COMPOSITION AND METHOD

Abstract

A composition is described, having a relative permittivity, preferably a real part ε′γ of a relative complex permittivity εγ, substantially inversely proportional to the square of a frequency of an alternating electrical field, preferably due to an electromagnetic field, for example applied thereto and/or propagating therethrough.

Claims

1. A composition having a relative permittivity, preferably a real part ε′.sub.r of a relative complex permittivity ε.sub.r, substantially inversely proportional to the square of a frequency of an alternating electrical field, preferably due to an electromagnetic field, for example applied thereto and/or propagating therethrough.

2. The composition according to any previous claim, comprising a dielectric liquid and/or a neutral liquid, for example comprising an amide, a sulfonamide, a phosphoramide, an alcohol, a nitrile, a ketone, a pyridine, a pyrrolidine, a piperidine, a furan, a sulfoxide and/or an alkylene carbonates.

3. The composition according to any previous claim, comprising a gelator.

4. The composition according to any previous claim, comprising dispersed particles.

5. The composition according to any previous claim, having a static relative permittivity of at least 100, preferably at least 1,000, more preferably at least 10,000 and/or having a static relative permittivity of at most 20, preferably at most 10, more preferably at most 5 at a frequency of 10 GHz.

6. The composition according to any previous claim, comprising an amide, for example having a functional group R.sub.1—NH—CO—R.sub.2 and/or having an isobutyramide functional group R.sub.1—NH—CO—CH(CH.sub.3).sub.2.

7. The composition according to any previous claim, comprising an ionic liquid.

8. The composition according to claim 7, wherein the ionic liquid comprises a imidazolium ionic liquid, for example C.sub.2C.sub.1im BF.sub.4, C.sub.2C.sub.1im CH.sub.3CO.sub.2, C.sub.2C.sub.1im EtOSO.sub.3, C.sub.2C.sub.1im N(CN).sub.2, C.sub.2C.sub.1im OTf, C.sub.2C.sub.1im NTf.sub.2, C.sub.2C.sub.1im SCN, C.sub.2C.sub.1im DEP, C.sub.2C.sub.1im L-Lac, C.sub.2C.sub.1im FeCl.sub.4, C.sub.2OHC.sub.1im OTf, C.sub.2OHC.sub.1im PF.sub.6, C.sub.3C.sub.1im I, C.sub.3C.sub.1im NTf.sub.2, C.sub.3OC.sub.1im PF.sub.6, C.sub.4C.sub.1im BF.sub.4, C.sub.4C.sub.1im SCN, C.sub.4C.sub.1im L-Lac, C.sub.4C.sub.1im MeOSO.sub.3, C.sub.4C.sub.1im N(CN).sub.2, C.sub.4C.sub.1im NTf.sub.2, C.sub.4C.sub.1C.sub.1im NTf.sub.2, C.sub.6C.sub.1im BF.sub.4, C.sub.6C.sub.1im NTf.sub.2, C.sub.6C.sub.1im PF.sub.6, C.sub.6C.sub.1im I, C.sub.8C.sub.1im NTf.sub.2 and/or C.sub.8C.sub.1im Cl.

9. The composition according to any of claims 7 to 8, wherein the ionic liquid comprises a choline ionic liquid, for example [Ch] HCO.sub.2, [Ch] L-Ala, [Ch] L-Cys, [Ch] L-His, [Ch] L-Lac, [Ch] L-Lys, [Ch] L-Phe, [Ch] L-Pro, [Ch] L-Tryp, [Ch] Cl/Urea and/or [ACh] NTf.sub.2, preferably [Ch] L-Ala.

10. The composition according to any of claims 7 to 9, wherein the ionic liquid comprises a phosphonium ionic liquid and/or a sulfonium ionic liquid, for example aP.sub.4443 HCO.sub.2, aP.sub.4443 CH.sub.3CO.sub.2, aP.sub.4443 L-Lac, aP.sub.4443 L-Val, P.sub.4441 MeOSO.sub.3, P.sub.666(14) Cl, P.sub.666(14) NTf.sub.2 and/or S.sub.222 NTf.sub.2.

11. The composition according to any of claims 7 to 10, wherein the ionic liquid comprises an ammonium ionic liquid, for example N.sub.HHH2 NO3, N.sub.HHH(2OH) HCO.sub.2, N.sub.HHH(2OH) CH.sub.3CO.sub.2, N.sub.HHH(2OH) L-Lac, N.sub.(2OH)(2OH)(2OH)1 MeOSO.sub.3, N.sub.(2OH)(2OH)(2OH)1 L-Pro, N.sub.8881 NTf.sub.2, N.sub.122(2O1) BF.sub.4 and/or DIMCARB.

12. The composition according to any of claims 7 to 11, wherein the ionic liquid comprises a pyrrolidinium ionic liquid, a pyridinium ionic liquid and/or a piperidinium ionic liquid, for example C.sub.4C.sub.1Pyrr OTf, C.sub.4C.sub.1Pyrr N(CN).sub.2, C.sub.4C.sub.1Pyrr FAP, C.sub.4-3-MePyrr NTf.sub.2, C.sub.6py BF.sub.4, C.sub.6py NTf.sub.2, C.sub.4 Mepy MeOSO.sub.3 and/or C.sub.3C.sub.1 Pip NTf.sub.2.

13. The composition according to any of claims 4 to 12, wherein the particles comprise a metal and/or a metal compound, for example a pure or unalloyed metal, an alloy thereof, an inorganic compound such as a ceramic comprising the metal, a metal chalcogenide, or an organometallic comprising the metal and/or mixtures thereof.

14. The composition according to claim 13, wherein the metal is a transition metal, for example a first row, a second row or a third row transition metal.

15. The composition according to any of claims 4 to 14, wherein the particles have a regular, such as spherical, cuboidal or rod, shape and/or an irregular, such as spheroidal, flake or granular, shape.

16. The composition according to any of claims 4 to 15, wherein at least 50% by weight of the particles have a diameter at most 200 μm, at most 100 μm, at most 50 μm, at most 25 μm, at most 15 μm, or at most 10 μm and/or wherein at least 1% by weight of the particles have a diameter of at least 10 nm, at least 100 nm, at least 1 μm, at least 2.5 μm, at least 5 μm, at least 7.5 μm, or at least 10 μm.

17. The composition according to any of claims 4 to 16, comprising from 1 to 60 wt. %, preferably from 5 to 50 wt. %, more preferably from 10 to 45 wt. %, most preferably from 15 to 40 wt. %, for example from 25 to 35 wt. % particles, for example 33 wt. %.

18. The composition according to any of claims 4 to 17, wherein the particles are in suspension.

19. The composition according to any previous claim, comprising and/or is a gel, for example a hydrogel, a nanocomposite hydrogel, an organogel and/or a xerogel.

20. The composition according to any previous claim, having a dynamic viscosity in a range from 10 to 1,000 Pa.Math.s at 25° C.

21. The composition according to claim 1, comprising an electrically insulating polymer, dispersed particles and a binder/plasticizer.

22. The composition according to claim 21, wherein the polymer is a fluoro-polymer (such as PVDF and its co-polymers e.g. PVDF-HFP), carbopols, an acrylic such as PMMA, a polystyrene, a polyester, an epoxy, a polyamide, a silicone, a polyvinylpyrrolidone, a polyvinyl chloride, a polypropylene, a polyethylene, a polysiloxane, a polyimide, a polyacrylonitrile, a sulfur polymer, a synthetically functionalised polymeric material or a mixture thereof.

23. The composition according to any of claims 21 to 22, wherein the particles comprise a metal and/or a metal compound, for example a pure or unalloyed metal, an alloy thereof, an inorganic compound such as a ceramic comprising the metal, a metal chalcogenide, or an organometallic comprising the metal and/or mixtures thereof.

24. The composition according to any of claims 21 to 23, wherein the binder comprises and/or is a solvent, for example an organic solvent, an aprotic solvent and/or a polar solvent such as propylene carbonate, and/or a plasticizer such as dimethylcarbonate, diethylcarbonate, dibutyl phthlalate, tetramethylurea, triethylphosphate and trimethylphosphate.

25. A method of providing a composition having a relative permittivity, preferably a real part ε′.sub.r of a relative complex permittivity ε.sub.r, substantially inversely proportional to the square of a frequency of an alternating electrical field, preferably due to an electromagnetic field, for example applied thereto and/or propagating therethrough, wherein the method comprises: including a solid phase in a liquid phase; and optionally solidifying, at least in part, the liquid phase, thereby providing the composition.

26. An electronic device comprising a composition according to any of claims 1 to 24.

27. Use of dispersed powders in a dielectric matrix as an antenna.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0124] For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

[0125] FIG. 1A shows a graph of relative permittivity ε.sub.r as a function of frequency f of electromagnetic radiation of an ideal material; and FIG. 1B shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation between 0 and 18 GHz for a composition according to an exemplary embodiment and for a comparative example;

[0126] FIG. 2A shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation for acetone, propan-2-ol, NMF and DMF; and FIG. 2B shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation for N-methylpropionamide, N-(cyanomethyl)formamide, N-(2-hydroxyethyl)formamide and formamide;

[0127] FIG. 3A shows a photograph of permittivity measurement set-up/procedure; FIG. 3B shows a comparison of the real parts of relative complex permittivities (ε′.sub.r) for dielectric reference liquids DMSO, MeOH and EtOH between the data from the NPL and this work (experimental) up to 5 GHz; FIG. 3C shows a comparison of the real part of relative complex permittivity (ε.sup.′.sub.r) of H.sub.2O at 25° C. up to 18 GHz between the literature and this work (experimental); FIG. 3D shows a graphical example of the extrapolation procedure used to determine the static dielectric constants.

[0128] FIG. 4 shows structures, names and abbreviations of ionic liquids;

[0129] FIG. 5 shows procedures and characterisation data for prepared ionic liquids;

[0130] FIG. 6 shows spectra acquired for the ionic liquids;

[0131] FIG. 7A shows nomenclature for abbreviated IL cations; and FIG. 7B shows structures and abbreviations for complex anions;

[0132] FIG. 8A shows the effect of increasing chain length on the real part of the permittivity ε′.sub.r for (a) C.sub.nC.sub.1im BF.sub.4 and (b) C.sub.nC.sub.1im NTf.sub.2 IL series; and FIG. 8B shows the effect of a methyl group on the complex permittivity of butyl-imidazolium NTf.sub.2 ILs;

[0133] FIG. 9 shows complex permittivity spectra of Choline AA IL series;

[0134] FIG. 10 shows the effect of increasing H.sub.2O concentration on the complex permittivity spectra of (a) N.sub.HHH(2OH) HCO.sub.2 and (b) N.sub.HHH(2OH) CH.sub.3CO.sub.2;

[0135] FIG. 11A shows a method of providing a composition, according to an exemplary embodiment;

[0136] FIG. 11B shows a method of providing a composition, according to an exemplary embodiment; and FIG. 11C shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation for the compositions of FIGS. 11A and 11B;

[0137] FIG. 12A shows a method of providing a composition, according to an exemplary embodiment;

[0138] FIG. 12B shows a method of providing a composition, according to an exemplary embodiment;

[0139] FIG. 12C shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation for the composition of FIG. 12A; and FIG. 12D shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation for the compositions of FIGS. 12A and 12B; and

[0140] FIG. 13A shows a method of providing a composition, according to an exemplary embodiment;

[0141] FIG. 13B shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation for the composition of FIG. 13A.

DETAILED DESCRIPTION OF THE DRAWINGS

[0142] FIG. 1A shows a graph of relative permittivity ε.sub.r as a function of frequency f of electromagnetic radiation between 0 and 30 GHz of an ideal material, in which the relative permittivity ε.sub.r ∝1/f.sup.2 (i.e. ideal behaviour). In this way, the effective wavelength λ of the electromagnetic radiation in the ideal material is constant as a function of the frequency f of the electromagnetic radiation between 0 and 30 GHz, as also shown in FIG. 1A. FIG. 1A also shows the effective wavelength λ of the electromagnetic radiation in free space.

[0143] FIG. 1B shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation between 0 and 18 GHz for a composition according to an exemplary embodiment, labelled ‘Liquid 1+Filler’ particles, and for a comparative example, labelled ‘Liquid 2+Filler’ particles. Shown also are the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation between 0 and 18 GHz for ‘Liquid 1’ and for ‘Liquid 2’. In FIG. 12C, between 0.04 and 0.08 GHz, the real part ε′.sub.r of the relative complex permittivity of the composition is reduced by 4 times from about 50 to about 12.5. That is, the exemplary embodiment approaches the desired behaviour of the ideal material of FIG. 1A.

[0144] Liquid 1 is an ionic liquid (IL) Ch L-ala (cholinium L-alaninate).

[0145] Liquid 2 is DMSO (dimethylsulfoxide).

[0146] The filler particles are precipitated copper particles, having a diameter of about 10 μm.

[0147] The composition ‘Liquid 1+Filler particles’ was prepared by dispersing the copper particles therein. The viscosity of Liquid 1 is sufficiently high to suspend the copper particles therein for a period of time.

[0148] The composition ‘Liquid 2+Filler particles’ was prepared by dispersing the copper particles in a mixture of warmed Liquid 2 and a small amount (<2 wt. %) of synthesised gelator molecules. After cooling, the mixture (i.e. Liquid 2 including the gelator) gelates and immobilises the dispersed copper particles within the self-assembled gel matrix.

Example 1—Liquid+Liquid+Gelator (L+L+G)

[0149] Generally, Composition II may comprise two organic liquids from which variation of the ratios of each liquid produces a new mixture which has a permittivity inversely proportional to the square of the frequency in a given range, or closer to it. This liquid composite may be gelated or “hardened” using organo-gelator additives or polymer additives (heating may be required).

[0150] An example of L+L+G composite material is described below. Example 1 is an example of Composition II. Scheme 1 (FIG. 11A) describes the synthesis and composition, and FIG. 11C displays the complex permittivity spectra which demonstrates a new complex permittivity can be achieved by varying the ratios of liquids 1 & 2.

[0151] FIG. 11C shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation between 0 and 2 GHz for a composition according to an exemplary embodiment, particularly showing curves for N-methylisobutyramide (labelled ‘Liquid 1’), N-methylpropionamide (labelled ‘Liquid 2’), a composition according to an exemplary embodiment comprising N-methylisobutyramide, N-methylpropionamide and a gelator (labelled ‘L+L+G’) and a composition according to an exemplary embodiment comprising N-methylisobutyramide, N-methylpropionamide, Cu particles and a gelator (labelled ‘L+L+G+P’). This Gelated Composite shows an E′-f trace different from that of either Liquid 1 or Liquid 2. Addition of certain particles (such as copper) can increase the overall dielectric constant. This can be used to tune the permittivity. See difference between dashed trace vs dotted trace (i.e. for ‘L+L+G’ vs “L+L+G+P”). Composite materials which satisfy the inverse square rule will benefit from the addition of high dielectric constant particles (such as copper), especially when approaching higher frequencies (up to 30 GHz).

[0152] The composition labelled ‘L+L+G’ was prepared, as shown in FIG. 12, by mixing N-methylisobutyramide (0.9 g, 8.90 mmol) and N-methylpropionamide (1.8 g, 20.67 mmol) to create a desired permittivity. LMWG R—C.sub.12C.sub.12 (as described above) (10 mg, 0.021 mmol) was added to 1 mL of the liquid mixture, which was then heated to 70° C. with stirring until all LMWG had dissolved 5-10 mins). The mixture was allowed to cool to room temperature, upon which a solid composite material was formed.

Example 2— Liquid+Liquid+Gelator+Particles (L+L+G+P)

[0153] Generally, composition III may comprise two organic liquids from which variation of the ratios of each liquid produces a new mixture which has a permittivity inversely proportional to the square of the frequency in a given range, or closer to it. This liquid composite may be gelated or “hardened” using organo-gelator additives or polymer additives (heating may be required) which would then be able to host a variety of particles, which can also contribute to altering the overall permittivity of the composite material.

[0154] An example of L+L+G+P composite material is described below. Example 2 is an example of Composition III. Scheme 2 (FIG. 11B) describes the synthesis and composition, and FIG. 11C displays the complex permittivity spectra which demonstrates how addition of metal particles (in this example copper) can increase the overall dielectric constant Vs frequency.

[0155] The composition labelled ‘Liquid 1+Liquid 2+Cu Particles Gelated Composite’ was prepared, as shown in FIG. 12, by mixing N-methylisobutyramide (0.9 g, 8.90 mmol) and N-methylpropionamide (1.8 g, 20.67 mmol) to create a desired permittivity. LMWG R—C.sub.12C.sub.12 (as described above) (10 mg, 0.021 mmol) was added to 1 mL of the liquid mixture, which was then heated to 70° C. with stirring until all LMWG had dissolved 5-10 mins). Copper powder (precipitated copper particles, having a diameter of about 10 μm) (33.3 wt. %) was added to the hot mixture and stirred for 5 mins. The mixture was allowed to cool to room temperature, upon which a solid composite material was formed.

Example 3—Liquid+Solid+Gelator (L+S+G)

[0156] Comprises an organic solid dissolved in an organic liquid from which variation of the amount of dissolved solid produces a new liquid mixture which has a permittivity inversely proportional to the square of the frequency in a given range, or closer to it. This liquid composite may be gelated or “hardened” using organo-gelator additives or polymer additives (heating may be required).

[0157] An example of L+S+G composite material is described below. Example 3 is an example of Composition II. Scheme 3 (FIG. 12A) describes the synthesis and exact composition, and FIG. 12C displays the complex permittivity spectra which demonstrates how addition of an organic solid (in this example NMLA) to a liquid (in this example NMIBA) can tune the complex permittivity close to the calculated curve, where the permittivity is inversely proportional to the square of the frequency (f) in a given range.

[0158] FIG. 12C shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation between 0 and 0.2 GHz for a composition according to an exemplary embodiment, particularly showing curves for N— N-methylisobutyramide (labelled ‘Liquid NMIBA’), a composition according to an exemplary embodiment comprising N-methyllauramide and N-methylisobutyramide (labelled ‘Liquid (NMIBA)+Solid (NMLA) Mixture’), a composition according to an exemplary embodiment comprising N-methyllauramide, N-methylisobutyramide and a gelator (labelled ‘Liquid+Solid Gelator Composite’) and a calculated inverse square curve. The ‘Liquid (NMIBA)+Solid (NMLA) Mixture’ composition and the ‘Liquid+Solid Gelator Composite’ show quasi-ideal behaviour. Particularly, these compositions have a permittivity inversely proportional to the square of the frequency (f) in a given range.

[0159] The composition labelled ‘Liquid (NMIBA)+Solid (NMLA) Mixture’ was prepared, as shown in FIG. 12A, by mixing N-methyllauramide and N-methylisobutyramide.

[0160] The composition labelled ‘Liquid+Solid Gelator Composite’ was prepared, as shown in FIG. 12A, from the composition labelled ‘Liquid+Solid Mixture’ by adding the gelator R—C.sub.12C.sub.12, as shown below, mixing for about 10 minutes at 70° C. and cooling to room temperature.

[0161] Gelator R—C.sub.12C.sub.12:

##STR00003##

Example 4—Liquid+Solid+Gelator+Particles (L+S+G+P)

[0162] Generally, Composition III comprises an organic solid dissolved in an organic liquid from which variation of the amount of dissolved solid produces a new liquid mixture which has a permittivity inversely proportional to the square of the frequency in a given range, or closer to it. This liquid composite may be gelated or “hardened” using organo-gelator additives or polymer additives (heating may be required) which would then be able to host a variety of particles, which can also contribute to the overall permittivity of the composite material.

[0163] An example of L+S+G+P composite material is described below. Example 4 is an example of Composition III. Scheme 4 (FIG. 12B) describes the synthesis and exact composition, and FIG. 12D displays the complex permittivity spectra which demonstrates the permittivity boost effect from addition of metal particles (in this example copper), which can be tuned as required by varying the quantity.

[0164] FIG. 12D shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation between 0 and 10 GHz for a composition according to the exemplary embodiment, described above, comprising N-methyllauramide and N-methylisobutyramide (labelled ‘NMIBA+NMLA’), and a composition according to an exemplary embodiment comprising N-methyllauramide and N-methylisobutyramide and additionally comprising Cu particles (labelled ‘NMIBA+NMLA+Cu’). The Cu particles enhance the experimentally-determined real part ε′.sub.r of the relative complex permittivity while showing quasi-ideal behaviour.

[0165] The composition labelled ‘NMIBA+NMLA+Cu’ was prepared, as shown in FIG. 12B.

Example 5—Polymer+Particles+Liquid (Po+P+L)

[0166] Generally, Composition IV comprises a polymer (insulating matrix material) mixed with particles or a combination of particles of different types (filler material) and also a liquid additive which may behave as a solvent or plasticizer binding the composite material together (heating may be required), all at which certain ratios of materials correspond to a desired complex permittivity inversely proportional to the square of the frequency in a given range, or close to it. The complex permittivity of these composite materials can be tuned by varying the ratios of starting materials. Examples of Po+P+L composite materials are described below. Example 5 is an example of Composition III. Scheme 5 (FIG. 13A) describes the synthesis and exact compositions, and FIG. 13B displays the complex permittivity spectra which demonstrate how increasing the wt. % of carbon filler particles can increase the overall permittivity and sharpen the relaxation.

TABLE-US-00001 TABLE Composite compositions (wt. %) Composite Composite Composite Composite A B C D PVDF 34 34 34 34 BTO 34 27 24 17 Carbon 0 7 10 17 PC 32 32 32 32

[0167] FIG. 2A shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation between 0 and 18 GHz for acetone, propan-2-ol, NMF and DMF.

[0168] FIG. 2B shows a graph of the experimentally-determined real part ε′.sub.r of the relative complex permittivity as a function of frequency f of electromagnetic radiation between 0 and 18 GHz for N-methylpropionamide, N-(cyanomethyl)formamide, N-(2-hydroxyethyl)formamide and formamide.

[0169] N-methylpropionamide shows quasi-ideal behaviour (i.e. ε′.sub.r ∝1/f.sup.2).

[0170] Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above.

[0171] Experimental Methods and Preparation of Ionic Liquids

0. Introduction

[0172] The behaviour of molecules under the influence of electromagnetic waves at frequencies >1 THz has been extensively studied in the field of physical chemistry and closely related subjects using techniques such as infra-red and UV-visible spectroscopy. However, the influence of radio and microwave frequencies on the behaviour of molecules, such as relative complex permittivity, is comparatively less researched or considered, especially in the field of chemistry. The relative complex permittivity of a material provides useful information on the dielectric relaxation and absorption processes that are prevalent in a wide frequency range. Information relating to polarity, conductivity, dielectric loss, ionic/dipolar relaxations and atomic/electronic resonances can be deduced from the complex permittivity spectrum of a given material [2]. Relative complex permittivity ε.sub.r can be defined as in Equation (1), where ε is the absolute permittivity and ε.sub.0 is the vacuum permittivity (often referred to as the permittivity of free space). The value of ε.sub.0 is fixed at 8.85×10.sup.−12 F/m. The real and imaginary parts of the relative complex permittivity are experimentally measured and denoted by ε′ and ε″, respectively, and ω describes the dependence upon frequency and is called angular frequency which is ω=2πf.

[00003]$\begin{array}{cc}{\epsilon }_r\left(\omega \right)=\frac{\epsilon \left(\omega \right)}{{\epsilon }_0}={\epsilon }^{\prime }\left(\omega \right)-i{\epsilon }^{″}\left(\omega \right)& \mathrm{Equation}\left(1\right)\end{array}$

[0173] Complex permittivity is a very important material property for electromagnetics and related subjects. Data derived from relative complex permittivity measurements have been utilized in applications, such as liquid antennas, radio-frequency and microwave devices, lithium batteries, carbon nanotube (CNT) microwave absorbers and even field-deployable sensors for the detection of improvised explosives. The availability of accurate permittivity data, both static (i.e. 0 Hz, or direct current (DC)) and as a function of frequency, for ionic liquids (ILs) and other solvents, liquids and materials, will play a vital role in the development and discovery of new technologies and spectroscopic techniques.

[0174] Dielectric loss describes the inherent dissipation of electromagnetic energy by a material and can be defined by the loss tangent tan δ.sub.e as in Equation (2). It is typically frequency-dependent and parametrized using the real and imaginary parts of the relative complex permittivity as described in Equation (1). For time-varying electromagnetic fields, the electromagnetic energy is typically viewed as waves propagating either through free space, in a transmission line, or through a waveguide. Dielectrics are often used in all of these environments to mechanically support electrical conductors and keep them at a fixed separation. In these scenarios, the electromagnetic energy is often dissipated in the form of heat.

[00004]$\begin{array}{cc}\tan {\delta }_e=\frac{{\epsilon }^{″}\left(\omega \right)}{{\epsilon }^{\prime }\left(\omega \right)}& \mathrm{Equation}\left(2\right)\end{array}$

[0175] The relative permittivity of a material at 0 Hz is referred to as the static relative permittivity (often called the static dielectric constant) and is normally derived by extrapolation of values measured at higher frequencies. It can also be measured through techniques such as capacitance measurements of a capacitor at DC (0 Hz). There are many literature sources that report static dielectric constants of organic molecules, solvents, binary mixtures but far fewer for ILs.

[0176] ILs are generally described as salts completely comprised of ions with melting points lower than 100° C., although the term room temperature ionic liquid (RTIL) has been more commonly used recently and invokes a much lower melting temperature. First reported over 100 years ago (ethylammonium nitrate, melting point 12° C.), ILs have received a huge increase in interest over the last two decades, with over 5000 publications in 2016 alone. ILs boast highly desirable properties, such as a wide liquid range, extremely low volatility, good conductivity, excellent electrochemical windows, tunable polarity, thermal stability and low flammability. ILs have been described as “designer solvents” and due to the anionic/cationic combinatorial nature of their preparation, ILs can be customized to almost any desired requirements. Thousands of articles utilizing ILs have been reported in the fields of energy, materials, nano-area, electrochemistry and catalysis with novel and more niche applications arising regularly. Recent articles with novel applications of ILs include battery technologies, metal-organic frameworks (MOFs), separation of rare-earth minerals and an ever increasing amount of research in the field of catalysis.

[0177] The static dielectric constants of ionic liquids have been reported with updated values found in more recent publications. There appear to be some discrepancies within the literature over the accuracy of values derived using various measurement techniques, such as microwave dielectric spectroscopy and polarity-sensitive fluorescent molecular probes. This variation has reportedly been linked to their inherent conductivity which can essentially short-circuit the system when directly measuring ε using fluorescent probes. To the best of our knowledge, there are no studies of complex permittivity (i.e. frequency dependent dielectric constants) reported for pure ILs so far, except for the original IL N.sub.HHH2 NO.sub.3. This study provides data on the dielectric behaviour of ILs as a function of frequency.

[0178] Static dielectric constants are a useful indicator of polarity, often presented along with dipole moment or polarity index values. However, complex permittivity (over a broad frequency range) can provide additional information vital to many chemical and electrical engineers. Recently we have demonstrated, for the first time, that ILs can be used to fabricate dielectric resonator antennas (DRAs) with their performance inherently linked to their frequency-dependent dielectric constants and the loss factor derived from the real and imaginary parts of the permittivity spectrum (Equation 2).

[0179] Herein we report the experimentally derived complex permittivity data of 64 imidazolium, choline, phosphonium, ammonium, pyrrolidinium, pyridinium and piperidinium ILs in the range of 1 to 18 GHz and extrapolated static values at 0 Hz (measured down to 30 KHz). The information disclosed will be of value when selecting a material for engineering applications which require precise electromagnetic properties.

1. General Information

[0180] All synthetic reactions were carried out under an atmosphere of N.sub.2(g) using standard techniques, all chemicals were purchased commercially and used as received without further purification (unless otherwise stated), and all solvents were of HPLC grade. .sup.1H, .sup.13C, .sup.31P and .sup.19F NMR spectra were recorded on a Bruker 400 MHz spectrometer at 298 K using deuterated solvents as specified for each compound.

2. Materials

[0181] High purity samples of C.sub.2C.sub.1im BF.sub.4, C.sub.2C.sub.1im CH.sub.3CO.sub.2, C.sub.2C.sub.1im EtOSO.sub.3, C.sub.2C.sub.1im N(CN).sub.2, C.sub.2C.sub.1im OTf, C.sub.2C.sub.1im NTf.sub.2, C.sub.2C.sub.1im SCN, C.sub.2C.sub.1im DEP, C.sub.2C.sub.1im FeCl.sub.4, C.sub.3C.sub.1im I, C.sub.3C.sub.1im NTf.sub.2, C.sub.4C.sub.1im BF.sub.4, C.sub.4C.sub.1im SCN, C.sub.4C.sub.1im MeOSO.sub.3, C.sub.4C.sub.1im N(CN).sub.2, C.sub.4C.sub.1im NTf.sub.2, C.sub.4C.sub.1C.sub.1im NTf.sub.2, C.sub.6C.sub.1im NTf.sub.2, C.sub.6C.sub.1im PF.sub.6, C.sub.6C.sub.1im I, P.sub.4441 MeOSO.sub.3, P.sub.666(14) Cl, P.sub.666(14) NTf.sub.2, S.sub.222 NTf.sub.2, N.sub.HHH2 NO.sub.3, N.sub.8881 NTf.sub.2, N.sub.122(2O1) BF.sub.4, C.sub.4C.sub.1Pyrr OTf, C.sub.3C.sub.1Pip NTf.sub.2 were purchased from loLiTec, Germany. High purity samples of C.sub.6C.sub.1im BF.sub.4, C.sub.4C.sub.1Pyrr N(CN).sub.2, C.sub.4C.sub.1Pyrr FAP, C.sub.4-3-MePyrr NTf.sub.2, C.sub.6py BF.sub.4, C.sub.6py NTf.sub.2 and C.sub.4-3-Mepy MeOSO.sub.3 were purchased from Merck, UK. C.sub.2OHC.sub.1im OTf, C.sub.2OHC.sub.1im PF.sub.6 and C.sub.3OC.sub.1im PF.sub.6 were purchased from Solchemar, Portugal. N.sub.(2OH)(2OH)(2OH)1 MeOSO.sub.3 was purchased from Fluka, UK. C.sub.2C.sub.1im L-Lac and C.sub.4C.sub.1im L-Lac were purchased from Acros Organics.

3. Permittivity Measurement Set-Up/Procedure

[0182] FIG. 3A shows a photograph of permittivity measurement set-up/procedure.

[0183] The composite materials are characterised and evaluated in terms of the permittivity, conductivity, thermal and mechanical stability as a function of the frequency, temperature and RF/microwave power level. Most material characterisation measurements are undertaken at room temperature. For this work, the measurement temperature is extended down to −40° C. and up to +70° C. to cover extreme conditions. The Keysight Technology high temperature probe can withstand a wide temperature range (−40° C. to +200° C.) and is used for this measurement. The measurement setup for high temperature measurements is shown in FIG. 3A where the sample under test and the probe can be tuned to the desired temperature. The temperature is maintained constant during the measurement and a digital thermal probe is used to monitor the temperature. The system is computer controlled using LabVIEW®. This measurement system is most suitable for liquid materials. For solid materials, the measurement accuracy could be an issue if an air-gap between the probe and sample is introduced during the measurement, thus an alternative measurement system to cover solid materials and a computer controlled measurement and real time data collecting system, using either LabVIEW or MATLAB®, is employed.

[0184] In more detail, there are a number of methods for permittivity measurements with their pros and cons. One of the most suitable methods for liquid material measurement is the open-end coaxial probe approach which has been used for our study. It gives a high degree of accuracy over a wide frequency range. We used an Agilent N9917A FieldFox Microwave Vector Network Analyser (VNA) and a Keysight 85070E Dielectric Probe Kit. The measurement frequency range of the instrument was set from 30 kHz to 18 GHz with 1001 data points. The radio-frequency output power level of the VNA was set to 0 dBm. A 3-point calibration method was employed which was comprised of open-circuit (in air), short-circuit (calibration block) and 25° C. distilled water measurements and as per the manufacturer's guidelines. Each unknown sample was measured using multiple probes and after repeated calibrations to confirm identical permittivity spectra were obtained. All measurements were conducted at 25° C. (unless otherwise stated) under an inert nitrogen atmosphere. The liquid samples were dried in a vacuum oven for >24 h prior to use and held using 10 mL standard lab glass vials with PTFE lined caps. The probe was immersed into the IL samples with a depth of 20-30 mm from the liquids surface. The relative complex permittivities of the compounds, including the real and imaginary parts, were measured accordingly.

[0185] To ensure reliability of results before measuring unknown liquid samples, compounds with well-defined relative complex permittivity spectra found in the literature were first measured. These reference liquids include dimethylsulfoxide (DMSO), methanol (MeOH) and ethanol (EtOH), the dielectric constants of which have been described to a high degree of accuracy by the National Physical Laboratory (NPL, U.K.) up to a frequency of 5 GHz. As shown in FIG. 3B, our experimentally measured results are in excellent agreement with the literature.

[0186] A further comparison with the literature data, recreated from the tables in the literature, can be seen in FIG. 3C, using H.sub.2O measured at 25° C. up to 18 GHz, which is also the high frequency limit of our presented data. Again our data shows excellent agreement with the literature across the broad measured frequency range.

[0187] In order to validate the accuracy of our results, the same measurement and extrapolation procedures were employed for selected common liquid materials with static dielectric constants well defined in the literature by a myriad of sources. The static dielectric constants derived using our method were found to be in excellent agreement with the literature sources; a comparison between the experimental and literature values is shown in Table 1. The reference values presented in Table 1 are the highest and lowest published static dielectric constants available; however, it should be noted that the vast majority of the literature values generally lie somewhere between. The liquids chosen include those with negligible contributions from the imaginary part ε″ of the complex permittivity (i.e. acetonitrile) and those with considerable imaginary contributions (i.e. propylene carbonate). It should also be noted that our experimental values of these liquid materials are also in excellent agreement with those available in the literature at higher frequencies, which further confirms the validity of the measured results within this paper.

TABLE-US-00002 TABLE 1 Comparison of experimental static dielectric constants derived in this study with values found in the literature. Static ε′ Static ε′ Liquid Experimental Literature H.sub.2O 78.3 77.9-79.5 DMSO 46.2 45.9-48.8 Acetone 20.1 19.1-20.8 DMF 38.1 36.7-40.2 Acetonitrile 35.8 34.3-37.0 Propylene Carbonate 64.9 63.0-65.5

[0188] The dielectric constants presented in Table 2, Table 3, Table 4, Table 5, Table 6 (vide infra) within the frequency range of 1-18 GHz are taken directly from the experimentally measured data. The reported dielectric constants in the tables at 0 Hz (static dielectric constants) were extrapolated from the experimental data (measured down to the low frequency of 30 KHz, which is the approximate accuracy range of the Keysight 85070E Dielectric Probe Kit [39]). The polynomial curve fitting (PCF) method of MATLAB 2017 was then employed to extrapolate the values. An example of this low frequency extrapolation can be seen in FIG. 3D, where the solid line represents the experimental data and the markers indicate the extrapolated values.

[0189] Full details of the 64 commercially available and synthesized ILs used in this study are presented below, including the full structures, chemical names, abbreviated names, synthetic procedures and characterisation data. Also included are individual spectra for all ILs measured containing both real ε′ and imaginary ε″ parts of the relative complex permittivity ε.sub.r.

4. Structures, Names and Abbreviations of Ionics Liquids

[0190] FIG. 4 shows structures, names and abbreviations of ionic liquids. All abbreviated names state the Cation+first followed by the Anion-.

5. Procedures and Characterisation Data for Prepared Ionic Liquids

[0191] FIG. 5 shows procedures and characterisation data for prepared ionic liquids.

6. Relative Complex Permittivity Spectra (ε.SUB.r.)—Real (ε′) and Imaginary Parts (ε″)

[0192] FIG. 6 shows spectra acquired for the ionic liquids. The spectra are experimental data without extrapolation or interpolation performed.

7. References

[0193] [1] R. Bini, C. Chiappe, V. L. Mestre, C. S. Pomelli, T. Welton, A rationalization of the solvent effect on the Diels—Alder reaction in ionic liquids using multiparameter linear solvation energy relationships, Org. Biomol. Chem. 6 (2008) 2522. doi:10.1039/b802194e. [0194] [2] N. Oberleitner, A. K. Ressmann, K. Bica, P. Gartner, M. W. Fraaije, U. T. Bornscheuer, F. Rudroff, M. D. Mihovilovic, From waste to value-direct utilization of limonene from orange peel in a biocatalytic cascade reaction towards chiral carvolactone, Green Chem. 19 (2017) 367-371. doi:10.1039/c6gc01138a. [0195] [3] Q.-P. Liu, X.-D. Hou, N. Li, M.-H. Zong, Ionic liquids from renewable biomaterials: synthesis, characterization and application in the pretreatment of biomass, Green Chem. 14 (2012) 304-307. doi:10.1039/C2GC16128A. [0196] [4] S. De Santis, G. Masci, F. Casciotta, R. Caminiti, E. Scarpellini, M. Campetella, L. Gontrani, Cholinium-amino acid based ionic liquids: a new method of synthesis and physico-chemical characterization, Phys. Chem. Chem. Phys. 17 (2015) 20687-20698. doi:10.1039/C5CP01612F. [0197] [5] A. Xu, X. Guo, Y. Zhang, Z. Li, J. Wang, Efficient and sustainable solvents for lignin dissolution: Aqueous choline carboxylate solutions, Green Chem. 19 (2017) 4067-4073. doi:10.1039/c7gc01886j. [0198] [6] Z. Li, X. Liu, Y. Pei, J. Wang, M. He, Design of environmentally friendly ionic liquid aqueous two-phase systems for the efficient and high activity extraction of proteins, Green Chem. 14 (2012) 2941. doi:10.1039/c2gc35890e. [0199] [7] Q.-P. Liu, X.-D. Hou, N. Li, M.-H. Zong, SUPP MAT—Ionic liquids from renewable biomaterials: synthesis, characterization and application in the pretreatment of biomass, Green Chem. 14 (2012) 304. doi:10.1039/c2gc16128a. [0200] [8] Y. Zhang, S. Zhang, X. Lu, Q. Zhou, W. Fan, X. Zhang, Dual Amino-Functionalised Phosphonium Ionic Liquids for CO 2 Capture, Chem.—A Eur. J. 15 (2009) 3003-3011. doi:10.1002/chem.200801184. [0201] [9] X. L. Yuan, S. J. Zhang, X. M. Lu, Hydroxyl Ammonium Ionic Liquids: Synthesis, Properties, and Solubility of SO2, J. Chem. Eng. Data. 52 (2007) 596-599. doi:10.1021/je060479w. [0202] [10] N. Iranpoor, H. Firouzabadi, Y. Ahmadi, Carboxylate-based, room-temperature ionic liquids as efficient media for palladium-catalyzed homocoupling and sonogashira-hagihara reactions of aryl halides, European J. Org. Chem. (2012) 305-311. doi:10.1002/ejoc.201100701. [0203] [11] J. Sun, N. V. S. N. M. Konda, R. Parthasarathi, T. Dutta, M. Valiev, F. Xu, B. A. Simmons, S. Singh, One-pot integrated biofuel production using low-cost biocompatible protic ionic liquids, Green Chem. 19 (2017) 3152-3163. doi:10.1039/C.sub.7GC01179B.

8. Results and Discussion

[0204] Due to the cationic/anionic nature of IL structures and the huge number of possible combinations, including their associated long chemical names, an abbreviation system is often used. FIG. 7A gives a structural breakdown of the abbreviations used for IL cations in this study. 1-Ethyl-3-methylimidazolium, for example, has previously been abbreviated as EMIM; however, using the ethyl/methyl description would not suffice for some alkyl chains, such as propyl/pentyl. More recently a numbered R-group carbon system has been implemented and we have tried to stick to this naming system herein, e.g. EMIM now described as C.sub.2C.sub.1im.

[0205] For anion abbreviations, structural or molecular formulae work for most common anions, such as tetrafluoroborate (BF.sub.4) or formate (HCO.sub.2). FIG. 7B gives the structures associated with more complex anions and their abbreviations. When the source of the anion is an amino acid (AA), the standard 3-letter abbreviations have been used, i.e. L-Alaninate written as L-Ala.

[0206] Air and moisture stable imidazolium ILs are some of the most studied and commercially available classes of IL. From the results presented in Table 2, we can see the effect that increasing the imidazolium alkyl chain length has on the dielectric constant. In two of the most common IL series, C.sub.nC.sub.1im with BF.sub.4 anions and also with NTf.sub.2 anions, there is a significant drop in the overall permittivity when increasing alkyl chain length and therefore molecular weight, which can be further seen in FIG. 8A. Literature sources have shown that increasing alkyl chain length on imidazolium ILs results in an increase in viscosity, and this reduction in ionic mobility also correlates to a decrease in the overall polarity as well as electrical conductivity.

TABLE-US-00003 TABLE 2 Dielectric constants of imidazolium ionic liquids as a function of frequency f (GHz). Ionic liquid 0.sup.a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 C.sub.2C.sub.1im BF.sub.4 12.9 11.7 11 10.5 10.2 9.9 9.7 9.5 9.4 9.2 9.1 9 8.9 8.7 8.5 8.3 8.1 7.9 7.7 C.sub.2C.sub.1im 16.3 13 11.5 10.7 10.1 9.7 9.4 9.1 8.9 8.7 8.5 8.4 8.3 8.2 8.1 8 7.9 7.9 7.9 CH.sub.3CO.sub.2 C.sub.2C.sub.1im 24.4 13.3 11.1 10.1 9.4 9 8.7 8.4 8.2 7.9 7.8 7.7 7.5 7.4 7.4 7.3 7.2 7.1 7.1 EtOSO.sub.3 C.sub.2C.sub.1im 12.5 11.8 11.1 10.5 9.9 9.5 9.1 8.9 8.6 8.4 8.3 8.1 8 7.9 7.8 7.7 7.7 7.6 7.5 N(CN).sub.2 C.sub.2C.sub.1im OTf 19.3 14.1 12.7 11.6 10.9 10.3 9.9 9.6 9.3 9.1 8.8 8.6 8.4 8.2 7.9 7.6 7.4 7.1 6.9 C.sub.2C.sub.1im 13.8 11.5 9.9 8.9 8.3 7.8 7.4 7.1 6.9 6.7 6.5 6.4 6.3 6.2 6.1 6 6 5.9 5.9 NTf.sub.2 C.sub.2C.sub.1im 15.1 13.8 12.8 11.9 11.2 10.7 10.2 9.9 9.6 9.3 9.2 9 8.8 8.7 8.6 8.5 8.4 8.3 8.3 SCN C.sub.2C.sub.1im 8.1 6.8 6.3 6 5.9 5.8 5.8 5.8 5.8 5.8 5.8 5.8 5.8 5.8 5.7 5.7 5.6 5.6 5.5 DEP C.sub.2C.sub.1im L- 12.1 8.5 7.6 7.2 7 6.9 6.9 6.8 6.8 6.8 6.8 6.8 6.8 6.8 6.7 6.6 6.6 6.5 6.5 Lac C.sub.2C.sub.1im 13.1 9.6 8.5 7.9 7.5 7.3 7.2 7 6.9 6.8 6.8 6.7 6.6 6.6 6.5 6.3 6.2 6.1 6 FeCl.sub.4 C.sub.2OHC.sub.1im 20.7 11.6 9.8 8.8 8.3 8.1 7.9 7.7 7.6 7.5 7.4 7.3 7.2 7.2 7.1 6.9 6.7 6.6 6.6 OTf C.sub.2OHC.sub.1im 9.9 7.9 7.3 7 6.8 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.6 6.5 6.5 6.5 PF.sub.6 C.sub.3C.sub.1im I 5.5 5.2 5.2 5.2 5.2 5.2 5.1 5.1 5.1 5.1 5.1 5.1 5.1 5.1 5.1 5.1 5.1 5.1 5.1 C.sub.3C.sub.1im 9.4 8.1 7.2 6.7 6.3 6.1 6 5.9 5.8 5.7 5.7 5.6 5.5 5.5 5.4 5.3 5.2 5.1 5 NTf.sub.2 C.sub.3OC.sub.1im 8.6 7.6 7.1 6.8 6.7 6.6 6.5 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.3 6.2 6.2 6.1 6.1 PF.sub.6 C.sub.4C.sub.1im BF.sub.4 9.7 8.8 8.1 7.7 7.5 7.3 7.2 7.2 7.1 7.1 7.1 7 7 6.9 6.9 6.8 6.7 6.5 6.4 C.sub.4C.sub.1im 9.7 8.4 7.7 7.3 7 6.9 6.8 6.7 6.7 6.7 6.7 6.7 6.6 6.6 6.5 6.5 6.4 6.3 6.2 SCN C.sub.4C.sub.1im L- 6.6 5.6 5.4 5.3 5.3 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 Lac C.sub.4C.sub.1im 11.9 8.7 7.8 7.3 7.1 7 6.9 6.8 6.8 6.8 6.7 6.7 6.7 6.7 6.6 6.5 6.4 6.4 6.4 MeOSO.sub.3 C.sub.4C.sub.1im 10.3 9.4 8.7 8.2 7.9 7.6 7.5 7.4 7.3 7.2 7.1 7.1 7 6.9 6.8 6.6 6.5 6.4 6.3 N(CN).sub.2 C.sub.4C.sub.1im 9.2 8.2 7.4 6.8 6.4 6.2 6 5.9 5.8 5.7 5.6 5.5 5.5 5.5 5.4 5.4 5.4 5.4 5.4 NTf.sub.2 C.sub.4C.sub.1C.sub.1im 8.8 7.5 6.7 6.2 5.9 5.7 5.6 5.5 5.4 5.3 5.2 5.2 5.2 5.1 5.1 5.1 5.1 5.1 5.1 NTf.sub.2 C.sub.6C.sub.1im BF.sub.4 8.4 7.5 6.9 6.5 6.3 6.1 5.9 5.8 5.7 5.6 5.6 5.5 5.5 5.4 5.4 5.3 5.3 5.3 5.3 C.sub.6C.sub.1im 8.5 7.6 6.8 6.2 5.9 5.6 5.4 5.3 5.1 5 4.9 4.9 4.8 4.7 4.7 4.6 4.6 4.6 4.6 NTf.sub.2 C.sub.6C.sub.1im PF.sub.6 7.1 6.4 5.9 5.6 5.5 5.3 5.2 5.1 5 4.9 4.9 4.9 4.8 4.8 4.7 4.7 4.7 4.7 4.7 C.sub.6C.sub.1im I 4.3 4.3 4.3 4.3 4.3 4.3 4.4 4.4 4.5 4.5 4.6 4.6 4.7 4.7 4.7 4.7 4.7 4.8 4.8 C.sub.8C.sub.1im 6.9 6 5.4 5.1 4.8 4.7 4.6 4.5 4.4 4.3 4.3 4.2 4.2 4.2 4.2 4.2 4.1 4.1 4.1 NTf.sub.2 C.sub.8C.sub.1im Cl 3.2 3.3 3.5 3.6 3.6 3.7 3.7 3.8 3.8 3.9 4 4 4.1 4.1 4.1 4.1 4.2 4.2 4.3 .sup.aStatic dielectric constants derived by polynomial extrapolation; see experimental section, FIG. 3A and Table 1 for details.

[0207] The 2-position of imidazolium ILs are known to be reasonably acidic. This inherent acidity and the ability to generate NHC-type carbenes have been utilized in transition metal catalysis, although any non-innocent side reactions of ILs would generally be considered unwanted in the vast majority of IL applications. FIG. 8B shows the negligible effect of 2-position methylation on the permittivity of butyl-imidazolium ILs with NTf.sub.2 anions. An insignificant change in the overall permittivity spectrum and polarity can be deduced, implying that if required, one may substitute for a similar yet more chemically stable IL without sacrificing a polar environment.

[0208] Table 3 shows the relative complex permittivity measurements of choline ([Ch]) amino-acid (AA) or related carboxylic acid ILs. The general trend revealed implies again, that increasing molecular weight results in a decrease in frequency-dependent dielectric constants. This trend is observable in FIG. 9. Individual viscosity and conductivity values for specific ILs vary considerably across multiple literature sources. For example, Ch L-Pro has published viscosity values ranging between 0.5 and 10.6 Pa.Math.S and conductivity values from 0.3 to 7.5 μS cm.sup.−1. Thus, although there appears to be a correlation between increased molecular weight and decreased dielectric constants (also see FIG. 8A), any correlation of dielectric constants with the literature values of viscosity or conductivity should be cautious.

TABLE-US-00004 TABLE 3 Dielectric constants of choline ionic liquids as a function of frequency. All measurements performed at 298 K, under an inert N.sub.2(g) atmosphere. See data for imaginary parts of the dielectric spectra and full plotted graphs. Ionic liquid 0.sup.a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 [Ch] HCO.sub.2.sup.b 24.7 17.4 14.6 13.2 12.4 11.8 11.4 11 10.8 10.7 10.5 10.4 10.3 10.2 10.1 10 9.9 9.9 9.9 [Ch] L-Ala 31.4 10.6 9.4 8.7 8.3 8 7.8 7.6 7.4 7.3 7.3 7.2 7.1 7.1 7 7 6.9 6.9 6.9 [Ch] L-Cys 9.5 7.3 6.8 6.5 6.3 6.2 6.1 6 6 5.9 5.9 5.9 5.9 5.9 5.9 5.9 5.9 5.9 5.9 [Ch] L-His 9.1 7.1 6.6 6.3 6.1 5.9 5.8 5.7 5.7 5.6 5.6 5.6 5.5 5.5 5.5 5.5 5.5 5.5 5.5 [Ch] L-Lac 20.4 9.6 8.6 8 7.7 7.4 7.2 7.1 7 6.9 6.8 6.7 6.7 6.6 6.6 6.5 6.5 6.5 6.5 [Ch] L-Lys 9.8 8.3 7.4 7 6.7 6.5 6.4 6.2 6.1 6.1 6 6 5.9 5.9 5.9 5.9 5.8 5.8 5.8 [Ch] L-Phe 14.8 7.6 6.9 6.5 6.3 6.1 6 5.9 5.8 5.8 5.7 5.7 5.7 5.7 5.6 5.6 5.6 5.6 5.5 [Ch] L-Pro 12.6 8.2 7.4 7 6.7 6.5 6.4 6.3 6.2 6.1 6.1 6 6 6 6 6 6 6 6 [Ch] L-Tryp 7.7 6.4 6 5.8 5.6 5.5 5.4 5.3 5.3 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.1 [Ch]Cl/Urea.sup.c 9.8 8.9 8.3 7.8 7.5 7.3 7.2 7.1 7 6.9 6.9 6.8 6.7 6.7 6.6 6.6 6.6 6.6 6.6 [ACh] NTf.sub.2 17.5 8.5 7.4 6.9 6.5 6.2 6.1 5.9 5.9 5.8 5.7 5.6 5.6 5.6 5.6 5.6 5.5 5.5 5.6 .sup.aStatic dielectric constants derived by polynomial extrapolation; see experimental section, FIG. 3A and Table 1 for details; .sup.bMeasured at 311 K (liquid state of compound); and .sup.cCholine chloride:Urea in a 1:2 ratio, deep eutectic solvent (DES).

[0209] The complex permittivities of these completely organic [Ch] AA ILs have extremely small contributions from their imaginary parts ε″, which give these liquids somewhat unique properties among ILs (see data for full spectra displaying ε′ & ε″). Compared to the commonly used imidazolium series, they exhibit extremely low loss (see Equation (2)) and very low electrical conductivity σ.sub.ac, which can be calculated as in Equation (3), where ε′.sub.rε.sub.0 represents the real part of the absolute permittivity.

[00005]$\begin{array}{cc}{\sigma }_{ac}={\mathrm{\omega \epsilon }}_r^{\prime \left(\omega \right){\epsilon }_0}\tan {\delta }_e& \mathrm{Equation}\left(3\right)\end{array}$

[0210] The phosphonium ILs found in Table 4 all exhibit very little dielectric relaxation with stable permittivities over the full range measured up to 18 GHz. The dielectric constants are small and lie between ≈3-8, and the phosphonium series also possess extremely small imaginary parts and therefore dielectric loss. This is in contrast to the sulfonium IL included in Table 4, which shows some dielectric relaxation from low to high frequencies and also has more contributions from the imaginary part. Therefore, this sulfonium IL can be considered a more dielectrically lossy and electrically conductive material.

TABLE-US-00005 TABLE 4 Dielectric constants of phosphonium & sulfonium ionic liquids as a function of frequency f (GHz). All measurements performed at 298 K, under an inert N.sub.2(g) atmosphere. See data for imaginary parts of the dielectric spectra and full plotted graphs. Ionic liquid 0.sup.a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 aP.sub.4443 6.4 5.8 5.4 5.2 5.1 5 4.9 4.9 4.8 4.8 4.8 4.8 4.8 4.8 4.8 4.8 4.8 4.8 4.8 HCO.sub.2 aP.sub.4443 5.5 5.3 5.1 5 4.9 4.8 4.7 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 CH.sub.3CO.sub.2 aP.sub.4443 L- 5.1 4.9 4.7 4.6 4.5 4.4 4.4 4.4 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 Lac aP.sub.4443 L- 8.3 6.8 6.2 5.8 5.6 5.4 5.3 5.2 5.1 5.1 5 5 4.9 4.9 4.9 4.9 4.8 4.8 4.8 Val P.sub.4441 5.5 5.2 5.1 5 5 4.9 4.9 4.9 4.9 4.9 4.9 4.9 4.9 4.9 4.9 4.9 4.9 4.9 4.9 MeOSO.sub.3 P.sub.666(14) 3.6 3.5 3.5 3.4 3.3 3.3 3.2 3.2 3.2 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3 Cl P.sub.666(14) 4.9 4.3 4 3.8 3.7 3.6 3.5 3.5 3.4 3.4 3.4 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 NTf.sub.2 S.sub.222 NTf.sub.2 14 11.7 10.2 9.2 8.5 8.1 7.7 7.4 7.2 7 6.9 6.7 6.6 6.5 6.4 6.4 6.3 6.2 6.2 .sup.aStatic dielectric constants derived by polynomial extrapolation; see experimental FIG. 3A and Table 1 for details.

[0211] Table 5 presents the results for ammonium based ILs. It appears that N.sub.HHH2 NO.sub.3, the original IL discovered in 1914, has one of the largest overall complex permittivities across the full frequency range. It has a comparatively large static dielectric constant, which is smaller still than its imaginary part (see supplementary data), and therefore is extremely lossy, especially at frequencies <9 GHz. It has, however, been used as a conductive solvent in electrochemistry; so the large imaginary contribution is expected, Equation (3). N.sub.HHH(2OH) HCO.sub.2 exhibits similar properties to N.sub.HHH2 NO.sub.3 and shows a sharp dielectric relaxation as the frequency increases and with reports in the literature stating a conductivity of =0.34 S/m, the large imaginary contribution is again rationalized and expected. There is a contrast between this formate IL and the very closely related N.sub.HHH(2OH) CH.sub.3CO.sub.2 which differs only from a change in anion from formate to acetate. The broadband permittivity of the latter is significantly lower, and the material is much less electrically conductive and lossy. This could be explained by the vastly different viscosities, with the formate analogue having a much greater fluidity compared to the viscous acetate IL. The extra mobility of charge carriers obviously contributes to the conductivity, and the extra methyl group located within the anion could be said to contribute to the reduction in the overall polarity. The rest of the ammonium series progresses as expected, with additional alkyl groups of increasing length, and also with hydroxyl functionality, significantly reducing the dielectric relaxation and imaginary contributions. This is highly likely due to the increase in mass and the hydrogen bonding contributions both intra- and inter-molecularly from the —OH and —NH groups where relevant.

TABLE-US-00006 TABLE 5 Dielectric constants of ammonium ionic liquids as a function of frequency f (GHz). All measurements performed at 298 K, under an inert N.sub.2(g) atmosphere. See supplementary data for imaginary parts of the dielectric spectra and full plotted graphs. Ionic Liquid 0.sup.a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 N.sub.HHH2 NO3 25.4 20 16.6 14.5 13.3 12.5 11.9 11.5 11.1 10.8 10.6 10.4 10.2 10.1 9.9 9.8 9.7 9.6 9.5 N.sub.HHH(2OH) 80.5 28.9 19.2 15.4 13.3 12.3 11.5 10.8 10.4 10.1 9.9 9.6 9.3 9.2 9.2 9 8.9 8.9 8.9 HCO.sub.2 N.sub.HHH(2OH) 8.7 6.8 6.3 6 5.8 5.7 5.7 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 CH.sub.3CO.sub.2 N.sub.HHH(2OH) L- 8.3 7 6.3 5.9 5.6 5.4 5.2 5.1 5 5 5 5 5 5 5 5 5 5 5 Lac N.sub.(2OH)(2OH)(2OH)1 24.9 11.6 9.9 9 8.6 8.2 8 7.7 7.6 7.5 7.4 7.2 7.2 7.2 7.2 7.1 7.1 7.1 6.8 MeOSO.sub.3 N.sub.(2OH)(2OH)(2OH)1 6.4 5.9 5.6 5.4 5.3 5.2 5.2 5.1 5 5 5 5 5 5 4.9 4.9 4.9 4.9 4.8 L-Pro N.sub.8881 NTf.sub.2 5.4 4.4 4.1 3.9 3.8 3.7 3.6 3.6 3.5 3.5 3.5 3.5 3.4 3.4 3.4 3.4 3.4 3.4 3.4 N.sub.122(2O1) BF4 8.8 7.5 7 6.8 6.7 6.6 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.4 6.3 6.3 6.3 DIMCARB.sup.b 12.6 9.7 8.6 8 7.6 7.3 7.2 7 6.9 6.8 6.7 6.7 6.7 6.7 6.6 6.6 6.6 6.6 6.6 .sup.aStatic dielectric constants derived by polynomial extrapolation; see experimental FIG. 3A and Table 1 for details; .sup.bDIMCARB:Dimthylammonium dimethylcarbamate in a 1:1 ratio, distillable ionic liquid.

[0212] Table 6 presents the results for pyrrolidinium, pyridinium and piperidinium ILs which exhibit similar dielectric properties. They lie within the range of ε′≈5-10 with the highest found for C.sub.4C.sub.1Pyrr OTf. The other results are as expected with the largest imaginary contributions found for ILs with anions linked to greater electrical conductivities, i.e. OTf, N(CN).sub.2 and NTf.sub.2.

TABLE-US-00007 TABLE 6 Dielectric constants of pyrrolidinium, pyridinium and piperidinium ionic liquids as a function of frequency f (GHz). All measurements performed at 298 K, under an iert N.sub.2(g) atmosphere. See data for imaginary parts of the dielectric spectra and full plotted graphs. Ionic Liquid 0.sup.a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 C.sub.4C.sub.1Pyrr 10.5 9.1 8.2 7.7 7.4 7.2 7 6.9 6.8 6.8 6.7 6.6 6.6 6.5 6.4 6.3 6.1 6 6 OTf C.sub.4C.sub.1Pyrr 8.1 7.8 7.3 7 6.8 6.7 6.6 6.5 6.4 6.4 6.4 6.4 6.4 6.3 6.2 6.1 6 6 6 N(CN).sub.2 C.sub.4C.sub.1Pyrr 5.3 5 4.7 4.5 4.4 4.4 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.2 4.2 4.2 FAP C.sub.4-3- 7.9 7.1 6.5 6.1 5.9 5.7 5.6 5.6 5.5 5.5 5.4 5.4 5.4 5.3 5.2 5.1 5 5 5 MePyrr NTf.sub.2 C.sub.6py 5.7 5.5 5.4 5.3 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 Bf.sub.4 C.sub.6py 6 5.6 5.2 5 4.8 4.7 4.7 4.7 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.5 4.5 4.5 4.5 NTf.sub.2 C.sub.4-3- 9.5 7.6 7.1 6.8 6.7 6.6 6.5 6.5 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.3 6.2 6.2 6.2 Mepy MeOSO.sub.3 C.sub.3C.sub.1Pip 6.8 5.9 5.5 5.2 5 4.9 4.9 4.8 4.8 4.8 4.8 4.8 4.8 4.7 4.7 4.6 4.6 4.6 4.6 NTf.sub.2 .sup.aStatic dielectric constants derived by polynomial extrapolation; see experimental, FIG. 3A and Table 1 for details.

[0213] Values for extrapolated static dielectric constants (0 Hz frequency) found in the literature often vary. Table 7 shows a comparison between values derived in this study with those for the same ILs found within multiple literature sources.

TABLE-US-00008 TABLE 7 Comparison of IL static dielectric constants derived in this study with values found in the literature. Static ε′ Static ε′ Lit. IL Exp. [Ref] C.sub.2C.sub.1im BF.sub.4 12.9 12.8 12.9 13.6 C.sub.4C.sub.1im BF.sub.4 9.7 11.2 11.7 12.2 13.9 14.1 C.sub.2C.sub.1im NTf.sub.2 13.8 12.0 12.3 12.3 14.0 C.sub.3C.sub.1im NTf.sub.2 9.3 11.8 13.3 C.sub.4C.sub.1im NTf.sub.2 9.3 11.5 11.6 14.0 15.0 13.7 C.sub.2C.sub.1im N(CN).sub.2 12.5 11.7 C.sub.4C.sub.1im N(CN).sub.2 10.3 11.3 C.sub.6C.sub.1im PF.sub.6 7.1 8.9 15.5 N.sub.HHH2 NO.sub.3 25.4 26.2 26.3 S.sub.222 NTf.sub.2 14.0 13.2 15.8

[0214] The comparison of static dielectric constants shows a varied agreement with the literature. Some ILs are in good agreement. Our experimental value for S.sub.222 NTf2 of 14.0 lies neatly between the two reported literature values of 13.2 and 15.8, which were from the same laboratory. Other ILs, such as C.sub.2C.sub.1im BF.sub.4, C.sub.2C.sub.1im N(CN).sub.2 and N.sub.HHH2 NO.sub.3, are in good to excellent agreement with multiple literature sources, and values for N.sub.HHH2 NO3 are also in excellent agreement at higher frequencies. It can be noted that our values for higher order alkyl imidazoliums (such as C.sub.4C.sub.1im BF.sub.4 and NTf.sub.2) are often slightly lower than those of the literature.

[0215] It has been previously reported that the addition of water to ILs has little effect on the permittivity and that at IL weight fractions above 0.75, the static dielectric constant is rather insensitive to the water content. This assertion was deduced using N.sub.HHH(2OH) HCO.sub.2 as an example which already possesses a static permittivity close to that of H.sub.2O; thus the effect of water might be less visible. However, this may not be the case with other ILs with typically significantly lower static values, especially the protic ionic liquids (PILs) or those capable of hydrogen bonding. Indeed, significant changes in the permittivity spectra are observed. As shown in FIG. 10, the effect of increasing concentrations of H.sub.2O on the complex permittivity spectra of the PILs N.sub.HHH(2OH) formate and acetate is clearly visible. Although the effect on the static permittivity with respect to the formate analogue is negligible (80.5 for neat IL and 82.6 for a 50:50 IL:H.sub.2O mixture), the overall complex permittivity spectra are significantly altered when the concentration of highly polar H.sub.2O is increased. The effect is more dramatic on both the static and complex permittivity for the acetate analogue, which possesses a significantly lower static dielectric constant in its anhydrous form (8.7 for neat IL and 75.2 for a 50:50 IL:H.sub.2O mixture).

[0216] The most common route for IL synthesis (especially the well-researched imidazoliums) is via a metathesis reaction involving a halide salt precursor. This can result in trace halides remaining in the final ILs which cannot be removed by treatment at elevated temperatures and reduced pressures. Halide content of ILs has been shown to have an effect on the physicochemical properties, e.g. increasing viscosity and decreasing density. In order to determine whether residual halide content affects IL complex permittivities, we prepared and measured C.sub.2C.sub.1im NTf.sub.2 with varying concentrations of halide (<50 ppm, 250 ppm and 500 ppm). No change in the complex permittivity was observed, even when an extreme excess was added. Residual water content and temperature have much more significant effects on the complex permittivity and these should be of primary concern when measuring the dielectric constant of ILs or when applied to electronic devices exploiting permittivity. Several literature sources provide experimental methods for the quantitative halide determination of ILs, including microfluidic electrochemical devices and specialised capillary electrophoresis techniques.

9. Conclusions on ILs

[0217] A comprehensive comparison of permittivities was performed with common liquids and solvents, which have been well-defined in the literature from a multitude of sources, using the identical measurement and extrapolation procedure utilized for ILs in this study. Our data are in excellent agreement with the myriad of complex permittivity literature sources. The comparison of static permittivities for ILs between this study and the literature shows agreement ranging from excellent to poor. There are many factors for this disagreement, such as purity and age of samples, water content, the dielectric measurement technique used, the calibration procedure employed, and temperature sensitivity.

[0218] 64 ILs were selected in this study. These include 28 imidazolium ILs which are the most commonly used and reported cations, paired with a wide variety of anions, and 11 ILs comprising a choline cation with mostly amino acid anions, which generally exhibit higher viscosities when compared to more common ILs and are of interest to those exploiting green chemistry materials or when bio-compatibility is desirable. A sulfonium, 8 phosphonium and 9 ammonium cations were also chosen as these represent a significant portion of the other commonly available ILs classes. Finally, 8 pyrrolidinium, pyridinium and piperidinium ILs were included as these can form a wide variety of useful low-viscosity liquids and are easily synthetically customizable. We feel that the selected ILs represent a wide range of common and lesser known materials that exhibit a variety of thermal, physicochemical, bio-compatible and electromagnetic properties.

[0219] The frequency-dependent dielectric constants of all 64 ILs were found to be within the range 3-30 (with the exception of N.sub.HHH(2OH) HCO.sub.2) with most following a known general trend of dielectric relaxation when approaching higher frequencies. This relaxation is reversed, albeit to a limited degree, when considering the ILs C.sub.6C.sub.1im I and C.sub.8C.sub.1im Cl. The slight increase in the dielectric constant from 0 to 18 GHz (of 0.5 for C.sub.6C.sub.1im I and 1 for C.sub.8C.sub.1im Cl) may result from the presence of single halide anions (I— or Cl—) which are present in both materials and will be investigated further. Another general observed trend throughout all the series is the reduction in overall permittivity when the mass or alkyl chain length is increased. This feature is best visualized through the imidazolium and choline series (FIG. 8A, FIG. 9). FIG. 8B shows the effect that functionalization of the 2-position of imidazolium cations has on polarity. Thus, addition of a methyl group to this position does not significantly reduce c′ for C.sub.4C.sub.1C.sub.1im NTf.sub.2 compared to C.sub.4C.sub.1im NTf.sub.2. In relation to the imaginary contributions of the complex permittivity spectra, it can be said that ILs with high electrical conductivities have large imaginary parts, which is in accordance with the literature for these inherently linked properties.

[0220] We have also shown that the dielectric constants of ILs can be rather sensitive to the concentration of water and maintaining water content as low as possible is necessary for accurate measurement. This could be a reason for the previously reported high static values for the ILs N.sub.HHH(2OH) acetate/lactate. We also determined that residual halides in the ILs examined does not affect the observed complex permittivity.

[0221] From the full range of ILs and their relative complex permittivity spectra (available in the supplementary data), it may be possible to select liquid materials with desired requirements. ILs with more stable dielectric relaxations across the full frequency range have also been identified, i.e. C.sub.6py BF.sub.4 or ILs with multiple large alkyl groups such as P.sub.666(14) Cl. This phosphonium chloride IL (P.sub.666(14) Cl) has other beneficial properties such as a temperature-stable complex permittivity and a wide liquid range (−69.8° C. to 350° C.), ideal for applications that require extreme temperatures and stable dielectric performance. It has recently been exploited by our groups as the resonating material in a miniaturised ultra-wideband DRA, the first example of an ionic liquid RF device. If one's requirements involve a more conductive medium, ILs with large imaginary contributions may be considered; these include C.sub.2C.sub.1im N(CN).sub.2, S.sub.222 NTf.sub.2 and N.sub.HHH2 NO.sub.3. The presented complex permittivities, in combination with other reported physicochemical/thermophysical IL data, such as viscosity, density, conductivity, heat capacity and phase transition temperatures, can be of great use when selecting an IL with specific properties. In particular, the availability of a wide range of data will inform the application of ILs in catalysis, separation, electronics or perhaps a more comprehensive description of solvent properties.

[0222] Notes

[0223] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0224] All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and/or steps are mutually exclusive.

[0225] Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0226] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.