Analysis of the Cybotactic Region of Two Renewable LactoneâWater

Article pubs.acs.org/JPCB

Analysis of the Cybotactic Region of Two Renewable Lactone−Water Mixed-Solvent Systems that Exhibit Synergistic Kamlet−Taft Basicity Alif Duereh,† Yoshiyuki Sato,† Richard Lee Smith, Jr.,†,‡ and Hiroshi Inomata*,† †

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

ABSTRACT: Kamlet−Taft solvatochromic parameters (polarity, basicity, acidity) of hydrogen bond donor (HBD)/acceptor (HBA) mixed-solvent systems, water (H2O)−γ-valerolactone (GVL), methanol (MeOH)−GVL, ethanol (EtOH)−GVL, H2O−γ-butyrolactone (GBL), MeOH−GBL, and EtOH−GBL, were measured over their entire composition region at 25 °C using UV−vis spectroscopy. Basicity of H2O−GVL and H2O−GBL systems exhibited positive deviation from ideality and synergism in the Kamlet−Taft basicity values. The cybotactic region around each indicator in the mixed-solvent systems was analyzed with the preferential solvation model. Both H2O−GVL and H2O−GBL mixed-solvent systems were found to be completely saturated with mutual complex molecules and to have higher basicity than pure water because water prefers to interact with GVL or GBL molecules rather than with itself. Formation of H2O−GVL and H2O−GBL complex molecules via specific hydrogen bond donor−acceptor interactions were confirmed by infrared spectroscopy. In MeOH−GVL or MeOH−GBL mixed-solvent systems, MeOH molecules prefer self-interaction over that with GVL or GBL so that synergistic basicity was not observed. Synergistic basicity and basicity increase for various functional groups of ten mixed-solvent (water−HBA solvent) systems can be quantitatively explained by considering electrostatic basicity and a ratio of the partial excess HBA solvent basicity with the HBA solvent molar volume that correlate linearly with the preferential solvation model complex molecular parameter (f12/1). Analysis of the cybotactic region of indicators in aqueous mixtures with the preferential solvation model allows one to estimate the trends of mixed-solvent basicity.

1. INTRODUCTION

to specific interactions that the GBL or GVL molecule has with the monomer.21,22 Mixtures of solvents allow control of Kamlet−Taft polarity (π*), basicity (β), and acidity (α) parameters through the choice of the mixed-solvent composition.23 In this work, “ideality” in a Kamlet−Taft solvatochromic parameter is defined as the variation of the value that is directly proportional to a constant multiplied by the bulk composition of the binary mixture. On the other hand, an increase in one or more of the Kamlet−Taft parameters in the mixture over the values of both pure solvents Kamlet−Taft parameters is referred to as “synergism” in this work. Synergism in Kamlet−Taft parameters has been reported for many HBD−HBA solvent mixtures,15,23−30 due to the occurrence of HBD−HBA complex molecules that have either higher or lower polarity, higher or lower basicity, or higher or lower acidity than both of the pure solvent values. Kamlet−Taft solvatochromic parameters of water−GBL or water−GVL solvent mixtures most likely exhibit synergism in

Lactones are cyclic esters formed by the removal of a water molecule from carboxylic acid and hydroxyl functional groups of a hydroxyl acid molecule that widely occur in nature. Four and five carbon (C4 and C5) lactones, γ-butyrolactone (GBL) and γ-valerolactone (GVL), are readily derived from biomass resources and are considered to be platform chemicals because they are both versatile and relatively safe.1−10 While GBL and GVL have been proposed for use as biofuels or chemicals,7−9,11 they also have many potential applications as molecular solvents.12,13 Both GBL and GVL have high Kamlet− Taft polarity (π*) and basicity (β) and are hydrogen bond acceptor (HBA) solvents5,6 such that when combined with a hydrogen bond donor (HBD) solvent, the mixed-solvent system can take on properties that are highly favorable for a given separation or reaction.14−20 For example, water−GVL mixtures can be used to fractionate birchwood biomass that is attributed to the high affinity that GVL has for lignin.14,20 In the preparation of engineering plastic precursors, water−GBL and water−GVL solvent mixtures are found to be able to replace environmentally undesirable N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP) solvents that are attributed © 2016 American Chemical Society

Received: March 25, 2016 Revised: April 24, 2016 Published: April 25, 2016 4467

DOI: 10.1021/acs.jpcb.6b03090 J. Phys. Chem. B 2016, 120, 4467−4481

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The Journal of Physical Chemistry B their Kamlet−Taft parameters due to the electron-donor and electron-acceptor interactions of the lactone−water molecules that lead to the formation of complex molecules.15,23−29,31−34 Infrared (IR) spectroscopy is one technique to investigate intermolecular interaction between binary solvents.35−43 To confirm the formation of complex molecules of water with GVL and with GBL, H2O−GVL and H2O−GBL mixtures were studied with attenuated total reflectance (ATR)-infrared spectroscopy over the entire composition range of the mixtures. The objectives of this work were to examine the variation of Kamlet−Taft parameters of water−GBL and water−GVL binary mixed-solvent systems as the bulk mole fraction of the HBA solvent (GBL or GVL) is changed over the full range of compositions and to provide a qualitative description of chemical species in the cybotactic region of the solvatochromic indicators so that synergistic basicity of these systems can be better understood. Comparisons are shown for the methanol−GBL, ethanol−GBL, methanol−GVL, and ethanol−GVL mixed-solvent systems that do not exhibit synergism. Special emphasis is placed on Kamlet− Taft basicity because this parameter affects solvent choice for many separations and reactions.5,44−51 The cybotactic region of a Kamlet−Taft dye indicator in a mixed-solvent system consisting of solvent 1 (S1) and solvent 2 (S2) is more complicated than that in a pure solvent system because S1−indicator, S2−indicator, S1−S2 (complex)− indicator interactions exist for the case of a binary mixed-solvent system, resulting in preferential solvation and differences in bulk and local composition.15,31,52−55 In this work, the preferential solvation model proposed by Rośes and co-workers32,33,56−59 is used to relate the preferential solvation of Kamlet−Taft solvatochromic indicators in the mixed-solvent systems to local composition in the cybotactic region of Kamlet−Taft indicators for polarity, basicity, and acidity. Relative spectral shifts with composition are used to support results provided by the model. Comparisons are made for other HBD (methanol and ethanol)− lactone solvent mixtures measured in this work and for other water−HBA mixtures in the literature so that conclusions can be reached for understanding synergistic basicity in water−lactone mixed-solvent systems. The formation of complex molecules that leads to synergistic mixed-solvent basicity is discussed in terms of local composition, relative spectral shifts in the UV−vis data, the free energy of hydrogen bonding interactions calculated by the electrostatic basicity (βH) and acidity (αH) functional group contributions reported by Hunter et al.,60−63 and the relative charge transfer of the solvents.

phenolate (97%, indicator 6) were obtained from SigmaAldrich Co. The molecular structures of the indicators are given in previous work.21 The indicators and solvents were handled carefully to prevent contamination by humidity or degradation by light, but were used as received without further purification. 2.2. Kamlet−Taft Solvatochromic Indicators. In this work, six indicators were used to determine Kamlet−Taft solvatochromic parameters. The indicators used for polarity (π*) were N,N-dimethyl-4-nitroaniline (indicator 1) and 4-nitroanisole (indicator 2). Polarity (π*) values were normalized to have its values of zero for cyclohexane and one for N,N-dimethyl sulfoxide.64 The indicators used for basicity (β) were 4-nitroaniline (indicator 3) and 4-nitrophenol (indicator 4). Basicity (β) values were normalized to assume a value of 1 for hexamethylphosphoramide.65 The indicators used for acidity (α) are 2,6-diphenyl4-(2,4,6-triphenylpyridinio) phenolate (indicator 5) and 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio) phenolate (indicator 6). Acidity (α) values were normalized to assume a value of 1 for MeOH.66 Calculation of Kamlet−Taft solvatochromic parameters for mixed-solvent systems in this work and those in the literature was based on procedures reported by Marcus as follows:15,25,26,31,32,54,67−69 indicator 1

π* = 0.314 × (27.52 − vmax1)

(1)

indicator 2

π* = 0.427 × (34.12 − vmax2)

(2)

indicator 3 * β = 0.358 × (31.10 − vmax3) − 1.125 × πaverage

(3)

indicator 4 * β = 0.346 × (35.04 − vmax4) − 0.57 × πaverage

(4)

indicator 5 * α = 0.0649 × E T(30) − 2.03 − 0.72 × πaverage

E T(30) kcal mol−1 = 28 591.5/λmax5 (nm)

indicator 6

vmax5 = 0.98vmax6 − 2.61

(5) (6) (7)

The vmax is the maximum absorption wavenumber of indicator in term of kiloKaiser (1kK = 1000 cm−1 = 10 000/λmax (nm)) obtained from the UV−vis spectrophotometer. The π*average is the average value of π* from indicators 1 and 2. The maximum absorption wavenumber (vmixture) of the solvatochromic indicators in the solvent mixtures were evaluated by numerically smoothing curves using the first-order derivative of absorption from Origin software (Microcal, version 9.1) suggested from the literature.70 Indicator 5 is insoluble in water and aqueous mixtures, while indicator 6 is soluble in water and some aqueous binary mixedsolvent systems.70,71 The α parameter can be directly calculated from indicator 5 using eq 5. Indicator 6 is used for determining acidity (α) of aqueous binary mixed-solvent systems. For aqueous binary mixed-solvent systems, vmax6 from indicator 6 is converted to vmax5 with eq 770 and then used in eqs 5 and 6 to obtain the α value. 2.3. UV−Vis Spectra. The six HBD (1)−HBA (2) solvent mixtures studied in this work were the following: (i) H2O−GBL, (ii) H2O−GVL, (iii) MeOH−GBL, (iv) MeOH−GVL, (v) EtOH−GBL, and (vi) EtOH−GVL. Mixed-solvent compositions

2. EXPERIMENTAL SECTION 2.1. Materials. Distilled water (HPLC grade), deionized water, methanol (HPLC grade), and ethanol (HPLC grade) were purchased from Wako Pure Chemical Industries Ltd. (Japan). The γ-butyrolactone (GBL) and γ-valerolactone (GVL) had a purity of 99% and were obtained from Sigma-Aldrich Co. Distilled water (HPLC grade) was used for Kamlet−Taft solvatochromic measurements with UV−vis spectra (section 2.3). Deionized water was used for ATR-spectra (section 2.4) and density measurement (section 2.5). Solvatochromic indicators N,N-dimethyl-4-nitroaniline (98%, indicator 1) and 4-nitroanisole (98%, indicator 2) were purchased from Tokyo Chemical Industry Co. Ltd. (Japan). The 4-nitroaniline (99%, indicator 3), 4-nitrophenol (99.5%, indicator 4), 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio) phenolate (90%, indicator 5), and 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio) 4468


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The Journal of Physical Chemistry B for Kamlet−Taft solvatochromic (K-T) measurements were prepared using a microbalance (Mettler Toledo, model ax 504) to within an uncertainty in mass fraction of ±2 × 10−4 g/g with constant increment of weight composition of 0.1. A total of 11 weight compositions for K-T measurements were prepared for each mixed-solvent system, except for H2O−GBL and H2O−GVL mixtures for which 12 weight compositions were prepared. To confine the maximum UV−vis absorbance of the indicators to be within a 0.5−1.2 absorbance unit range, the concentration of each indicator in the mixed-solvent systems was adjusted to be 5 × 10−5 mol dm−3, except for indicators 5 and 6 that were prepared at 1 × 10−4 mol dm−3. A double-beam UV−vis spectrophotometer (Jasco, model V-530) used in the measurements was calibrated using holmium(III) in perchloric acid standard from NIST72 with an uncertainty (2σ) in maximum absorption wavelength (λmax) of ±0.8 nm. All UV−vis spectra of the solvatochromic indicators in the HBD-HBA solvent mixtures were measured in a 1 cm path length of quartz cell with an encapsulation case at constant temperature 25 ± 0.1 °C using a temperature controller (Jasco, ETC 505). All UV−vis spectra were replicated by three scans with the standard deviation being less than 0.2 nm and UV−vis spectra of the solvent background of identical composition were subtracted from the measured UV−vis spectra. The maximum absorption wavenumbers (vmaxture, kK) of the indicators were used to determine Kamlet−Taft solvatochromic parameters (π*, β, α) for binary solvents with eqs 1−6. 2.4. ATR-Infrared Spectra. Water−lactone mixed-solvent systems (H2O−GVL and H2O−GBL) were prepared by mass using a microbalance (Mettler Toledo, model ax 504) at corresponding mole fractions of 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 to within an uncertainty of ±3 × 10−4 mole fraction without solute indicators. All infrared spectra of the H2O−GVL and H2O−GBL were measured with an FT-IR spectrophotometer (Jasco, model 6300) in 0.07 cm−1 wavenumber resolution equipped with an ATR single reflection attachment (Jasco, ATR PRO 450-S) at room temperature (ca. 22 ± 1 °C) from 600 to 4000 cm−1. The angle of incidence was 45° with Ge crystal. 2.5. Density Measurements. Mixed-solvent compositions for density measurements were prepared with a microbalance (Mettler Toledo, model ax 504) at the corresponding mole fraction of 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 with an uncertainty of ±3 × 10−4 mole fraction. Densities of the solvent mixtures were measured with a vibrating tube method (Anton Paar, SVM3000) with a standard uncertainty of temperature (u(T)) of ±0.02 K and density (u(ρ)) of ±0.5 kg m−3. The densities of the solvent mixtures over the entire composition ranges at 25 °C are given in Table S1 (Supporting Information).

Here I(S1), I(S2), and I(S12) represent the indicator (solute) solvated by solvent 1 (S1) or solvent 2 (S2), or by mutual complex molecule (S12), respectively, and m represents the number of solvating molecules in the cybotactic region of the indicator. At equilibrium, two preferential solvation parameters ( f 2/1 and f12/1) are related to solvent exchange (eqs 8 and 9). The f 2/1and f12/1 parameters defined by eqs 10 and 11 represent the tendency of the indicator to be preferentially solvated by the solvent 2 (S2) and the mutual complex molecule (S1−S2) in the presence of solvent 1 (S1), respectively, when the f 2/1and f12/1 parameters are greater than unity. The solute−solvent and solvent−solvent interactions in the cybotactic region of indicator can be determined by f 2/1, f12/1, and f12/2 preferential solvation parameters, respectively, as

f12/1 =

f12/2 =

I(S1)m + mS2 ←→ I(S2 )m + mS1 I(S1)m +

f12/1 m m S2 ←→ I(S12 )m + S1 2 2

(10)

L X12 /X1L

(X 2bulk /X1bulk )m /2 L X12 /X 2L

(X 2bulk /X1bulk )m /2

(11)

=

f12/1 f2/1

(12)

To obtain f 2/1 and f12/1 preferential solvation parameters, experimental UV−vis spectra of maximum absorption wavenumbers (vmixture in term of kiloKaiser) of the indicator over the entire range of mixed-solvent compositions were fitted using eq 13 with the number of solvent molecules in the cybotactic region set equal to 2 (m = 2) as suggested in the literature.32,33,56−59 vmixture = v1 +

f2/1 (v2 − v1)(X 2bulk )2 + f12/1 (v12 − v1)[(1 − X 2bulk )X 2bulk ] (1 − X 2bulk )2 + f2/1 (X 2bulk )2 + f12/1 [(1 − X 2bulk )X 2bulk ]

(13)

After obtaining f 2/1 and f12/1 preferential solvation parameters, local compositions of solvent 1 type (XL1 ), solvent 2 type (XL2 ), and 1−2 complex molecule (XL12) in the cybotactic region of each indicator were determined as a function of bulk composition (Xbulk i ) using eqs 14−16. X1L =

(1 − X 2bulk )2 (1 − X 2bulk )2 + f2/1 (X 2bulk )2 + f12/1 (1 − X 2bulk )X 2bulk (14)

X 2L

3. THEORY 3.1. Preferential Solvation Model. Solute−solvent and solvent−solvent interactions in the cybotactic region of each indicator occur in a mixed-solvent system as mentioned in the Introduction. The probable chemical species can be elucidated with the preferential solvation model,32,33,56−59 which is a theory based on solvent exchange as follows: f2/1

X 2L /X1L bulk (X 2 /X1bulk )m

f2/1 =

=

f2/1 (X 2bulk )2 (1 − X 2bulk )2 + f2/1 (X 2bulk )2 + f12/1 (1 − X 2bulk )X 2bulk (15)

L X12

=

f12/1 (1 − X 2bulk )X 2bulk (1 − X 2bulk )2 + f2/1 (X 2bulk )2 + f12/1 (1 − X 2bulk )X 2bulk (16)

L X1bulk + X 2bulk = X1L + X 2L + X12 =1

(8)

(17)

32,33,56−59

The preferential solvation model was applied to correlate experimental data of Kamlet−Taft solvatochromic parameters measured from indicators with eqs 18−20. Fitting of eqs 18−20 to experimental data provided the Kamlet−Taft

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especially at low GVL and GBL bulk mole fractions (water-rich, X2 ≈ 0.01−0.20) for which β increased rapidly. Synergistic basicity of averaged values from indicators 3 and 4 was observed for the H2O−GVL and H2O−GBL mixed-solvent system studied in this work (Figure 1), whereas synergistic basicity of averaged values from the indicators 3 and 4 was not observed for measured alcohol−GVL and alcohol−GBL mixedsolvent systems measured in this work (Figure 2). K-T parameter (π*, β, α) for the water−GVL (Figure 1a−c) and water−GBL (Figure 1d−f) had similar trends with bulk mole fraction because the functional groups of GVL (ester) are the same as those of GBL. K-T parameters (π*, β, α) for the methanol−GVL and ethanol−GVL (Figure 2d−f) and methanol−GBL and ethanol−GBL (Figure 2d,e) had similar trends with bulk mole fraction because the interactions (alcohol−ester) were between the same types of functional groups. Solvent combinations of GVL with H2O, MeOH, or EtOH can be expected to have similar respective trends for the case when GBL is used instead of GVL. The presence and absence of synergistic effects on basicity (β) in mixed-solvent systems reported in this work are further analyzed using the preferential solvation model in the following section. In this work, analyses of the cybotactic region are shown for H2O−lactone and alcohol−lactone systems with detailed analyses for all systems being given in the Supporting Information (section B, Figures S1−S9). Analyses of the cybotactic region of H2O−acetonitrile mixed-solvent systems using literature data26 (Figures S10 and S11, Supporting Information) were made for comparison. 4.1. Preferential Solvation of Indicators in MixedSolvent Systems. Fitting of spectral data with the preferential solvation model, eq 13, for the mixed-solvent systems of all indicators (Table 1) gave correlation coefficients, R2, that were higher than 0.97 and generally above 0.99 (Table 1). The preferential solvation parameters (f 2/1, f12/1, and f12/2) tabulated in Table 1 showed the existence of complex molecules in each mixed-solvent system. Detailed statistics and maximum absorption wavenumber (v12) for each mixed-solvent system and each indicator are given in Table S9 (Supporting Information). Since vmixture values for all mixed-solvent systems and all indicators (Tables S3−S8, Supporting Information) deviated from ideality as explained in the Introduction, the preferential solvation of the indicators in the mixed-solvent systems occurs and can be assessed with the preferential solvation parameters (Table 1). Indicators 1 and 2 (Table 1) are sensitive to polarity (π*) of the solvent with no specific interaction with solvent.65 The preferential solvation parameter values (f 2/1) of indicator 1 for polarity in H2O−GBL and H2O−GVL mixed-solvent systems (Table 1) were higher than 1, indicating that the indicators prefer to be solvated in the HBA solvent (GBL and GVL) than in water. On the other hand, the f 2/1 values for indicator 1 for polarity in H2O−GBL and H2O−GVL solvent systems (Table 1) were lower than 1, indicating that the indicators prefer to be solvated in water than in the HBA solvent (GBL and GVL). Indicator 1 is more hydrophobic than indicator 2 because indicator 1 has two methyl groups, while indicator 2 has one methyl group. Thus, indicator 1 is more sensitive to changes in the polarity of its environment than indicator 2 for H2O−GBL and H2O−GVL mixed-solvent systems (Figure 1). Indicators 3 and 4 (Table 1) are sensitive to polarity (π*) and basicity (β) because indicator 3 has amino (NH2) functional group and indicator 4 has hydroxyl (OH) functional group in which both NH2 and OH functional groups can interact with HBA and/or HBD solvents.65 The preferential solvation

solvatochromic parameters of the 1−2 complex molecules (π*12, β12, α12) for the solvent system.

{

* * + f (πpure2 * − πpure1 * )(X 2bulk )2 πmixture = πpure1 2/1

}

* − πpure1 * )[(1 − X 2bulk )X 2bulk ] + f12/1 (π12

{

/ (1 − X 2bulk )2 + f2/1 (X 2bulk )2

}

+ f12/1 [(1 − X 2bulk )X 2bulk ]

(18)

{

βmixture = βpure1 + f2/1 (βpure2 − βpure1)(X 2bulk )2

}

+ f12/1 (β12 − βpure1)[(1 − X 2bulk )X 2bulk ]

{

/ (1 − X 2bulk )2 + f2/1 (X 2bulk )2

}

+ f12/1 [(1 − X 2bulk )X 2bulk ]

(19)

{

αmixture = αpure1 + f2/1 (αpure2 − αpure1)(X 2bulk )2

}

+ f12/1 (α12 − αpure1)[(1 − X 2bulk )X 2bulk ]

{

/ (1 −

X 2bulk )2

+

f2/1 (X 2bulk )2

}

+ f12/1 [(1 − X 2bulk )X 2bulk ]

(20)

In summary, the preferential solvation model with experimental UV−vis spectra (vmixture) of each indicator was used to determine solute−solvent and solvent−solvent interactions and local compositions via f 2/1 and f12/1 parameters using preferential solvation parameters in eqs 14−16, so that the cybotactic region of each Kamlet−Taft indicator could be analyzed and related to trends in the mixed-solvent polarity, basicity, and acidity. The f 2/1 and f12/1 parameters were obtained by fitting experimental vmixture with eq 13 using nonlinear regression by Origin software (Microcal, version 9.1) with the Levenberg−Marquardt algorithm.

4. RESULTS AND DISCUSSION Experimental maximum absorption wavenumbers (vmax) for six indicators and experimental Kamlet−Taft solvatochromic (K-T) parameters for pure solvents used in this work at 25 °C are tabulated in section A (Table S2) in Supporting Information and were in accordance with literature values. The average deviation of maximum wavelength (λmax) between experimental and literature values with the same indicators (Table S2) was 1.3 nm, which was higher than the experimental uncertainty of this work (0.8 nm) and may be due to variations in experimental conditions, including temperature, measurement resolution, and material purities. Experimental vmixture and K-T parameters for all binary mixedsolvent systems are summarized in Tables S3−S8 (Supporting Information). Figure 1 shows Kamlet−Taft solvatochromic parameters (π*, β, α) obtained from six indicators for the H2O−GVL and H2O−GBL mixed-solvent system as a function of component 2 (GVL or GBL). Due to the higher polarity (π*) and acidity (α) of water over that of GVL and GBL, polarity (π*), which is the average of indicators 1 and 2 (solid line, Figure 1a,d), and acidity (α) from indicator 6 (solid line, Figure 1c,f) decreased with increasing GVL and GBL content (X2), while basicity (β), which is the average of indicators 3 and 4 (solid line, Figure 1b,e), showed positive changes and synergism with increasing component 2 in water, 4470


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Figure 1. Kamlet−Taft solvatochromic parameters (polarity (π*), basicity (β), acidity (α)) for the water−γ-valerolactone (H2O−GVL) (a−c) and water−γ-butyrolactone (H2O−GBL) (d−f) mixed-solvent systems at 25 °C: (a and d) experiment polarity (π*) of H2O−GVL and H2O−GBL systems from indicator 1 (△, n, n-dimethyl-4-nitroaniline), indicator 2 (○, 4-nitroanisole), and average values from indicators 1 and 2 (■); (b and e) experiment basicity (β) of H2O−GVL and H2O−GBL systems from indicator 3 (◊, 4-nitroaniline), indicator 4 (⬡, 4-nitrophenol) and average values from indicators 3 and 4 (■); (c and f) experimental acidity (α) from indicator 6 (■, 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio) phenolate). Lines are correlated with experimental polarity, basicity, and acidity using eqs 18−20 (Tables S5 and S8, Supporting Information).

solvent (GBL and GVL) than in the water probably due to hydrophobicity. The preferential solvation parameter values ( f12/1) of all indicators for all solvent mixtures were greater than zero (Table 1), indicating that HBA and HBD solvents used in this work tend to form complex HBA−HBD molecules by hydrogen bond donor and hydrogen bond acceptor interactions.50,51 Since basicity for H2O−GBL and H2O−GVL systems displays synergism, the cybotactic region of indicators for basicity was analyzed in detail in the next section. 4.2. Cybotatic Region in the Water−Lactone MixedSolvent Systems. Local compositions of the solvent mixtures around an indicator for basicity in the H2O−GVL mixed-solvent systems were calculated as a function of bulk composition using eqs 14−16 and plotted in Figure 3. Figure 3 shows Kamlet−Taft basicity (β), the local composition (XL1 , XL2 , XL12) in the cybotactic region of indicator 3 (4-nitroaniline), and maximum wavelengths (λmax’s) for the H2O−GVL mixed-solvent system as a function of GVL bulk mole fraction. Schematic graphical representations of molecules (S1 or S2), self-complexes (S1−S1), and mutual

parameter values (f 2/1) for indicators 3 and 4 in all mixed-solvent systems (Table 1) were higher than 1, indicating that the indicators prefer to be solvated in the HBA solvent (GBL and GVL) rather than in the HBD solvents (H2O, MeOH, EtOH). These results can be explained by specific indicator−solvent interactions because indicators 3 and 4 have amino (NH2) and hydroxyl (OH) functional groups that preferentially interact with an HBA solvent (GBL and GVL) molecule.22,63,65,73 Indicators 5 and 6 (Table 1) are sensitive to polarity (π*) and acidity (α) due to specific interactions with the phenolate oxygen atom.66,71 The preferential solvation parameter values (f 2/1) for indicator 5 in MeOH-GBL and MeOH-GVL solvent systems (Table 1) were lower than one, indicating that the indicators prefer to be solvated in MeOH than in the HBA solvent (GBL and GVL) because the phenolate oxygen of indicator 5 preferentially interacts with the MeOH molecule in accordance with our previous work.22 The preferential solvation parameter values (f 2/1) for indicator 6 for acidity in H2O−GBL and H2O−GVL solvent systems (Table 1) were higher than 1, indicating that the indicators prefer to be solvated in the HBA 4471


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Figure 2. Kamlet−Taft solvatochromic parameters (polarity (π*), basicity (β), acidity (α)) for γ-valerolactone (GVL) with methanol (MeOH) and ethanol (EtOH) and for γ-butyrolactone (GBL) with MeOH and EtOH at 25 °C: (a) experimental polarity of MeOH−GVL (red ▲) and EtOH−GVL (blue ◆) average values from indicators 1 and 2; (b) experimental basicity of MeOH-GVL (red ▲) and EtOH−GVL (blue ◆) average values from indicators 3 and 4; (c) experimental acidity of MeOH−GVL (red ▲) and EtOH−GVL (blue ◆) obtained from indicator 5; (d) experimental polarity of MeOH−GBL (black ●) and EtOH−GBL (green ■) average values from indicators 1 and 2; (e) experimental basicity of MeOH−GBL (black ●), EtOH−GBL (green ■) average values from indicators 3 and 4; (f) experimental acidity of MeOH−GBL (black ●), EtOH−GBL (green ■) obtained from indicator 5.

complexes (S1−S2) in the cybotactic region of indicator 3 (I3) at selected bulk mole fractions of GVL are shown in Figure 3. Although self-complex (S1−S1) molecules for pure water, methanol or ethanol are not considered in the preferential solvation model, self-complex (S1−S1) molecules are able to form in the cybotactic region in pure water, pure methanol, or pure ethanol systems.74,75 Therefore, the local composition of self-complex (S1−S1) molecules is independent of the indicators, and their existence is recognized in the schematic images of the mixed-solvent systems. Beginning from pure H2O composition (X2 = 0) in Figure 3, the indicator has two electron-acceptor positions63,73 from the amino functional group (NH2) that accepts lone pair electrons from a single solvent molecule (S1 or S2) or an associated selfcomplex H2O−H2O molecule (S1−S1);74,75 thus, indicator 3 responds to the basicity of the solvent or solvent mixture (electron donor). Although the nitro functional group (NO2) of indicator 3 can donate its lone pairs of electrons to the hydrogen atom of H2O and cause a bias in the basicity measurement, especially for amphiprotic solvents (e.g., water and alcohol),

this effect is not considered in this work due to limitations of the spectroscopic method used and in the model. When a small amount of GVL solvent was added to pure H2O (X2 = 0.01), the maximum absorption wavelengths (λmax’s) of the indicator (Figure 3c) moved to longer wavelengths (red shift) relative to pure H2O, implying that energy required for electrons to be in the excited state of the mixture was lower than that of pure H2O. The H2O−GVL mixture showed synergism such that λmax of the H2O−GVL mixture was higher than that of the pure solvents, which implies that a large number of complex H2O−GVL molecules exist compared with monomeric forms of the solvents. These results are consistent with local compositions shown (Figure 3b) for the cybotactic region of the H2O−GVL system (Figure 3, X2 = 0.01) in that mutual complex H2O−GVL molecules (S1−S2) are formed that cause an increase in basicity. Schematically, the graphic (Figure 3, X2 = 0.01) shows that a complex molecule appears in the cybotactic region of the indicator for an arbitrarily chosen GVL composition of X2 = 0.01. When the amount of GVL in water (Figure 3, X2 > 0.01) was increased, the λmax and local mole fraction (XL12) for 4472


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Table 1. Fitting Parameters ( f 2/1, f12/1, and v12) from the Preferential Solvation Model (Equation 13) with Solvent Mixtures of Hydrogen Bond (HBD) Donor Solvents (Component 1) with Hydrogen Bond (HBA) Acceptor Solvent (Component 2) Using Spectral Data of the Solvatochromic Indicators in Tables S3−S8 (Supporting Information) at 25°Ca indicator solvent system, (HBD(1)−HBA(2)) H2O−GBL MeOH−GBL EtOH−GBL H2O−GVL MeOH−GVL EtOH−GVL H2O−GBL MeOH−GBL EtOH−GBL H2O−GVL MeOH−GVL EtOH−GVL H2O−GBL MeOH−GBL EtOH−GBL H2O−GVL MeOH−GVL EtOH−GVL H2O−GBL MeOH−GBL EtOH−GBL H2O−GVL MeOH−GVL EtOH−GVL MeOH−GBL EtOH−GBL MeOH−GVL EtOH−GVL H2O−GBL H2O−GVL a

f 2/1

f12/1

f12/2

Indicator 1 (N,N-Dimethyl-4-nitroaniline) for Polarity (π*) 2.05 5.86 2.86 1.07 3.93 3.67 1.26 3.24 2.57 2.13 7.78 3.65 0.25 4.14 16.6 1.52 4.35 2.86 Indicator 2 (4-Nitroanisole) for Polarity (π*) 0.44 1.91 4.34 0.70 3.21 4.59 0.96 4.25 4.43 0.03 4.67 155.7 0.91 3.43 3.77 0.75 4.93 6.57 Indicator 3 (4-Nitroaniline) for Basicity (β) 46.9 27.5 0.59 1.36 5.12 3.76 2.27 7.84 3.45 325.0 213.4 0.66 2.76 7.89 2.86 5.00 25.51 5.10 Indicator 4 (4-Nitrophenol) for Basicity (β) 6.49 42.0 6.47 1.37 4.17 3.04 1.11 8.35 7.52 53.5 252.6 4.72 1.68 5.70 3.39 2.69 13.72 5.10 Indicator 5 (2,6-Diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate) for Acidity (α) 0.14 1.20 8.57 1.27 10.7 8.43 0.89 3.77 4.24 1.50 5.23 3.49 Indicator 6 (2,6-Dichloro-4-(2,4,6-triphenyl-1-pyridinio)phenolate) for Acidity (α) 1.33 8.54 6.42 5.43 14.1 2.60

R2b

Nc

0.998 0.998 0.996 0.995 0.995 0.999

12 11 11 12 11 11

0.990 0.996 0.998 0.982 0.998 0.999

12 11 11 12 11 11

0.997 0.999 0.986 0.971 0.997 0.991

12 11 11 12 11 11

0.997 0.996 0.999 0.971 0.996 0.997

12 11 11 12 11 11

0.999 0.999 0.999 0.999

11 11 11 11

0.998 0.997

12 12

HBD solvents are methanol (MeOH), ethanol (EtOH), and water (H2O). HBA solvents are γ-butyrolactone (GBL) and γ-valerolactone (GVL). R = coefficient of determination. cN = number of experimental data.

b 2

Figure 4 shows a schematic diagram of the cybotactic region of indicator 4 (4-nitrophenol) for the H2O−GVL system along with the local composition and basicity as a function of bulk mole fraction of GVL. Indicator 4 has one electron-acceptor position from a hydroxyl functional group (OH) that can accept a lone pair of electrons from HBA or HBD solvents;52,62 thus, indicator 4 is used to measure electron donor (basicity) of HBA or HBD solvent systems. Analysis of the cybotactic region of indicator 4 for the H2O−GVL system (Figure 4) can be considered to be similar to that of indicator 3 for H2O−GVL (Figure 3). However, the relative increase of basicity in the H2O−GVL system for indicator 4 compared with the basicity of pure water is lower than that for indicator 3 because indicator 4 has one electron-acceptor position (OH functional group), while indicator 3 has two electron-acceptor positions (NH2 functional group). Thus, indicator 3 is more sensitive to solvent basicity (electron donor) than indicator 4. Although the relative increase in basicity for indicator 3 is higher than that for indicator 4, the basicity value obtained from

the H2O−GVL system rapidly increased and reached a peak at X2 ≈ 0.08. A rapid increase of λmax and XL12 and peak maximum in the XL12 (Figure 3b) imply that the cybotactic region is saturated with mutual complex H2O−GVL molecules (S1−S2, Figure 3b, X2 = 0.1) which results in a rapid increase in Kamlet− Taft basicity (β, Figure 3a, X2 = 0. 1) and saturation of the cybotactic region with complex molecules as shown schematically (Figure 3). At GVL mole fractions X2 > 0.2, the λmax and XL12 for H2O−GVL systems (Figure 3b,c) rapidly decreased. The cybotactic region contains a GVL molecule (S2) and a complex molecule (S1−S2) because the GVL molecule directly interacts with the amino functional group of indicator 3 due to high local concentrations of GVL molecules that allow its competition with the complex as proposed in the solvent-competition model.50,52 Thus, the basicity of H2O−GVL systems (X2 ≈ 0.5−1.0) tends to approach the basicity of pure GVL with the basicity of H2O−GVL mixtures being relatively flat until the bulk composition reaches that of pure GVL (Figure 3a). 4473


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Figure 3. Water−γ-valerolactone (GVL) mixed-solvent system with 4-nitroaniline (indicator 3) at 25 °C showing (a) Kamlet−Taft basicity; (b) local mole fraction (cybotactic region) of water (XL1 ), GVL (XL2 ), and complex molecule H2O−GVL (XL12); and (c) maximum wavelength of absorption of indicator 3, all plotted versus the bulk mole fraction of GVL (X2). Schematic graphical images around the plots show the cybotactic region around indicator (I3) that can contain a water molecule (S1), a GVL molecule (S2), an associated water complex molecule (S1−S1), or a water−GVL complex molecule (S1−S2). Symbols: unfilled circles, selected bulk mole fractions for graphical images; filled squares, compositions of the spectral measurements. Line in part a is calculated with eq 19, and lines in part b are calculated with eqs 14−16 using fitted parameters from the preferential solvation model (Table 1).

indicator 3 is lower than that obtained from indicator 4 that can be attributed to the difference in the basicity and acidity of the functional groups in the indicators. The hydrogen bond donor parameter (αH) for hydroxyl functional group (OH) for indicator 4 (αH = 2.7, Figure S12, Supporting Information) is higher than that for the aniline functional group (NH2) in indicator 3 (αH = 2.1, Figure S12).52,62 Thus, the interaction of indicator 4 to solvent basicity is stronger than that of indicator 3. In practice, the basicity values of these two indicators are averaged as shown in Figure 1b to reduce the bias of indicator specific (e.g., indicator-centric) interactions. Analyses of the cybotactic regions of indicators 3 and 4 for the H2O−GBL mixed-solvent system is available in Supporting Information (Figures S1 and S2) and was similar to that of the H2O−GVL mixed-solvent system. The intermolecular interactions of water with GVL and with GBL were further investigated by ATR-FTIR spectroscopy (Figures S13−S16). The CO stretching band of GBL and GVL and OH stretching band of water (Figures S13 and S14) was red-shifted when water was added to lactones (GBL and GVL), indicating that water interacted with lactones to form complex molecules via specific interactions of the carbonyl (CO) groups of the lactone and the hydrogen atom of water according to IUPAC definition of hydrogen bond interactions.76

Infrared measurement techniques are not readily applied to analyze the cybotactic region of the indicator because low indicator concentrations are required to obtain reliable Kamlet− Taft solvatochromic parameters, even though some reports have analyzed systems that have high indicator concentrations to better understand solvent−indicator interactions.77,78 However, the formation of complex molecule of lactones with water measured by ATR-FTIR spectroscopy confirms the local composition images (Figures 3 and 4) of the preferential solvation model. Molar excess volumes (VE) calculated from densities of the mixtures (Table S1) for the H2O−GBL and H2O−GVL mixedsolvent systems had negative values (section D, Figures S17 and S18) at mole fractions of GVL and GBL lower than 0.8 (X2 < 0.8), indicating that GVL and GBL solvents have specific interactions with water that allow the lactone to be incorporated into the solution structure of water. When bulk mole fractions of GVL and GBL were greater than 0.8 (X2 > 0.8), VE values of the mixtures were greater than 1, implying that GVL or GBL prefers to have self-interactions over mutual interactions with water. These results are consistent with the UV−vis results in the cybotactic region for H2O−GVL (Figures 3 and 4) and for H2O−GBL (Figures S1 and S2) at high bulk mole fractions of 4474


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Figure 4. Water−γ-valerolactone (GVL) mixed-solvent system with 4-nitrophenol (indicator 4) at 25 °C showing (a) Kamlet−Taft basicity; (b) local mole fraction (cybotactic region) of water (XL1 ), GVL (XL2 ), and complex molecule H2O−GVL (XL12); and (c) maximum wavelength of absorption of indicator 4, all plotted versus the bulk mole fraction of GVL (X2). Schematic graphical images around the plots show the cybotactic region around indicator (I4) that can contain a water molecule (S1), a GVL molecule (S2), an associated water complex molecule (S1−S1), or a water−GVL complex molecule (S1−S2). Symbols: unfilled circles, selected bulk mole fractions for graphical images; filled squares, compositions of the spectral measurements. Line in part a is calculated with eq 19, and lines in part b are calculated with eqs 14−16 using fitted parameters from the preferential solvation model (Table 1).

When the amount of GVL added to the methanol was increased (Figure 5, X2 > 0.01), λmax and XL12 for the MeOH− GVL mixed-solvent system gradually increased. However, the cybotactic region of the MeOH−GVL system is never fully occupied with complex H2O−GVL molecules at compositions up to X2 = 0.1 (Figure 5), so that the basicity of MeOH−GVL mixtures gradually decreased. At GVL mole fractions X2 > 0.2, λmax and XL12 gradually increased (Figure 5c,b) and reached a peak at X2 ≈ 0.38. The cybotactic region for MeOH−GVL most likely contains a GVL molecule (S2) and a complex molecule (S1−S2), because the GVL molecule (S2) directly interacts with the amino functional groups of indicator 3 as noted in the literature61,63 and there was a high local concentration of GVL molecules according to the preferential solvation model. Thus, the basicity of the mixed-solvent system (X2 ≈ 0.5−1.0) approached the basicity of pure GVL as GVL was added to the mixed-solvent system (Figure 5a). Analyses of the cybotactic regions of indicators 3 and 4 for the MeOH−GVL, MeOH−GBL, EtOH− GVL, and EtOH−GBL mixed-solvent systems are available in Supporting Information (Figures S3−S9) and were similar to those of the MeOH−GVL mixed-solvent system (Figure 3).

GVL and GBL greater than 0.8 (X2 > 0.8), indicating that the cybotactic region contains only single GVL molecules or GBL molecules. 4.3. Cybotatic Region in the Alcohol−Lactone MixedSolvent System. Figure 5 shows Kamlet−Taft basicity (β), local composition (XL1 , XL2 , XL12) in the cybotactic region of indicator 3 (4-nitroaniline), and maximum wavelengths (λmax’s) for the MeOH−GVL mixed-solvent system as a function of GVL bulk mole fraction. Schematic graphical representations of molecules (S1 or S2), self-complexes (S1−S1), or mutual complexes (S1−S2) in the cybotactic region of the indicator (I3) are shown in Figure 5. Beginning from pure MeOH composition (X2 = 0) in Figure 5, single MeOH molecule (S1) and self-complexing MeOH molecule (S1−S1) interact with the NH2 group of indicator 3. When a small amount of GVL solvent was added to pure MeOH (X2 = 0.01), λmax (Figure 5c) moved slightly to longer wavelengths (red shift) relative to that of pure MeOH. The complex MeOH−GVL molecule was not present in the cybotactic region of MeOH−GVL mixtures at low mole fractions of GVL (Figure 5, X2 = 0.01) which can be attributed to the basicity of the mixture being similar to that of pure MeOH. 4475


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Figure 5. Methanol (MeOH)−γ-valerolactone (GVL) mixed-solvent system with 4-nitroaniline (indicator 3) at 25 °C showing (a) Kamlet−Taft basicity; (b) local mole fraction (cybotactic region) of MeOH (XL1 ), GVL (XL2 ), and complex molecule MeOH-GVL (XL12); and (c) maximum wavelength of absorption of indicator 3, all plotted versus the bulk mole fraction of GVL (X2). Schematic graphical images around the plots show the cybotactic region around indicator (I3) that can contain a methanol molecule (S1), a GVL molecule (S2), an associated MeOH complex molecule (S1−S1), or a MeOH−GVL complex molecule (S1−S2). Symbols: unfilled circles, selected bulk mole fractions for graphical images; filled squares, compositions of the spectral measurements. Line in part a is calculated with eq 19, and lines in part b are calculated with eqs 14−16 using fitted parameters from the preferential solvation model (Table 1).

4.4. Analysis of Synergistic Basicity in the Water− Lactone Systems. For understanding the synergistic basicity observed for H2O−GVL and H2O−GBL mixtures (Figure 1), comparison of complexes in the cybotactic region was made between the H2O−GVL (Figure 3) mixed-solvent system that exhibits synergism and the MeOH−GVL (Figure 5) mixedsolvent system that does not exhibit synergism in Kamlet−Taft basicity. Hunter et al.49−52 developed a method to estimate molecular basicity−acidity group contributions and proposed a competitive solvation model for HBA−HBD mixed-solvent systems considering hydrogen bond acceptor (βH) and hydrogen bond donor parameters (αH). The βH and αH and values from Hunter et al.63 for the indicators and solvents used in this work are given in the Supporting Information (Figure S12) and are used in the following discussion. From the Gibbs energy of hydrogen bonding theory of Hunter,63 the interaction (ΔΔGH‑bond) of methanol with GVL is positive63 as determined from the βH and αH values of GVL and MeOH. On the other hand, the interaction (ΔΔGH‑bond) of GVL with water is negative as determined from the βH and αH values of

GVL and water. Therefore, methanol prefers to undergo selfinteraction (MeOH−MeOH complex molecule formation) rather than hydrogen bonding with GVL, while the water molecule prefers to favors hydrogen bonding with the GVL molecule since ΔΔGH‑bond for GVL−water is negative.63 However, ΔΔGH‑bond and βH and αH values63 are based on gas-phase conditions; thus, their values assume no solution interactions. Another reason to support the occurrence of the mutual complex H2O-GVL molecule at low mole fractions of GVL (S1−S2, Figure 3, X2 = 0.01) is due to hydrophobicity of the aromatic ring of indicator 3 and indicator 4. Water prefers to interact with GVL rather than to interact with the aromatic ring of the indicator, which is in accordance with the preferential solvation model that shows high f 2/1 values for indicators 3 and 4 in H2O−GVL and H2O−GBL mixtures (Table 1), indicating that the indicators 3 and 4 prefer solvent component 2 (GVL or GBL) over solvent 1 (H2O). Synergistic basicity was observed when the cybotactic region was fully saturated with 1−2 complex molecules (e.g., H2O−GVL, X2 = 0.2, Figures 3 and 4). The basicity of the 1−2 complex 4476


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system for indicators 3 and 4 (Figures S10 and S11, Supporting Information), reaches a maximum at X2 ≈ 0.1 that results in a rapid increase in Kamlet−Taft basicity, which is similar to the trends found for the H2O−GVL mixed-solvent system (Figures 3 and 4). Analysis of the literature data with the preferential solvation model and methods of this work showed the formation of complex molecules and a peak in their local composition (Figures S10 and S11). The presence of synergistic basicity (S) and structure enhancement of water (W) for various functional groups of HBA solvents with water (Table 2) can be discussed in terms of electrostatic hydrogen bond acceptor parameters (βH), infinite E, ∞ dilution partial excess basicities ( β2̅ ), infinite dilution partial molar excess volumes (V2̅ E, ∞), and preferential solvation parameters (f 2/1 and f12/1). Due to hydrophobicity of cosolvents in aqueous mixtures,80−82 water prefers to form a water−complex network by hydrogen bonding or clathrate-like structures around hydrophobic functional groups of cosolvent that cause an increase in synergistic polarity (π*)15,24,56 and result in positive values of the partial molar excess volume (V1̅ E )80 of water in water-rich compositions (X2 < 0.2). The presence of synergistic basicity and absence of a water structure enhancement in water−HBA mixed-solvent systems (Table 2) seems to occur when βH values of HBA solvents are similar to that of water (low Δ values) and excess partial molar volume of water (V1̅ E ) have negative values80 over the entire composition range (Figures S17c−S22c, Supporting Information). On the other hand, the absence of synergistic basicity and the presence of water structure enhancement are observed when water is combined with an HBA solvent that has a high βH value (e.g., 2-pyrrolidinone (2-PYR), n-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO)) as shown by the Δ values in Table 2 and the V1̅ E values that are positive for water-rich compositions (X2 < 0.2, Figures S23c−S26c). Synergism of HBA solvents in aqueous systems can be explained by the formation of complex molecules with water due to similarity of βH values and thus V1̅ E of water and V2̅ E of HBA solvent are negative in water-rich compositions (X2 < 0.4, Figures S17 and S22), indicating free volume in liquid solutions due to formation of hydrogen bonds. On the contrary, in mixed-solvent systems that

molecules (H2O−GVL) is expected to be higher than that of pure water and pure GVL because the 1−2 complex molecule has two specific sites for donating lone pair electrons. On the other hand, synergistic basicity was not found in the MeOH−GVL system (X2 = 0.2, Figure 5), although a 1−2 complex molecule is formed. One reason for the lack of synergistic basicity in the MeOH−GVL system is that the cybotactic region is not fully occupied with 1−2 complex molecules, and another reason is due to the strength of the solvent−solvent interaction. Water has a higher charge separation than that of methanol;79 thus, the solvent−solvent interaction of water with GVL should be higher than that of methanol with GVL. Indicators 3 and 4 exhibit bias in determining basicity in water and alcohol solvents, and this can also be expected for H2O−GVL and H2O−GBL systems. However, regardless of the bias of the indicators, synergistic basicity exists, and the analyses presented in this work are in accordance with actual solvent effects because it has been demonstrated that H2O−GVL and H2O−GBL mixtures can be used to fractionate biomass14,20 and can be used to prepare soluble precursor of engineering plastics21 in which pure components of those mixed-solvent systems are ineffective for fractionating biomass or for preparing engineering plastics. 4.5. Analysis of Synergistic Basicity in Water− Hydrogen Bond Acceptor Systems. Synergistic basicity in ten water−HBA mixed-solvent systems has been reported in the literature,15,24,26,85 where it has been shown that synergistic basicity can occur when an HBA solvent and water are combined (Table 2). Table 2 shows electrostatic hydrogen bond acceptor parameters (βH) obtained from the literature,63 maximum Kamlet−Taft basicity (Δβmax), infinite dilution partial excess E, ∞ basicity (β2̅ ) calculated from preferential solvation model (eq 19, details given in section D in Supporting Information), basicity trends for synergistic basicity (S) and water enhanced structure (W), partial molar excess volume (Vi̅ E, ∞, details also given in section D), and preferential solvation parameters (f 2/1and f12/1) obtained by fitting literature basicity data15,24,26,85 with eq 19, except for H2O−GVL and H2O−GBL that were measured in this work, for various functional groups of HBA solvents in aqueous mixture systems. One example for water−acetonitrile mixtures reported in the literature,26 λmax and β for the H2O−acetonitrile mixed-solvent

Table 2. Basicity Increase of Mixed-Solvent Systems Containing Hydrogen Bond Donor Solvent (HBD) Water (Component 1) and Hydrogen Bond Acceptor (HBA) Solvents (Component 2) According to Maximum Synergistic Basicity (Δβmax) at Bulk Composition (X2) Compared with the Electrostatic Hydrogen Bond Acceptor Parameter (βH) of Hunter,63 Infinite Dilution E, ∞ Partial Excess Basicity ( β2̅ ), and Infinite Dilution Partial Molar Excess Volume (Vi̅ E, ∞)

Δ = βH2 − βHH2O = βH2 − 4.5. bMole fraction of component 2 (X2) for the maximum deviation in basicity from ideality (Δβmax = βmixture − βideality = βmixture − (X1βpure1 + X2βpure2)). cPositive deviation of basicity from ideality (+), synergistic basicity (S), water enhanced structure (W). d 2 R = coefficient of determination. eAsterisks refer to measurements from this work. Component 2: acetonitrile (ACN), γ-valerolactone (GVL), γ-butyrolactone (GBL), tetrahydrofuran (THF), 1,4-dioxin (DI), acetone (Ace), 2-pyrrolidinone (2-PYR), n-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO). a

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E, ∞

have water structure enhancement, there is a lack of synergistic basicity because complex molecule formation of water with HBA solvent is unfavorable due to hydrophobicity and the differences in βH values (high Δ values). Thus, water prefers to form self-complex molecules or possibly to form clathrate-like structures,74,80−82 especially at water-rich compositions (X2 < 0.2) that cause synergistic polarity (π*).15,24,56 Basicity (β) of water−HBA mixed-solvent systems (Table 2) shows a steep increase in their values for water-rich compositions (X2 < 0.2, Figures S17a−S26a). To compare the increase in basicity of these mixtures with increasing HBA solvent composition (component 2), a hypothesis was formulated that the increase in solution basicity should be related to molecular size and interaction of the mixture solvents (e.g., hydrogen bond and dipole moment).

composition is higher when HBA solvents have higher β2̅ , higher f 2/1, f12/1 values, and more negative V2̅ E, ∞ values because at infinite dilution, higher negative values of V2̅ E, ∞ are related to increased free volume of one molecule of HBA solvent that can interact with water.84 For example, GVL solvent has a higher E, ∞ β2̅ , a more negative V2̅ E, ∞ and higher f 2/1 and f12/1 values than those of GBL solvent (Table 2); thus, the increase of basicity of water−GVL mixed-solvent systems is higher than that for the water−GBL mixed-solvent system (Figures S17a and S18a). E, ∞ values with V2̅ E, ∞ values among different Comparison of β2̅ functional groups does not show correlation as expected (Table 2) that can be attributed to differences in interactions of water with E, ∞ each functional group of the HBA solvent. The ratio of β2̅ /V2 is a complicated function in terms of the f 2/1 parameter (Figure 6b, E, ∞ β2̅ /Vpure2 (mol cm−3) = 0.0142f12/1 − 0.0169, R2 = 0.60), but correlation is probably possible for mixtures having the same functional groups. Thus, complex molecule formation and the similar electronic basicity value (βH) between HBA solvent and water determine whether synergistic basicity is possible and the HBA solvent’s basicity per molecular volume determine the magnitude of the synergistic basicity. Some HBA solvents (component 2 in Table 2) in alcohol-mixed systems do not show synergistic basicity (e.g., MeOH−ACN,67 MeOH−THF,22 MeOH−Ace67) due to reasons described in section 4.3 for the cybotatic region in the methanol−γ-valerolactone mixed-solvent system.

5. CONCLUSIONS The cybotactic regions of water (H2O)−γ-valerolactone (GVL), methanol−GVL, ethanol−GVL, water−γ-butyrolactone (GBL), methanol−GBL, and ethanol−GBL were studied with UV−vis spectroscopy as a function of mixed-solvent composition and analyzed with the preferential solvation model. The formation of water−lactone complex molecules via specific hydrogen bond donor−acceptor interactions in the mixed-solvent systems was confirmed with ATR-IR spectroscopy. Synergistic Kamlet−Taft basicity in water-lactone mixedsolvent systems becomes a maximum when water−lactone complex molecule formation is favored and saturates in the cybotactic region of the indicator. Synergistic basicity for hydrogen bond acceptor (HBA) solvents in water can be found when the βH value of the solvent is similar to that of water. On the contrary, synergistic basicity does not occur when water is combined with an HBA solvent that has a high βH value that can be attributed to the low concentration of mutual complex H2O−HBA molecules. Preferential solvation model parameter ( f12/1) can be used to estimate an increase in basicity or rank the basicity increase of water−HBA mixture solvents when used with the HBA solvent molar volume. The H2O−HBA mixed-solvent systems that have E, ∞ a higher β2̅ , higher f 2/1, f12/1 values, and more negative V2̅ E, ∞ values show higher increases in solution basicity than other HBA solvents. The trend of basicity increase for a given HBA solvent E, ∞ /Vpure2 and can be described by a linear function between β2̅ the f12/1 parameter of the preferential solvation model. Kamlet− Taft parameters, when used in the context of partial molar properties, can be used to analyze mixed-solvent effects when combined with the preferential solvation model. Lactones, such as, GVL or GBL, when mixed with water, have wide application for processing biomass and polymer systems.

Figure 6. Relationship between the ratio of the infinite dilution of partial E, ∞ excess basicity ( β2̅ ) and the molar volume (Vpure2) of hydrogen bound acceptor (HBA) solvents (component 2, in Table 2) with the preferential solvation model parameters: (a) f12/1 parameter (R2 = 0.99) and (b) f 2/1 parameter (R2 = 0.60). HBA solvents: acetonitrile (red ●, ACN), γ-valerolactone (green ■, GVL), γ-butyrolactone (blue ▲, GBL), tetrahydrofuran (aqua ◆, THF), 1,4-dioxin (magneta ⬢, DI), acetone (orange ▼, Ace), 2-pyrrolidinone (●, 2-PYR), n-methyl-2-pyrrolidone (black and yellow ★, NMP), dimethylformamide (gray ◀, DMF), dimethyl sulfoxide (red ⬟, DMSO).

Figure 6a shows infinite dilution partial excess basicity divided E, ∞ /Vpure2 ) that is by the molar volume of pure component 2 ( β2̅ a linear function with f12/1 parameter: β2̅

E, ∞

/V2(mol cm 3) = 0.0027f12/1 − 0.0138

(21) E, ∞

Thus, infinite dilution partial excess basicity ( β2̅ ) and infinite dilution partial molar excess volume (V2̅ E, ∞) can be qualitatively compared for the same functional groups of HBA solvents in Table 2. Within the same functional groups of lactone (GVL and GBL), ether (THF and DI), and cyclic amide (2-PYR and NMP) in Table 2, the increase in basicity of water with HBA 4478


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Superscripts

ASSOCIATED CONTENT

∞ bulk E L H

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b03090. Tables containing experimental, density, wavenumber (vmixture) of indicators, and Kamlet−Taft solvatochromic parameters for all binary solvent mixtures; and figures containing Kamlet−Taft solvatochromic parameters along with local composition in the cybotactic region of indicators, ATR-infrared spectra of water−lactone mixed-solvent systems, and partial molar excess volume and excess basicity of mixed-solvent systems (PDF)

■

Subscripts

1 2 11 12 mixture pure

■

AUTHOR INFORMATION

Corresponding Author

*Phone (fax): +81-22-795-7282. E-mail: [email protected]. tohoku.ac.jp. The authors declare no competing financial interest.

ACKNOWLEDGMENTS Support from the Japanese government (MEXT) for a doctoral study scholarship (A.D.) is gratefully acknowledged. ABBREVIATIONS AND SYMBOLS

Abbreviations

ACN Ace DI DMF DMSO EtOH GBL GVL HBA HBD H2O K-T MeOH NMP 2-PYR

acetonitrile acetone 1,4-dioxin dimethylformamide dimethyl sulfoxide ethanol γ-butyrolactone γ-valerolactone hydrogen bond acceptor solvent (component 2) hydrogen bond donor solvent (component 1) water Kamlet−Taft solvatochromic parameters methanol n-methyl-2-pyrrolidone 2-pyrrolidinone

Latin Symbols

f 2/1 f12/1 f12/2 M W X

HBD solvent molecule, solvent type 1 HBA solvent molecule, solvent type 2 complex HBD−HBD solvent molecule pair complex HBD−HBA solvent molecule pair mixture property pure property

REFERENCES

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Notes

■ ■

infinite dilution property bulk composition excess property local composition electrostatic hydrogen bond acceptor or donor parameter

preferential solvation parameter, according to eq 10 preferential solvation parameter, according to eq 11 preferential solvation parameter, according to eq 12 molecular weight weight fraction of solvent i mole fraction of solvent i

Greek Symbols

α β β̅ λ

acidity Kamlet−Taft solvatochromic parameter basicity Kamlet−Taft solvatochromic parameter partial basicity Kamlet−Taft solvatochromic parameter maximum absorption wavelength of indicator in units of nanometer (nm) v maximum absorption wavenumber of indicator in units of kiloKaiser ρ density π* polarity Kamlet−Taft solvatochromic parameter V molar volume V̅ partial molar volume 4479


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