Synthesis and Properties of Deep Eutectic Solvents: Comparison
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The use of deep eutectic solvents (DES) is on the rise worldwide because of the astounding properties they offer, such as simplicity of synthesis and utilization, low-cost, and environmental friendliness, which can, without a doubt, replace conventional solvents used in heaps.

  • deep eutectic solvents
  • extraction
  • organic compounds
  • green sample preparation
  • analytical chemistry

1. Introduction

Over the last decades, conventional solvents and extraction techniques—for example, methanol as a solvent or liquid–liquid extraction as a technique—tend to be replaced by new, simpler, inexpensive, and environmentally friendly approaches. The movement of “Green Analytical Chemistry” (GAC) and its 12 principles, presented by P. Anastas in 1998, has especially inspired scientists to switch to greener and alternative solutions for extracting compounds from different matrices [1]. Thus, developing new and sustainable solvents to match the above criteria is of high interest to many researchers globally [2,3,4,5,6,7,8][2][3][4][5][6][7][8]. A new group of organic salts that possess melting points below 100 °C, called ionic liquids (ILs), have emerged and demonstrate properties that could replace volatile organic solvents. However, ionic liquids pose some problems, such as toxicity, stability, biodegradability, and expense regarding their synthesis.
That is where deep eutectic solvents (DES) emerged in 2001 as the perfect candidate for replacing ILs, demonstrating cheaper and easier synthesis, while their environmentally friendliness is much more evident [9,10,11,12][9][10][11][12]. Deep eutectic solvents have low melting points due to low lattice energy formed by their large and asymmetrical ions. Habitually, they are formed by the combination of metal salt, called the hydrogen bond donor (HBD), with a quaternary ammonium salt, the hydrogen bond acceptor (HBA). Thus, hydrogen bonds are formed by charge delocalization, and the mixture’s melting point is lower than the separate components of the DES [13,14][13][14]. Hydrophilic and hydrophobic DES and natural deep eutectic solvents (NADES) can, without a doubt, be applied to analytical chemistry in different fields to extract molecules from a variety of samples [4,8,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29][4][8][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].
In Figure 1, the main scheme of how DES are used in extraction is presented.
Figure 1.
Representative scheme of the typical development of an extraction method by using deep eutectic solvents.

2. Synthesis of Deep Eutectic Solvents

The most common way to synthesize DES is by mixing two or three cheap and safe salts and heating them up in order to obtain a homogenous solution. Molten salts at the ambient temperature can also be prepared through the mixture of metal salts with quaternary ammonium salts [30,31,32][30][31][32]. There are four main types of deep eutectic solvents: type I, where a metal chloride is combined with a quaternary salt, type II, where a hydrated metal chloride is merged with a quaternary salt, type III, where an HBD is put together with an HBA quaternary salt, and type IV, where an organic HBD compound reacts with a metal chloride [2,33][2][33]. Table 1 summarizes the four types of the DES, while in Figure 2, the most frequently used HBA and HBD are presented.
Figure 2.
Structures of some common HBAs and hydrogen bond donors HBDs used in the synthesis of deep eutectic solvents.
Table 1.
The four types of Deep Eutectic Solvents and their formulas.

Type

Formula

Terms

DES

Molar Ratio

Extraction Technique

Recovery (%)

LOD 1

Reference

I

Cat+X− + zMClx

M = Zn, Sn, Ga, In, Al, Fe

II

Cat+X− + zMClx · yH2O

M = Cr, Cu, Fe, Ni, Co

III

Cat+X− + zRZ

Z = COOH, CONH2, OH

IV

MClx + RZ

M = Al, Zn and Z = CONH2, OH
Cat+ = any phosphonium, ammonium or sulfonium cation, X− = a Lewis base, often a halide anion, MClx = metal chloride, RZ = organic compound.
Choline chloride (ChCl) is the most widely used HBA due to its low cost, low toxicity and biodegradability than has the advantage of reacting with safe and inexpensive HBDs, such as carboxylic acids, urea, or glycerol [32,34][32][34]. Additionally, type III acidic deep eutectic solvents are synthesized from quaternary organic salts with Brønsted acids (for example, citric acid) and type IV acidic DES from Lewis acids (for instance, zinc chloride or bromide) [35].

3. Properties of Deep Eutectic Solvents

Physicochemical properties of deep eutectic solvents derive from the various interactions between an HBA and HBD, which depend on the molar ratios, the organic compounds themselves, and the nature of the interactions (π-π and/or hydrogen bonding, anion exchange, weak non-covalent interactions) [36]. The properties that need to be considered when using DES and/or NADES are density, acidity, conductivity, viscosity, volatility, the melting and freezing point, and surface tension. Additionally, the toxicity, cost, thermal stability, and biodegradability should not be passed over [2,37][2][37]. The value of pH that results from the combination of HBA and HBD can drastically affect the extraction efficiency of the target compounds, so this parameter ought to not be overlooked as well [38]. As described by El Achkar et al. [39] in a thorough review about the properties of DES, viscosity is an important parameter to be studied, among others, since it severely impacts the extraction efficiency of a developed method. Water content is an ambiguous parameter since, for some, it might seem as an impurity, while others rely on it and add water on purpose to expose the reliability of the DES. Molar ratio and temperature of extraction can impact the isolation of organic compounds as well, and they are studied in almost all developed methods of extraction. Consequently, by knowing the parameters that can affect the extraction with the usage of DES, one can exploit their high tunability [40].

4. Extraction of Organic Compounds

In Table 2, the methods developed for the extraction of the organic compounds from pesticides, fungicides, herbicides, flavonoids, phenolic and other bioactive compounds are summarized and presented.

Table 2.

Summary and analytical performance of the methods discussed.

Sulfonamide

ChCl: ethylene glycol

1:2

PT-SPE

91.0–96.8

0.01 μg/mL

[93][83]

Analgetic

Betaine: oxalic acid

1:2

SA-DES-ME

94.2–107.1

14.9 μg/L

[94][84]

Antibiotics

ChCl: glycerol

1:2

MIPs-SPE

87.0–91.2

N/R 2

[95][85]

Antivirals

TBAC: p-aminophenol

1:2

LLE

90–96

1.0–1.3 μg/L

[96][86]

Antimalarial

Menthol: fenchyl alcohol

1:1

Stirring

101

N/R

[97][87]

NSAID 3

Menthol: acetic acid

Diff. 4

LLE

80

N/R

[98][88]

Antibiotics

TBAC: butanol

1:1

SPE, DLLME

84–99

0.32–0.86 ng/g

[99][89]

Tocotrienols, tocopherols

ChCl: malonic acid

1:1

LLE

93.0–99.8

N/R

[100][90]

α-tocopherols

ChCl: sucrose

1:2

LLE

N/R

N/R

[101][91]

α-, γ-, δ-tocopherols

ChCl: p-cresol

1:2

Vortex

77.6

N/R

[102][92]

Parabens

Menthol: decanoic acid

2:1

LLME

69.1–78.5

0.6–0.8 mg/mL

[103][93]

PAHs

Menthol or thymol with fatty acids

Diff.

Stirring

70–91

2–90 ng/kg

[105][94]

VOCs

ChCl: urea

1:3

MAE, HS-SPME

N/R

N/R

[106][95]

Polysaccharides

ChCl: 1,4-butanediol

N/R

UAE

N/R

N/R

[107][96]

Polysaccharides

ChCl: 1,4-butanediol

1:5

MAE

91.2

N/R

[108][97]

Polysaccharides

ChCl: 1,2-propanediol

1:2

UAE

N/R

N/R

[109][98]

Polysaccharides

ChCl: glycerol

1:2

Thermal treatment

60.3

N/R

[110][99]

Natural pigments

Citric acid: glucose

1:1

SPE

88.5–94.4

0.25–0.37 mg/L

[111][100]

Curcuminoids

ChCl: lactic acid

1:1

UAE

N/R

N/R

[112][101]

Curcuminoids

ChCl: phenol

1:4

VA-LLME

96–102

2.86 μg/L

[113][102]

Pigment

Different types

Diff.

UAE

N/R

N/R

[114][103]

Pigment

ChCl: ethylene glycol

Diff.

N/R

N/R

N/R

[115][104]

Terpene trilactones

Betaine: ethylene glycol

Diff.

UAE

99.4

N/R

[116][105]

Benzophenone-type UV filters

Menthol: decanoic acid

1:1

AA-DLLME

88.8–105.9

0.05–0.2 ng/mL

[117][106]

Organic solvents

Menthol: capric acid

2:1

LLE

N/R

N/R

[118][107]

Tanshinones

ChCl: -1,3-butanediol

N/R

BMAE

96.1–103.9

5–8 ng/mL

[119][108]

Catechins

Different types

Diff.

LLE

82.7–97.0

N/R

[120][109]

Essential oils

ChCl: L-lactic acid

1:3

MAE

N/R

N/R

[121][110]

Essential oils

ChCl: oxalic acid

1:1

MAE

N/R

N/R

[122][111]

Steviol glycosides

Tetraethylammonium chloride: ethylene glycol

1:2

UAE

N/R

N/R

[123][112]

Group of Compounds

DES

Molar Ratio

Extraction Technique

Recovery (%)

LOD 1

Reference

Triazole fungicides

ChCl: 4-chlorophenol

1:2

HS-SDME

94–97

0.82–1.0 mg/L

[41]

Pesticides

Menthol: dichloroacetic acid

1:2

DLLME

56–86

0.32–1.2 ng/g

[42]

Pesticides

ChCl: p-chlorophenol

1:2

LPME

56–93

0.13–0.31 ng/mL

[43]

Neonicotinoid pesticides

Menthol: dodecanoic acid

2:1

LLE

80

N/R 2

[44]

Pesticides

ChCl: decanoic acid

1:2

DLLME

64–89

0.9–3.9 ng/mL

[45]

Acidic pesticides

ChCl: ethylene glycol

1:2

DLLME-SBSE

76–90

7–14 ng/L

[47][46]

Pesticides

TBAC: dichloroacetic acid

1:1

DLLME

81–94

0.09–0.27 ng/mL

[48][47]

Fungicides

Menthol: decanoic acid

1:1

UA-DLLME

72–109

0.75–8.45 μg/mL

[49][48]

Herbicides

ChCl: butyric acid

1:2

LLE-DLLME

70–89

2.6–8.4 ng/kg

[50][49]

Flavonoids

ChCl: 1,4-butanediol

1:5

UAE

N/R

0.07, 0.09 μg/mL

[56][50]

Flavonoids

Glycerol: L-proline

2:5

UAE, SPE

81–87

N/R

[57][51]

Isoflavones

ChCl: citric acid

1:1

UAE

64.7–99.2

0.06–0.14 μg/mL

[58][52]

Flavonoids

ChCl: 1,2-propanediol

1:4

UAE

86.7–98.9

0.05–0.14 μg/mL

[59][53]

Flavonoids

ChCl: oxalic acid: ethylene glycol

1:1:3

LLE

N/R

1.11–1.40 mg/g

[60][54]

Flavonoids

ChCl: acetic acid

1:2

UAE

N/R

1.11–11.57 mg/g

[61][55]

Flavonoids

Betaine: D-mannitol

N/R

UAE

91.7–95.8

0.14–0.17 μg/mL

[62][56]

Flavonoids

Different types

Diff. 3

UAE

93.8–107.7

0.14–0.22 μg/mL

[63][57]

Bioactive compounds

ChCl: malic acid

1:1

UAE

N/R

N/R

[64][58]

Bioactive compounds

ILs, ChCl based DES

Diff.

Reflux

N/R

N/R

[65][59]

Anthocyanins

ChCl: glycerol:citric acid

0.5:2:0.5

UAE

N/R

0.02 mg/L

[66][60]

Anthocyanins

ChCl: malic acid

Diff.

UAE

N/R

0.15–0.28 mg/L

[67][61]

Anthocyanins

Lactic acid: glucose

1:2

Stirring

N/R

N/R

[68][62]

Phenolic metabolites

Different types

Diff.

N/R

75–97

N/R

[71][63]

Phenolics

ChCl: oxalic acid

1:1

MAE, UAE

N/R

0.05–0.37 mg/L

[72][64]

Phenolics

ChCl: 1,2-propanediol

1:1

Vortex

N/R

N/R

[73][65]

Phenolics

ChCl: lactic acid

1:2

MAE, HUE, UAE

N/R

N/R

[74][66]

Phenolics

ChCl + phenolics

N/R

RDSE

66–87

10–60 μg/L

[75][67]

Polyphenols

ChCl: ethylene glycol

1:4

SLE

N/R

N/R

[77][68]

Phenolics

Lactic acid: glucose

5:1

UAE

86.0–109.6

0.6–89.1 ng/g

[78][69]

Polyphenols

Lactic acid: glucose

5:1

SLE

N/R

N/R

[79][70]

Phenolics

ChCl: malic acid

1.5:1

P-UAE

N/R

N/R

[80][71]

Phenolics

Glycerol: citric acid: glycine

Diff.

UAE

N/R

N/R

[81][72]

Flavonoids

ChCl: glycerol

1:1

UAE

95

N/R

[82][73]

Polyphenols

Lactic acid: glycine:water

3:1:3

UAE

N/R

N/R

[84][74]

Phenolic acids

ChCl: 1,3-butanediol

1:6

MAE

79.2–86.0

N/R

[85][75]

Phenolics

ChCl: glycerol

1:2

MAE

77.8–83.8

0.15–0.78 μg/mL

[86][76]

Phenolics

Tetramethyl ammonuium chloride:urea

1:4

UAE

97.3–100.4

N/R

[87][77]

Flavanones

Betaine: ethanediol

1:4

Heated LLE

97.0–101.6

N/R

[88][78]

Bioactive compounds

ChCl: propylene glycol

1:1

UAE

N/R

N/R

[89][79]

Phenolics

ChCl: acetic acid

1:2

Thermo-shaking

N/R

N/R

[90][80]

Phenolics

TBAC: hydroquinone

1:2

LLE/DLLME

74–89

0.13–0.42 ng/mL

[91][81]

Phenols

L-proline: decanoic acid

1:4.2

Stirring

57–62

N/R

[92][82]

1 Limit of Detection, 2 Not reported, 3 Different molar ratios tested.

In Table 3, all the methods developed for the extraction of the organic compounds from pharmaceutical compounds, preservatives, polycyclic aromatic hydrocarbons, volatile organic compounds, pollutants, polysaccharides, pigments, terpenes and other organic compounds are summarized, and their analytical performances are demonstrated.
Table 3.
Summary and analytical performance of the methods discussed.

Group of Compounds

1 Limit of Detection, 2 Not reported, 3 Non-steroidal anti-inflammatory drugs, 4 Different molar ratios tested.

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