Intradiffusion, Density, and Viscosity Studies in Binary Liquid Systems

Article pubs.acs.org/jced

Intradiffusion, Density, and Viscosity Studies in Binary Liquid Systems of Acetylacetone + Alkanols at 303.15 K Xiaojuan Chen,† Renquan Hu,† Huajie Feng,‡ Liuping Chen,*,† and Hans-Dietrich Lüdemann§ †

KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, People’s Republic of China § Institut für Biophysik und Physikalische Biochemie, Universität Regensburg, D-93040 Regensburg, Germany S Supporting Information *

ABSTRACT: Intradiffusion coefficients of acetylacetone (AcAc) and methanol/ethanol/1-propanol/1-butanol were measured in binary liquid mixtures over the whole concentration range at 303.15 K by the 1H diffusion-order spectroscopy (DOSY) nuclear magnetic resonance (NMR) method based pulse field gradient (PFG). The solvent effect on the enol-keto tautomeric equilibrium as well as differences in intradiffusion coefficients (D) between two tautomers were also studied. The experimental results show that from methanol to 1-butanol, the differences in D between the enol and keto tautomer vary from 5 % to 26 % at different concentrations of AcAc. The densities and viscosities of binary liquid mixtures of AcAc with the above four alkanols have also been determined at 303.15 K and employed to calculate the excess molar volumes and deviations in viscosity over the entire range of mole fractions. Isotherms of VE as a function of mole fraction of AcAc show positive deviations in methanol and ethanol but negative deviations in 1-propanol and 1-butanol, whereas all isotherms of Δη as a function of mole fraction of AcAc record negative deviations. The VE and Δη are fitted to a Redich− Kister type equation. The measured results are interpreted in terms of molecular interactions in the solutions.

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INTRODUCTION Acetylacetone is a widely used chelating reagent in synthesis chemistry for its excellent ability to form coordination complex with metals.1 However, as one member of β-diketones, much more attention has been devoted to its keto−enol tautomerism, which is regarded as an important mechanism in both organic and biological chemistry and for drug action.2 It is investigated that the electric structure of substituents, temperature, and nature of solvent have been the factors influencing the move of the enol−keto tautomerism equilibrium.3 For this reason, acetylacetone and its derivatives have been the subject of experimental and theoretical studies4−14 in which molecular structure, hydrogen bonding interaction, and keto−enol tautomeric equilibrium in pure liquid and solvents are extensively investigated using a large variety of different methods, including electron diffraction4 and X-ray diffraction5 measurements, IR,6 Raman,7 NMR spectroscopies,8−10 quantum-chemical calculations,11−14 and some other techniques. Besides, much work has been done with respect to excess properties of binary mixtures containing AcAc, which exhibits various situations when different substances possessing different polarity and molecular structure are added to AcAc.15−23 To the best of our knowledge, the majority of previous work was concerned about the thermodynamics properties, and few researchers focused on the dynamic issues of AcAc until Chen24 © 2012 American Chemical Society

et al. measured self-diffusion coefficients D for neat acetylacetone by the PFG-NMR method for a wide range of temperatures and pressures. They observed D difference between two tautomers: the keto have lower D values at all temperatures than enol and the differences appear to have no pressure dependence and weak temperature dependence, which is attributed to the geometrical effects that are not very sensitive to changes of T and density. While in another case25 also by this group, the self-diffusion coefficient for the cis conformer of neat liquid N-methylformamide (NMF) has been found at T ≤ 280 K to be 17 % lower than the trans conformer, and the difference becomes smaller with temperature increasing, which is regarded to be caused by the strong electrostatic interaction between conformers and such an interaction would be excluded by certain steric effects. That was verified by the smaller D difference between two conformers of IMPPA.24 Work mentioned above has been in great effort to investigate what kind of structural difference may lead to measurable differences of D. With the similar interest, we endeavor to study the intradiffusion coefficient differences between AcAc tautomers in several polar solvents and learn which type of interaction Received: January 12, 2012 Accepted: July 23, 2012 Published: August 2, 2012 2401

dx.doi.org/10.1021/je3000553 | J. Chem. Eng. Data 2012, 57, 2401−2408

Journal of Chemical & Engineering Data

Article

Table 1. Intradiffusion Coefficients of Methanol/Ethanol and Tautomers of Acetylacetone (AcAc) and the Equilibrium Constants of AcAc at T = 303.15 K D/10−10m2·s−1

D/10−10m2·s−1

mole fraction of AcAc

methanol

enol/keto

Ke

ethanol

enol/keto

Ke

0 0.0500 0.1000 0.3000 0.5000 0.7000 0.9000 0.9500 1.0000

26.30 20.88 9.29 8.05 6.53 7.90  21.09 

/ 21.55/19.34 7.98/7.62 7.31/6.49 6.88/6.14 7.48/6.70 / 15.68/14.31 18.99/16.03

 0.46 0.45 0.34 0.31 0.28  0.28 0.26

11.50 11.67 12.28 15.44 17.06 18.37 20.23 22.06 

/ 13.40/12.33 15.06/14.20 17.63/15.81 19.91/16.37 21.95/18.20 19.83/17.46 18.36/16.27 18.99/16.03

 0.23 0.24 0.26 0.27 0.26 0.25 0.25 0.26

Table 2. Intradiffusion Coefficients of 1-Propanol/1-Butanol and Tautomers of Acetylacetone (AcAc) and the Equilibrium Constants of AcAc at T = 303.15 K D/10−10m2·s−1

D/10−10m2·s−1

mole fraction of AcAc

1-propanol

enol/keto

Ke

1-butanol

enol/keto

Ke

0 0.0500 0.1000 0.3000 0.5000 0.7000 0.9000 0.9500 1.0000

6.72 6.70 7.40 9.96 12.97 15.03 16.97 19.49 

/ 11.90/9.78 12.21/10.17 14.62/12.69 16.65/14.19 16.69/14.80 17.36/15.31 17.38/15.24 18.99/16.03

 0.18 0.18 0.22 0.22 0.24 0.24 0.24 0.26

4.87 5.27 6.18 8.56 11.63 14.11 15.66 16.44 

/ 9.99/7.93 11.26/9.20 13.97/11.82 16.44/13.67 18.14/14.99 18.08/14.88 17.84/15.52 18.99/16.03

 0.15 0.16 0.19 0.21 0.23 0.25 0.25 0.26

2

between AcAc and solvent molecules might be responsible for this difference. The PFG NMR method building on the pioneering experiment of Stejksal and Tanner26 in 1965 has been a reliable, noninvasive method to obtain diffusion coefficients of molecules or complexes in solution, which provides abundant information about intermolecular interactions. More recently, diffusion-ordered NMR spectroscopy (DOSY)27 employing PFG sequences has become a more powerful tool to investigate heterogeneous mixtures and complex interactions,28−31 such as biological samples, polymers, protein−ligand interactions, ligand−receptor binding, aggregation of supramolecules or assembles, hydrogen bonding, etc. In this work, four primary alkanols with different hydrogenbonding ability are chosen to mix with AcAc, and a series of 1H DOSY experiments by application of a Doneshot pulse sequence32 have been carried out to study how the hydrogen bond forming/breaking interactions impact the diffusion properties of AcAc as well as difference of D between two tautomers. In addition, the thermodynamic excess properties have also been obtained for better understanding of the intermolecular interaction in a solution of AcAc and alkanols.

S(Gzi) = S(0) exp(−Dγ2δ (Gzi)2 (Δ − δ /3))

(1)

where S(Gzi) and S(0) are the signal intensities obtained with gradients Gzi and 0, respectively, D is the diffusion coefficient, γ is the gyromagnetic constant, δ is the gradient pulse duration, and Δ is the diffusion delay. The sequence used in our DOSY experiments is a one-shot pulse sequence32 which possesses several advantages compared with traditional sequences, such as allowing diffusion measurement in a single shot, using as little as one transient per gradient value, minimizing eddy current effects and fieldfrequency lock disturbance, etc. The sample was filled into capillary (o.d. 1.2 mm, i.d. 1 mm) with both ends closed, and the capillary was inserted and fixed in the middle of a 5 mm sample tube full of heavy water with purity of 99.96 % as the deuterium locking solvent. All the intradiffusion measurements were operated on a Varian Mercury Plus 300 MHz spectrometer with a probe capable of producing the gradient, g, up to 30 G·cm−1. All the samples were nonspinning during the experiment. The value of 1 to 3 ms was used for δ, 100 to 400 ms used for Δ and g was varied from 4.6 G·cm−1 to 16 G·cm−1 in 12 increments. The combination of g, δ, and Δ were chosen to generally obtain 90 % to 95 % total signal attenuation throughout the experiment. Other parameters included the following with typical values: a sweep width of 3600 Hz, 16 384 points for Fourier transform, 1 transient, and an acquisition time plus a delay of (3 to 5) T1. The temperature was controlled to ± 0.1 K using the air-bath controller on the NMR spectrometer. The overall error of intradiffusion coefficients was estimated to ± 5 %, and the reproducibility was better than ± 2 %. The intradiffusion coefficients of components and the equilibrium constants were listed in Tables 1 and 2.

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EXPERIMENTAL SECTION Materials. Methanol with mass purity of 99.9 % was acetylacetone, anhydrous ethanol, 1-propanol, and 1-butanol with mass purity > 99.5 % were obtained from Guangzhou Chemical Reagent Factory. All above reagents were directly used without further purification. Intradiffusion Experiments. In the DOSY PFG NMR experiments, the diffusion coefficients are calculated according to the Tejskal−Tanner formula:26 2402



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Density and Viscosity Experiments. Densities of the pure liquids and mixtures were measured with an Anton Paar DMA 4500 vibrating-tube densimeter in which the temperature was controlled automatically within ± 0.01 K. The apparatus was calibrated with deionized doubly distilled water and dry air. The uncertainty of density measurements was 0.05 kg·m−3. Viscosities of pure liquids and mixtures were measured using an Ubbelohde viscosimeter which was calibrated with distilled water first, and the temperature was maintained in a water bath thermostat with a fluctuation not exceeding 0.01 K. An electrical stopwatch with a precision of 0.01 s was used. At least three readings were taken for the flow time with repeatability within ± 0.05 s. The viscosities of all mixtures were calculated from the average flow time. The uncertainty of viscosity results was estimated within ± 0.003 mPa·s. Values measured for pure alkanols are compared to literature values33−40 listed in Table 3. The measured density and viscosity data for binary liquid mixtures were collected in Table 4.

Table 4. Densities, Excess Molar Volumes, Viscosities, and Viscosity Deviations with the Mole Fraction (x1) of AcAc for the Binary System (x1 AcAc + (1 − x1)Methanol/Ethanol/1Propanol/1-Butanol) at 303.15 K ρ x1 0 0.0502 0.1002 0.3002 0.5000 0.6983 0.9004 0.9511 1 0 0.0500 0.0997 0.3000 0.5000 0.6989 0.9000 0.9480 1

Table 3. Comparison of Experimental Densities, ρ, and Viscosities, η, of Pure Liquids with Literature Values at 303.15 K ρ/g·cm−3

η/mPa·s

exp

lit

exp

lit

methanol ethanol

0.78185 0.78139

0.506 0.975

1-propanol

0.79559

1-butanol

0.80201

0.78182a 0.7811c 0.78115d 0.79548e 0.7956f 0.80190g 0.8024c

0.503b 0.987c 0.987d 1.713e 1.72f 2.271h 2.262c

1.718 2.268

0 0.0500 0.1000 0.2999 0.4999 0.6999 0.8994 0.9487 1

a

Reference 33. bReference 34. cReference 35. dReference 40. Reference 36. fReference 39. gReference 37. hReference 38.

e

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RESULTS AND DISCUSSION Intradiffusion Studies. Intradiffusion Coefficients. Intradiffusion coefficients of components in all mixtures as the function of the mole fraction of AcAc(x1) were plotted in Figure 1. As inspection of Figure 1 shows in mixture of AcAc with methanol, the intradiffusion coefficients of both tautomers and methanol have the minimum values at x1 = 0.5 and the intradiffusion coefficient of AcAc reaches its maximum value at x1 = 0.7 when mixed with ethanol, while in 1-propanol and 1butanol solutions, intradiffusion coefficients of all the components appear as an increasing trend. As a kind of hydrogen-donor, alkanol is capable of forming an intermolecular hydrogen bond with both tautomers that leads to an increase of the hydrodynamic radii (rH). According to the Stokes−Einstein equation41 which combines the structural property of the diffusing particle, i.e., the hydrodynamic radii (rH) with the self-diffusion coefficient (D), the viscosity of solvent is also an important factor influencing D, and both rH and viscosity are inversely proportional to D. It is measured that the viscosities of components increase in the following order (shown in Table 4):

0 0.0509 0.1003 0.3007 0.5004 0.7000 0.8998 0.9504 1

g·cm

η −3

mPa·s

Δη

VE cm ·mol 3

−1

AcAc (1) + Methanol (2) 0.78185 0.506 0 0.80495 0.510 −0.088 0.82451 0.516 −0.150 0.88106 0.547 −0.302 0.91610 0.579 −0.300 0.94011 0.611 −0.256 0.95734 0.645 −0.120 0.96075 0.655 −0.063 0.96377 0.664 0 AcAc (1) + Ethanol (2) 0.78139 0.975 0 0.79762 0.896 −0.058 0.81215 0.825 −0.077 0.86107 0.702 −0.101 0.89882 0.655 −0.097 0.92856 0.644 −0.059 0.95324 0.651 −0.028 0.95844 0.657 −0.017 0.96379 0.663 0 AcAc (1) + 1-Propanol (2) 0.79559 1.718 0 0.80688 1.453 0.007 0.81775 1.271 0.016 0.85743 0.922 0.058 0.89238 0.769 0.069 0.92335 0.697 0.062 0.95104 0.673 0.028 0.95744 0.667 0.016 0.96396 0.664 0 AcAc (1) + 1-Butanol (2) 0.80201 2.268 0 0.81109 1.941 0.013 0.81971 1.700 0.035 0.85368 1.142 0.115 0.88646 0.887 0.132 0.91820 0.751 0.097 0.94880 0.692 0.037 0.95630 0.678 0.027 0.96372 0.670 0

mPa·s 0 −0.004 −0.005 −0.006 −0.007 −0.005 −0.003 −0.002 0 0 −0.064 −0.119 −0.180 −0.164 −0.113 −0.043 −0.022 0 0 −0.212 −0.342 −0.479 −0.422 −0.283 −0.098 −0.051 0 0 −0.246 −0.408 −0.646 −0.582 −0.398 −0.139 −0.071 0

interaction to determine D, so intradiffusion coefficients of enol, keto, and alkanol all increase with the drop of viscosities as AcAc concentration increases. In ethanol solution, things seem a little different. It is observed from Figure 1b that when x1 < 0.7, viscosity gains a dominant effect on D values of AcAc; while x1 > 0.7, the molecular association between AcAc and ethanol overwhelms the viscosity effect to reduce D although a little increase of viscosity gives the similar contribution at the same time. This could be verified by the intradiffusion behavior of ethanol, when x1 > 0.7, its intradiffusion coefficient remains the increasing trend, not affected by the viscosity increase of the system. We deduce that the self-aggregation structure between ethanol molecules may be destroyed by forming intermolecular hydrogen bonds with AcAc, which makes ethanol diffuse

CH3OH < AcAc < C2H5OH < 1‐C3H 7OH < 1‐C4 H 9OH

In solvents with low polarity such as 1-propanol and 1butanol, because of their weak molecular associating abilities, the viscosity plays a dominant role over the intermolecular 2403



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Figure 1. Intradiffusion coefficients of alkanols and tautomers of AcAc in a binary system (x1 AcAc + (1 − x1)methanol (a)/ethanol (b)/1-propanol (c)/1-butanol (d)) at T = 303.15 K and p = 0.1 MPa. Δ, alkanol; ■, enol; ●, keto.

D of enol and keto tautomer derives, in our work is 1.18 for neat liquid AcAc, means the enol tautomer diffuses more quickly than keto by 18 %, a little higher than older result 14 % at 297 K by Chen et al.24 It was supposed that the more compact structure of the enol tautomer attributed to the formation of an intramolecular hydrogen bond makes it be more mobile. From methanol to 1-butanol, the differences in D between the enol and keto tautomer vary from 5 % to 26 % at different concentrations of AcAc. The differences in methanol are lower than 18 %, probably because the strong intermolecular interaction between methanol and enol destroys the intramolecular hydrogen bonds formed by enol itself, which reduces the structure discrepancy between two tautomers; and the differences in D between the enol and keto tautomer show higher than 18 % in 1-butanol, possibly because the dominating dispersion force over the dipole−dipole interaction between AcAc and 1-butanol stabilizes the conjugated structure of enol and thus enlarges the structure discrepancy of two tautomers. Enol−Keto Equilibrium. Regarding solvent influence on thermodynamic properties of mixtures, the tautomeric equilibrium constant, Ke determined by the equation Ke = [keto]/[enol], was also studied. The temperature and pressure influence upon the Ke value have been extensively investigated, and Ke appears independent of pressure but sensitive to temperature. It seems that lower temperature favors the enol tautomer for stabilizing the intramolecular hydrogen bond, Ke

more freely. An interesting point found from Figure 1b is the different changing trend for D between enol and keto in the AcAc rich region (x1 > 0.95): D of enol increases while D of keto still remains decreasing, probably because there are too many tautomers surrounding the ethanol molecules in the region; such a “crowding phenomenon” enables enol free from the intermolecular hydrogen bonds to form its own intramolecular hydrogen bonds again; however, for keto, because of its larger dipole moment than enol, the stability of the intermolecular hydrogen bond with ethanol is much stronger and not so easy to be influenced by the “crowding phenomenon” . Because of the strongest hydrogen bonding ability among all four alkanols, methanol has the most special D values with the mole fraction of AcAc increasing. It is noticed that when methanol and AcAc are mixed in a 1:1 composition (x1 = 0.5), intradiffusion coefficients of both components reach the lowest values at the same time. The result quite agrees with the estimation calculated with the ab initio method42 that the two components might form the most stable solute−solvent complex with the most and strongest hydrogen bonds. The special molecular interaction gives such overwhelming contribution to intradiffusion coefficient that the viscosity effect could be insignificant. Intradiffusion Coefficient Difference. The ratio (Denol/ Dketo)T, from which the difference of intradiffusion coefficients 2404



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Table 5. Coefficients Ai and Standard Deviations σ of Mixtures of AcAc + Alkanols at 303.15 K coefficients binary system

property

A0

A1

A2

A3

σa

methanol

VE/cm3·mol−1 Δη/mPa·s VE/cm3·mol−1 Δη/mPa·s VE/cm3·mol−1 Δη/mPa·s VE/cm3·mol−1 Δη/mPa·s

−1.22624 −0.02430 −0.35251 −0.64806 0.28518 −1.65083 0.53040 −2.31549

−0.28619 −0.00236 −0.20201 −0.37019 −0.00445 −0.98800 0.16500 −1.34671

−0.47501 −0.03193 −0.36952 −0.35612 −0.05015 −1.22607 −0.18045 −1.13483

0.08605 −0.02069 −0.26697 −0.21595 −0.12076 −1.11517 −0.33012 −0.80725

0.0066 0.0009 0.0095 0.0036 0.0021 0.0089 0.0036 0.0040

ethanol 1-propanol 1-butanol a

For σ(VE) in cm3·mol−1; for σ(Δη) in mPa·s.

deviation, Δη, are fitted to a Redlich−Kister equation44 by the least-squares method:

was measured 0.27 for pure AcAc by Burdett et al. at 306.15 K,43 0.22 by Chen et al. at 297 K,24 and in our work 0.26 at 303.15 K. What’s more, the value varies with the different alkanols used, as we can see from Tables 1 and 2 that Ke is decreasing in the following order: CH3OH > C2H5OH > 1C3H7OH > 1-C4H9OH. Much work has been accomplished concerning solvent dependence on Ke for various β-dicarbonyls, like AcAc, measured Ke at 306.15 K was 0.35 in methanol solution while 0.22 in absolute ethanol at x1 = 0.1 by Rogers et al.,9 and the value in our work is 0.45 and 0.24, respectively, at 303.15 K. It is known that the dielectric constant, namely, polarity of alkanol decreases with the chain lengthening. Alkanol with higher polarity not only disrupts the intramolecular hydrogen bonds of enol but also stabilizes the keto by electrostatic interactions, which undoubtedly increases the fraction of keto in AcAc and finally results in larger Ke; as for the concentration influence, Ke increases with dilution of AcAc in the lower alkanol because the more content of the lower alkanol exists in solution, the more effect on destruction of the stability of the enol will be, which moves the equilibrium toward the keto, and Ke decreases with dilution in the higher alkanol for a similar reason. Excess Properties Studies. The excess molar volumes (VE) were evaluated from densities using the equation: VE =

x1M1 + x 2M 2 xM xM − 1 1 − 2 2 ρ ρ1 ρ2

k

y E = x(x − 1) ∑ Ai (1 − 2x)i i=0

and the standard deviations are calculated by the following equation45 1/2 E E 2 σ(y E ) = ⎡⎣∑ (yobs − ycal ) /(n − m)⎤⎦ E

(2)

Figure 2. Excess molar volumes VE for the binary system (x1AcAc + (1 − x1) methanol/ethanol/1-propanol/1-butanol at 303.15 K. ●, methanol; ○, ethanol; ▲, 1-propanol; Δ, 1-butanol.

(3)

The excess molar volumes VE show negative values in systems of AcAc + CH3OH and AcAc + C2H5OH but positive values in systems of AcAc + 1-C3H7OH and AcAc + 1C4H9OH (Figure 2). It was suggested46 that VE is the result of contributions from several opposing effects, which may be divided into three types: physical, chemical, and structural effects. Physical effects make a positive contribution to VE; chemical and structural effects make a negative contribution. The negative VE values in AcAc + CH3OH and AcAc + C 2H 5OH indicate that the chemical contributions are dominant. The VE values for AcAc + CH3OH are more negative than AcAc + C2H5OH, probably because methanol has a much stronger ability than ethanol to form the intermolecular hydrogen bonds between the unlike molecules (AcAc +

where ρ, ρw and t, tw are the densities and flow times of the liquid sample and distilled water, respectively, ηw is the viscosity of distilled water at 303.15 K. The viscosity deviation is calculated by the equation:

Δη = η − x1η1 − x 2η2

(6)

where y represents either V or Δη, Ai is the coefficient of eq 5, n is the total number of experimental points, and m is the number of coefficients. All the coefficients (Ai) for VE and Δη are presented in Table 5; plots of VE and Δη versus the mole fraction of acetylacetone (x1) are shown in Figures 2 and 3, respectively. E

where ρ is the density of the binary system, x1, ρ1, M1 and x2, ρ2, M2 are the mole fractions, densities, and molecular weights of pure components 1 and 2, respectively. From the values of densities and the efflux times, the dynamic viscosities (η) of the liquid sample are calculated using the equation: η /ηw = ρt /ρw tw

(5)

(4)

where η, η1, and η2 represent the viscosities of the liquid mixture and the pure components 1 and 2, respectively, x1 and x2 are the mole fractions of pure components 1 and 2. The densities and viscosities of the mixture of AcAc with methanol, ethanol, 1-propanol, and 1-butanol at 303.15 K are presented in Table 4. Excess molar volume, VE, and viscosity 2405



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of the intramolecular hydrogen bond and movement to keto in the tautomeric equilibrium; in order to form intermolecular hydrogen bonds, enol turns into a more extended structure instead of the former compact structure, which reduces the structure discrepancy between two tautomers and thus the measurable D difference. It has been learned that the intermolecular hydrogen-bond forming interaction not only makes the intradiffusion coefficients of AcAc and methanol achieve the minimum values at x1 = 0.5 at the same time but also produces the most negative VE and nearly zero Δη, validating this kind of special interaction between unlike molecules from another aspect. With the length of the carbon chain of alkanol being longer, the polarity of the alkanol becomes lower and dispersion forces between the like molecules gradually replace the special interaction to be the overwhelming factor impacting D, VE, and Δη, respectively. Since a decrease in the polarity of alkanols weakens the ability of forming intermolecular hydrogen bonds with AcAc, there is a good chance for enol to form its own intramolecular hydrogen bond again, which favors the move toward enol, as well as enlarges the structure discrepancy of two tautomers and thus the difference of their diffusion behavior that can be explored by the NMR technique.

Figure 3. Viscosity deviations Δη for binary system (x1AcAc + (1 − x1) methanol/ethanol/1-propanol/1-butanol) at 303.15 K. ●, methanol; ○, ethanol; ▲, 1-propanol; Δ, 1-butanol.

alkanol), which produces a much more compact structure. Oppositely, the positive values in AcAc + 1-C3H7OH and AcAc + 1-C4H9OH show that interaction between like molecules overweighs interaction between unlike molecules.16 The VE values for AcAc + 1-C3H7OH is less positive than AcAc + 1C4H9OH, probably because the self-association of alkanols in propanol may be more easily destroyed than in 1-butanol when AcAc is added. As for viscosity deviations, Δη, all of the mixtures exhibit negative values and the values show the following decreasing order (see Figure 3):

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ASSOCIATED CONTENT

* Supporting Information S

NMR spectrum (Figure 1S), effect of interconversion rate on diffusion measurements, NMR experimental details (Figures 2S and 3S), and equation for calculation of keto−enol equilibrium constant (Figure 4S). This material is available free of charge via the Internet at http://pubs.acs.org.

AcAc + CH3OH > AcAc + C2H5OH > AcAc + 1‐C3H 7OH

■

> AcAc + 1‐C4 H 9OH

According to Tangeda et al.,47 dispersion and dipolar interaction contribute to produce negative values of Δη; whereas charge transfer, hydrogen bonding interactions, and other chemical forces leading to the formation of complex species between unlike molecules result in positive values of Δη. For methanol, Δη appears almost zero, which can be attributed to the dominating special interactions (hydrogen bond interactions) between unlike molecules. This kind of special interaction is gradually replaced by dispersion and dipolar interaction between alkanols in the mixture of AcAc with ethanol, 1-propanol, and 1-butanol, which makes negative contributions to Δη; and the higher the alkanol is, the more negative the contribution will be. Three types of interaction16 are illustrated when mixing AcAc and alkanols: (a) keto−enol, (b) keto−alkanol, and (c) enol− alkanol. We estimate in our work that in AcAc + 1-C3H7OH and AcAC + 1-C4H9OH, the keto−enol interaction gains far more predominant effects, while in AcAc + CH3OH and AcAc + C2H5OH, the above three interactions may simultaneously exist.

AUTHOR INFORMATION

Corresponding Author

*Phone: +8620 84115559. Fax: +8620 84112245. E-mail: [email protected]. Funding

The work has been funded by the National Natural Science Foundation of China (Grant No. 20173074) and the Natural Science Foundation of Guangdong Province (Grant 031583). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) El-Hendawy, A. M. Osmium(II) bipyridine (bpy) complexes containing O,O-donor ligands and X-ray crystal structure of the acetylacetonato (acac) complex [Os(bpy)2(acac)](PF6). J. Mol. Struct. 2011, 995, 97−102. (2) Giannini, F.; Devia, C.; Rodrıguez, A.; Enriz, R.; Suvire, F.; Baldoni, H.; Furlan, R.; Zacchino, S. The importance of keto-enol forms of arylpropanoids acting as antifungal compounds. Molecules 2000, 5, 580−582. (3) Rappoport, Z., Ed. The Chemistry of Enols; John and Wiley: New York, 1990. (4) Iijima, K.; Ohnogi, A.; Shibata, S. The molecular structure of acetylacetone as studied by gas-phase electron diffraction. J. Mol. Struct. 1987, 156, 111−118. (5) Boese, R.; Antipin, M. Y.; Blaeser, D.; Lyssenko, K. A. Molecular crystal structure of acetylacetone at 210 and 110 K: is the crystal disorder static or dynamic? J. Phys. Chem.B 1998, 102, 8654−8660. (6) Roubin, P.; Chiavassa, T.; Verlaque, P.; Pizzala, L.; Bodot, H. FTIR study of UV-induced isomerization of intramolecularly hydrogenbonded carbonyl compounds isolated in xenon matrixes. Chem. Phys. Lett. 1990, 175, 655−659.

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CONCLUSIONS Solvent dependence upon the intradiffusion coefficient D, the excess molar volume VE, and the viscosity deviation Δη have been completely studied in this work along with solvent influence on the enol−keto tautomeric equilibrium of AcAc and the difference of D between two tautomers. Methanol is capable of forming the strongest intermolecular hydrogen bond among four alkanols with both tautomers of AcAc, and the strength of the hydrogen bond greatly surpasses the intramolecular hydrogen bond formed by enol itself, resulting in destruction 2406



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Intradiffusion, Density, and Viscosity Studies in Binary Liquid Systems

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