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Does Synergism in Microscopic Polarity Correlate with Extrema in Macroscopic Properties for Aqueous Mixtures of Dipolar Aprotic Solvents? Alif Duereh,† Yoshiyuki Sato,† Richard Lee Smith, Jr.,*,†,‡ Hiroshi Inomata,† and Fabio Pichierri§ †

Research Center of Supercritical Fluid Technology, Graduate School of Engineering, Tohoku University, Aramaki Aza Aoba 6-6-11, Aoba-ku, Sendai 980-8579, Japan ‡ Graduate School of Environmental Studies, Tohoku University, Aramaki Aza Aoba 6-6-11, Aoba-ku, Sendai 980-8579, Japan § Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 6-6-07, Aoba-ku, Sendai 980-8579, Japan S Supporting Information *

ABSTRACT: Aqueous mixtures of dipolar aprotic solvents (acetonitrile, γ-valerolactone, γ-butyrolactone, tetrahydrofuran, 1,4-dioxane, acetone, pyridine, N-methyl-2-pyrrolidone, N,Ndimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide) show synergism in microscopic polarity and extrema in macroscopic viscosity (η) and molar excess enthalpy (HE) in water-rich compositions that correlate with solvent functional group electrostatic basicity (βH2 ). Microscopic polarities of aqueous solvent mixtures were estimated by measuring the spectral shift (λmax) of 4-nitroaniline with UV−vis spectroscopy at 25 °C. Dynamic viscosities (η) and densities were measured for eight aqueous dipolar aprotic mixtures over the full range of compositions at (25 to 45) °C. The λmax, η, and HE values of the aqueous mixtures showed a linear trend with increasing electrostatic basicity of the solvent functional groups that is attributed to the size and strength of the hydration shell of water. Density functional theory (DFT) calculations were performed for 1:3 complexes (solvent: (H2O)3) and it was found that aqueous mixtures with high basicity have high binding energies and short hydrogen bonding distances implying that the size and strength of the hydration shell of water is proportional to functional group basicity. Consideration of functional group basicity of dipolar aprotic solvents allows one to relate synergism in microscopic polarity to extrema in macroscopic properties for a wide range of aqueous dipolar aprotic solvent mixtures.

1. INTRODUCTION Dipolar aprotic solvents (e.g., dimethyl sulfoxide (DMSO), dimethylacetamide (DMA), N-methyl-2-pyrrolidone (NMP)) are chemical compounds that have high Kamlet−Taft dipolarity/polarizability and basicity and zero Kamlet−Taft acidity.1,2 Dipolar aprotic solvents have a large dielectric constant (ca. > 15), a large permanent dipole moment (ca. > 1.6 D), and are highly protophilic.3 When dipolar aprotic solvents are combined with water, they form versatile mixtures for dissolution of biomass,4−6 processing of active pharmaceutical ingredients (APIs),7−9 polymerizations,10,11 and catalytic reactions12−14 with one reason for their solvent power being due to synergism in Kamlet−Taft solvatochromic parameters.15−17 Dipolar aprotic solvents, which are denoted as hydrogen bond acceptor (HBA) solvents in this work, form highly nonideal solutions when added to water and exhibit strong negative deviations from Raoult’s law for the case of dimethyl sulfoxide (DMSO)−water18,19 that is attributed to strong dissociation of water that gives high pKa values.20,21 Micro© 2017 American Chemical Society

scopic polarities (spectral shifts) and macroscopic properties (viscosity and molar excess enthalpy) of aqueous dipolar aprotic mixtures show extrema in their values in water-rich compositions when compared with their ideal solution values. The reason for the extrema in both microscopic and macroscopic properties of water−dipolar aprotic solvent mixtures in water-rich compositions can be attributed to the solution structure of water,22−26 which forms a hydration shell surrounding the dipolar aprotic solvent−water complex.27−30 The hydration shell of water is of longer range and is of higher polarity than that of pure water.28 Lotze et al.26 reported that the water fraction according to local composition in H2O− DMSO mixtures is higher than that in H2O−acetone mixtures in water-rich compositions using mid-infrared spectroscopy. Those results imply that the hydration shell of water in H2O− DMSO mixtures is larger than that in H2O−acetone mixtures. Received: April 12, 2017 Revised: May 24, 2017 Published: May 30, 2017 6033

DOI: 10.1021/acs.jpcb.7b03446 J. Phys. Chem. B 2017, 121, 6033−6041

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The Journal of Physical Chemistry B

Figure 1. Water−hydrogen bond acceptor (HBA) mixed-solvent systems as a function of mole fraction of HBA solvents (x2) at 25 °C showing microscopic properties: (a) Kamlet−Taft solvatochromic parameters (dipolarity/polarizability (π*) and basicity (β)) and (b) 1H NMR diffusivity of water; and macroscopic properties: (c) dynamic viscosity (η) and (d) molar excess enthalpy (HE). HBA solvents: acetone (Ace) and dimethyl sulfoxide (DMSO). The π* and diffusivity data were taken from ref 15 and ref 19, respectively. The η and HE data were taken from ref 40 and refs 56,65, respectively. Unfilled (upside-down triangle) and filled (pink pentagon) symbols represent properties of H2O-Ace and H2O−DMSO, respectively. Half-filled symbols (Figure 1a) represent β data of H2O-Ace and H2O−DMSO mixtures, while unfilled and filled symbols represent π* data of H2O-Ace and H2O−DMSO mixtures.

valerolactone (GVL), γ-butyrolactone (GBL), tetrahydrofuran (THF), 1,4-dioxane (DI), acetone (Ace), pyridine (Pyr), Nmethyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and dimethyl sulfoxide (DMSO) listed in order (low to high) of their βH2 values. Viscosity and density of the aqueous dipolar aprotic mixtures were measured according to their availability in the literature and molar excess enthalpy data were collected from available references. Gibbs energies of solvent hydration (ΔGhyd) from the literature were considered in the analyses, because they represent the energy during the solvation process and can be considered to be a microscopic property.35,36 In this work, it is shown that synergism in microscopic polarity correlates with extrema in macroscopic properties for a wide range of water-HBA solvent mixtures. UV−vis spectroscopy, macroscopic property measurements, and density functional theory (DFT) calculations are used to support the hypothesis explored in this work.

The hydration shell of water observed in H2O−DMSO mixtures is clearly related to the basicity of the dipolar aprotic solvent. Considering these two aqueous dipolar aprotic solvent mixtures (H2O−DMSO and H2O−acetone), the electrostatic basicity of the sulfoxide functional group (βH2 = 8.9)31 for DMSO is higher than that of carbonyl functional group for acetone (βH2 = 5.8),31 while both DMSO and acetone solvents have similar numbers of hydrophobic methyl groups. For example, H2O−DMSO mixtures show higher Kamlet−Taft dipolarity/polarizability (Figure 1a), and lower diffusivity of water (Figure 1b), higher viscosity (Figure 1c), and more negative molar excess enthalpy (Figure 1d) than those of H2O−acetone mixtures. The hypothesis explored in this work is that the addition of dipolar aprotic solvents having higher basicity than water strengthens the formation of the hydration shell of water and that the enlargement and strengthening of the hydration shell are directly related to both microscopic polarity and macroscopic properties. In the data analysis, the UV−vis spectral shift of 4-nitroaniline indicator was used to quantify changes in the microscopic polarity of aqueous dipolar aprotic mixtures, because this indicator responds to both the dipolarity/ polarizability and hydrogen-bonding basicity of the solvent environment in the cybotactic region32,33 and its usable range for the solvent mixtures studied is suitable. The 4-nitroaniline indicator allows characterization of the solvent environment in terms of combined dipolarity/polarizability and hydrogenbonding basicity in an analogous way that Reichardt’s dye is used for ET(30) polarity,34 which is used to characterize both dipolarity/polarizability and acidity of a solution. HBA solvents chosen for study were according to functional group electrostatic basicities (βH2 ) reported by Hunter,31 even though the solvents might not be considered to be strictly dipolar aprotic according to the IUPAC definition.3 The HBA solvents studied in this work were acetonitrile (ACN), γ-

2. EXPERIMENTAL SECTION 2.1. Materials. Purities and sources of solvents used for viscosity, density, and solvatochromic measurements are listed in Table S1 (Supporting Information). The microscopic polarity of the solvent mixtures was estimated with solvatochromic indicator 4-nitroaniline (99%), which was obtained from Sigma-Aldrich Co. and used without further purification. 2.2. UV−vis Measurements. The absorption wavelengths of 4-nitroaniline indicator in H2O-HBA mixtures were measured using a double-beam UV−vis spectrophotometer (Jasco, model V-530) with an uncertainty (2σ) in maximum absorption wavelength (λmax) of ±0.4 nm, except for H2O-GVL and H2O-GBL data that were obtained from our previous work.16 6034


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solvent molecule:(H2O)3) were chosen. In these complexes, the functional group (CO, SO, O) of each HBA solvent molecule forms two H-bonds, one with each neighboring water. Further, the same orientation within the cyclic H-bonded structure surrounding the functional group of each HBA solvent molecule was adopted so that a quantitative comparison of the binding energies can be made. These 1:3 complexes can be thought as formally obtained by replacing one water molecule of the experimentally observed water tetramer47 with one HBA solvent molecule. The (static) DFT calculations made with the present molecular models are thus able to capture the essence of the functional group hydration as seen, for example, in molecular dynamics simulations for H2O− DMSO mixtures.48

The H2O-HBA mixtures were prepared using a microbalance (Mettler Toledo, model ax 504) and the concentration of 4nitroaniline indicator in the mixtures was adjusted to be (3 to 5) × 10−5 mol·dm−3 so that maximum UV−vis absorbance of the indicators could be within a 0.5−1.2 absorbance unit range. All UV−vis spectra of the solvatochromic indicator in the H2OHBA solvent mixtures were measured in a 1 cm path length quartz cell that was controlled at 25 ± 0.1 °C using a temperature controller (Jasco, ETC 505) and were replicated by three scans with the standard deviation being less than 0.2 nm. 2.3. Density and Viscosity Measurements. Dynamic viscosities and densities of eight aqueous mixtures of γvalerolactone (GVL), γ-butyrolactone (GBL), tetrahydrofuran (THF), acetone (Ace), pyridine (Pyr), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and N-methyl2-pyrrolidone (NMP) solvents were measured in this work. Data for aqueous mixtures of acetonitrile (ACN),37 1,4-dioxane (DI),38,39 and dimethyl sulfoxide (DMSO)40 were obtained from the literature. Water content of the pure solvents was confirmed to be lower than 1000 ppm by coulometric Karl Fischer titration with Coulomat AG (Fluka) as an anolyte solution and Coulomat CG (Fluka) as a catholyte solution. Aqueous-HBA mixtures were prepared in mass fraction using a microbalance (Mettler Toledo, model ax 504) with an uncertainty of ±1 × 10−4 g and then were converted to mole fraction with an uncertainty of ±2 × 10−4 mol. Densities (ρ) and dynamic viscosities (η) of the solvent mixtures were measured concurrently using a Stabinger viscometer (Anton Paar, SVM3000) with a standard uncertainty of temperature (u (T)) of ±0.02 K, density (u (ρ)) of ±0.5 kg·m−3 and viscosity of (ur (η)) of 0.35%. Before performing the viscosity and density measurements, the viscometer was calibrated with certified reference fluids that had a range of densities of (761.1−827.9) kg·m−3 and a range of viscosities of (0.9142−50.1890) mPa·s over the temperature range of 20 to 60 °C. The uncertainties in densities (u (ρ)) and viscosities (ur (η)) obtained from the calibration were 0.67 kg· m−3 and 0.12%, respectively. The density (ρ) and dynamic viscosity (η) measurements of the aqueous-solvent mixtures at each corresponding mole fraction were performed over a temperature range of 25 to 45 °C with quadruplicate measured values being the average of two cycles of temperature (low-tohigh and high-to-low). 2.4. DFT Calculations and Molecular Models. All DFT calculations in this study were performed with the Gaussian 09 software package.41 The B3LYP hybrid functional of Becke42 in combination with the Pople-style 6-31G(d,p) basis set43 was employed in the geometry optimizations of the HBA-water complexes investigated herein. The geometry optimizations were performed in an implicit water environment by using the polarization continuum model (PCM) of Tomasi and coworkers within the integral equation formalism (IEF-PCM).44 The binding energy (Be) between each HBA solvent molecule and the water molecules was calculated by including the basisset superposition error (BSSE) according to the counterpoisecorrection method of Boys and Bernardi.45 A positive value of Be indicates that a stabilizing interaction is operative within each complex. Visualization of the results was carried out with the GaussView graphical interface.46 To make comparisons of the H-bonding distances and binding energies without considering the entire hydration shell around the HBA solvent molecule, 1:3 complexes (HBA

3. McALLISTER VISCOSITY MODEL The McAllister viscosity model49−51 was developed by considering the activation energy of molecular motion that can be expressed as the interaction of i-j molecules given by eq 1: κmix = x14 ln κ1 + 4x13x 2 ln κ1112 + 6x12x 2 2 ln κ1122 + 4x1x 2 3 ln κ2221 ⎡ (3 + M w2 /M w1) ⎤ + x 2 4 ln κ2 − ln[x1 + x 2M w2 /M w1] + 4x13x 2 ln⎢ ⎥ ⎣ ⎦ 4 ⎡ (1 + M w2 /M w1) ⎤ ⎡ (1 + 3M w2 /M w1) ⎤ 3 + 6x12x 2 2 ln⎢ ⎥ + 4x1x 2 ln⎢ ⎥ ⎣ ⎦ ⎣ ⎦ 2 4 + x 2 4 ln[M w2 /M w1]

(1)

where the κmix, κ1, and κ2 parameters are the kinematic viscosity of the mixture, pure 1 and pure 2 components, respectively. The κ1112, κ1122, and κ2221 parameters are binary interaction parameters, representing the interaction of component 1 molecules around molecules of component 2. For example, the κ1112 parameter represents the interaction of one molecule of component 2 surrounded by three molecules of component 1 that has a similar hydration shell structure. Thus, the binary interaction parameters can imply formation of a hydration shell.49 Several models were tested for correlating the maxima observed in viscosities of H2O-HBA mixtures, but the McAllister viscosity model was able to correlate experiment viscosity data with the least number of parameters. The binary interaction parameters (κ1112, κ1122, and κ2221) were obtained by fitting experimental κmix data with eq 1 and using Origin software (Microcal, version 9.1) with the Levenberg− Marquardt algorithm. In the discussion of the mixture viscosity data (Section 4.2), a maximum-difference viscosity (ΔηDmax) was defined by eq 2 as D max Δηmax = ηmixture − (x1Δηpure1 + x 2Δηpure2)

(2)

where ηmixture and ηpure are dynamic viscosities of the mixture and pure components, respectively. The ηmax mixture value is the taken as the maximum viscosity over the entire composition range (0 ≤ x2 ≤ 1) for the H2O-HBA mixture at the given temperature and atmospheric pressure.

4. RESULTS AND DISCUSSION 4.1. Synergism in Microscopic Polarity of Solvent Mixtures. Figure 1 shows microscopic properties (Figures 1a,b) and macroscopic properties (Figures 1c,d) compiled from the literature. The Kamlet−Taft dipolarity/polarizability (π*) and basicity (β) parameters were obtained from the 6035


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The Journal of Physical Chemistry B literature15,16 as averaged values from multiple indicators for H2O (1)−HBA (2) mixtures (Figure 1a and Figure S1) and show clear synergism in either π* or β that seems to correspond to changes in microscopic (Figure 1b) and macroscopic (Figures 1c,d) properties. DMSO−water mixtures show larger changes in macroscopic properties than acetone− water mixtures. The maximum absorption wavelength (λmax) of the solvatochromic indicator, 4-nitroanisole, which responds only to dipolarity/polarizability, shows synergism in the spectral shifts (Figure S1) for aqueous HBA solvent mixtures. It can be concluded that for water-rich compositions, changes in both solvent-mixture dipolarity/polarizability and basicity seem to be related to the trends in the macroscopic properties shown in Figure 1. Figure 2 shows maximum absorption wavelengths (λmax) of 4-nitroaniline indicator in 11 H2O−HBA mixtures based on

4.2. Extrema in Thermodynamic Properties. Figure 3 shows viscosities of eight H2O−HBA mixtures measured in this work along with three mixtures reported in the literature. All systems exhibited maxima in viscosity at water-rich compositions (x2 < 0.5). To discuss the data in Figure 3, ΔηDmax values of the H2O−HBA mixtures (eq 2) were tabulated against βH2 as shown in Table 1. The ΔηDmax values of the H2O−HBA mixtures tended to increase with increasing βH2 values (Table 1). Figure S2 (Supporting Information) shows molar excess enthalpy (HE) data for 11 H2O−HBA mixtures obtained from the literature.52−60,65 The 11 H2O−HBA mixtures had minima in HE values in water-rich compositions (Figure S2). Minimum HE values of the H2O−HBA mixtures (−HEmax-, Figure S2) are tabulated in Table 1. With increasing βH2 values of the HBA solvents (Table 1), the −HEmax values of the H2O−HBA mixtures became more negative. The more negative HE values indicate stronger hydrogen bonding interactions in both the complex H2O−HBA structures and in hydration shell of water. Figure S3 (Supporting Information) shows partial molar excess volumes of waters (V̅ E1 ) for all H2O−HBA mixtures at water rich compositions (x2 < 0.15). The H2O−DMF, H2O− DMA, H2O−NMP, H2O−DMSO mixtures that have high βH2 values (βH2 > 8.3, Figure S3) show positive V̅ E1 values, indicating formation of a hydration shell of water according to Marcus’ criterion.61 The reason for the maxima in viscosities of the H2O−HBA mixtures can be attributed to the size and strength of hydration shell of water as the maximum viscosity (ηmax) for a H2O−HBA mixture represents the most bulkiest complexstructure. The κ1112 parameter values from the McAllister viscosity model (eq 1) for all H2O−HBA mixed-solvent systems (Table S2) showed values higher than unity, implying that the most bulkiest complex-structure occurred in water-rich compositions and the κ1112 values (Table S2) increased with increasing βH values. The general trend of increasing βH2 values of HBA solvents is discussed in detail in the next section along with polarity, Gibbs energy of solvent hydration, viscosity, and molar excess enthalpy. 4.3. Correlation of Properties. The hypothesis proposed in the introduction is discussed by considering correlation between microscopic polarity and macroscopic properties. The λmax values (Table 1 and Figure 2) measured in this work were used to represent microscopic polarity of the H2O−HBA mixtures. Gibbs energies of solvent hydration (ΔGhyd) of H2O−HBA mixtures were obtained from the literature35,36 (Table 1). Since viscosity depends on molecular size, the ΔηDmax values (Table 1) were divided by the HBA solvent molar volume (V2) to examine the trends with functional group basicity. Figure 4 shows a plot of λmax, ΔGhyd, ΔηDmax, and −HEmaxvalues against βH2 for the H2O−HBA mixtures. The λmax, ΔGhyd, ΔηDmax, and −HEmax values of the H2O−HBA mixtures (Figure 4) showed the following linear relationships with the βH2 parameter (detailed statistics in Table S3 and Figure S4):

Figure 2. Maximum absorption wavelength (λmax) of 4-nitroaniline indicator in water (component 1) with a hydrogen bond acceptor (HBA) solvent (component 2) at 25 °C. Inset: x2 < 0.3. HBA solvents ordered in terms of their Hunter basicity (βH2 ) values (low to high): acetonitrile (red circle, ACN), γ-valerolactone16 (green square, GVL), γ-butyrolactone16 (blue triangle, GBL), tetrahydrofuran (light blue diamond, THF), 1,4-dioxane (pink octagon, DI), acetone (orange upside-down triangle, Ace), pyridine (red half-filled octagon, PYR), Nmethyl-2-pyrrolidone (yellow star, NMP), N,N-dimethylformamide (gray sideway triangle, DMF), N,N-dimethylacetamide (white octagon, DMA), and dimethyl sulfoxide (pink pentagon DMSO).

measurements in this work. All mixtures (Figure 2) exhibited a red-shift and a maximum in λ for water-rich compositions (x2 < 0.3, Figure 2 (inset)), indicating that the H2O−HBA mixtures in water-rich compositions have higher microscopic polarity than that of pure water. The values of λmax were larger than the uncertainty of the measurements, so that synergism in microscopic polarity due to both dipolarity/polarizability and basicity, is certain to exist. The trends of the spectra for the H2O−HBA mixtures (Figure 2) can be attributed to the size and strength of the hydration shell of water, since the measurements show that the microscopic polarities of the H2O−HBA solvent mixtures were higher than those of pure water. Thus, the UV/vis results are evidence that there is strengthening of the hydration shell at water-rich compositions. The values of λmax for H2O-HBA mixtures are compiled in Table 1 with Hunter electrostatic basicity (βH2 ) values along with microscopic and macroscopic properties that are discussed in later sections. The λmax values obtained from Figure 2 were in order of the HBA solvent βH2 values (Table 1). Details on the UV−vis measurements including statistics are given in Section A (Supporting Information).

λmax (nm) = 1.344( ±0.220) ·β2H − 376.5( ±1.5)

(3)

ΔG hyd(kJ·mol−1) = −4.93(± 0.88)·β2H + 9.33(± 5.89) (4) D Δηmax (mPa ·s)/V 20(cm 3·mol−1)

= 0.0073( ± 0.0011)·β2H − 0.0299(± 0.0076) 6036

(5)


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Table 1. Maximum Absorption Wavelength (λmax) of 4-Nitroaniline Indicator, Binding Energy, Hydrogen-Bonding Angle, Hydrogen-Bonding Distance, and Dipole Moment from DFT Calculations on the 1:3-Complexes of Water with HBA Solvents, Gibbs Energies of Solvent Hydration (ΔGhyd) Obtained from Literature,35,36 Maximum-Difference Viscosity (ΔηDmax, eq 2), and Molar Excess Enthalpy (HE) of Solvent Mixtures Containing Water (Component 1) and Hydrogen Bond Acceptor (HBA) 31 Solvents (Component 2) along with the Electrostatic Hydrogen Bond Acceptor Parameter of Hunter (βH 2 ) and Molar Volume 0 of Pure Component 2 (V2) at 25 °C

a

Maximum absorption wavelength of 4-nitroaniline in pure water is 380 nm. bObtained data at x2 = 0.30 due to broad spectra (λmax.) cAsterisks refer to measurements from this work. Component 2 is acetonitrile (ACN), γ-butyrolactone (GBL), γ-valerolactone (GVL), tetrahydrofuran (THF), 1,4dioxane (DI), acetone (Ace), pyridine (Pyr), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N-methyl-2-pyrrolidone (NMP), or dimethyl sulfoxide (DMSO). dObtained data at 30 °C for H2O-GVL mixture and 27 °C for DMF−GVL mixture.

molar excess enthalpy that is most likely due to the size and strength of the hydration shell. 4.4. DFT Modeling. The DFT-optimized geometries for hydrated clusters of six different HBA solvent molecules as 1:3 complexes (HBA solvent molecule: (H2O)3) are shown in Figure 5. Table 1 provides a tabulation of the binding energy, Be, the H-bonding angle (H···O···H) involving the functional group of the HBA solvent molecule, the H-bonding distance for the shorter length, and dipole moment of the 1:3-complexes. The DMSO:(H2O)3 complex had the highest value of Be (92.0 kJ·mol−1) that was characterized by the shortest H-bonding distance (1.747 Å), whereas those with the lowest values of Be (85.8−87.0 kJ·mol−1) had longer H-bonding distances (1.789− 1.828 Å). The DMF:(H2O)3 complex had an intermediate value of Be (90.0 kJ·mol−1) and was characterized by an Hbonding distance of 1.762 Å. The H2O−HBA complexes having the higher binding energies and the shorter H-bonding distances, i.e., the complexes with the stronger interaction, were those with the HBA solvent molecules exhibiting higher basicity (Table 1). However, although both DMA and DMF have the same basicity value of 8.3 (Table 1), the DMA:(H2O)3 complex had a Be value which was 3.0 kJ·mol−1 smaller than that calculated for the DMF:(H2O)3 complex. A possible explanation for this outlier is attributed to the additional methyl group of DMA which interacts repulsively with one water molecule within the cluster (Figure 5). In general, the increase in the binding energies for the 1:3 complexes with solvent basicity (Table 1) was in accordance with the increase in Gibbs energies of solvent hydration (ΔGhyd) (Figure 4b and Table 1). 4.5. Other Support. IR Spectra. In the literature, infrared measurements have been made on some of the H2O−HBA solvent mixtures (H2O−ACN, H2O−THF, H2O−Ace, and H2O−DMSO) studied in this work.25,26,62,63 Since deuterium oxide (D2O) can react with water to form HOD bonds with water, IR-spectra of HOD stretching is an indicator of the strength of hydrogen bonds in the hydration shell of water.62,64

Figure 3. Dynamic viscosity (η) of water-hydrogen bond acceptor (HBA) mixed-solvent systems as a function of mole fraction of HBA solvents (component 2, x2) at 25 °C. Symbols represent experimental data, while lines are correlated with the McAllister viscosity model (eq 1). HBA solvents ordered in terms of their Hunter basicity (βH2 ) values (low to high): acetonitrile37 (red circle, ACN), γ-valerolactone (green square, GVL), γ-butyrolactone (blue triangle, GBL), tetrahydrofuran (light blue diamond, THF), 1,4-dioxane38 (pink octagon, DI), acetone (orange upside-down triangle, Ace), pyridine (red half-filled octagon, PYR), N-methyl-2-pyrrolidone (yellow star, NMP), N,N-dimethylformamide (gray sideways triangle, DMF), N,N-dimethylacetamide (white octagon, DMA), and dimethyl sulfoxide (pink pentagon, DMSO).40 E Hmax (Jmol−1 ) = − 739(± 55)·β2H + 3554(± 370)

(6)

Values in the parentheses of eqs 3−6 are parameter standard errors. Proportionality between both microscopic properties (λmax and ΔGhyd) and macroscopic properties (ΔηDmax and −HEmax) of the H2O−HBA mixtures (Figure 4) is strong support of the hypothesis that high basicity HBA solvents with water form larger hydration shells than low basicity HBA solvents. Thus, synergism in microscopic polarity of H2O− HBA mixtures shows correlation with extrema in viscosity and 6037


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Figure 4. Correlation of electrostatic hydrogen bond acceptor parameters (βH2 ) of hydrogen bond acceptor (HBA) solvents with microscopic properties: (a) maximum absorption wavelength (λmax) of 4-nitroaniline indicator (R2=0.80) and (b) Gibbs energies of solvent hydration (ΔGhyd) (R2=0.86); and macroscopic properties: (c) the ratio of maximum-difference viscosity and the molar volume (ΔηDmax/V02) (R2 = 0.83) and (d) molar excess enthalpy (HE) (R2 = 0.95) at 25 °C according to eqs 3−6. HBA solvents (component 2) listed in Figure 2 and Table 1.

Figure 5. Optimized geometries of 1:3 complexes of one hydrogen bond acceptor (HBA) solvent molecule with three water molecules. HBA solvents are (a) γ-valerolactone, (b) tetrahydrofuran, (c) acetone, (d) N,N-dimethylformamide, (e) N,N-dimethylacetamide, and (f) dimethyl sulfoxide. Distances in Å and angles in degrees.

Figure S5 (Supporting Information) shows HOD stretching spectra for H2O−ACN, H2O−THF, H2O−Ace, and H2O− DMSO mixtures. The HOD stretching spectra of the mixtures (Figure S5) exhibit a blue-shift over the entire range of compositions, so that addition of an HBA solvent to water causes breaking of hydrogen bonds in the hydration shell due to the formation of donor (H2O)-acceptor (HBA) bonds. However, as expected, the H2O−DMSO mixture (Figure S5) exhibits a relatively lower blue-shift than other H2O−HBA mixtures (H2O−ACN, H2O−THF, H2O−Ace), implying that the hydration shell of water changes less for HBA solvents having high βH2 values, because the hydration shell for HBA solvents having a high basicity functional group must be larger than that for HBA solvents having a low basicity functional

group. Since the microscopic polarity affects both size and strength of the hydration shell, an HBA solvent with high functional group basicity will exhibit larger extrema in their macroscopic properties subject to molecular size (entropic) effects. 4.6. Extensions. Although the hypothesis examined in this work has been analyzed based on properties mostly taken at 25 °C, the hypothesis is expected to apply to other temperatures because maximum-departure viscosity (ΔηDmax) of H2O−HBA mixtures at 35 °C and at 45 °C show a linear relationship with βH2 as shown in Figure S6 (Supporting Information). Other microscopic properties (e.g., NMR shift and neutron diffraction) macroscopic properties (e.g., diffusivity and refractive index) of H2O−HBA mixtures related to the 6038


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The Journal of Physical Chemistry B Be DI DMA DMF DMSO GBL GVL H HBA HBD NMP Pyr THF

hydration shell of water can be expected to vary in proportion to the βH2 parameter. Enlargement of the hydration shell of water with increasing basicity of the HBA solvent or changes in the hydration shell with the HBA solvent composition are important for understanding biomolecule solvation (e.g., proteins and DNA), since many biomolecules have similar functional groups as those of the simple molecular solvents studied in this work.29

5. CONCLUSIONS Synergism in microscopic polarity (λmax) of 11 H2O−HBA mixtures correlates with extrema in macroscopic properties (viscosity and molar excess enthalpy) of H2O−HBA mixtures and shows a linear trend with functional group basicity (βH2 ) of the HBA solvent. Correlation of the microscopic polarity with extrema in macroscopic properties is related to the size and strength of the hydration shell of water. By considering functional group basicity (βH2 ) of an HBA solvent, it is possible to relate trends in microscopic polarity to extrema in macroscopic properties of water−HBA solvent mixtures. Further, for water−HBA solvent mixtures, it is possible to infer changes in microscopic polarity or changes in the size and strength of the hydration shell of water from variations in the extrema of macroscopic properties derived from functional group basicity.

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LATIN SYMBOLS ΔGhyd Gibbs energies of solvent hydration Mw molecular weight x mole fraction of solvent

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GREEK SYMBOLS β Hunter electrostatic basicity β Kamlet−Taft basicity λmax maximum absorption wavelength in unit of nm V molar volume V̅ partial molar volume η dynamic viscosity κ kinematic viscosity κ1112 binary interaction parameter, according to eq 1 κ1122 binary interaction parameter, according to eq 1 κ1222 binary interaction parameter, according to eq 1 ρ density π* Kamlet−Taft dipolarity/polarizability H

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b03446. Material sources and purities, binary interaction parameters from the McAllister model, detailed statistics of standard errors from eqs 3−6, λmax of 4-nitroaniline indicator in pure solvents and in the H2O-HBA mixtures measured in this work at 25 °C, dynamic viscosities (η) of eight H2O-HBA mixtures measured in this work and three H2O-HBA mixtures obtained from the literature at temperatures from 25 to 45 °C, and property comparisons (PDF)

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binding energy 1,4-dioxane N,N-dimethylacetamide N,N-dimethylformamide dimethyl sulfoxide γ-butyrolactone γ-valerolactone molar enthalpy hydrogen-bond acceptor solvent (component 2) hydrogen-bond donor solvent (component 1) N-methyl-2-pyrrolidone pyridine tetrahydrofuran

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SUPERSCRIPT E excess property SUBSCRIPT 1 HBD solvent 2 HBA solvent mix mixture property max maximum property

AUTHOR INFORMATION

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Corresponding Author

* Tel (Fax): +81-22-795-5863; E-mail: [email protected]. ac.jp.

REFERENCES

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ORCID

Alif Duereh: 0000-0003-0170-8601 Richard Lee Smith Jr.: 0000-0002-9174-7681 Fabio Pichierri: 0000-0003-3165-4456 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge partial financial support of this research from a JSPS Grant in Aid Scientific Research (B), contract No. 25289272 (Japan). The authors also wish to acknowledge support of the Research Center of Supercritical Fluids.

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ABBREVIATIONS Ace acetone ACN acetonitrile 6039


Article

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