Intradiffusion,density and viscosity studies in binary liquid systems of acetyla

Xiaojuan Chen ,Zhaoxun Lian ,Haimin Zhong ,Liuping Chen ,*

1 School of Chemistry and Chemical Engineering,Sun Yat-sen University,Guangzhou 510275,China

2 School of Chemistry and Chemical Engineering,Henan Institute of Science and Technology,Xinxiang 453003,China

Keywords:Intradiffusion coef ficient Density Viscosity Acetylacetone 1H DOSY PFG NMR method

ABSTRACT Intradiffusion coef ficients of acetylacetone(AcAc)and DMF/DMSO/benzene in binary systems over the entire concentration range at 303.15 K were determined by 1H diffusion-order spectroscopy(DOSY)nuclear magnetic resonance(NMR)method based pulse field gradient(PFG).The densities and viscosities ofthe above three binary systems at303.15 K were also studied and employed to calculate the excessmolarvolumes(V E)and deviations in viscosity(Δη).Besides,experiments were carried outat 333.15 K for the system ofAcAc+DMF.The solventand temperature effect upon the difference in D between enol and keto tautomers,the tautomeric equilibrium and excess properties(V E and Δη)were discussed as well.Isotherms of V E as a function of mole fraction of AcAc(x1)show positive deviations in benzene but negative deviations in DMF and DMSO,whereas isotherms of Δη as a function of x1 record positive deviations in DMF but negative in benzene and DMSO.V E values show more negative and Δη values are less positive in the system of AcAc+DMF at 333.15 K compared to 303.15 K.The V E and Δη were fitted to a Redlich-Kister type equation and the measured results were interpreted concerning molecular interactions in the solutions.

1.Introduction

As members ofβ-diketones,acetylacetone(AcAc)and its derivatives are always of great interest in experimental and theoretical studies for their importance being not only an excellent chelating reagent in synthesis chemistry but also a typical researching object of the keto-enol tautomerism[1-10].Many factors such as temperature,pressure,substituent groups and solvent properties,etc.are proved to affect the move of the keto-enol tautomerism[11].Besides,excess properties of binary mixtures containing AcAc are also studied,which shows different situations when AcAc dissolved in solvents with different polarities[12-18],but mixtures of AcAc with N,N-dimethyl formamide(DMF)and dimethyl sulfoxide(DMSO)have not been reported yet.All in all,most of researchers focused on thermodynamic properties of AcAc in the previous work,and few of them have paid attention to dynamic issues until Chen[19]et al.observed the different self-diffusion coef ficients of two tautomers in pure AcAc under variable temperatures and pressures by NMR method.

Based on the work mentioned above,the solvent in fluence upon the diffusive behavior of AcAc was further investigated[20].Intradiffusion coef ficients(D)of enol and keto tautomers of AcAc in four protonic solvents:methanol/ethanol/1-propanol/1-butanol were measured at 303.15 K by application of1H DOSY PFG NMR method.It is observed that enol diffuses more quickly than keto by 18%or so in pure AcAc for its compact structure by forming the intramolecular hydrogen bond.When dissolved in methanol,the structure is subjected to a great destruction because the strong intermolecular hydrogen bond is formed between AcAc and methanol,and the destruction is alleviated with the chain length of alkanol lengthening.What's more,differences of D between enol and keto are smaller than 18%in methanol while larger than 18%in 1-butanol due to different intermolecular interactions between AcAc and alkanols with different polarities.And the interactions are also verified by excess molar volumes(VE)and viscosity deviations(Δη)of the binary systems.

As continuation of our work on the solvent dependence,we present here the experimental data on D,VEand Δη when mixing AcAc with three aprotic solvents:DMF/DMSO/benzene.In addition,the temperature in fluence is studied for the system of AcAc+DMF.In a word,we are trying to explore differences of D between enol and keto tautomers of AcAc and to learn which type of molecular interaction would be responsible for these differences.

2.Experimental

2.1.Equipment and materials

Nuclear Magnetic Resonance Spectrometer(Mercury-plus 300;Varian,America),vibrating-tube densimeter(DMA 4500;Anton Paar,Austria),analytical balance(AL 204;Mettler Toledo,Switzerland)and Ubbelohde viscometer(Ф 0.55 mm;Shanghai Glass Instrument Factory,China)were used in experiments.

Dimethyl sulfoxide and benzene with mass purity of 99.9%were purchased from Alfa Aesar Chemical Company;acetylacetone and N,N-dimethyl formamide with mass purity＞99.5%were obtained from Guangzhou Chemical Reagent Factory.All above reagents were directly used without further puri fication.

2.2.Intradiffusion experiments

In the DOSY PFG NMR experiments[21],self-diffusion coef ficients(intradiffusion coef ficients)ofeach componentare calculated according to the Stejskal-Tanner formula[22]:

where S(Gzi)and S(0)are the signal intensities obtained with gradients Gziand 0,respectively,D is the intradiffusion coef ficient,γ is the gyromagnetic constant,δis the gradientpulse duration andΔis the diffusion delay.The sequence used in our DOSY experiments is a one-shot pulse sequence[23].

The sample was filled into a capillary(o.d.1.2 mm,i.d.1 mm)with both ends sealed,and the capillary was inserted and fixed in the middle of 5 mm sample tube full of heavy water with mass 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 non-spinning during the experiment.

The experimental procedure and parameter setting on the NMR spectrometer have been fully illustrated in our previous paper[20].The temperature was controlled to±0.1 K using the air-bath controller on the NMRspectrometer.The overallerrorofintradiffusion coef ficients was estimated to±5%,and the reproducibility was better than±2%.The measured D of each component and the equilibrium constant,Ke,calculated from the experimental data were listed in Tables 1 and 2.

Table 1 Intradiffusion coef ficients(D)of DMF and tautomers of acetylacetone(AcAc)and equilibrium constants(K e)between tautomers at different AcAc concentrations and temperatures,p=0.1 MPa

2.3.Density and viscosity experiments

Densities of the pure liquids and mixtures were measured with an Anton Paar DMA 4500 vibrating-tube densimeter in which thetemperature was controlled automatically within±0.01 K.The apparatus was calibrated with the deionized doubly distilled water and dry air.The uncertainty and reproducibility of density measurements were±5×10-5g·cm-3and 1 × 10-5g·cm-3,respectively.

Table 2 Intradiffusion coef ficients(D)of DMSO/Benzene and tautomers of acetylacetone(AcAc)and equilibrium constants(K e)between tautomers at different AcAc concentrations at 303.15 K,p=0.1 MPa

All binary mixtures were prepared by mass on an analytical balance(AL 204;Mettler Toledo,Switzerland)with an uncertainty of±1×10-4g.The mole fraction of each mixture was obtained with an uncertainty of±2×10-4from the measured masses of the components,and all the mixtures were completely miscible over the whole composition range.The excess molar volumes were calculated from composition-density data with an uncertainty better than±0.002 cm3·mol-1.

Dynamic viscosities of pure liquids and mixtures were measured using an Ubbelohde viscometer which was calibrated with the distilled water(Marsh,1987).The apparatus was submerged in a thermostatic bath at the required temperature with a resolution of±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,and the results were averaged.Since all flow times were suf ficiently slow and the capillary radius(0.55 mm)was far less than its length(105 mm),the kinetic energy and end corrections were found to be negligible,respectively.The dynamic viscosities ofthe liquid sample are calculated by the following equation[24]:

where ρ,ρwand t,tware densities and flow times of the liquid sample and the distilled water,respectively,ηwis the dynamic viscosity of the distilled water at 303.15 K or 333.15 K.The uncertainty of viscosity results was estimated within ±0.003 mPa·s.

Densities and dynamic viscosities measured for pure DMSO,DMF and benzene were compared to literature values[25-29]listed in Table 3.

Table 3 Comparison of experimental densities,ρ,and dynamic viscosities,η,of pure liquids with literature values

3.Results and Discussions

3.1.Intradiffusion studies

3.1.1.Intradiffusion coef ficients

Intradiffusion coef ficients of components in allmixtures as the function ofthe mole fraction of AcAc(x1)were plotted in Fig.1.According to the Stokes-Einstein equation[30],given the temperature(T)to be constant,the self-diffusion coef ficient(D)of solute is determined by the viscosity of mixture(η)and the hydrodynamic radii of diffusing molecule(rH)that represented by the molecular interaction,and both η and rHare inversely proportional to D.

DMSO and DMF are both aprotic solvents with high polarities,so both of them own good abilities of accepting proton from hydrogen bond proton donors to form intermolecular hydrogen bonds.

For the mixture of AcAc+DMSO,it is shown in Table 4 that the viscosity of the mixture drops so dramatically that it becomes a predominantfactor leading to the increase of D of all components,comparing to the molecular interaction between DMSO and AcAc.An interesting point found in Fig.1(b)is that the intradiffusion coef ficient of enoltautomer is nearly the same as thatofDMSO,itseems thatthey diffuse together as a solute-solventcomplex due to the strong intermolecular interaction between them,which is quite similar with the situation in the mixture of AcAc+methanol in our previous work[20].

While for AcAc+DMF system,D of each component and viscosities of mixture do not display the contrary changing tendency like AcAc+DMSO,for the modi fication in viscosity is not so remarkable to gain the predominance to determine D.Instead,the intermolecular interaction especially the intermolecular hydrogen bonding interaction has played a more importantrole.We can see from Fig.1(a)thatthere is a minimum D for enol and keto tautomer respectively at x1=0.3,but a maximum value for DMF at this point.It has been studied that the dipole-dipole interactions in pure DMF are very strong and a dramatic local ordered structure[31,32]has been contained.But the addition of AcAc has greatly destroyed the structure,and thus set more DMF molecules free from the self-associations in itself,leading to the increase in D ofDMF in the lowerconcentration region.Contrarily,the behaviorofthe AcAc seems totally different.On one hand,DMF forms strong intermolecular hydrogen bonds with enol,which de finitely damages the more compact structure of enol formed by its intramolecular hydrogen bonding;on the other hand,DMF has the dipole-dipole interaction with keto,which has an effect to increase rHof keto,so D of both tautomers decrease.We also find that x1=0.3 should be a turning point,after that point,D of both tautomers increase and that of DMF decreases oppositely.Thatis probably because when more and more AcAc molecules accumulate in system,the intermolecular interaction will be weakened due to the crowded environment in the solution.

It presents different situation for AcAc+DMF system at 333.15 K.First of all,all components have greater D than at 303.15 K,mainly because the higher temperature accelerates the molecular motion;secondly,the climax of D of DMF moves from the point x1=0.3 to 0.5 although the similarity of the changing trend,for viscosities of mixture appear nearly no change until x1=0.5 and the intermolecular interaction has the preponderance over viscosity modi fication to determine D of components;besides,we can see from Fig.1(d)that the peak of DMF looksboarderthan 303.15 K,probably because the dominantintermolecular hydrogen bonding at303.15 Kis replaced by the combination of interactions such as hydrogen bonding interactions,dipole-dipole interactions between tautomers,self-associations in DMF etc.Lastly and most importantly,minimums of D of tautomers at 303.15 K disappear at 333.15 K,and D of enol tautomer keeps increasing with the addition of AcAc while the situation of keto appears a little more complicated.

Fig.1.Intradiffusion coef ficients of DMF/DMSO/Benzene and tautomers of AcAc in binary systems containing:(a)DMF,(b)DMSO,(c)benzene at T=303.15 K,and(d)DMF at T=333.15 K,p=0.1 MPa.Δ,solvent;■,enol;●,keto.

Table 4 Densities,excess molar volumes,dynamic viscosities,and viscosity deviations with the mole fraction ofAcAc(x1)for the binary system(x1 AcAc+(1-x1)DMF/DMSO/Benzene)at 303.15 K and 333.15 K

Benzene is a solvent different from DMSO and DMF for its weak polarity.We can see from Fig.1(c)that,almost all D values of components decrease with AcAc increasing.This tendency is mainly induced by two factors.One is the modi fication of viscosities in mixtures,which seems to be obvious in the lower concentration range,the other might be certain interaction between AcAc and benzene,which is gaining more dominance in the higher concentration range.Some researchers[33]have suggested by calculation that benzene comes much closer to molecules with the higher polarity and interacts more strongly with them,and according to some experimental results[34],the attraction may exist between the π-electrons of benzene and hydrogen atoms of water in a hydrogen-bond-like con figuration.In our study for AcAc+benzene,we estimate that this sort of con figuration may also exist between benzene and enol tautomer.

3.1.2.Intradiffusion coef ficient difference

The ratio(Denol/Dketo)T,from which the difference in D of enol and keto tautomer derives,in our work is 1.18 at 303.15 K and 1.13 at 333.15 K for neat liquid AcAc,namely,the enol tautomer diffuses more quickly than keto by 18%at 303.15 K and 13%at 333.15 K,respectively.The difference in D at 333.15 K is learned smaller than that at 303.15 K,indicating that the higher temperature may destroy the intramolecularhydrogen bond ofenoltautomerand shorten the discrepancy of D of two tautomers more easily.As far as the solventin fluence is concerned,differences in D between two tautomers are measured below 18%in the most of concentrations at 303.15 K(except for x1=0.7 and 0.9 in DMF,x1=0.9 in DMSO)and below 13%in all concentrations at 333.15 K.It is proposed that three solvents are capable of forming intermolecular hydrogen bonds with enol tautomer more or less,which causes the destruction to intramolecular hydrogen bonds of enol tautomer and then decreases the structural discrepancy between two tautomers.To be speci fic,DMF and DMSO can undoubtedly accept the hydroxy proton from enol tautomer easily due to their great polarities,while benzene,possessing a considerable molecular quadrupole moment,is also able to come close to enol tautomer and form a hydrogen-bond-like con figuration.

3.1.3.Enol–keto equilibrium

The tautomeric equilibrium constant,Ke,is calculated by the equation:Ke=[keto]/[enol],where[keto]and[enol]denotes the concentration of keto and enol tautomer,respectively.As is shown in Tables 1 and 2,Keis decreasing in the following order:DMF(333.15 K)＞DMSO(303.15 K)＞DMF(303.15 K)＞benzene(303.15).With regard to the temperature affect,the higher temperature will destabilize the enol tautomer greatly by making destruction to its intramolecular hydrogen bond,and move the equilibrium to the keto tautomer and consequently produces larger Ke.Regarding the solvent affect,the polarity descending order is as follows:DMSO＞DMF＞benzene,and it is obvious that Kedecrease in the same order as the polarity of solvents.It is known that the solvent with higher polarity not only disrupts the intramolecular hydrogen bonds of enol but also stabilizes keto by electrostatic interactions,which undoubtedly increases the fraction ofketo in AcAc and finally results in larger Ke.As forthe concentration in fluence,Keincreases with dilution of AcAc in the solvent with the higher polarity,like DMSO and DMF,because the more content of this solvent exists in solution,the more effect on destruction of the stability of enol will be,and Kedecreases with dilution in solvent with the lower polarity like benzene for a similar reason.

3.2.Excess properties studies

The excess molar volume(VE)is evaluated from the density using the equation:

where x1and x2are the mole fractions,M1and M2are the molar masses,and ρ1and ρ2are the densities of pure components 1 and 2,respectively.The viscosity deviation(Δη)is calculated by the equation:

where η1, η2are viscosities of components 1 and 2,respectively.Quantities without subscripts refer to the mixture.Measured densities,dynamic viscosities,together with calculated excess molar volumes and viscosity deviations of binary systems are presented in Table 4.

Excess molar volumes and viscosity deviations have been fit to the Redlich-Kister equation[35]:

where yEdenotes either VEorΔη,x1is the mole fraction ofcomponent1,and Aiare the fitting coef ficients determined by the least-squares method.Coef ficients that best fit the experimental results are summarized in Table 5.The standard deviation reported in Table 5 is computed by the following expression[36]:

where n is the number of experimental points and m is the number of coef ficients,Aiin Eq.(5).

Excess molar volumes(VE)and viscosity deviations(Δη)are expected to show molecular interactions of binary mixtures from another aspect.It was suggested[37]that VEis the result of contributions fromseveral 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.From Fig.2,the negative deviations of VEfor AcAc+DMF and DMSO indicate that the chemical forces have the dominance,while the positive deviations for AcAc+benzene show the predominance of the dispersion force.It is also observed that the negative deviations of VEslightly decrease with the increased temperature in AcAc+DMF,probably because the effect of volume contraction induced by the selfassociation of DMF overweights the effect of volume increase resulted from the breakdown of intermolecular hydrogen bonds between AcAc+DMF at the higher temperature.

Fig.2.Excess molar volumes V E for the binary system(x1 AcAc+(1-x1)DMF/DMSO/Benzene)at 303.15 K:,DMSO and 333.15 K:○,DMF.

According to Tangeda and Nallani[38],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.The actual values depend upon the dominantfactor.Fig.3 shows thatthe dominance ofdispersion interaction in mixture of AcAc+benzene gives a negative contribution to Δη and the hydrogen bonding interactions or other chemical forces in AcAc+DMF give a positive contribution to Δη.While for AcAc+DMSO,the deviations of Δη and VEboth appear negative(Fig.2 and 3),not completely following the general rule.It is estimated that the non-interaction part[39]such as the molecular size or the molecular shapes changing,etc.might be another important factor to in fluence Δη in AcAc+DMSO.

4.Conclusions

Fig.3.Viscosity deviations Δη for the binary system(x1 AcAc+(1-x1)DMF/DMSO/Benzene)at 303.15 K:,DMSO and 333.15 K:○,DMF.

The combination of transport properties including diffusion and viscosity and the thermodynamics property in the form of VEcan give an overall understanding about intermolecular interaction between AcAc and solvents.And both the solvent and temperature in fluence have been taken into consideration.

DMF and DMSOare aprotic solvents with high polarity whereas benzene is a kind of non-polar solvent.The strong molecular interaction plays the predominant role to determine D values of components in DMF,while the modi fication ofviscosity replaces molecularinteractions to take primary responsibility for the change of D in DMSO.The hydrogen-bonding-like con figuration is estimated to form in benzene,so the modi fication of viscosity and the certain intermolecular interactions might have roughly equivalent effects on D.The difference in D between enol and keto tautomer,and the tautomeric equilibrium constant also show solvent dependence.The polarity of solvent greatly determines the strength of intermolecular hydrogen bonds between enoland the solventand consequently determines the extentofdestruction to intramolecular hydrogen bonds in enol.The destruction has not only reduced the structural discrepancy between enol and keto,but also destabilized the enol and moved the equilibrium towards keto.

Furthermore,the dominance of special interactions in AcAc+DMF results in the negative VEand positive Δη,while the predominance of dispersion force in AcAc+benzene gives positive VEand negative Δη reversely.But both VEand Δη exhibit negative values in DMSO,not completely complying the general rule due to the size effect.

From inspection upon system of AcAc+DMF,the higher temperature is proved to disrupt the intramolecular hydrogen bonds of enol more intensely and then reduce the discrepancy of D between two tautomers to a smaller level,moreover,itmoves the tautomeric equilibrium towards keto more easily and produces more negative VEand less positiveΔη,from which we can conclude thatthe stabilization ofhydrogen bonds are greatly diminished under high temperature.

Nomenclature