Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure-property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.

Deep Eutectic Solvents: A Review of Fundamentals and Applications.

Ionic liquids (ILs) and deep eutectic solvents (DESs) have been suggested as eco-friendly alternatives to organic solvents. A trace amount of water is often unavoidable as impurity, and water is also added on purpose to reduce their problematically high viscosity and lower their high price. Understanding the distinct effects of water on the properties of ILs/DESs is highly important. In this review, we collect published experimental and theoretical results for IL/DES–H2O systems at varied water concentrations and analyze them. Results from mechanistic studies, thermodynamic modelling and advanced experiments are collected and critically discussed. Six commonly studied IL/DES–H2O systems were selected to map experimental observations onto microscopic results obtained in mechanistic studies. A great variety of distinct contours of the excess properties can be observed over the entire compositional range, indicating that the properties of IL/DES–H2O systems are highly unpredictable. Mechanistic studies clearly demonstrate that the added H2O rapidly changes the heterogeneous 3D structures of pure ILs/DESs, leading to very different properties and behaviour. There are similarities between aqueous electrolytes and IL/DES solutions but the bulky and asymmetric organic cations in ILs/DESs do not conform to the standard salt dissolution and hydration concepts. Thermodynamic modelling previously assumes ILs/DESs to be either a neutral ion-pair or completely dissociated ions, neglecting specific ion hydration effects. A new conceptual framework is suggested for thermodynamic modelling of IL/DES–H2O binary systems to enable new technologies for their practical applications.

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The peculiar effect of water on ionic liquids and deep eutectic solvents

Deep Eutectic Solvents

Deep eutectic solvents (DESs) are a mixture of a salt and a molecular hydrogen bond donor, which form a eutectic liquid with a depressed melting point. Quantum mechanical molecular dynamics (QM/MD) simulations have been used to probe the 1 : 2 choline chloride–urea (ChCl : U), choline chloride–ethylene glycol (ChCl : EG) and choline chloride–glycerol (ChCl : Gly) DESs. DES nanostructure and interactions between the ions is used to rationalise differences in DES eutectic point temperatures and viscosity. Simulations show that the structure of the bulk hydrogen bond donor is largely preserved for hydroxyl based hydrogen bond donors (ChCl:Gly and ChCl:EG), resulting in a smaller melting point depression. By contrast, ChCl:U exhibits a well-established hydrogen bond network between the salt and hydrogen bond donor, leading to a larger melting point depression. This extensive hydrogen bond network in ChCl:U also leads to substantially higher viscosity, compared to ChCl:EG and ChCl:Gly. Of the two hydroxyl based DESs, ChCl:Gly also exhibits a higher viscosity than ChCl:EG. This is attributed to the over-saturation of hydrogen bond donor groups in the ChCl:Gly bulk, which leads to more extensive hydrogen bond donor self-interaction and hence higher cohesive forces within the bulk liquid.

Nanostructure, hydrogen bonding and rheology in choline chloride deep eutectic solvents as a function of the hydrogen bond donor

Physicochemical properties of choline chloride-based deep eutectic solvents and excess properties of their pseudo-binary mixtures with 1-butanol

Densities of aqueous mixtures of (choline chloride+ethylene glycol) and (choline chloride+malonic acid) deep eutectic solvents in temperature range 283.15–363.15K

The apparent molar volumes (VΦ) of l-lysine monohydrochloride, l-histidine, and l-arginine in water and in aqueous of sodium acetate, potassium acetate, and calcium acetate solutions have been dete...

Volumetric Behavior on Interactions of α-Amino Acids with Sodium Acetate, Potassium Acetate, and Calcium Acetate in Aqueous Solutions

Benzocaine drug (BZC) is an active component of various nonprescription drugs and used for numb teething treatments. The interaction of BZC with bovine serum albumin (BSA) has been studied using fluorescence, synchronous fluorescence, UV-Vis, circular dichroism (CD), and molecular docking analysis. The results revealed that BZC has a strong affinity to quench the intrinsic fluorescence of BSA in terms of a static quenching mechanism under physiological conditions. The fluorescence quenching data revealed that the quenching constants are (KSV) 4.10, 3.30, and 2.35 × 104 L mol−1 at 298, 304, and 310 K, respectively. The binding constants (Kb) at three different temperatures (298, 304, and 310 K) were found to be 6.02, 3.72, and 1.10 × 105 L mol−1, respectively. The thermodynamic parameters ∆H° and ∆S° have been estimated to be − 70.67 and − 128.9 J mol−1 K−1, respectively, thereby, indicating that hydrogen bonding and Van der Waals forces play major role in the interaction of BSA–BZC. Moreover, the negative values of ΔG° − 32.30, − 31.50, − 30.68 kJ mol−1 at 298, 304, 310 K, respectively, indicate the spontaneity of the interaction. FRET analysis proved high probability of energy transfer from BSA to the drug molecule. Molecular docking and displacement studies indicated that BZC was bound to the Sudlow’s site II through hydrogen bonding and Van der Waals interactions.

Investigation of Conformational Changes of Bovine Serum Albumin upon Binding with Benzocaine Drug: a Spectral and Computational Analysis

Exploring the intermolecular interactions and contrasting binding of flufenamic acid with hemoglobin and lysozyme: A biophysical and docking insight.

Intermolecular interaction study of human serum albumin (HSA) with two anthraquinones i.e. danthron and quinizarin has been performed through fluorescence, UV-vis and CD spectroscopy along with doc...

Saeeda Naqvi

Papers

Densities of aqueous mixtures of (choline chloride+ethylene glycol) and (choline chloride+malonic acid) deep eutectic solvents in temperature range 283.15–363.15K

Volumetric Behavior on Interactions of α-Amino Acids with Sodium Acetate, Potassium Acetate, and Calcium Acetate in Aqueous Solutions

Investigation of Conformational Changes of Bovine Serum Albumin upon Binding with Benzocaine Drug: a Spectral and Computational Analysis

Exploring the intermolecular interactions and contrasting binding of flufenamic acid with hemoglobin and lysozyme: A biophysical and docking insight.

Probing the intermolecular interactions into serum albumin and anthraquinone systems: a spectroscopic and docking approach.