Introduction

Room temperature ionic liquids (RTILs) are with us for just over a century, when 104 years ago Walden described the properties of ethylammonium nitrate, C₂H₅NH₃⁺ NO₃^– [1]. This salt melts at 14 ºC [2] and in the molten state at 25 ºC it has a very low vapor pressure, ~5.3 Pa [3]. Evans et al. [4] pointed out the similarity of this molten salt to water, in terms of the thermodynamics of solubilities of inert gases and the ability of surfactants to form micelles in them. Molten ethylammonium nitrate forms a three-dimensional hydrogen bonded network, since the number of donor hydrogen atoms and acceptor sites on the oxygen atoms are well matched. Another unconventional RTIL, trifluoromethanesulfonic acid monohydrate, H₃O⁺ CF₃SO₃^– (hydronium trifluoromethanesulfonate), was first characterized by Gramstad and Haszeldine [5] as a white stable solid with a melting point of 34 ºC. Hydrogen bonding again plays a dominant role in the structure and function of its melt. This acid is analogous to a salt hydrate, where the cation is hydrated by a definite number of water molecules (one in this case) whereas the anion is not hydrated in this manner.

Molten salt hydrates may be considered to form a sub-group of room temperature ionic liquids. Many of them have melting points around room temperature, for instance [6]: lithium chlorate trihydrate, Li(H₂O)₃⁺ ClO₃^–, t_m = 8 °C, potassium fluoride tetrahydrate, K(H₂O)₄⁺ F^–, t_m = 18.5 °C, dipotassium hydrogenphosphate hexahydrate [K(H₂O)₃]₂²⁺ HPO₄^2–, t_m = 13 °C, and zinc chloride trihydrate, Zn(H₂O)₃²⁺ 2Cl^–, t_m­ = 6.5 ⁰C. Other salt hydrates may still melt congruently at somewhat above room temperature, but still below 100 ⁰C.

Deep eutectic solvents (DESs), introduced by Abbott et al. in 2003 [7] are another sub-group of room temperature ionic liquids (although non-ionic DESs have since been described). Deep eutectic solvents have freezing points below 25 ℃ (set as an arbitrary upper limit) and are binary compositions of two components (contrasting ordinary RTILs that are single compounds), each of which has a melting point above that of the deep eutectic solvent, hence they are eutectics. The DESs are characterized by one component (generally a salt, choline chloride being a well-known example) being a hydrogen bond acceptor and the other component (for example an amide, such as urea or a carboxylic acid) being a hydrogen bond donor. As solvents they should be non-inflammable, non-toxic, have low vapor pressures, and be friendly to the environment (‘green’, bio-degradable) in order to be useful for industrial processes.

When certain salt hydrates are mixed with water at definite ratios they form deep eutectic solvents, hence have very low freezing points (remembering that one of the components, water, has t_m = 0 °C). The ions of the salt hydrate act as the hydrogen bond acceptors and the water acts as the hydrogen bond donor. Such mixtures, when the salt is appropriately chosen (non-toxic and inexpensive), are ‘green’ in the above-mentioned sense and have low viscosities. Examples of such solvents are the 2:3 mixture of KF∙4H₂O (t_m­ = 18.5 ⁰C) and water, the freezing point of the eutectic is t_{m eutectic} = –40 ⁰C and the 1:1 mixture of ZnCl₂∙3H₂O (t_m­ = 6.5 ⁰C) and water, t_{m eutectic} = –62 ⁰C.

Deep eutectic solvents have found many applications, such as reaction media, for biomass and biodiesel processing, extraction of bioactive materials, nanotechnology, and sorption of obnoxious gases. They should be able to dissolve a variety of solutes, be these organic substances, metal oxides, or substances of other kinds. The present review deals with the specific sub-class of deep eutectic solvents comprising salt hydrates and water, i.e. ionic liquid-based mixed solvent systems, their properties and applications.

Properties of Salt Hydrate/Water Eutectics

The composition of the salt hydrate/water eutectics is generally expressed in several manners: the mass fraction of the salt hydrate w_esh, the mole fraction of the salt hydrate x_esh, and the molality of the salt in the eutectic mixture m_es. As prepared from the anhydrous salt with added water, the mass fraction of the anhydrous salt w_eas is generally the primary quantity; interconversion among these measures is as follows.

            w_esh = [1 + n(M_w/M_as)]/[1 + n(M_w/M_as) + (1–w_eas)/w_eas]                                     (1)

            x_esh = (w_esh/M_sh)/[(w_esh/M_sh) + (1– w_esh)/M_w]                                                                  (2)

where M_sh = M_ah + nM­_w is the molar mass of the salt hydrate, M_ah being that of the anhydrous salt that has n water molecules in the crystalline hydrate and M_w is the molar mass of water. The molality of the salt in the eutectic mixture is:

            m_es = (w_esh/M_sh)/(1– w_esh)                                                                                             (3)

the molar mass of the salt hydrate M_sh being given in kg mol^–1. The molar masse M_e of the deep eutectic solvent is:

            M_e = x_eshM_sh + (1 – x_esh)M_w                                                                                          (4)

The first property of the salt hydrate/water eutectics to be considered is their freezing points, showing them to be true eutectics. These are presented in Table 1 for a variety of systems. Also shown there is the depth of the eutectic temperature, Δt_me/⁰C, which is the distance between the eutectic melting point, t_me/⁰C, and the temperature at the eutectic (mole fraction) composition, x_esh, on the straight line connecting the melting point of ice, 0 ⁰C, and that of the salt hydrate, t_msh/⁰C. Some of these distances are very large indeed: for Ca(ClO₄)₂⸱6H₂O/ice it is 90 ⁰C, for MgBr₂⸱6H₂O/ice it is 105 ⁰C, for KOH⸱H₂O/ice it is 113 ⁰C, and for Mg(ClO₄)₂⸱6H₂O/ice it is 137 ⁰C.

The molar volumes V_e of the deep eutectic solvents are then obtained from the measured densities ρ_e at a given temperature as V_e = M_e/ρ_e. The isobaric expansibility is α_P = –ρ_e^–1(∂ρ_e/∂T)_P = V_e^–1(∂V_e/∂T)_P. There are only few data regarding the compressibilities of these deep eutectic solvents, and these are the adiabatic (constant entropy) quantities κ_S, obtained from measured speeds of sound v_e and densities ρ_e as κ_S = 1/v_e²ρ_e. These volumetric data for the salt hydrate/ice deep eutectic solvents are from [8,9], the density range of the eutectic being less than twofold: 1.102 for lithium acetate < ρ_e < 2.118 for calcium bromide eutectics.

The osmotic coefficients φ_e at 25 °C for the eutectic molalities m­_e of the eutectic mixtures are mainly from [10] with some data from [11-15, 17, 18]. The salt dissociates into υ ions and the derived water activities a_W are then:

            lna_We = –M­_waterυm_eφ_e                                                                                                    (5)

where the molar mass of water M­_water is in kg mol^–1 and its vapor pressures p_We = p_W⁰a_We, where p_W⁰ is the vapor pressure of water at 25 °C. The vapor pressures of the eutectic mixtures are quite small, ranging from 0.93 for potassium hydroxide < p_We < 2.79 for lithium nitrate eutectics. There are only few other thermodynamic properties that have been reported for the deep eutectic solvents made up from salt hydrates and ice, but among them are the surface tensions σ [9].

The viscosities η_e of the salt hydrate/ice eutectics generally follow the Arrhenius expression:

            ln(η_e/mPa∙s)_ = A_η + B_η/(T/K)                                                                         (6)

and the values at 25 °C of η_e as well as the coefficients A_η and B_η were known are shown in Table 2 [8,9]. Fewer data have been reported regarding the specific conductances of these aqueous mixtures for the eutectic composition, and the available data [8,9] at 25 °C, are shown in Table 2 too.

The polarizabilities of the constituting ions of the eutectic salts do not vary appreciable from their crystalline state to their aqueous solutions and the molar refractions of the ions and the water in the deep eutectic solvent mixtures are additive. The molar refractions of the ions R_D+ and R_D– [22] with that of water, R_DW = 3.84 cm³ mol^–1 at 25 ⁰C (measured at the sodium D-line frequency), yield the molar refraction of the solution of the aqueous salt hydrate C_pA_q∙nH₂O, corresponding to the eutectic mole fraction x_e:

            R_De = x_e(pR_D+ + qR_D– + nR_DW) + (1 – x_e)R_DW                                                               (7)

The molar refraction R_D of the salt eutectics mixtures with ice reflects mainly their molar volumes because of the narrow range of the refractive index n_D values. These that can be back-calculated from the R_D data according to the Lorentz-Lorenz expression:

            R_D = V(n_D² ­– 1)/(n_D² + 2)                                                                                             (8)

. The polarizability α of the deep eutectic solvents is obtained from the molar refraction as:

            α/nm³ ≈ 3.96x10^–4(R_D/cm³ mol^–1)                                                                                (9)

Some of the eutectic mixtures of salt hydrates with ice are highly basic, namely those involving KOH and NaOH. Others, involving anions of weak acids, are expected to be mildly basic, namely those involving KF, K₂HPO₄, LiCH₃CO₂, Mg(CH₃CO₂)₂, and NaCH₃CO₂. On the other hand mixtures involving strongly hydrolysable cations are expected to be acidic: those involving Al(NO₃)₃ and FeCl₃ and also ZnCl₂ [9]. The acidity of aqueous 59 mass% LiBr, i.e., molten LiBr⸱3.35H₂O, was assessed by means of the Hammett acidity function as quite acidic H₀ = –4.83 [21].

It is of interest to compare the viscosities of the salt hydrate/ice eutectics with those of other solvents. For most of the former the dynamic viscosity at 25 ⁰C is η_e/mPa∙s < 10 (exceptions are those involving calcium nitrate, magnesium acetate, manganese and zinc chlorides, Table 3). For conventional deep eutectic solvents typical values at 25 ⁰C are η_e/mPa∙s > 100, and in many cases > 1000. Popular and commercial deep eutectic solvents based on choline chloride have η_e/mPa∙s > 700 with urea (Reline), >300 with glycerol (Glyceline), >800 with malonic acid (Maline), and even with ethylene glycol (Ethaline) it is >40 [8]. Typical room temperature ionic liquids have at 25 ⁰C values η_e/mPa∙s > 30 as for 2-methyl-3-methylimidazolium tetrafluyoroborate, methylsulfate or bis(trifluoromethylsulfonyl)imide and their higher homologues, or 1-alkylpyridinium or 1-methyl-1-alkylpyrrolidinium salts with various anions [6].

Applications of Salt Hydrate/Water Eutectics

No specific uses of the eutectics of salt hydrates with water have been reported, but several applications of molten salt hydrates, with or without the addition of water, or of moderately or highly concentrated aqueous salt solutions with compositions near the eutectics have been published.

Applications of molten salt hydrates towards the dissolution and treatment of cellulose and lignocellulosic biomass are well documented. Molten LiClO₄⸱3H₂O and ZnCl₂⸱4H₂O cause structural changes in cellulose allowing its swelling and dissolution and the effect of the water content is dealt with in [23]. In addition to swelling and dissolution the decomposition of cellulose was affected in molten MgCl₂⸱6H₂O and in Mg(ClO₄)₂⸱6H₂O, whereas molten Na(CH₃CO₂⸱3H₂O and CaCl₂⸱6H₂O had no effect. It is the concerted effect of the cations and the water that causes cellulose to react [24]. The ZnCl₂⸱3H₂O/ice eutectic composition has a large enough salt concentration to enable dissolution of the cellulose, its hydrolysis to glucose, and the hydrogenation of the latter to produce isosorbide that is readily separated from the reaction mixture as the product, permitting the recycling of the aqueous molten salt hydrate solvent [25]. The lignocellulosic biomass was converted to furfural and 5-hydroxymethylfurfural in slightly acidified molten LiBr⸱3.25H₂O [21], although the eutectic has more water, the composition being LiBr⸱5H₂O, freezing point t_m = –70.3 ⁰C [26].

Rather than aiming at the decomposition of the cellulose to useful low molecular weight products, the regeneration of cellulose in useful forms after the dissolution of the biomass has also employed molten salt hydrates with water. This subject was reviewed, where the use of a concentrated (>48.5 mass%) aqueous Ca(SCN)₂ solution, corresponding to a <4-hydrate, was emphasized. The molten lithium salt hydrates LiClO₄⸱3H₂O and LiSCN⸱2H₂O as is ZnCl₂⸱3H₂O are capable of dissolving cellulose, but molten LiNO₃⸱3H₂O and ZnCl₂⸱2H₂O and ZnCl₂⸱4H₂O are not. These facts and the interactions involved are rationalized according to the amount of water in the inner coordination sphere of the cation [27]. The eutectic of aqueous sodium hydroxide has t_m = –33.4 ⁰C at 20 mass% salt, and this [28] as well as other compositions, e.g., 8 to 10 mas% [29] have been used for the dissolution of cellulose at low temperatures, ­–5 to 4 ⁰C. Contrary to the previously mentioned use of molten lithium bromide hydrate LiBr⸱3H₂O for the preparation of low molecular weight derivatives of cellulose, this solvent can also be employed for the preparation of regenerated cellulose film [30].

The dissolution of starch by molten salt hydrates has had fewer reports than that of cellulose, aqueous zinc chloride at >29.6 mass% (< 18.6 H₂O:ZnCl₂) was mentioned in [31] for this purpose.

A quite different application of aqueous salt hydrate mixtures is as electrolytes for rechargeable batteries as reviewed in [32], where, however, emphasize is placed on the electrode materials rather than on the electrolytes. One advantage of these aqueous systems is their non-flammability, making them safer than batteries with organic solvents for the electrolytes. Another advantage is the larger conductivity of the aqueous electrolyte systems. An early description of such batteries is [33], where 5 M aqueous LiNO₃ served as the electrolyte. The “water-in-salt” concept was promoted in [34] for lithium-based batteries, nearing the salt hydrate entity, but without specifying the salts. Molten lithium bis(trifluoromethylsulfonyl)amide hydrate, with 2.6 water molecules per formula unit, t_m = 19.9 ⁰C, corresponding to a 21 mol kg^–1 aqueous solution of the salt, was specified in [35], where the addition of 3 mol% of the magnesium salt contributed to the electrochemical stability of the battery.

Another quite different application of salt hydrate eutectics is for cold storage and air conditioning. This use is based on phase change materials (PCMs), a widely used one is Glauber’s salt, Na₂SO₄⸱10H₂O, having t_m = 32.4 ⁰C and a high latent heat of fusion of 254 kJ kg^–1 or 377 MJ m^–3 [36]. Earlier problems with sub-cooling of this salt hydrate were solved by using borax as a nucleating agent. For sub-zero cold storage other inorganic salt/water eutectics were found to be useful, with listed melting points –62 < t_m < –1.8 and latent heats of fusion of 116 < Δ_m­H/kJ kg^–1 < 314, generally increasing with t_m. Commercial eutectic salt/water mixtures with similar properties are also listed [37].

The concept of “solvent-in-salt”, that fits well the salt hydrate/water eutectics, has been most recently promoted in [38]. Lithium salts: LiClO₃⸱3H₂O, t_m = 8 ⁰C, and LiNO₃⸱3H₂O, t_m = 30 ⁰C, as well as zinc salts: ZnCl₂⸱4H₂O, t_m < 25 ⁰C, and Zn(NO₃)₂⸱6H₂O, t_m = 37 ⁰C were pointed out, as well as the system Ca(NO₃)₂⸱4H₂O + KNO₃ that is liquid at 25 ⁰C.

Possible future applications include the treatment at low temperatures of heat-sensitive materials that require or may advantageously use an aqueous medium.

References

Table 1. Salt hydrates forming with ice deep eutectic solvents, showing the eutectic composition: mass fraction of the anhydrous salt w_eas, mole fraction of the salt hydrate, x_esh, the freezing points of the eutectics, t_me, the freezing points of the salt hydrates, t_msh, and the eutectic distance, Δt_me [8,9].

Table 2. The viscosities η_e of salt hydrates eutectics with ice at 25 ⁰C [8], their Arrhenius function parameters

A_η and B_η (eq. (6)), and their specific condutances κ at 25 ⁰C [8,9].

^a Ref. [18]. ^b Ref. [19].