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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Effect of Water on the Activation Thermodynamics of Deep Eutectic Solvents Based on Carboxybetaine and Choline Misha Rumyantsev, Sergey Rumyantsev, and Ivan Yu. Kalagaev J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b01218 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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The Journal of Physical Chemistry

Effect of water on the activation thermodynamics of deep eutectic solvents based on carboxybetaine and choline Misha Rumyantsev*, Sergey Rumyantsev, Ivan Yu. Kalagaev Nizhny Novgorod State Technical University n.a. R.E. Alekseev, 24 Minin St., 603950 Nizhny Novgorod, Russia *Corresponding author. 606026, Dzerzhinsk, Nizhny Novgorod region, Gaidar st., 49 Tel./fax: +7 8313 344730, E-mail address: [email protected]

ABSTRACT Thermodynamic characteristics as well as peculiarities of hydrogen bonding in aqueous solutions of new deep eutectic solvents (DES) based on synthetic carboxybetaine have been studied. Cheap starting reagents (acrylic acid and 2-dimethylaminoethanol), the simplicity of preparation, and high yields are the advantages of the proposed betaine. Analyzes of the calculated stabilization energies and NBO delocalization energies revealed the absence of preferential association in aqueous reline solutions tested for the comparative purpose. On the other hand it was demonstrated that water preferentially associates with betaine molecules as this type of interaction was proved to possess the highest stabilization energy among the considered complexes. These data were used to discuss the anomalous behavior of the experimentally determined enthalpy and entropy factor of a viscous flow. Thus, sharp increase in the values of T∆SA and ∆HA was demonstrated for the betaine-based solutions with 30–40 wt% of water. The observed thermodynamic peculiarities were explained in terms of structurization of the betaine-based DES caused by strong H-bonding between betaine and water molecules and were supported with experimental IR spectroscopy measurements.

INTRODUCTION In contrast to ionic liquids, deep eutectic solvents (DES) offer several advantages which are simple preparation, low-cost of the reagents and thus the resulting solvents, chemical inertness with respect to water, biodegradability and much lower toxicity.1 High viscosity is among the peculiarities typical for ionic liquids and DES. A mixture of DES with more fluid and less viscose solvents may afford improved physicochemical properties of the system. It was reported that moderating the amount of water in DES one can control absorption capacity and phase separation of the system and create shape-specific 2

synthesis of nanoparticles. Aqueous mixtures of DES, in this respect, have garnered increased attention owing to the proposition that the aqueous mixtures of DES may form a class of useful “hybrid green” 3

systems. The effect of water on DES was also postulated to be one of the most significant unanswered questions in the field.4 Using various experimental techniques it was reported that the addition of water decreases the viscosity of DES mixtures and improve transport properties with little effect on other properties.

3d, 5

Recently, Ma et al. demonstrated dependencies of the molar enthalpies of mixing (Hm) for

binary systems of choline chloride (ChC)/urea DES with different molar ratios of the components from water content.

1b

According to the published data Hm for the reline (ChC:urea 1:2) exhibits the “S-shaped”

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behavior. It implies that the mixing process for the binary reline-water mixtures is endothermic at first, and E

then it changes to exothermic with increasing DES fraction. The excess molar energy of activation (A ) was calculated from the measured viscosities on the basis of Eyring’s absolute reaction rate theory by Xie et al.5b The values were found to be negative in the whole region at low temperatures (298.15 K to 313.15 K), but they change to positive at high temperatures (318.15 K to 333.15 K) in the water-rich region. The negative excess molar activation energy was discussed in terms of strong interaction between ChC/urea and water. The effect of temperature and addition of water on solvatochromic probe behavior within three DES formed on the basis of choline chloride was shown by Pandey.

3c

It was concluded that aqueous

mixtures of ethaline (ChC/ethylene glycol) and glyceline (ChC/glycerol) possess relatively more interspecies H-bonds as compared to aqueous mixtures of reline, where interstitial accommodation of water within the reline molecular network appears to dominate. Complex reorganization of H-bonds between water molecules and DES fragments is the key feature for the observed effects and was studied separately via experimental and computational methods. Thus, molecular dynamics simulations were carried out to investigate the behavior of reline and its equimolar mixture with water.1c Analysis of atom– atom radial and spatial distribution functions and of the H-bond network reveals the microheterogeneous structure of these complex liquid mixtures. In concentrated reline the structure was governed by strong Hbonds of the trans- and cis-H atoms of urea to the chloride ion. In hydrous reline, water competes for the anions, and the H atoms of urea were found to have similar propensities to bond to the chloride ions and oxygen atoms of urea and water. Quantum chemical calculations were performed on three popular DES including reline in order to elucidate the molecular interactions, charge transfer interactions, and 6

–

thermodynamics associated with these systems. Sum of the bond orders of the choline–Cl interactions were postulated to correlate directly with the melting temperatures of the corresponding DES. Differences in the vibrational entropy changes upon DES formation were found to consist with the trend in the overall entropy changes upon DES formation. (Carboxy)betaines constitute a unique class of zwitterionic compounds possessing both cationic and anionic (carboxylate residue) sites per one molecule and combining attractive properties for use in the fields of chemistry and biology. The latest published data indicate that these substances are of particular interest in the field of organocatalysis.7 One of the potential areas of application that is almost unexplored to date is the use of low molecular weight carboxybetaines as components of environmentally friendly solvents of a new type, such as DES. It is desirable to use carboxybetaines with additional functional groups similar to the hydroxymethyl group in choline chloride. In line with the sustainable chemistry concept, this strategy will make it possible to obtain valuable products (e.g., catalytic “green” solvents) from cheap raw chemicals and even waste products that require special methods of disposal. In this paper we use activation thermodynamics, thermodynamic IR spectroscopy measurements and quantum chemical calculations to test structure organization of the new DES based on synthesized carboxybetaine as a function of water content at various temperatures (T = 243–360 K). Thermodynamic peculiarities of the reline and choline iodide + urea aqueous solutions were also discussed. EXPERIMENTAL SECTION

Materials


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Commercial choline chloride (ChC) ≥ 99%, choline iodide (ChI) 98%, urea 99%, ethylene glycol (EG) (analytical grade) and acetonitrile (HPLC grade), deuterium oxide (D2O) D=99.9 from Cambridge isotope were used without further purification. Powder KBr and LiF were dried under reduced pressure prior to the experiments. Acrylic acid (AA) was distilled under reduced pressure prior to use. Commercial 2dimethylaminoethanol (DMAE) was distilled under reduced pressure and stored over 4Ǻ molecular sieves. DMAE concentration determined via isothermal potentiometric titration was equal to 98%.

Synthesis of betaine 3-((2-hydroxyethyl)dimethylammonio)propanoate (Bet) 10 g (0.1 mol) of DMAE was added to the stirred solution of AA (7.56 g, 0.1 mol) dissolved in 30 ml of MeCN at ambient temperature. The resulting solution was placed in a simple water bath at ambient temperature. The stirring was stopped after 30 min and the mixture allowed to stay for a few hours. The precipitated white solid was filtered off, washed with small portions of MeCN and dried under reduced pressure at 40 °C to give the desired betaine in nearly quantitative yield as a white powder. Almost no double bonds were observed in the final product when the standard bromide-bromate titration method was used. M.p. 117 °C. 1H NMR (400 MHz, d2o) δ 4.05 (dd, J = 9.8, 5.1 Hz, 1H), 3.65 (t, J = 7.8 Hz 1H), 3.5 (t, J = 5.1 Hz 1H), 3.15 (s, 3H), 2.70 (t, J = 7.3 Hz, 1H) ppm. Spectra and structure of the Bet are given in the Supporting information.

Synthesis of betaine with excess of AA (ABet) 3.15 g (35 mmol, 1 equiv) of DMAE was added to the 5 g (69 mmol, 2 equiv) of AA under vigorous stirring at ambient temperature. The reaction is exothermic, thus the mixture was chilled to the room temperature using a water bath. The viscose mixture solidifies typically in 1-2 days depending on the ambient temperature. The resulting ABet was used without further purification. 1H NMR (400 MHz, d2o) δ 6.28 (dd, J = 17.3, 1.2 Hz, 1H), 6.20 – 6.08 (m, 1H), 5.91 – 5.82 (m, 1H), 4.03 (dd, J = 6.9, 2.2 Hz, 1H), 3.66 (dd, J = 19.0, 11.4 Hz, 1H), 3.47 (dd, J = 12.7, 7.8 Hz, 1H), 3.13 (s, 3H), 2.74 (t, J = 7.7, 1H) ppm.

Typical procedure for the preparation of DES Prescribed amounts of powder urea (2 equiv) and choline (1 equiv) or betaine (1 equiv) were placed in a round bottom recovery flask. These components were mixed using a rotary evaporator at 70-80 °C for 1−2 h under reduced pressure until a colorless clear liquid was formed. The resulting DES was kept in a sealed bottle at room temperature. Using this procedure four DES were prepared: betaine:urea (BetU), acrylic acid/betaine:urea (ABetU), reline (ChCU) and choline iodide:urea (ChIU).

Typical procedure for the preparation of DES/water mixtures Prescribed amounts of DES and water were placed in a round bottom recovery flask. The mixture was homogenized using a rotary evaporator at 40 °C for 1h at normal pressure. The resulting mixtures were kept at room temperature in flasks sealed with a rubber septa.

Determination of the freezing points Freezing points of the DES/water mixtures were determined in accordance with ASTM D1177. The accuracy was within ±3 °C.



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Devices and analysis 1

NMR spectra were recorded using an Agilent DD2 400 spectrometer at 400 MHz for H. Chemical shifts 1

(δ) are given in parts per million (ppm) relative to residual solvent signals (DMSO-d6: H – 2.5 ppm; D2O: 1

H – 4.79 ppm).8 Spectra were recorded at 25 °C.

Purity of the starting volatile chemicals was determined by gas chromatography using a Chromos GC1000 chromatograph equipped either with a VertiBond AqWAX capillary column (length: 60 m, I.D.: 0.32 mm, film: 0.50 µm) or ValcoBond VB-1 capillary column (length: 60 m, I.D.: 0.32 mm, film: 0.50 µm) and flame ionization detector. Steady-shear flow parameters for the DES solutions were measured at various temperatures and shear -1

rates in the range from 0.084 to 79.2 s using a Brookfield DV2T viscometer, equipped with the smallsample adapter and a SC4-18 spindle. Tested solutions were filtered through a synthetic filter prior to the experiments. The duration of each experiment was no less than 2 min in order to ensure that steady state at definite shear rate was reached. Reduced viscosity values were calculated based on the standard protocol described previously.9 FTIR spectroscopy measurements were performed on a Shimadzu IRAffinity-1 spectrometer at various temperatures using high stability temperature controller (4000 series) equipped with electrical heating jacket and a liquid sample cells with ZnSe windows (Specac). Samples were thermostated for at least 10 -1

minutes at each temperature before the recording of the spectra. Resolution was set to 2 cm and 30 scans were accumulated.

Analytical methods Thermodynamic parameters of a viscous flow, viz. entropy (∆SA), enthalpy (∆HA) and free energy (∆GA) were calculated from the Frenkel-Eyring-Kobeko relation based on the experimentally obtained apparent viscosity values:

η = A0 exp

∆G A

1

RT

-3

where A0 is the preexponential factor and according to Kobeko has a constant value equal to 10 for all 10

liquids , η is the apparent viscosity, mPas The free energy of activation of viscous flow ∆GA was calculated directly from the above equation. After linearization of Eq. 1 the value of the activation enthalpy 10-11

∆HA can be determined as the derivative at each point of the function lnη = f(1/T) (slope of the line):

ln(η ) =

∆H A RT

+ ln(A 0 ) +

∆S A R

2

The entropy factor T∆SA can then be determined from the classical relationship:

T ∆S A = ∆H A − ∆GA

3


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Enthalpy of the hydrogen bond reorganization ∆H0 was determined on the basis of the equilibrium constant K given as ratio of infrared absorptions of the bands corresponds to the hydrogen bonded betaine-based complex (product ABet) and hydrogen bonded urea-based complex (AU):

ln( K ) = ln(

where

ε ABet ∆H 0 ∆S 0 + )=− − ln( U ) AU RT R ε Bet

εU and ε Bet

4

are the molar absorptivity coefficients for the urea and betaine-based complexes and

can be used to calculate entropy of the hydrogen bond reorganization ∆S0. ∆H0 can be determined as the slope of the linear function ln(ABet/AU)=1/T. Theoretical methods Quantum mechanics calculations were performed using the Gaussian 03 program.

12

All geometry

parameters of the studied systems were fully relaxed by searching for the energy minimum. Minnesota 13

hybrid M06-2X DFT functional based on the meta-GGA approximation

with the 6-31+G(d,p) basis set

was used for local minima calculations. A preliminary optimization procedure with the B3LYP/3-21G* level of theory or MMFF94s molecular mechanics with steepest descent algorithm as implemented in Avogadro 1.0.1 was conducted in some cases. To facilitate interpretation of the experimental IR spectra protons of the water molecules were changed to deuterium atoms with consecutive geometry optimization and vibrational analysis. A relaxed potential energy scan was also employed to test the proton transfer from water to nucleophile. Length of the Intermolecular hydrogen bond was used as scanning variable. Individual molecules and the appropriate complexes were optimized both by a condensed-phase 14

simulation using conductor-like polarizable continuum model

with water as solvent as implemented in

Gaussian 03. Vibrational frequency calculations were conducted to confirm the local minima obtained and to determine zero-point vibrational energies and thermal corrections to the Gibbs free energies. The scope of the natural bond orbital (NBO) analysis applied in this study has been described by Weinhold et al.15 The mathematical and historical background of NBO methods can be found elsewhere.15e On the basis of the NBO conception, H-bond strength was determined as delocalization energy E(2) (e.g., electron transfer from donor to acceptor orbital) by the second-order perturbation theory. Along with NBO second-order perturbation theory used to analyze the existence of intermolecular interactions topological properties of the electron density were characterized using the “atoms in molecules” (AIM) methodology 16

and used to verify H-bond formation.

Bond critical points (BCPs) were located as extrema in the

electron density where the gradient vector vanishes. This method has already been successfully 17

employed to study noncovalent interactions such as conventional hydrogen bonds bonds.

and also dihydrogen

18

Association energy ∆E was calculated as the difference between the total energy of any considered complex and the sum of the energies of individual units (molecules) of this complex (Ei):

n

∆E = Ecomplex − ∑ Ei

5

i =1


19


The basis set superposition error (bsse) was calculated using the Boys and Bernardy counterpoise algorithm,

20

thus providing determination of the corrected stabilization energy ∆Estab:

Estab = ∆E + bsse

6

RESULTS AND DISCUSSION

Quantum chemical calculations and FTIR spectroscopy Quantum chemical calculations using Minnesota hybrid M06-2X functional were performed to study energetic and geometry parameters of various complexes containing ChC and synthesized 3-((2hydroxyethyl)dimethylammonio)propanoate (Bet). Figure 1 shows optimized geometries of the ChC and Bet complexes with water and urea. Owing to the geometry restrictions one molecule of urea can form two H-bonds with H-bond acceptor site of the studied charged molecules. Therefore, in this study we used to compare complexation ability of one urea molecule with two water molecules acting as single specie due to the strong homo-association. All of the studied systems demonstrate strong H-bond driven stabilization expressed in terms of stabilization energy (∆Estab). Results of the energetic calculations and also geometrical and frequency data are summarized in Tables 1 and 2. Data obtained for the ChC complexes indicate that the highest stabilization energy was observed for the system with two water -1

molecules H-bonded with chlorine anion (∆Estab=34.38 kJmol ). However one can observe just a slight energetic profit for the association of ChC with water versus association of ChC with urea. Thus, ∆Estab for the ChCU complex was equal to 33.16 kJmol-1 and hence ∆∆Estab was determined to be 1.22 kJmol-1 (3.5% deviation). This result is somewhat surprising as it was recognized that urea and analogues molecules are very efficient in anion binding,21 so that these molecules are widely used nowadays as organic small molecule catalysts.22 As can be seen from Table 2 complexation of either urea or two water molecules with OH-group of ChC was found to be less efficient as the derived ∆Estab was equal to 22.35 and 22.54 kJmol-1 correspondingly.


Page 6 of 29

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Figure 1. Geometries of choline chloride (ChC), betaine (Bet) and urea (U) complexes optimized at M062X/6-31+G(d,p)/CPCM level of theory. H-bonds are shown by dashed lines. Table 1. Lowest frequencies and energies for the individual components constituting DES–water solutions calculated at the M062X/6-31+G(d,p)/CPCM level of theory. Molecular

Water

Urea

Betaine

Choline

system

chloride

E+zpe, au Lowest frequency, cm

-76.38194

-225.14137

-555.7542

-788.78459

1597.86

365.96

49.45

14.5

-

1

Table 2. Stabilization energies (∆Estab), vibrational frequencies, delocalization energy (E(2)) and H-bond length (dHB) for the betaine and choline chloride complexes optimized at the M062X/6-31+G(d,p)/CPCM level of theory. Molecular

E+zpe, au

-

∆E, kJmol

bsse, au

1

system

∆Estab,

E(2), -1

kJmol

dHB, Ǻ -1

Lowest frequency, cm-

kcalmol

1

Bet_2W Bet_U

-708.54051 -780.91253

58.88 44.51

0.00321 0.00139

50.46 40.86


12.76

1.8

27.37

1.66

10.95

1.87

22.91 15.09


Bet_U2 ChC_2W ChC_2W

-780.90422

22.7

-941.5648

42.88

-941.5605

31.58

0.00157 0.00324 0.00345

18.57 34.38 22.54

2 ChC_U ChC_U2 U_2W U_2W2

U_2W3

-1013.93997 -1013.93599 -377.91794

36.77 26.33 33.31

-377.92115

41.72

-377.90972

11.72

0.00137 0.00152 0.00371 0.00297

0.00219

33.16 22.35 23.57 33.92

5.97

Page 8 of 29

15.11

1.87

6.52

2.1

2.53

2.28

4.52

2.45

11.18

2.26

5.68

2.6

8.43

1.92

6.27

2.44

6.31

2.44

5.43

2.05

5.5

2.08

6.17

2.05

15.12

1.94

16.14

1.78

6.49

2.06

6.28

2.06

9.7

2.06

9.91

2.06

29.31 33.18 30.78 19.9 13.58 37.92 30.96

27.73

The localized character of the natural bond orbitals serves as an effective tool for analyzing the donor/acceptor ability of the molecular systems. Thus, using second-order perturbation theory for NBO donor–acceptor interactions, given by E(2), it was demonstrated that within the complex of choline chloride with two water molecules, delocalization energy for the H-bond interaction between localized lone pair orbital of chlorine anion (nCl‾) and antibonding orbital of the O-H bond (δ*OH) E(2)nCl‾→δ*OH was equal to –1

–1

4.52 and 11.18 kcal mol , and the summarized E(2) (ΣE(2)) was therefore equal to 15.7 kcal mol . The analogues delocalization energies for the urea complex (ChC_U, Figure 1) were almost equal demonstrating the symmetry in the Cl‾urea association (E(2)nCl‾→δ*NH = 6.27 and 6.31 kcal mol–1). The total delocalization energy derived from these interactions was approximately 20% less in contrast with ΣE(2) observed for the ChC_2W complex (12.58 kcal mol–1 versus 15.7 kcal mol–1). To explain the discrepancies in the difference between the stabilization energy and the delocalization energy, we turned to more detailed analysis of the ChC_U complex via calculation of the bond critical points (BCPs) within the framework of AIM methodology. As seen in Figure 2 four more noncovalent interactions were established for the considered ChC_U complex. Thus, two protons (H10 and H8, see Figure 2) of the methyl groups of choline tend to form CHO H-bond with carbamide oxygen and methylene protons (H18 and H17) form CHN bond with carbamide nitrogen atoms (N22 and N23 correspondingly). Analysis of the delocalization energies revealed that although nN→δ*CH orbital interactions are weak (E(2)=0.6 kcal mol–1), nO→δ*CH interaction is approximately two times stronger and equal to 1.2 kcal mol–1. Assuming these findings the resulting total E(2) for ChC_U was calculated to be 16.18 kcal mol

–1

which is

close to the total energy obtained for the ChC_2W (2.9% deviation) complex and thus explains the nature of the close stabilization energies observed for these two systems.


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Figure 2. Bond critical points (purple dots) for the optimized ChC_U complex. H-bonds are shown by dashed lines. More pronounced association effects were observed for the betaine-based systems. As can be seen (Table 2) carboxylate function demonstrates better ability to associate with both urea and water molecules (complexes Bet_U and Bet_2W). As a result of these interactions H-bond length (dHB) formed between urea and carboxylate group or water and carboxylate group decreases and the calculated stabilization energy increases simultaneously. Thus, dHB for each of two H-bonds (Bet_U complex) was equal to 1.87 Ǻ in contrast to the values observed for the ChC_U complex (dHB = 2.44 Ǻ). As a result calculated ∆Estab for Bet_U was equal to 40.86 kJmol-1. It is important to note that difference in the -1

stabilization energies between Bet_U and ChC_U given by ∆∆Estab was determined to be 7.7 kJmol

(~19% deviation). NBO analysis support these findings as the determined E(2)nO→δ*NH for the betainebased complex was equal to 10.95 and 15.1 kcal mol

–1

–1

(ΣE(2) = 26.05 kcal mol

–1

versus 12.58 kcal mol

for ChC_U). In addition to the observed increase in the value of stabilization energy for the urea–betaine interaction in comparison with urea–choline interaction, the association of betaine with two water molecules proved to be energetically much more favorable in comparison with the competing interaction of urea and betaine. Thus, ∆Estab was determined to be 50.46 kJmol-1. Delocalization energies for the appropriate nO→δ*OH interactions were equal to 12.76 and 27.37 kcal mol –1

versus 26.05 kcal mol

–1

for Bet_U and 15.7 kcal mol

–1

–1

(ΣE(2)=40.13 kcal mol

for ChC_2W). One can expect that strong

association between nucleophilic center of the betaine and water molecule can lead to the protonation of the carboxylate function. This type of transformation can in turn lead to the chemical reorganization in the system with subsequent change in the thermodynamic properties of the studied solutions. To test this assumption we performed the relaxed scan of the potential energy surface mowing the very acidic proton of the water towards nucleophile with a step-size equal to 0.025 Ǻ and simultaneous optimization of the resulting geometry using M062X/6-31+G(d,p)/CPCM level of theory. Results of the performed calculations are shown in Figure 3. As can be seen this process demonstrates the steadily growth of the electronic energy under given conditions. This observation indicates that the studied process is energetically unfavorable under the studied conditions.



Page 10 of 29

Figure 3. Scan of total energy along the reaction coordinate for the proton transfer from water to carboxylate oxigen within the Bet_2W complex. Labile proton is marked with purple color. H-bonds are shown by dashed lines. As a general trend computations also revealed that association of urea and water with OH-group of both betaine and choline molecules was much less efficient than the discussed interaction between H-bond donors and charged functions (chlorine anion or carboxylate group). Observing the data on stabilization energies calculated for the urea–water complexes one can resume -1

that the values of ∆Estab vary substantially. Thus, the lowest stabilization energy (∆Estab=5.97 kJmol ) was observed for the system with two water molecules serving as H-bond donors and interacting with nitrogen atoms of urea (U_2W3, see Figure 1). On the other hand, the highest stabilization energy -1

(∆Estab=33.92 kJmol ) was observed for the complex in which one water molecule serves as H-bond donor (OHOH interaction) and the second molecule serves as H-bond acceptor forming two symmetric H-bods with carbamide protons (OHN interaction, U_2W2). This value was very close to the values of stabilization energies determined for the association of choline chloride with water (ChC_2W, ∆Estab=34.38 kJmol-1) and urea (ChC_U, ∆Estab=33.16 kJmol-1) indicating the absence of preferential association in aqueous reline solutions. These data are also in line with previously published data on molecular dynamics study of the reline–water mixtures as it was postulated that H atoms of urea have similar propensities to bond to the chloride ions and the oxygen atoms of both urea and water.1c On the other hand, it can be concluded that formation of the complex between urea and water is less favorable for the betaine-based solutions. In this systems water preferentially associates with betaine molecules as this type of interaction was proved to possess the highest stabilization energy among the considered systems. To supplement the results of the discussed computer modeling we turned to IR spectroscopy -1

experiments. In the region of characteristic amide–A and O–H bands (3100-3500 cm ) multiple signals arising from associations between urea, choline, betaine and water were observed, thus we were forced to use deuterium oxide instead of H2O to study the effect of betaine and choline on the association state


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of water in DES. Due to the isotope shift using D2O in such conditions allows one to observe O–D bond -1

stretching vibrations in the region of 2300-2700 cm , thus avoiding undesirable overlaying of the stretching bands typical for the 3100-3500 cm

-1

region. To facilitate interpretation of the recorded

experimental IR spectra complexes of urea, choline and betaine with water were recalculated via the same level of theory (M062X/6-31+G(d,p)/CPCM) using D2O instead of H2O. Figure 3S shows fragments of the IR spectra of reline and betaine-based DES containing 5 wt% of D2O in the region of O–D stretching band recorded at 276 K. It can be seen that O-D stretching frequency of D2O in the mixture -1

with betaine-based DES is red shifted by approximately 20 cm with respect to the reline–D2O mixture. These data confirm previously discussed results of the quantum chemical calculations and allow one to conclude that in DES based on betaine, water forms stronger H-bonds in contrast to DES based on choline. Figure 4 shows C=O stretching region for the betaine-based DES containing 30 wt% of D2O as a function of temperature. The observed pronounced influence of the temperature on the absorbance of the ν(CO) bands indicates that the system is strongly hydrogen bonded and the decrease in the values of the IR absorbance with increase in temperature indicates the decrease in the degree of association in the studied system. Our preliminary studies based on close investigation of the IR spectra of all individual components of the betaine-based and choline-based DES and also their mixtures with D2O with various weight fractions of water using both the squished drop method and the matrix isolation method (e.g., see -1

Figures 4S-6S) revealed that ν(CO) observed at 1590 cm corresponds to the betaine associated with -1

D2O (analog of the complex Bet_2W, Figure 1 and calculated value was equal to 1589 cm ), 1625-1610 cm-1 was found to be the region typical for the C=O stretching vibrations of the urea molecule associated with D2O (analog of the complex U_2W2 represented in Figure 1, calculated value was equal to 1625 cm 1

-1

) and band observed at around 1680 cm

was attributed to the homo-associates of urea.

-

23

Deconvolution of the experimental spectra using Lorentz procedure allowed us to calculate the area of the listed bands (Figure 5A). Assuming that no isolated domains are presented in the system one can consider the following equilibrium process of the H-bond reorganization in the simplified DES-water complexes: [UD2O]Bet ↔ [BetD2O]U. Here [UD2O] and [BetD2O] reflect preferential association (Hbonding) of D2O with betaine and urea respectively within the betaine-based mixture. Thus the forward reaction indicates the breakage of the H-bond between urea and D2O and consecutive formation of new H-bond of the betaine–D2O type. Enthalpy of the discussed H-bond reorganization was determined from the slope of the linear function ln(A1590/A1625)=1/T (see Figure 5B) using ν(CO) IR absorptions at 1590 cm1

(products) and at 1625 cm-1 (initial state) as described in the experimental section. It was calculated

from the graphical data that H-bond reorganization in the system caused by shift in preferential association of D2O with urea to the association with betaine is exothermic as the experimentally obtained enthalpy was equal to -9.2 kJ mol-1. These data confirm that betaine–water interaction possesses the highest stabilization energy among the studied systems.



Figure 4. Fragmented experimental IR spectra of betaine-based DES with 30 wt% of D2O recordered at various temperatures.

Figure 5. A: Deconvolution of the ν(CO) region of the IR spectra of betaine-based DES containing 30 wt% of D2O recordered at 275 K. B: Relationship between ln(A1590/A1625) and 1/T obtained after deconvolution of the experimental IR spectra of betaine-based DES with 30 wt% of D2O. Thermodynamic parameters of a viscose flow Calculated on the basis of the experimentally determined viscosity values thermodynamic parameters serve as an efficient method to test peculiarities of the association or structurization in a liquid systems. Therefore, in this work we focus on determination of the thermodynamic parameters of a viscose flow (∆GA, ∆HA, T∆SA) in order to study and explain the differences in H-bond organization in various DES + water mixtures. It is assumed that entropy is a function directly linked with the degree of order in a liquid systems, and is highly sensitive to fluctuations in the association–dissociation equilibrium within the systems with strong specific interactions. When multiple types of strong competing interactions such as solute-solute and solute-solvent interactions realized in a liquid phase entropy of the system can change dramatically. With respect to protocol described in the experimental section entropy factor (T∆SA) was


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calculated and plotted as a function of temperature for reline with different content of water ranging from 5 to 60 wt% (Figure 6).

Figure 6. Left: T∆SA as a function temperature for reline solutions with various content of water. Right: Slopes of the corresponding curves plotted as a function of water content. Observing the trends obtained for the reline solutions one can make several conclusions. It can be seen that in general T∆SA values do not change significantly with temperature, however more precise observation reveals that the slop of the curves change with increase in the content of water. Thus, the -3

-3

slops for the reline containing 5, 30 and 60 wt% of water were equal to 33.410 , 0.510 and -16.710

-3

respectively (Figure 6, Right). These data show that more concentrated solutions of reline tend to decrease the entropy values with decrease in temperature. Positions of the curves depicted in Figure 6 also demonstrate that average ‹T∆SA› decrease with increase in the content of water in the studied system.

Figure 7. Left: T∆SA as a function temperature for Bet-urea DES with various content of water. Right: Average T∆SA as a function of water content for betaine and choline-based DES. Figure 7 (Right) shows the average entropy factor as a function of water content for reline as well as Bet and choline iodide-based DES. At low water content (5 wt%) the values of ‹T∆SA› are positive and relatively high and equal to 10.7 kJmol-1 for reline solution. The positive values are evidence that the



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initial state is more ordered than the activated one which is typical for the strongly associated systems with high degree of structurization. Nevertheless, when the content of water reaches 30 wt.% the values of ‹T∆SA› become negative for reline solutions. As can be seen the values of the activation entropy are negative for all of the studied aqueous mixtures of the choline iodide-based DES (ChIU). However, the absolute values of ‹T∆SA› are not high and close to zero for ChIU. This indicates that addition of water molecules breaks the initially (prior to the activation) strong structure for the studied ChIU systems. A different trend was observed for the aqueous DES solutions based on the synthesized carboxybetaine. Thus, in contrast to choline-based mixtures betaine-based solutions demonstrate anomalous entropic behavior. It can be seen that the value of ‹T∆SA› for the solution with 5 wt.% of water is substantially -

-1

higher (30.7 kJmol 1) than the analogues value for reline aqueous solution (10.7 kJmol ). These values demonstrate that at low water content the structurization of BetU mixture is characterized with more ordered structure formed via strong H-bonds which are readily breaks under deformation caused by a viscous flow. However, the most prominent effect was observed when the content of water reached 30 wt%. At this point one can observe sharp increase in the values of ‹T∆SA›. The entropy factor remains high (13.3-10 kJmol-1) in the range of water content 30-40 wt% and drops only for the system with fraction of water equal to 50 wt%. The observed phenomenon can be attributed to the formation of new strongly associated domains in the aqueous solutions of DES. These data are in line with the results of quantum-chemical calculations reported and discussed in a previous section. Using DFT methods it was demonstrated that carboxybetaine tends to form strong associates with two water molecules stabilized via H-bonds. In contrast to carboxybetaine, choline chloride shows no preferential association between water or urea molecules and hence the real aqueous solutions do not show pronounced anomalies in the entropy function. To supplement the picture we turned to the calculated apparent heat of activation ∆HA. This parameter is known to be the measure of intensity of the intermolecular interactions in liquids and is particularly sensitive to the structure formation in the system. In general it is assumed that unlike the values of ∆HA at high temperatures, the enthalpy change is much higher at low temperatures, which indicates an ordered and strong initial structure of the studied system. In this paper we used a wide range of temperatures to test the prepared solutions (243-360 K). For all of the studied systems fine linearity with average R2 equal to 0.996 was observed when lnη was plotted as a function of 1/T indicating applicability of the used approach towards ∆HA determination. Figure 8 shows activation enthalpy as a function of water content for the reline and BetU aqueous solutions. As can be seen, at low water content both of the studied solutions characterized with high positive ∆HA values, however the enthalpy change observed for the BetU solution was approximately 26 kJmol-1 higher than ∆HA determined for the reline solution.


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Figure 8. Left: Average Gibbs free energies as a function of water content for various DES. Right: ∆HA as a function of water content for betaine and choline-based DES. The high positive values of activation enthalpy are typical for the concentrated solutions with advanced intermolecular association. For the strongly associated systems the energy of non-covalent intermolecular interactions (H-bonding in our case) is high and the higher is the energy of intermolecular interactions the more energy one could apply to cause structural reorganization in the solution. The high degree of association in the studied systems in the initial (unactivated state) is somewhat analogues to the endothermic melting of the glassy materials, where one need to apply heat to disrupt strongly associated structure. Therefore, the enthalpy of the activated state (HA) is high which in turn leads to the high values of the experimentally determined ∆HA. Decrease in ∆HA with dilution (increase in the water content) demonstrates the simultaneous decrease in the structurization of the reline solutions. However, one can observe increase in the ∆HA values for the BetU solutions with 30 wt% of water. The anomalous behavior of the enthalpy complements the described anomalies in the behavior of entropy and indicates the strengthening of structural organization in the carboxybetaine-based DES containing water due to the appearance of new stable complexes (molar ratio DES:water was approximately equal to 1:6). Figure 8 also shows ∆GA as a function of water content. It can be seen that no anomaly in the behavior of the free energy as a function of water content was observed, which can be explained in terms of enthalpy-entropy compensation regular for the liquid systems.24 Positive values of the activation free energy imply the existence of specific interactions between molecules. Along with observation of ∆GA as a function of water content one can compare the appropriate values for various DES at definite water content. The highest ∆GA was observed for the BetU systems (34.2 kJmol-1 at 5 wt% of water). ∆GA for -1

the choline-based DES (ChCU and ChIU) was equal to 28.6 and 26.1 kJmol

correspondingly.

Therefore, it can be concluded that the values of ∆GA for the betaine-based DES with 5 wt% of water are higher than the appropriate values observed for both ChIU and reline DES. This effect is more pronounced at low water concentrations, i.e. at higher structurization of the solution. Figure 9 (A and B) shows apparent viscosity as a function of temperature for the studied DES containing 30 and 50 wt% of water. According to the obtained data it can be said that viscosity of the betaine-based DES with 30 wt% of water strongly depend on temperature. Calculating the viscosity increment given as a ratio of viscosities obtained at 269 K and 293 K (kη=η269/η293), it can be seen that kη possesses high value for the BetU solution at 30 wt% of water (kη=3.3). The value of kη obtained for the analogues solution based on reline was equal to 2.4. On the other hand much less kη values were obtained for the discussed



DES containing 50 wt% of water (2.3 for the BetU solution and 1.9 for the reline solution). Calculated reduced viscosity values plotted as a function of temperature (Figure 9C) support this position as the slope of the dependence obtained for the ChCU mixture was almost equal to zero while the slope calculated for the solution of BetU demonstrates the observable decrease in the values of viscosity indicating the highly associated nature of the betaine-based mixtures. The discussed data show that the specific structure organization realized in the BetU system with 30 wt% of water described previously in terms of activation thermodynamics leads to an increase in the viscosity of the corresponding solutions and a more pronounced temperature dependence of the viscosity, especially at low temperatures.

Figure 9. Dynamic viscosity plotted as a function of temperature for various DES with 50 wt% (A) and 30 wt% (B) of water and reduced viscosity as a function of temperature for reline and betaine-based DES with 30 wt% of water.

CONCLUSIONS In this paper we used to study peculiarities in the structure organization of some archetypical DES (e.g. reline) and new DES based on the synthesized carboxybetaine as a function of water content at various temperatures (T = 243–360 K) via activation thermodynamics expressed in terms of entropy, enthalpy and free energy of a viscose flow (∆GA, ∆HA, T∆SA), IR spectroscopic measurements and quantum chemical calculations. It was demonstrated that the average T∆SA value drops sharply with initial increase in water content (5→20 wt%) for all of the studied DES except for the one prepared from choline iodide. The anomalous reverse rise of T∆SA with further dilution of the betaine-based mixture was demonstrated. Thus, the entropy factor remains high in the range of water content 30-40 wt% and drops only for the system with fraction of water equal to 50 wt%. The anomalous behavior of T∆SA was complemented with analogues behavior of ∆HA for the betaine-based systems and indicates strengthening of the structure of the studied DES in presence of certain amounts of water. These data were in turn supplemented with quantum-chemical calculations using M062X functional and IR spectroscopy analysis. Hence, it was revealed that the formation of strong H-bonded complexes between carboxybetaine and water molecules gives rise to the observed thermodynamic effects.

ASSOCIATED CONTENT Supporting Information


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Cartesian coordinates for the discussed complexes and spectra of the synthesized carboxybetaine and DES are available free of charge on the ACS Publications website

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENTS Betaine synthesis and thermodynamic experiments were financially supported by The Ministry of Education and Science of the Russian Federation (a project part of the state task in the field of scientific activity, №10.2326.2017/PP); quantum chemical calculations were financially supported within the framework of the Program of development of Flagship University of Russia for Nizhny Novgorod State Technical University n.a. R.E. Alekseev.

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For table of Contents use only


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Figure 1. Geometries of choline chloride (ChC), betaine (Bet) and urea (U) complexes optimized at M062X/6-31+G(d,p)/CPCM level of theory. H-bonds are shown by dashed lines. 150x100mm (300 x 300 DPI)



Figure 2. Bond critical points (purple dots) for the optimized ChC_U complex. H-bonds are shown by dashed lines. 80x54mm (300 x 300 DPI)


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Figure 3. Scan of total energy along the reaction coordinate for the proton transfer from water to carboxylate oxigen within the Bet_2W complex. Labile proton is marked with purple color. H-bonds are shown by dashed lines. 70x68mm (300 x 300 DPI)



Figure 4. Fragmented experimental IR spectra of betaine-based DES with 30 wt% of D2O recordered at various temperatures. 70x68mm (300 x 300 DPI)


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Figure 5. A: Deconvolution of the ν(CO) region of the IR spectra of betaine-based DES containing 30 wt% of D2O recordered at 275 K. B: Relationship between ln(A1590/A1625) and 1/T obtained after deconvolution of the experimental IR spectra of betaine-based DES with 30 wt% of D2O. 80x61mm (300 x 300 DPI)



Figure 6. Left: T∆SA as a function temperature for reline solutions with various content of water. Right: Slopes of the corresponding curves plotted as a function of water content. 120x55mm (300 x 300 DPI)


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Figure 7. Left: T∆SA as a function temperature for Bet-urea DES with various content of water. Right: Average T∆SA as a function of water content for betaine and choline-based DES. 120x54mm (300 x 300 DPI)



Figure 8. Left: Average Gibbs free energies as a function of water content for various DES. Right: ∆HA as a function of water content for betaine and choline-based DES. 120x55mm (300 x 300 DPI)


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Figure 9. Dynamic viscosity plotted as a function of temperature for various DES with 50 wt% (A) and 30 wt% (B) of water and reduced viscosity as a function of temperature for reline and betaine-based DES with 30 wt% of water. 160x50mm (300 x 300 DPI)


Effect of Water on the Activation Thermodynamics ... - ACS Publications

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