Observation of cavitation dynamics in viscous deep eutectic solvents during power ultrasound sonication.

Deep eutectic solvents (DESs) are a class of ionic liquid with emerging applications in ionometallurgy. The characteristic high viscosity of DESs, however, limit mass transport and result in slow dissolution kinetics. Through targeted application of high-power ultrasound, ionometallurgical processing time can be significantly accelerated. This acceleration is primarily mediated by the cavitation generated in the liquid surrounding the ultrasound source. In this work, we characterise the development of cavitation structure in three DESs of increasing viscosity, and water, via high-speed imaging and parallel acoustic detection. The intensity of the cavitation is characterised in each liquid as a function of input power of a commercially available ultrasonic horn across more than twenty input powers, by monitoring the bubble collapse shockwaves generated by intense, inertially collapsing bubbles. Through analysis of the acoustic emissions and bubble structure dynamics in each liquid, optimal driving powers are identified where cavitation is most effective. In each of the DESs, driving the ultrasonic horn at lower input powers (25%) was associated with greater cavitation performance than at double the driving power (50%).

Overcoming passivation through improved mass transport in dense ionic fluids.

Deep Eutectic Solvents (DESs) have recently been shown to be part of a dense ionic fluid continuum between ionic liquids and concentrated aqueous brines. Charge transport was shown to be governed by fluidity, with no discontinuity between molar conductivity and fluidity irrespective of cation, charge density or ionic radius. By adjusting the activity of water and chloride ions, mass transport, speciation and reactivity can be altered. It has been shown that while brines provide a high chloride content at a lower viscosity than DESs, unlike DESs, brines are unable to prevent metal passivation due to their high water content. This results in the possibility to impart a level of selectivity towards metal dissolution (or passivation) when processing mixed metal materials. Forced convection can be used to avoid the issue of slow mass transport in viscous media, and the use of jets or targeted ultrasound are effective methods for overcoming this issue. High-powered ultrasound was applied to copper, cobalt, and aluminium electrodes undergoing anodic dissolution, and linear sweep voltammetry showed a linear current-voltage response at potentials anodic of the oxidation potential under sonication, with total charge passed being 5 to 134 times greater than under silent conditions. Application of ultrasound to silver and nickel electrodes displayed an initial linear current-voltage response, but the increased water content of the brines resulted in passivation. Mass transport throughout the bulk solution is governed by the forced convection imparted by the ultrasound and ionic species must only migrate across the electrical double layer. It is shown that the anodic dissolution of a range of metals classically expected to passivate, e.g. aluminium, can be significantly accelerated under insonation conditions.

Anion effect on the redox properties of copper ions in ionic liquids and deep eutectic solvents.

It has long been claimed that the anion of the DES or IL is critical for controlling the redox properties of metal ions. In this study we investigate the effect of different salt anions on the copper redox properties and speciation, and compare that with the effect of the different solvent anions, when a single copper salt is used in a range of solvents. It is shown that the effect of the solvent anion is much more significant than that of the salt anion on the redox properties. It is also found that copper species remain the same copper tetrachloride species despite the starting salt. An exception is seen for the copper(I) salt, which makes linear dichloride species, as well as the copper(II) acetate system, which displays concentration dependence. When the anion of the ionic liquid is changed, the copper species change correspondingly with the coordinating strength of the solvent anion, leading to a greater difference in redox response, which is due to the different species present. Thus, these speciation differences can be used to modify the redox potentials in the solution.

Concentrated Ionic Fluids: Is There a Difference Between Chloride-Based Brines and Deep Eutectic Solvents?

Deep Eutectic Solvents (DESs) have been lauded as novel solvents, but is there really a difference between them and concentrated aqueous brines? They provide a method of adjusting the activity of water and chloride ions which can affect mass transport, speciation and reactivity. This study proposes a continuum of properties across concentrated ionic fluids and uses metal processing as an example. Charge transport is shown to be governed by fluidity and there is no discontinuity between molar conductivity and fluidity irrespective of cation, charge density or ionic radius. Diffusion coefficients of iron(III) and copper(II) chloride in numerous concentrated ionic fluids show the same linear correlation between diffusion coefficient and fluidity. These oxidising agents were used to etch copper, silver and nickel and while the etching rate increased with fluidity for copper, etching of silver and nickel only occurred at high chloride and low water activity as passivation occurred when water activity increased. Overall, brines provide a high chloride content at a lower viscosity than DESs, but unlike DESs, brines are unable to prevent passivation due to their high water content. The results show how selective etching of mixed metal waste streams can be achieved by tuning chloride and water activity.

Iodine speciation in deep eutectic solvents.

Iodine has been shown to act as a good electrocatalyst for metal digestion in deep eutectic solvents (DESs) but little is known about its speciation or reactivity in these high chloride containing media. Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements were made at the iodine K-edge in a range of DESs with different glycolic or acidic hydrogen bond donors (HBDs), along with examining the effect of iodine concentration between 0.01 and 0.5 mol dm-3. Three groups of speciation were detected: mixed I2Cl-/I3- (glycol and lactic acid systems), mixed I3-/I2 (oxalic acid and urea systems), and singular I3- (levulinic acid system). UV-vis spectroscopy was used to confirm the speciation. Electrochemistry showed that iodine redox behaviour was unaffected by the changing speciation. Leaching data showed that metal oxidation was related not only to changing iodine speciation, but also the reactivity and coordination ability of the HBD.

23Na NMR T1 relaxation measurements as a probe for diffusion and dynamics of sodium ions in salt-glycerol mixtures.

Mixtures of sodium salts with oxygen-containing molecules are useful from the perspective of applications such as sodium ion batteries because they fill the gap between deep eutectic solvents and molten salt hydrates. In a previous work, the physical properties (such as diffusion coefficients, conductivity, viscosity, and glass transition temperature) of four salts, namely, Na2B4O7 · 10H2O, NaOAc · 3H2O, NaBr, and NaOAc, were measured with glycerol. Pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) was also used to measure self-diffusion coefficients of 1H-bearing species. However, the technique was not able to measure diffusion of sodium ions due to the very fast NMR relaxation rate of such species, resulting in loss of the PFG NMR signal. In the current work, this study is expanded using 23Na T1 relaxation measurements which, under certain assumptions, can be translated into diffusion coefficients. Analysis of the physical properties is then correlated with self-diffusion coefficient measurements to elucidate information about structure and ionic mobility. It is shown that NaOAc · 3H2O, NaBr, and NaOAc fit models for ionic conductivity and diffusion, which are consistent with ionic liquids where charge transport is limited by ionic mobility rather than the number of charge carriers. The waters of hydration of NaOAc · 3H2O do not appear to form a separate phase but are instead strongly coordinated to the cation. In contrast, Na2B4O7 · 10H2O appears to form a water-rich phase with enhanced sodium mobility.

Deep eutectic solvents-The vital link between ionic liquids and ionic solutions.

When selecting a solvent for a given solute, the strongly held idiom "like dissolves like", meaning that polar solvents are used for polar solutes, is often used. This idea has resulted from the concept that most molecular solvents are homogeneous. In a deep eutectic solvent (DES), however, both components can be ionic or non-ionic, polar or non-polar. By tuning the components, DESs can solubilize a wide variety of solutes, often mixing hydrophobic and hydrophilic components, and the mixture can be designed to control phase behavior. The liquids often contain significant short-length order, and preferential solvation of one component often occurs. The addition of small polar molecules such as water or alcohols results in non-homogeneous liquids, which have significantly decreased viscosity and increased ionic conductivity. Accordingly, the areas covered in this special issue focus on structure and dynamics, solvation, the mobility of charged species, and the ability to obtain controllable phase behavior by adding polar diluents or using hydrophobic DESs.

Globular and Fibrous Proteins Modified with Deep Eutectic Solvents: Materials for Drug Delivery.

Proteinaceous materials have numerous structures, many of which aid in the roles they perform. Some need to impart strength while others need elasticity or toughness. This study is the first to investigate the modification of both globular and fibrous protein, namely, zein, soy protein and gelatin, using deep eutectic solvents (DES) to form bioplastics, which may have application in drug delivery systems. The effects of DES content on the thermal and mechanical properties of the material were determined. Zein and soy are globular proteins, which both showed a significant change in the properties by the addition of DES. Both of these materials were, however, weaker and less ductile than the starch based materials previously reported in the literature. The material made from gelatin, a fibrous protein, showed variable properties depending on how long they were in contact with each other before pressing. Conductivity and NMR measurements indicate the existence of a continuous liquid phase, which are useful in the demonstrated application of transdermal drug delivery systems. It is shown that pharmaceutical DESs can be gelled with gelatin and this method is three times faster at delivering a pharmaceutical active ingredient across the skin barrier than from a corresponding solid formulation.

Brønsted acidity in deep eutectic solvents and ionic liquids.

Despite the importance of ionic liquids in a variety of fields, little is understood about the behaviour of protons in these media. The main difficulty arises due to the unknown activity of protons in non-aqueous solvents. This study presents acid dissociation constants for nine organic acids in deep eutectic solvents (DESs) using standard pH indicator solutes. The pKIn value for bromophenol blue was found by titrating the DES with triflic acid. The experimental method was developed to understand the acid-base properties of deep eutectic solvents, and through this study it was found that the organic acids studied were slightly less dissociated in the DES than in water with pKa values between 0.2 and 0.5 higher. pKIn values were also determined for two ionic liquids, [Bmim][BF4] and [Emim][acetate]. The anion of the ionic liquid changes the pH of the solution by acting as a buffer. [Emim][acetate] was found to be more basic than water. It is also shown that water significantly affects the pH of ionic liquids. This is thought to arise because aqueous mixtures with ionic liquids form heterogeneous solutions and the proton partitions into the aqueous phase. This study also attempted to develop an electrochemical pH sensor. It was shown that a linear response of cell potential vs. ln aH+ could be obtained but the slope for the correlation was less than that obtained in aqueous solutions. Finally it was shown that the liquid junction potential between two reference electrodes immersed in different DESs was dependent upon the pH difference between the liquids.

Bright metal coatings from sustainable electrolytes: the effect of molecular additives on electrodeposition of nickel from a deep eutectic solvent.

Organic and inorganic additives are often added to nickel electroplating solutions to improve surface finish, reduce roughness and promote uniform surface morphology of the coatings. Such additives are usually small molecules and often referred to as brighteners or levellers. However, there have been limited investigations into the effect of such additives on electrodeposition from ionic liquids (ILs) and deep eutectic solvents (DESs). Here we study the effect of four additives on electrolytic nickel plating from an ethyleneglycol based DES; these are nicotinic acid (NA), methylnicotinate (MN), 5,5-dimethylhydantoin (DMH) and boric acid (BA). The additives show limited influence on the bulk Ni(ii) speciation but have significant influence on the electrochemical behaviour of Ni deposition. Small concentrations (ca. 15 mM) of NA and MN show inhibition of Ni(ii) reduction whereas high concentrations of DMH and BA are required for a modest difference in behaviour from the additive free system. NA and MN also show that they significantly alter the nucleation and growth mechanism when compared to the additive free system and those with DMH and BA. Each of the additive systems had the effect of producing brighter and flatter bulk electrodeposits with increased coating hardness but XRD shows that NA and MN direct crystal growth to the [111] orientation whereas DMH and BA direct crystal growth to the [220] orientation.

Correction: Molecular and ionic diffusion in aqueous - deep eutectic solvent mixtures: probing inter-molecular interactions using PFG NMR.

Correction for 'Molecular and ionic diffusion in aqueous - deep eutectic solvent mixtures: probing inter-molecular interactions using PFG NMR' by Carmine D'Agostino et al., Phys. Chem. Chem. Phys., 2015, 17, 15297-15304.

Electrochemical fabrication of nanoporous copper films in choline chloride-urea deep eutectic solvent.

Nanoporous copper films were fabricated by a facile electrochemical alloying/dealloying process without the need of a template. A deep eutectic solvent made from choline chloride (ChCl) and urea was used with zinc oxide as the metal salt. Cyclic voltammetry was used to characterise the electrochemical reduction of zinc and follow Cu-Zn alloy formation on the copper substrate at elevated temperatures from 353 to 393 K. The alloy formation was confirmed by X-ray diffraction spectra. 3D, open and bicontinuous nanoporous copper films were obtained by in situ electrochemically etching (dealloying) of the zinc component in the Cu-Zn surface alloys at an appropriate potential (-0.4 V vs. Ag). This dealloying process was found to be highly temperature dependent and surface diffusion controlled, which involved the self-assembly of copper atoms at the alloy/electrolyte interface. Additionally, the effects of the deposition parameters, including deposition temperature, current density as well as total charge density on resulting the microstructure were investigated by scanning electron microscopy, and atomic force microscope.

Molecular and ionic diffusion in aqueous - deep eutectic solvent mixtures: probing inter-molecular interactions using PFG NMR.

Pulsed field gradient (PFG) NMR has been used to probe self-diffusion of molecular and ionic species in aqueous mixtures of choline chloride (ChCl) based deep eutectic solvents (DESs), in order to elucidate the effect of water on motion and inter-molecular interactions between the different species in the mixtures, namely the Ch(+) cation and hydrogen bond donor (HBD). The results reveal an interesting and complex behaviour of such mixtures at a molecular level. In general, it is observed that the hydroxyl protons ((1)H) of Ch(+) and the hydrogen bond donor have diffusion coefficients significantly different from those measured for their parent molecules when water is added. This indicates a clear and significant change in inter-molecular interactions. In aqueous Ethaline, the hydroxyl species of Ch(+) and HBD show a stronger interaction with water as water is added to the system. In the case of Glyceline, water has little effect on both hydroxyl proton diffusion of Ch(+) and HBD. In Reline, it is likely that water allows the formation of small amounts of ammonium hydroxide. The most surprising observation is from the self-diffusion of water, which is considerably higher that expected from a homogeneous liquid. This leads to the conclusion that Reline and Glyceline form mixtures that are inhomogeneous at a microscopic level despite the hydrophilicity of the salt and HBD. This work shows that PFG NMR is a powerful tool to elucidate both molecular dynamics and inter-molecular interactions in complex liquid mixtures, such as the aqueous DES mixtures.

EXAFS study into the speciation of metal salts dissolved in ionic liquids and deep eutectic solvents.

The speciation of metals in solution controls their reactivity, and this is extremely pertinent in the area of metal salts dissolved in ionic liquids. In the current study, the speciation of 25 metal salts is investigated in four deep eutectic solvents (DESs) and five imidazolium-based ionic liquids using extended X-ray absorption fine structure. It is shown that in diol-based DESs M(I) ions form [MCl2](-) and [MCl3](2-) complexes, while all M(II) ions form [MCl4](2-) complexes, with the exception of Ni(II), which exhibits a very unusual coordination by glycol molecules. This was also found in the X-ray crystal structure of the compound [Ni(phen)2(eg)]Cl2·2eg (eg = ethylene glycol). In a urea-based DES, either pure chloro or chloro-oxo coordination is observed. In [C6mim][Cl] pure chloro complexation is also observed, but coordination numbers are smaller (typically 3), which can be explained by the long alkyl chain of the cation. In [C2mim][SCN] metal ions are entirely coordinated by thiocyanate, either through the N or the S atom, depending on the hardness of the metal ion according to the hard-soft acid-base principle. With weaker coordinating anions, mixed coordination between solvent and solute anions is observed. The effect of hydrate or added water on speciation is insignificant for the diol-based DESs and small in other liquids with intermediate or strong ligands. One of the main findings of this study is that, with respect to metal speciation, there is no fundamental difference between deep eutectic solvents and classic ionic liquids.

Aluminium electrodeposition under ambient conditions.

The electrodeposition of aluminium is demonstrated using a eutectic mixture of aluminium chloride and urea. The mixture is shown to be conducting through the formation of both cationic ([AlCl2·urean](+)) and anionic (AlCl4(-)) species and electrodeposition is achieved through the cationic species. The use of a biphasic system with the ionic liquid and a protective hydrocarbon layer allows metal deposition to be carried out in an environment with ambient moisture without the need for a glove box. A direct comparison is made between the AlCl3:urea and imidazolium chloride:AlCl3 systems and the differences in speciation and mass transport manifest themselves in different deposit morphologies. Brighteners which work in the chloroaluminate system such as toluene and LiCl are shown to be ineffective in the urea based system and the reasons for these differences are ascribed to the mechanism of the anodic reaction which is rate limiting.

Speciation, physical and electrolytic properties of eutectic mixtures based on CrCl3·6H2O and urea.

The electrodeposition of chromium is a technologically vital process, which is principally carried out using aqueous chromic acid. In the current study, it is shown that eutectic mixtures of urea and hydrated chromium(III) chloride provide a liquid which reduces the toxicological issues associated with the current aqueous Cr(VI) electroplating solution. Using EXAFS, mass spectrometry and UV-Vis spectroscopy, it is shown that chromium is present predominantly as a cationic species. Conductivities are higher than for most comparable ionic liquids. It is shown that the electrodeposition of chromium is electrochemically reversible, with a current efficiency much higher than in aqueous electrolytes. Surface tension and density measurements indicate that hole theory is a valid model to describe transport properties in these liquids. Bulk Cr deposits are not macrocrystalline but they are generally crack-free. The deposits have a hardness of 600 ± 10 Vickers and, as such, are comparable to deposits from aqueous systems.

Influence of colourants on environmental degradation of plastic litter.

Plastic degradation and the resultant production of microplastics has an important effect on the environment and fauna across the world. This paper shows that the colourant incorporated into plastic formulations has a significant effect on the stability of plastics. A static experimental exposure of differently coloured polypropylene bottle tops from the same manufacturer to a moderate climate over 3 years showed that black, white and silver plastics were almost unaffected whereas the specific blue, green and especially red pigments used in this study were significantly degraded. The second part of the study collected littered HDPE plastic containers from a remote South African beach and analysed their condition as a function of the given manufacturing date stamp. Most items were black or white and samples up to 45 years old were found with relatively little environmental degradation other than mild abrasion. It appears that carbon and titanium dioxide colourants protect the HDPE polymer from photolytic degradation. While anthraquinone, phthalocyanine and diketopyrrolopyrrole pigments were found to enable UV light to degrade the polymer leading to brittle plastics, promoting the formation of microplastics, it is likely that other pigments that do not strongly absorb in the UV will result in similar degradation.

In situ electrochemical digital holographic microscopy; a study of metal electrodeposition in deep eutectic solvents.

This study has shown for the first time that digital holographic microscopy (DHM) can be used as a new analytical tool in analysis of kinetic mechanism and growth during electrolytic deposition processes. Unlike many alternative established electrochemical microscopy methods such as probe microscopy, DHM is both the noninvasive and noncontact, the unique holographic imaging allows the observations and measurement to be made remotely. DHM also provides interferometric resolution (nanometer vertical scale) with a very short acquisition time. It is a surface metrology technique that enables the retrieval of information about a 3D structure from the phase contrast of a single hologram acquired using a conventional digital camera. Here DHM has been applied to investigate directly the electro-crystallization of a metal on a substrate in real time (in situ) from two deep eutectic solvent (DES) systems based on mixture of choline chloride and either urea or ethylene glycol. We show, using electrochemical DHM that the nucleation and growth of silver deposits in these systems are quite distinct and influenced strongly by the hydrogen bond donor of the DES.

Ligand exchange in ionic systems and its effect on silver nucleation and growth.

The electrodeposition of metals from ionic solutions is intrinsically linked to the reactivity of the solute ions. When metal salts dissolve, the exchange of the anion with the molecular and ionic components from solution affects the speciation and therefore the characteristics of metal reduction. This study investigates the nucleation mechanism, deposition kinetics, metal speciation and diffusion coefficients of silver salts dissolved in Deep Eutectic Solvents. The electrochemical reduction of AgCl, AgNO3 and Ag2O is studied in 1 : 2 choline chloride : ethylene glycol and 1 : 2 choline chloride : urea. Cyclic voltammetry is used to evaluate electrochemical kinetics. Detailed analysis of chronoamperometric data shows that silver deposits form via multiple 3D nucleation with mass transport controlled hemispherical growth. The nucleation mechanism was found to be potential dependent, varying from progressive to instantaneous as the reduction potential becomes more cathodic. Diffusion coefficients are determined using three different methods. Trends are rationalised in terms of solvent viscosity and silver speciation analysis with EXAFS. The morphology of electroreduced silver is investigated with scanning electron microscopy and shows that deposits from the urea based liquid form more dense morphologies than those from the ethylene glycol based liquid.

A novel method for extracting metals from asteroids using non-aqueous deep eutectic solvents.

Extra-terrestrial mining and metal processing are vital for access to strategic metals for space exploration. This study demonstrates for the first time the catalytic dissolution of metals from meteorite proxies of metal-rich asteroids using a deep eutectic solvent (DES). DESs are of particular interest for extra-terrestrial mining as they can be designed to have relatively low vapour pressures and could potentially be made from organic waste products created in extra-terrestrial settlements. Three types of meteorites were investigated: two chondrites (H3, H5) and one iron (IAB-MG) meteorite. Chondrite samples were composed of silicates (olivine, pyroxene) with metal-rich phases occurring as native metal alloys, sulphides and oxides. Metallic Fe-Ni and troilite (FeS) are the most abundant metal-bearing phases in all three samples, particularly in the iron-rich meteorite. The samples were subjected to chemical micro-etching experiments with iodine and iron(III) chloride as oxidising agents in a DES formed from the mixture of choline chloride and ethylene glycol. Micro-etching experiments demonstrated that Fe-Ni rich phases are effectively leached out in this system, while other mineral phases remain unreactive.