.33 Indeed, a computational study performed by Sato et al.34 has suggested that the dissociation energy for [I3]− in ethanenitrile or methanol is smaller than in the gas state equilibrium structure, indicating that exchange reactions should proceed more easily in solution. Stated another way, the study implied that for solutions of [I3]−, the I2 constituent of the anion becomes isolated due to solvation and, therefore, dissociation of the anion is easier than in the gas state. Furthermore, the investigation by Sato et al.34 also suggested that the dissociation energy for [I3]− in methanol is smaller than in ethanenitrile, implying that anion dissociation should proceed more easily in methanol than ethanenitrile. In the current investigation, this phenomenon was observed in the negative mode electrospray ionisation mass spectra of ethanenitrile solutions of [C4mim][I3], and [C6mim][I3] (Table 4), i.e. signals for both [I3]− and I− were present.
Ionic liquid | Method | Anionb | Additional mass-peaksb | |
---|---|---|---|---|
a m/z values refer to 79Br and 35Cl. b (m/z, relative intensity %). c Recorded in CH3CN. d Recorded at the EPSRC MS Centre in CH3OH. e Recorded at the EPSRC MS Centre in 3-nitrobenzyl alcohol. f Observed in the full scan ESI mass spectrum. g Observed in the positive mode ESI mass spectrum. h Observed in the full scan LSI mass spectrum. | ||||
[C2mim][ClI2] | ESIc | [ClI2]− | [I3]− | [ICl2]− |
(289; 9) | (381; 100) | (197; 3) | ||
[C2mim][BrI2] | ESIc | [BrI2]− | [I3]− | [IBr2]− |
(333; 27) | (381; 100) | (287; 6) | ||
[C2mim][IBr2] | ESIc | [IBr2]− | [BrI2]− | [I3]− |
(287; 100) | (335; 13) | (381; 5) | ||
[C2mim][I7] | ESId | I− | — | — |
(127; 100) | ||||
[C2mim][I9] | LSIe | [I3]− | {[C2mim]I·4noba}− | {[C2mim]2I}− |
(381; 44) | (863; 100) | (349; 22);h | ||
{[C2mim]I2}− | ||||
(365; 5)h | ||||
[C4mim][ClI2] | ESIc | [ClI2]− | [I3]− | [ICl2]− |
(289; 14) | (381; 100) | (197; 8) | ||
[C4mim][BrI2] | ESIc | [BrI2]− | [I3]− | [IBr2]− |
(333; 38) | (381; 100) | (287; 5) | ||
[C4mim][IBr2] | ESIc | [IBr2]− | [BrI2]− | [I3]− |
(287; 100) | (335; 4) | (381; 3) | ||
[C4mim][I3] | ESIc | [I3]− | I− | {[C4mim]2I}− |
(381; 100) | (127; 14) | (405; 8)f | ||
[C4mim][I5] | LSIe | [I3]− | I− | {[C4mim]I2}− |
(381; 100) | (127; 30) | (393; 21)h | ||
[C4mim][I5] | ESIc | [I3]− | I− | {[C4mim]2I}− |
(381; 100) | (127; 13) | (405; 100)g | ||
[C4mim][I7] | ESId | I− | — | {[C4mim]I2}− |
(127; 100) | (393; 18)h | |||
[C6mim][BrI2] | ESIc | [BrI2]− | [I3]− | [IBr2]− |
(333; 38) | (381; 100) | (287; 5) | ||
[C6mim][IBr2] | ESIc | [IBr2]− | — | — |
(287; 100) | ||||
[C6mim][I3] | ESIc | [I3]− | I− | {[C6mim]2I}− |
(381; 100) | (127; 20) | (461; 5);f | ||
{[C6mim]I2}− | ||||
(421; 21)f |
To the best of our knowledge, no equilibrium data exist for higher polyiodides (> [I3]−) in ethanenitrile or methanol, but a similar dissociation mechanism to that of [I3]− anions is probable, since the local structures of polyiodide anions may be rationalised in standard coordination chemistry terms (Fig. 3). This postulate is supported by the negative mode electrospray ionisation scans of [C4mim][I5], recorded using ethanenitrile as infusion solvent, since signals for the intact anions are not observed; the only detectable anion peaks are [I3]− and I− (Table 4).
Liquid secondary ion mass full scan (both positive and negative) spectral data were also recorded for [C4mim][I5] in 3-nitrobenzyl alcohol, which showed strong signals for I− and [I3]−, but the remainder of the spectrum provided no conclusive structural information. The electrospray ionisation mass spectra of [C2mim][I7] and [C4mim][I7] recorded in methanol only exhibited strong signals for I−, while the full scan LSIMS mass spectra were too complex to allow any definite conclusions other than rearrangement possible under the experimental conditions.
Recent ab initio calculations have shown that mixed halide anions, [X–Y–X]−, where Y is heavier than X, are more stable than [Y–X–X]− isomers, both in the gas phase and in solution.35 As a result, [Y–X–X]− anions, if formed from X2 and Y−, are expected to isomerise12 to [X–Y–X]− anions via cleavage of the X–X bond. This phenomenon was observed in the mass spectra of [C2mim][ClI2], [C2mim][BrI2], [C4mim][ClI2], [C4mim][BrI2], [C6mim][IBr2], and [C6mim][BrI2], where each of the analysed compounds (Table 4) exhibited the expected anion fragment together with additional species formed by disproportionation, eqn (2).
2[YX2]− ⇌ [X3]− + [Y2X]− | (2) |
Ionic liquid | R I a | R I b | R I c |
---|---|---|---|
a Becke’s method,22 2002, estimated error ± 0.005. b Depth-measuring microscope,2 2003, estimated error: ± 0.002. c Fabry–Perot interferometer,24 2003, estimated error: ± 0.001. d Fabry–Perot interferometer,24 2004, estimated error: ± 0.001. | |||
[C2mim][ClI2] | 1.796 | — | — |
[C2mim][BrI2] | 1.833 | 1.833 | — |
[C2mim][IBr2] | — | 1.715 | — |
[C2mim][I7] | — | — | 2.010d |
[C2mim][I9] | — | — | 2.080d |
[C4mim][I3] | — | — | 1.700 |
[C4mim][ClI2] | 1.774 | 1.775 | — |
[C4mim][BrI2] | 1.810 | — | 1.805 |
[C4mim][IBr2] | — | 1.701 | — |
[C4mim][Br3] | 1.67 | 1.699 | — |
[C4mim][I5] | — | — | 1.910 |
[C4mim][I7] | — | — | 1.950 |
[C6mim][I3] | — | — | 1.880 |
[C6mim]I | 1.549 | — | 1.547 |
[C6mim][BrI2] | 1.768 | — | — |
[C6mim][IBr2] | — | 1.685 | — |
[C8mim]Br | 1.490 | — | — |
[C4mim][BF4] | 1.416 | 1.410 | — |
[C4mim][NTf2] | 1.451 | — | — |
[C4mim][OTf] | 1.433 | — | — |
The refractive index data indicate that the values remained essentially the same over a period of at least one year, which is encouraging for the storage of these ionic liquids as stable immersion fluids for optical mineralogical studies (Table 5). The refractive index measurements also show the reproducibility of the data, since three techniques were employed and only minor discrepancies exist.
Metathesis, or anion exchange, is one of the most common routes to prepare ionic liquids, shown in eqn (3) for the preparation of [Cnmim][BF4] salts.36 As a result, ionic liquids can contain large amounts of chloride if not removed by washing the product with water, which is also a contaminant if not removed. Such impurities are known to affect many physical properties of ionic liquids,37 including refractive index, as shown in this work.
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Refractive index data of ionic liquids are currently limited, with only a few values reported in the literature and none in the most recent data compilation.6 Since ionic liquid properties are acutely affected by the presence of impurities, especially chloride and/or water, it is essential that purity information always accompanies physical property data (Table 6). For example, Kim et al.38 give the refractive index of [C4mim][BF4] as 1.4227, but provide no purity information for the ionic liquid, minimizing the value of the published refractive index. In contrast, Sigma-Aldrich39 reports the refractive index of the same salt as 1.42 and the chloride and water contents as <10 mg kg−1 and <0.02% (w/w), respectively, which are very similar to the 12.6 mg kg−1 chloride and 0.03% (w/w) water contents of our sample (Table 6). Therefore, if Kim et al.’s38 refractive index value is disregarded, the standard deviations of the refractive index values of our [C4mim][BF4] sample, and that of Sigma-Aldrich, ±0.01, lie within experimental error of each other and thus are essentially the same.
Ionic liquid | Source | R I | Cl− (mg kg−1) | H2O (% w/w) | Other impurities (mg kg−1) |
---|---|---|---|---|---|
a Sigma-Aldrich 2004–2005 Catalogue. b Purity: 98.5% (NMR). c Not reported. d Purity: 98% (NMR). e Purity: 95% (NMR). | |||||
[C4mim][BF4] | QUILL | 1.410 | 12.6 | 0.03 | 0 |
Sigma-Aldricha,b | 1.42 | <10.0 | <0.02 | 15 000 | |
Kim et al.38 | 1.4227 | —c | —c | —c | |
[C4mim][NTf2] | QUILL | 1.451 | ca. < 5 | 0.01 | 0 |
Sigma-Aldricha,d | 1.428 | —c | <0.5 | 2000 | |
[C4mim][OTf] | Bonhôte et al.15 | 1.4271 | —c | 1.4 | —c |
QUILL | 1.433 | <7 | 0.03 | 0 | |
Sigma-Aldricha,e | 1.434 | —c | —c | 5000 | |
Bonhôte et al.15 | 1.4380 | —c | —c | —c |
Considering the data and purity information given in Table 6 for [C4mim][NTf2] and [C4mim][OTf] from various sources, it is again clear that water and chloride affect the refractive indices of ionic liquids. It must be mentioned here that at present, there is no published, standard method to determine the chloride content of [NTf2]− ionic liquids, although preliminary ion chromatography studies in our laboratories indicate that chloride contents of the salts can be determined at levels of ca. <5 mg kg−1.40
Fig. 4 is a plot of the refractive indices for all the investigated ionic liquids as a function of their total, anion and cation formula weights (FW). As expected, the salts containing the highest electron density anions viz. [C2mim][I7], [C2mim][I9] and [C4mim][I7], exhibit the highest refractive indices; 2.01, 2.08, and 1.95, respectively. Fig. 4 also indicates that the ionic liquid refractive indices are governed by a primary anion and secondary cation effect, i.e. the refractive indices of the compounds increase as the anion mass increases provided the 1-alkyl chain length of the cation remains short.
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Fig. 4 Plot of refractive indices versus cation, anion, and total formula weights of the investigated ionic liquids. |
The effect that shorter 1-alkyl chains have on the refractive indices of a series of salts with the same anion is illustrated in Fig. 5, which shows how the refractive indices of the [IBr2]− and [BrI2]− salts systematically decrease by 0.02 and 0.03 units, respectively, as the 1-alkyl chain length on the cations increases monotonically. Regression analyses of the refractive index vs. cation formula weights plots for the [IBr2]− and [BrI2]− salts (Fig. 5), gave respective coefficient of determination, R2, values of 0.9986 and 0.9938, indicating their potential application in predicting refractive indices of ionic liquids with the same anion.
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Fig. 5 Refractive indices versus cation, anion, and total formula weights of the [Cnmim][IBr2] and [Cnmim][BrI2] (n = 2, 4, or 6) ionic liquids. |
Similarly to the [IBr2]− ionic liquids, the salts with [C2mim]+ and [C4mim]+ cations and [I3]−, [I7]−, [ClI2]−, and [BrI2]− anions, exhibit refractive index increases that coincide with shortening the 1-alkyl chains (Table 5). However, a regular relationship between refractive index and anion mass is not observed for a series of ionic liquids with the same cation but different anions, viz. [C4mim]X (X = [I3], [I5] and [I7]), (Table 5). A large refractive index difference exists between [C4mim]I38 and [C4mim][I3] (ΔRI = 0.131), as well as between [C4mim][I3] and [I5] (ΔRI = 0.210), but the difference is much smaller between [C4mim][I5] and [I7] (ΔRI = 0.040). There were two possible explanations for this unexpectedly small change. Either (a) the philosophy that we had developed was incorrect or (b) the anion we had anticipated was not present. As polyhalides, in general, and polyiodides in particular, are known to dissociate according to eqn (4), we deliberately added an excess of I2 to the solution to force the equilibrium to the left. In the case of the anticipated [I7]− system, for example, this increased the observed value of RI from 1.95 to 2.01, reinforcing the design principle. Interestingly, an early 1903 RI value for liquid I2 was given as close to 2.08.41
[In]− ⇌ [In−2]− + I2 | (4) |
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Fig. 6 (a) Refractive index and (b) density vs. anion FW of [C4mim][Ix] (x = 1, 3, 5, or 7) salts (![]() ![]() ![]() |
Fig. 7 shows the images obtained for the four investigated minerals in (a) air and (b) water. In Fig. 7(a), the facets of the quartz, cubic zirconia, and orange beryl crystals are plainly visible due to extensive light reflection from their surfaces. The apparent “bending” of the edge of the cubic zirconia is due to surface tension of the water, Fig. 7(b), and ionic liquid, Fig. 8(a), near the side of the microbeaker, which distorts the liquids’ surfaces.
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Fig. 7 Images of quartz, orange beryl, brown corundum, and colourless cubic zirconia immersed in (a) air and (b) water. |
In Fig. 7(b), the facets of the minerals become less discernable since their immersion in water results in decreased light reflection from their surfaces, i.e. the facets of the sample crystals become increasingly difficult to discern with fluids of increasing refractive index. A refractive index match between the minerals and their immersion media is not evident in Fig. 7(a) or 7(b) due to the large refractive index mismatches between the minerals and their host media.
In contrast to Fig. 7(a) and 7(b), in Fig. 8(a), a close refractive index match between [C8mim]Br (RI = 1.49) and quartz (RI = 1.54), is evidenced by the almost invisible appearance of the quartz crystal, which is only discernable by its black inclusions.
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Fig. 8 Images of quartz, orange beryl, brown corundum, and colourless cubic zirconia immersed in (a) [C8mim]Br, and (b) [C4mim][Br3]. |
In Fig. 8(b), a refractive index match is again visible, this time between brown corundum (RI = 1.76) and [C4mim][Br3] (RI = 1.70), and where only the inclusion in the brown corundum remains visible. Fig. 8(a) and 8(b) represent typical examples of how earth scientists could locate inclusions in unpolished minerals without encumbrance by light reflection. An existing alternative immersion medium to [C4mim][Br3] is toxic diiodomethane saturated with sulfur (RI = 1.78). The odourless, transparent ionic liquid (with negligible vapour pressure) has a major health and safety advantage over the traditional fluid. Although the ionic liquids investigated with RI > 1.8 are opaque to the naked eye, they do transmit near infrared radiation; we thus continue our search for materials with RI > 1.8 that are transparent in the infrared.
Although we only studied about 20 examples of neoteric fluids, we have elaborated a design principle which will enable these fluids to be prepared with RI in the range 1.40–2.10, at intervals of 0.01 (either pure liquids or binary systems). Twenty ionic liquids mixed in binary fashion generate 400 novel immersion media, even with simple 1∶1 ratios. If ratios are changed in steps of 0.1, then 4000 new liquids will be generated.
In this work, we have potentially increased the number of RI matching media by approximately 700 liquids. As these fluids are ionic liquids, they have negligible vapour pressure, and, by virtue of this physical property, are more environmentally benign than the vast majority of existing high refractive index immersion fluids for optical mineralogy. In this work, we have thus not only made refractive index measurements more precise, but have also substantially reduced health risks to the experimental scientist.
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