Research Article pubs.acs.org/journal/ascecg
Unconventional Deep Eutectic Solvents: Aqueous Salt Hydrates Yizhak Marcus* Institute of Chemistry, The Hebrew University of Jerusalem, Edmund Safra Campus, Jerusalem 91904, Israel ABSTRACT: Neoteric solvents for sustainable chemical processes are continuously sought nowadays. Deep eutectic solvents (DES), i.e., liquids that are mixtures of two components that have freezing points considerably below the melting points of these components, constitute a possible solution for this search. Mixtures of certain congruently melting salt hydrates with water have freezing points considerably below the melting points of ice and of the salt hydrate. They should be considered as DES for applications in which their properties could be useful, namely their being “green” and nonflammable ionic liquids at very low temperatures. The eutectic points and eutectic temperature distances of such salt hydrate DES, calculated from literature data, are tabulated, and some illustrative phase diagrams are shown. Physical properties of the aqueous salt hydrate DES that are relevant to their possible applications are reported, and they compare favorably with those of conventional DES. The Brunauer−Emmett− Teller modeling procedure is applied to the case of the aqueous potassium fluoride tetrahydrate phase diagram. KEYWORDS: BET modeling, Deep eutectic solvents, Phase diagrams, Salt hydrates
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INTRODUCTION
Some aqueous salt hydrates, i.e., mixtures of ice and certain congruently melting salt hydrates, form liquid eutectics at low temperatures that may be deemed DES. They feature properties that commonly used or proposed DES should have, e.g., they are definitely nonflammable, they are nontoxic (heavy metal salt hydrates, such as Cd(NO3)2·4H2O, are avoided), they are inexpensive (expensive metal salt hydrates, such as CsF·H2O, are avoided), and they are readily reconstituted after use. So far, few applications of such DES are known, with dissolution and treatment of cellulose being one application. One attribute of DES is the eutectic distance, ΔTme, between the eutectic temperature Tme and the temperature at the eutectic mole fraction, xe, on the straight line connecting the melting temperatures of the two components, TmHBA and TmHBD. For the choline chloride−urea pair, this distance is quite impressive: ΔTme = 178 K.1 For the unconventional DES made of ice and a congruently melting salt hydrate, this ΔTme is smaller. Still, it is 137 K for ice−Mg(ClO4)2·6H2O.5 However, very low eutectic points can be reached, e.g., Tme = 199 K for the ice−Ca(ClO4)2·6H2O DES,5 compared with Tme = 285 K for choline chloride−urea1 and Tme = 207 K for choline chloride−ethylene glycol.6 Salt hydrates have previously been featured as components of DES, examples being Zn(NO3)2·6H2O−urea,7 CaCl2·6H2O− choline chloride,8 and CrCl3·6H2O−urea.9 Water has previously been featured as the HBD component with ionic liquids forming deep eutectics that could be used as alternatives to common organic solvents, examples being N-butyl-3-methylpyridinium 4-toluenesulfonate−water,10 1-butyl-1-methylpyrro-
Much of chemistry, be it in the laboratory or in industry, is carried out in solutions; hence, solvents play a crucial role for attaining successful chemical reactions and processes. In recent years, there has been a clear trend away from conventional organic solvents, and neoteric solvents have been sought for achievement of sustainable chemical processes. Such solvents include supercritical fluids, room-temperature ionic liquids, and deep eutectic solvents. They have their advantages as well as problematic aspects, and new types of solvents are therefore being nowadays extensively sought. Deep eutectic solvents (DES) are a group of materials that are liquid at ambient conditions and are binary compositions of two components, the freezing point of the DES being considerably below the melting points of the two components; hence, they are eutectics. The DES should also be nonflammable, nontoxic, and friendly to the environment (“green” and biodegradable) to be useful for industrial processes. DES are commonly constituted by a hydrogen-bond-accepting (HBA) component and a hydrogen-bond-donating (HBD) component at a certain molar ratio. The prototype DES is the one made up from choline chloride and urea at the molar ratio 1:2,1 such that the chloride anion plays the HBA role and urea the HBD role. The eutectic has a freezing point of 285 K. Conventional DES are constituted by a variety of quaternary ammonium salts and various amides, acids, and polyols, among other combinations. DES have been reviewed recently,2−4 such that their properties as “green” solvents are summarized. The applications of DES include use as (catalyzed) reaction media, biomass and biodiesel processing, metal electrodeposition and electropolishing, production of nanomaterials, capture of harmful gases, and extraction and separation technology. © 2017 American Chemical Society
Received: October 2, 2017 Published: October 26, 2017 11780
DOI: 10.1021/acssuschemeng.7b03528 ACS Sustainable Chem. Eng. 2017, 5, 11780−11787
Research Article
ACS Sustainable Chemistry & Engineering lidinium SCN−water,11 and 1-butyl-3-methylimidazolium 4toluenesulfonate.12 However, the combination of salt hydrates with water (ice) as deep eutectic solvents, dealt with in this paper, has not been considered before. Congruently, melting salt hydrates are mainly dealt with here because they can be considered to be independent substances, i.e., components in the thermodynamic sense. However, the extension of the present concept to noncongruently melting salts, such as CaCl2·6H2O or ZnCl2·3H2O, can be readily made. The molten salt hydrates themselves are a subclass of molten salts in general, with the hydrated cations being considered as independent entities,13 constituting the HBA component of the DES formed between them and water (ice), acting as the HBD component. Salt hydrates “provide a ‘natural end point’... a more realistic upper concentration limit for two-state models of aqueous electrolyte solutions”.14 In view of this, the DES described herein may be regarded as more nearly behaving as molten salts than as aqueous solutions.
Table 1. Salt Hydrates Forming DESa
RELEVANT PROPERTIES OF SALT HYDRATES Relevant properties of the salt hydrates dealt with in this study are shown in Table 1: melting points, Tm/K, and molar enthalpies of fusion, ΔmH/kJ mol−1. Where specified with “(c)”, the melting points are reported as pertaining to congruently melting hydrates.19 For a few other salt hydrates that would form DES with water no information regarding the melting point and the molar enthalpy of fusion could be found. Also included in Table 1 are the Brunauer−Emmett−Teller (BET) parameters at 298.15 K that should be useful for modeling the liquidus curves of the DES system.15,26,27 These parameters are r, the average number of sites for water molecules around the salt in concentrated aqueous solutions, and ε, the molar enthalpy of adsorption of water on these sites in excess of the molar enthalpy of condensation of water into itself.28,29 These quantities, valid for 298.15 K, are taken to be independent of the temperature due to the lack of relevant dependencies. The phase diagrams of many of the salts recorded in Table 1 have been reported.30 It should be noted that no information was found regarding whether the melting is congruent or not of the hydrates of the following salts listed in Table 1: Mg(CH 3 CO 2 ) 2 , MgBr 2 , Mg(ClO 4 ) 2 , Mn(NO 3 ) 2 , and NaCH3CO,2. The salts Al(NO3)3·9H2O, Co(NO3)·6H2O, MgCl2·6H2O, and MnCl2·6H2O melt incongruently, marked as (ic) in Table 1. No molar enthalpies of fusion of the listed hydrates of Ca(ClO4)2, LiI, and ZnCl2 were found, and the value found for the hydrate of KOH appears to have a large uncertainty. The Brunauer−Emmett−Teller parameters r and ε for LiCH3CO2 and NaCH3CO2 could not be determined because their reported water activities were not sufficiently low (
