First Evidence of Cyclodextrin Inclusion Complexes in a Deep Eutectic

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First evidence of cyclodextrin inclusion complexes in a deep eutectic solvent Tarek Moufawad, Leila Moura, Michel Ferreira, Herve Bricout, Sebastien Tilloy, Eric Monflier, Margarida Costa Gomes, David Landy, and Sophie Fourmentin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00044 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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ACS Sustainable Chemistry & Engineering

First evidence of cyclodextrin inclusion complexes in a deep eutectic solvent

Tarek Moufawad1,2, Leila Moura1, Michel Ferreira3, Hervé Bricout3, Sébastien Tilloy3, Eric Monflier3, Margarida Costa Gomes2, David Landy1, Sophie Fourmentin1, *

1Unité

de Chimie Environnementale et Interactions sur le Vivant (UCEIV), EA 4492 SFR Condorcet FR CNRS 3417, Université du Littoral-Côte d'Opale, 59140 Dunkerque, France. 2Laboratoire 3Univ.

de Chimie ENS Lyon, UMR CNRS 5182, 6 Allée Italie, 69007 Lyon, France.

Artois, CNRS, Centrale Lille, ENSCL, Univ. Lille, UMR 8181, Unité de Catalyse et de Chimie du Solide (UCCS), F-62300 Lens, France.

Full mailing address : Tarek Moufawad, David Landy, Sophie Fourmentin Unité de Chimie Environnementale et Interactions sur le Vivant (UCEIV), Université du LittoralCôte d'Opale, MREI 1, 145, Av Maurice Schumann, 59140 Dunkerque, France Leila Moura School of Chemistry and Chemical Engineering, Queen’s University of Belfast, Belfast BT9 5AG, Northern Ireland, United Kingdom. Michel Ferreira, Hervé Bricout, Sébastien Tilloy, Eric Monflier Unité de Catalyse et de Chimie du Solide (UCCS), Faculté Jean Perrin , Rue Jean Souvraz SP18, 62300 Lens, France Margarida Costa Gomes, Tarek Moufawad, Laboratoire de Chimie ENS Lyon, UMR CNRS 5182, 6 Allée Italie, 69007 Lyon, France

*

Corresponding author: Tel: +-33-3-28-65-82-54; Fax: +-33-3-28-23-76-05; e-mail: [email protected]

ACS Paragon Plus Environment

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Abstract Supramolecular host-guest inclusion complexes based on cyclodextrins (CDs) are generally studied in aqueous solutions or more rarely in the presence of a co-solvent. In this study, we investigate for the first time the ability of CDs to retain their host-guest properties in deep eutectic solvents (DESs). Five cyclodextrins were solubilized in mixtures of choline chloride:urea (ChCl:U) and these new solutions were characterized by measuring their viscosity and density as a function of temperature. The heat of dissolution of β-CD in ChCl:U was measured by calorimetry. The dissolution process is exothermic meaning that the interaction between ChCl:U and CD is favorable (enthalpy of dissolution of – 23.3 J/g). The ability of CDs to form inclusion complexes with various guests in ChCl:U was demonstrated using two different methods: UV-Visible spectroscopy and static headspace gas chromatography. On one hand, the experimental data obtained from the complexation between methyl orange and CDs follows the theoretical fitted curved for 1:1 inclusion complex. On the other hand, the observed volatility reductions of four volatile organic compounds in different mixture of ChCl:U:CD were correlated to the CD/guest formation constant values determined in water. These results will certainly contribute to enlarge the applications of CDs in the DESs field.

Keywords: cyclodextrin, eutectic solvents, densities, viscosities, supramolecular chemistry, inclusion complexes.


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Introduction Supramolecular chemistry is based on the creation of noncovalent bonds between chemical species, leading to the formation of well-organized structures1. These bonds are based on relatively weak intermolecular interactions which are often reversible and influenced by the microscopic structure. Cyclodextrins (CDs) play an important role on the advancement of supramolecular chemistry as they are the most widely used host molecular structures2. CDs are cyclic oligosaccharides obtained from the enzymatic degradation of starch. Native CDs consist of six (α-CD), seven (β-CD) or eight (γ-CD) glucopyranose units, bound by α-(1–4) linkages forming a truncated conical structure with different cavity size (Figure 1).

Figure 1: Representation of the 3 native cyclodextrins

The hydrophobic cavity of CDs, associated to their hydrophilic outer surface, enable them to encapsulate hydrophobic compounds to form host-guest inclusion complexes. The formation of inclusion complexes with CDs has been extensively studied in aqueous solution. Different factors affecting the complexation were assessed, as the temperature or the ionic strength. Few studies were nevertheless performed in the presence of co-solvents, like ethanol or DMSO, whose presence generally causes a decrease in the strength of the inclusion complex3. To the best of


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our knowledge, examples of inclusion complexes in non-aqueous media reported in the literature are scarce4,5. Deep eutectic solvents (DESs) recently emerged as a new generation of sustainable solvents6,7. DESs are mixtures of two compounds that form a eutectic point at a temperature well below that of the ideal mixture. To facilitate their use as solvents, it is convenient that the melting temperature is also below that of the temperature at which the dissolution process takes place (for example, room temperature) and, for practical reasons, at a range of compositions near the eutectic composition. The most popular DESs are generally mixtures of a hydrogen bond donor and a hydrogen bond acceptor, that at precise ratios lead to homogenous mixtures8. Because the DESs normally include salts, they can have similar physicochemical properties to those of ionic liquids but they are often cheaper, surely easier to produce and often prepared from mixing naturally occurring compounds, less toxic and mostly biodegradable. DESs have been used as extractant for organic and biologically active compounds, as media for organic, catalytic or enzymatic reactions or for solubilizing CO2, SO2 and volatile organic compounds9–13. Few studies have combined the use of CDs and DESs. The low-melting mixture of β-CD with Nmethylurea has been recently used as reaction media for Suzuki–Miyaura and Heck reactions14 and CD mixed with N,N’-dimethyl urea was taken as media for Tsuji-Trost and hydroformylation reactions11,15,16. The extraction of polyphenols using DESs and β-CD was reported, but no correlation could be made between the additions of β-CD to various DESs and the yield of extracted polyphenols17. Furthermore, a recent work investigated the solubilization of natives CDs and cucurbit[n]urils (CBs) in choline chloride:urea (1:2) DES. It was found that the solubility of natives CDs was significantly enhanced in choline chloride:urea compared to aqueous environment18. These authors also studied the interaction of methylviologen with CB[7], CB[8], βand γ-CD in the DES solution. However, except for cucurbit[n]urils, they were not able to prove complexation by titration methods using analytical techniques.


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We recently patented the use of DESs as absorbent for volatile organic compounds (VOCs)19. In this patent, the effect of CD on the solubilization of some VOCs in ChCl:U DES was also investigated by using static headspace gas-chromatography. We could observe a variation on the solubilization capacities of the DES depending on the amount of added CDs. Therefore, in this work, we have investigated the effect of CD addition in ChCl:U. Titration experiments were realized by UV-Vis spectrophotometry, in the case of methyl orange, and by static-headspace gas chromatography (SH-GC) in the case of VOCs (toluene, dichloromethane, limonene and tertbutylcyclohexane). The viscosity as well as density of these new solvents of CD in the DES were also determined and the dissolution of β-CD in ChCl:U was investigated using dissolution calorimetry.

Materials and methods Materials Choline chloride (98%), urea (99%), methyl orange (85%), toluene (99.8%), dichloromethane (99.8%), limonene (99%) and tert-butylcyclohexane (99%) were purchased from Sigma-Aldrich. α-CD, β-CD and γ-CD were provided by Wacker-Chemie (Lyon, France), low methylated-β-CD (CRYSMEB, DS = 4.9) and (HP-β-CD, DS= 5.6) were provided by Roquette Frères (Lestrem, France). Choline chloride was dried by placing it in an oven at 60°C for 2 weeks prior to use, the other compounds were used as received. The structures of the compounds used in this work are depicted in Figure 2.


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OR

ClO

RO

OR

HO O

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CH3 +

N

CH3

CH3

H2 N

O C

NH2

7

CRYSMEB (DS 4.9): R= -H or -CH3 HP--CD (DS 5.6): R= -H or -CH2-CH(OH)-CH3

Choline chloride

Urea

Figure 2: Structure of the compounds used in this work.

Preparation of the Choline chloride:urea DES Accurate amounts of dried choline chloride (ChCl) and urea (U) were mixed with respect to the molar ratio ChCl:U 1:2. Each mixture was stirred at 80°C until a transparent homogenous solution was obtained12. All samples were kept in sealed containers to avoid contact with atmospheric humidity that may affect the physical properties of the DES. The water content of each DES and of each solution of CD in the DES was determined using a coulometric Karl Fisher titrator (Mettler Toledo DL31). The values vary between 0.06 %w/w for ChCl:U to 4.18 %w/w for the solution of 10% CRYSMEB in ChCl:U.

Density and viscosity measurements Density was measured using a U shaped vibrating-tube densimeter (Anton Paar, model DMA 5000 M) operating in a static mode. All measurements were performed at atmospheric pressure and at temperatures between 303.15 K and 333.15 K. The factory calibration was used and verified before and after each measurement with air and tri distilled degassed water. The DMA 5000 densimeter performs an analysis with an estimated uncertainty in density and temperature of ±0.1 kg m-3 and ±0.001°C, respectively. The device has a built-in correction for liquids with a viscosity up to 700 mPa s, as in the present case.


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The viscosity of the solutions was measured using a falling-ball-based microviscosimeter (Lovis 2000 M/ME from Anton Paar) at atmospheric pressure and in the temperature range 303.15−333.15 K. The temperature was controlled to within 0.005 K and measured with an accuracy better than 0.02 K. A capillary tube of 1.8 mm diameter, previously calibrated as the function of temperature and angle of measurement with reference oils, was used for the measurements. The overall uncertainty on the viscosity was estimated to be 2%.

Dissolution calorimetry The heat of dissolution of β-CD in the DES was measured using a 25 mL precision solution semiadiabatic (i.e. isoperibol) solution calorimeter housed in a TAM III thermostat from TA instruments. The experiments were performed at 60˚C and the quantities of β-CD and DES were determined gravimetrically. The experimental procedure described previously was followed but special care was taken when filling the glass ampoule with dried samples of β-CD in order to avoid contamination with atmospheric water20,21. The glass ampoule was flame-sealed before introduction in the calorimeter. The experimental data obtained during the dissolution of β-CD in the DES was adjusted, as explained before to the equation20,21.



T t 

n0H  c   e t c  e t d T Cp  c  d





(1)

Where τd and τc are the characteristic times of dissolution and of thermal relaxation of the calorimeter, respectively; T∞ is the temperature when the calorimeter is in equilibrium with its surrounding thermostat; Cp is the heat capacity of the calorimeter and ΔH is the enthalpy of dissolution. The values of τc and Cp are determined from a calibration measurement performed in the solution after dissolution using a heat pulse from an electrical heater.


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UV-visible spectroscopy Spectra were recorded using a Perkin Elmer Lambda 2S double beam spectrometer and a quartz cell with optical path length of 10 mm at 298 K. Spectra were recorded between 510 and 520 nm. The control of temperature was achieved by the use of a thermostated bath linked to the cell holder (accuracy: ±0.1 °C). An algorithmic treatment was applied to the first derivatives of UV spectra in order to avoid any spectral influence of diffraction phenomena 22. A stock solution of ChCl:U with an adequate amount of methyl orange (MO) was prepared and then divided into aliquots. Different amounts of β-CD and HP-β-CD (0.5, 1, 1.5, 2, 2.5% wt) were then added to the vials. The vials were sealed and kept under stirring for 24 hours at 30°C to ensure homogenization of the samples.

Static headspace gas chromatography (SH-GC) A precise amount of ChCl:U was placed in 20 mL headspace vials. α-CD, β-CD,-CD, HP-β-CD and CRYSMEB were added to the DES at 2, 4, 6 and 10% (wt). After homogenization of the solutions, an equal amount of the volatile compounds (toluene, dichloromethane, limonene and tert-butylcyclohexane) was added to the samples. The sample were then kept under stirring for 24 hours to reach the equilibrium at 30°C. Then, the samples were left for 120 min in the headspace oven at 30°C prior to the analysis. All measurements were carried out with an Agilent G1888 headspace sampler coupled to a Perkin Elmer Autosystem XL gas chromatography equipped with a flame ionization detector and a DB624 column using nitrogen as carrier vector. The gas chromatography column temperature was set at 40˚C, 80˚C, 120˚C and 160˚C for dichloromethane, toluene, limonene and tert-butylcyclohexane, respectively. The transfer line temperature was set at 250˚C.


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Results and discussion DES characterization The density experimental values for ChCl:U and for the different ChCl:U:CD solutions are reported in Table S1 of the Supporting Information (SI). The values were adjusted to appropriate polynomial functions of the temperature and the parameters of the fits are listed in Table S2 of the SI. The values measured herein are compared with those published in the literature on Table S3 of the SI at normal pressure and at temperatures from 303.15 to 333.15 K. The deviations encountered are always lower than 0.1%, the larger differences being found at 303.15 K between our data and those of Abbott et al.8 and those of Leron and Li23 which are 0.09% higher than those reported herein. The ChCl:U:CD solutions are denser than the pure DES at all the temperatures studied, the density increasing when the concentration of CD increases (Figure 3a). The viscosity values are listed in Table S4 of the SI. In light of these values, the addition of CD to the DES does lead to weak variation in the viscosity of the liquid solution except for the 10% solutions of γ-CD and HP-β-CD for which a significant increase of the viscosity is observed at the temperatures covered (Figure 3b). For the solution with 10% of CRYSMEB a significant lower viscosity was measured in the temperature range covered. This lower viscosity could be explained by the higher amount of water in this solution (4.18%w/w) compared to the other CD solutions (from 1.26%w/w to 1.82%w/w).


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Figure 3: Experimental values for a) the density and b) the viscosity of the deep eutectic solvents based on choline chloride:urea alone

and with

α-CD, β-CD,

γ-CD,

HP-β-CD and

CRYSMEB at 2%

(solid line) and 10% (dashed line).

Dissolution calorimetry The temperature profile obtained when a sample of β-CD was dissolved in the DES is presented in Figure 4. The first peak corresponds to the CD dissolution and the second concerns the response of the calorimeter to a 2 J heat pulse thus serving to calibrate the heat capacity and relaxation tie of the apparatus.


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55

60

50

T /mK

45

50

40 35

T /mK

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30 25 0

50

100

150

200

t /min

40

30 0

250

500

750

t /min

Figure 4: Temperature profile of the solution calorimeter when a 290.880 mg sample of β-CD is dissolved at 60°C in 27.947 mL of DES6. The insert represents the detail of the fitting used to determine the enthalpy of dissolution (red line), the green line corresponding to the relaxation of the calorimeter and the blue line to the dissolution of CD (details given in the Supporting Information table S6).

It is observed that the dissolution of β-CD in the DES is an exothermic process with an enthalpy of dissolution of – 23.3 J/g (26.4 kJ/mol). Being an exothermic process means that the interactions between the DES and the CD are highly favorable but also means that an increase in temperature does not facilitate the dissolution, at least not in terms of the thermodynamics of the process. Titration experiments The formation of an inclusion complex could be investigated in aqueous solution by titration experiments using various analytical techniques (NMR, UV-Visible, HS-GC, ITC) 24–26. In the case of UV-visible spectroscopy, we could observe the variation of the guest absorbance in presence of increasing CD concentrations. In order to investigate if the different CDs are able to form inclusion complexes when dissolved in ChCl:U, we follow the variation of the area of the second derivatives of MO in ChCl:U in the absence and in the presence of different percentages of β-CD and HP-β-CD (Figure 5).


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Figure 5: Variation of the area of the second derivative of methyl orange for CHCl:U with different percentage of β-CD( ) and HP-β-CD( ), with the theoretical fitted curves (dashed lines) for a 1:1 inclusion complex.

As can be seen from Figure 5, the addition of CD to ChCl:U leads to a variation of the absorbance of MO. Moreover, this variation is in good agreement with the theoretical variation of this absorbance in the case of a 1:1 inclusion complex22. This agreement between experimental points and theoretical curves supports the assumption that a 1:1 inclusion complex is formed between MO and CD dissolved in the ChCl:U DES. These results observed at low CD percentage support the role of CD as a cage molecule for MO in DES. Assuming the existence of an inclusion complex, the formation constant (Kf) of MO with both CDs in ChCl:U could be determined using an algorithmic treatment22Error! Bookmark not defined.. The values listed in Table 1 are in good correlation with the values obtained in water, with the Kf value of HP-β-CD/MO inclusion complex being higher than the one with β-CD24. However, these Kf values are much lower in ChCl:U than in water. This could be due to the fact that MO is more


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soluble in ChCl:U than in water. Indeed, the main driving force of the formation of an inclusion complex is the better affinity of the guest for the hydrophobic CD cavity compared to the solvent, usually water. In addition, one cannot exclude that the cyclodextrin cavity is less available within the DES than in water, as each DES component could act as a potential competitor for inclusion. Table 1: Formation constant (M-1) of CD/MO in water and ChCl:U.

H 2O

ChCl:U

β-CD

250024

70

HP-β-CD

537324

141

SH-GC was used to study the effect of the addition of CDs to the ChCl:U DES, at higher concentrations than for the UV-vis study, on the retention of various volatile compounds. We chose guest molecules with a large range of Kf value in water (see Table 2), in order to evaluate if the selectivity of the CDs is maintained when dissolved in the ChCl:U DES. This selectivity is mainly due to the steric complementarity of the CD and guest, inclusion complexes being stronger when there is a good geometrical complementarity between the guest and the CD cavity 25.

Table 2: Formation constant (M-1) obtained for the various inclusion complexes in water and in DES (in brackets).

α-CD

β-CD

γ-CD

HP-β-CD

CRYSMEB

Dichloromethane

2127 (-)

927 (-)

- (-)

1027 (-)

927 (-)

Toluene

3828 (6)

14228 (11)

3328 (4)

16328 (66)

16528 (25)


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tert-Butylcyclohexane

248 (-)

409229 (11)

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18 (4)

203629

5577 (42)

(20)Error! Bookmark not defined. Limonene

128930 (2)

316230 (14)

11630 (7)

278730

366830

(80)Error!

(34)Error!

Bookmark not

Bookmark

defined.

not defined.

The addition of CD to an aqueous VOC solution lead to a decrease of the chromatographic peak area as a function of CD concentration31. As the peak area is proportional to the concentration of the compound in the gaseous phase, a decrease of the area is linked to a higher solubilization of the volatile compound in solution. If such a phenomenon is observed in ChCl:U, this could be related to the formation of an inclusion complex in the eutectic solvent. As can be seen in Figure 6, the obtained chromatographic peak area decreases in the presence of CDs except for dichloromethane. In this case (Figure 6a), the increased presence of CDs results to a slight increase of the chromatographic peak area. This result can be related to the low Kf values of CD/dichloromethane inclusion complex in water (Table 2). Indeed, dichloromethane is a small molecule that did not filled entirely the CD cavity27.


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Figure 6: Variation of the chromatographic peak area for (a) dichloromethane, (b) tert-butylcyclohexane, (c) limonene and (d) toluene in CHCl:U with various amount of CD ( α-CD; β-CD; γ-CD; HP-β-CD; CRYSMEB).

For tert-butylcyclohexane (Figure 6b), limonene (Figure 6c) and toluene (Figure 6d) we can clearly distinguish the selectivity between the different types of CD. The effect of β-CD and its derivatives in ChCl:U is more pronounced than for α-CD and γ-CD. This effect could be related to the greater Kf values of these compounds with β-CD and its derivatives compared to α-CD and γ-CD in water (Table 2). Indeed, the size of the α-CD cavity is too small for these guests while the one of γ-CD is too large to have a good space filling of the cavity29. The formation constant (Kf) of the four VOCs with CDs in ChCl:U were also determined (Table 2). Similarly to MO, these Kf values are much lower in ChCl:U than in water. We could also observe that the β-CD derivatives were slightly less affected than the native β-CD.


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It is noteworthy to mention, that a sample containing 10% (wt) of methyl α-D-glucopyranose was also prepared in order to determine if the solubilization properties of the CD solutions could be due to a “sugar effect” or to the presence of the cavity. No variation in the peak area was observed for this sample for all the studied VOCs. This result highlights the role of the cavity in the absorption enhancement of the examined volatile compounds, and consequently the formation of inclusion complexes between the CD and the guest molecule in the ChCl:U DES. Finally, we determined the maximum volatility reduction obtained in water and in DES in presence of CD, and reported them in Figure 7. As can be seen, a good correlation is observed between these values, supporting that the solubilizing mechanisms of VOCs induced by the presence of CD are analogous in water and DES. Such result constitutes another proof that CDs are able to form inclusion complexes within the ChCl:U DES.

Figure 7: Correlation between the volatility reduction obtained in DES and in water in the case of β-CD ([β-CD]=0.01M, CH2Cl2, toluene, limonene, tert-butylcyclohexane)


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McCune et al. 18 postulate that the enhanced solubility of natives CDs in ChCl:U results from the ability of the hydroxyl groups on the exterior of the CD to interact with the hydrogen-bonded network of the DES, which results in incorporation of the CDs within the bulk-liquid structure. In this paper, we demonstrated that CDs, although interacting with the DES components, retain their complexation properties in ChCl:U as shown through UV-Vis spectroscopy and static-headspace gas chromatography. Moreover, the selectivity of the different CDs towards various guest molecules is also maintained in these solvents. In summary, new solvents based on three natives CDs (α-, β-, γ-CD) and two modified β-CDs (HP-β-CD and CRYSMEB) solubilized in ChCl:U were presented and studied for the first time. The physical-chemical properties of the solvents were characterized by measuring their density, and viscosity and the heat of dissolution was measured for one of the native CDs. The dissolution of β-CD in the DES is exothermic, meaning that an increase of temperature is not thermodynamically favorable. Raising the temperature increases the fluidity of the DES thus facilitating mass transport and leading to a faster dissolution of CD. The complexation ability of CDs in ChCl:U DES was investigated using 2 different methods. Results obtained at low cyclodextrin concentrations for MO by the titration method using UV-Vis measurements are in good agreement with the theoretical fitted curves for a 1:1 inclusion complex. Furthermore, the solubilization of four volatile organic compounds was measured by SH-GC. The variation of the chromatographic peak area, related to the amount of solubilized compound, as a function of CD amount is in good agreement with their relative affinity for CDs in water. The selectivity of CDs towards the guests is preserved in ChCl:U, underlining the formation of inclusion complexes between CD and guest in ChCl:U. These news solvents take advantages of the DESs properties as well as the supramolecular properties of CDs (controlled release, solubilization, stability enhancement…). Therefore, the use of ChCl:U as solubilizing agent for


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dissolving high amount of CDs compared to water, especially in the case of β-CD, associated to a maintained molecular recognition could expand the applications of CDs in many fields.

Acknowledgements Authors are grateful to the French Environment and Energy Management Agency (ADEME) for the financial support of this project (CORTEA 1401C0035). T.M. acknowledges the financial support from both the ADEME thesis program and the PMCO (Pôle Métropolitain Côte d’Opale, France). M.C.G. acknowledges the financial support of the project IDEXLYON of the University of Lyon (ANR-16-IDEX-0005). The authors thank Mrs. L. Pison for her help in the measurement of the density and viscosity of the solutions studied in this work and Prof. A. Padua for his help with the fit of the calorimetric data.

Associated content Supporting information: densities, parameter, comparison of densities with literature, viscosities,

Vogel–Fulcher–Tammann (VFT) equation parameters, dissolution calorimetry parameters.

Author information Corresponding author E-mail: [email protected] Tel: +-33-3-28-65-82-54; Fax: +-33-3-28-23-76-05. ORCID Leila Moura: 0000-0002-7938-5892 Michel Ferreira : 0000-0003-4898-0290 Hervé Bricout: 0000-0002-4895-1636 Sébastien Tilloy: 0000-0002-3494-4734


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Eric Monflier: 0000-0001-5865-0979 Margarida Costa Gomes: 0000-0001-8637-6057 David Landy: 0000-0001-6297-6162 Sophie Fourmentin: 0000-0002-4334-0051

Present Address Leila Moura: School of Chemistry and Chemical Engineering, Queen’s University of Belfast, Belfast BT9 5AG, Northern Ireland, United Kingdom.

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Table of Contents

Adding cyclodextrins to a deep eutectic solvent leads to new green solvents with supramolecular properties.


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Figure 1: Representation of the 3 native cyclodextrins



Figure 2: Structure of the compounds used in this work.


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Figure 3: Experimental values for a) the density and b) the viscosity of the deep eutectic solvents based on choline chloride:urea alone and with α-CD, β-CD, γ-CD, HP-β-CD and CRYSMEB at 2% (solid line) and 10% (dashed line). 12x5mm (600 x 600 DPI)



Figure 4: Temperature profile of the solution calorimeter when a 290.880 mg sample of β-CD is dissolved at 60°C in 27.947 mL of DES6. The insert represents the detail of the fitting used to determine the enthalpy of dissolution (red line), the green line corresponding to the relaxation of the calorimeter and the blue line to the dissolution of CD (details given in the Supporting Information table S6).


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Figure 6: Variation of the chromatographic peak area for (a) dichloromethane, (b) tert-butylcyclohexane, (c) limonene and (d) toluene in CHCl:U with various amount of CD ( α-CD; β-CD; γ-CD; HP-β-CD; CRYSMEB). 11x8mm (600 x 600 DPI)


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First Evidence of Cyclodextrin Inclusion Complexes in a Deep Eutectic

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