Research Article | Open Access
Thermodynamic and Acoustic Study on Molecular Interactions in Certain Binary Liquid Systems Involving Ethyl Benzoate
Speeds of sound and density for binary mixtures of ethyl benzoate (EB) with N,N-dimethylformamide (NNDMF), N,N-dimethyl acetamide (NNDMAc), and N,N-dimethylaniline (NNDMA) were measured as a function of mole fraction at temperatures 303.15, 308.15 K, 313.15 K, and 318.15 K and atmospheric pressure. From the experimental data, adiabatic compressibility (), intermolecular free length (), and molar volume () have been computed. The excess values of the above parameters were also evaluated and discussed in light of molecular interactions. Deviation in adiabatic compressibilities and excess intermolecular free length () are found to be negative over the molefraction of ethyl benzoate indicating the presence of strong interactions between the molecules. The negative excess molar volume values are attributed to strong dipole-dipole interactions between unlike molecules in the mixtures. The binary data of , , and were correlated as a function of molefraction by using the Redlich-Kister equation.
The thermophysical study of esters is of increasing interest due to their wide range in flavouring, perfumery, artificial essences, and cosmetics. Esters are also important solvents in pharmaceutical, paint, and plastic industries . Recently, interest in the study of liquid mixtures containing esters as one of the components [2–5] has increased. These studies are of great significance because one can get information regarding structural changes that occur in pure ester because of mixing.
Among the different types of esters, ethyl benzoate (aromatic ester) is a polar solvent () and is not a strongly associated liquid. On the other hand, NNDMF, to some extent, is associated by means of dipole-dipole interactions. Significant structural effects are absent due to the lack of hydrogen bonds. Therefore, it acts as an aprotic protophilic solvent . NNDMAc is a dipolar aprotic solvent and is moderately structured . The N,N-dialkylamides have no significant intermolecular hydrogen bonding capability but are highly polar with high percentage of ionic character, making oxygen of C=O group strongly negative . NNDMA is a unassociated liquid . The interactions of ethyl benzoate with N,N-dialkylamides and N,N-dimethylaniline are important to enable analysis of their dissimilar geometric structures on mixture properties.
Rao  calculated the excess isentropic compressibilities for the binary mixtures of N,N-dimethylformamide and N,N-dimethylacetamide and various normal and branched esters. The negative excess values are attributed to the presence of n-n interactions between unlike molecules. Aminabhavi et al.  studied the physicochemical behavior of binary mixtures of ethyl benzoate with diethylene glycol and dimethyl ether. Rathnam et al.  reported the densities, viscosities, and refractive indices of ethyl benzoate with o-xylene, m-xylene, p-xylene, and ethyl benzene. Lien et al.  have measured the excess molar enthalpies of the binary mixture of ethyl benzoate with 1-Octanol at 298.15 K. Joshi et al.  measured the densities and viscosities of the binary mixture of bromoform with ethyl benzoate.
We report herein the experimental values of densities and ultrasonic velocities of mixtures of ethyl benzoate with NNDMF, NNDMAc, and NNDMA at temperatures 303.15, 308.15, 313.15, and 318.15 K covering the whole miscibility range to enable analysis of the effect of their dissimilar geometric structures on the mixture properties. The non-rectilinear behavior of ultrasonic velocity, compressibility, and other thermodynamic parameters of liquid mixtures reveal the strength of interactions. To get additional information about the nature and strength of molecular interactions, other parameters such as free length, adiabatic compressibility, and their excess parameters have been calculated in the liquid mixtures. These results were correlated with the Redlich-Kister polynomial equation  to derive the coefficients and standard deviation.
2. Materials and Experimental Details
Ethyl benzoate (SD Fine Chemicals, purity > 99%) was used without any further treatment. NNDMF, NNDMAc, and NNDMA were purchased from SD Fine or Merck and were purified by the recommended methods [16, 17]. Further, the purities were ascertained from their ultrasonic speed and density values at 303.15 K which agreed with the literature values (Table 1).
|Data taken from . bData taken from . cData taken from . dData taken from .|
Mixtures were prepared by mixing appropriate volumes of liquids in airtight bottles and weighed in a single-pan Mettler balance to an accuracy of ±0.001 mg. Preferential evaporation losses of solvents from mixtures were kept to a minimum as evidenced by repeated measurement of the physical properties over an interval of 2-3 days, during which no changes in physical properties were observed. The possible error in mole fractions is estimated to be around ±0.0001.
The densities of liquids and their mixtures were measured by a 25 mL specific gravity bottle, calibrated with redistilled water. The average uncertainty in measurement in the measured density is ±0.05 kgm−3. With the fluctuation of ±0.05 K, temperature was controlled by a water thermostat. Ultrasonic velocity was measured using the ultrasonic interferometer (Model M-83) provided by Mittal Enterprises, New Delhi. The values agree closely with the values given in the literature.
The experimental values of ultrasonic speeds (), densities (), adiabatic compressibilities (), molar volumes (), and free length () of pure ethyl benzoate, NNDMF, NNDMAc, NNDMA, and those of their binary mixtures over the entire composition range and at 303.15 K, expressed by mole fraction of ethyl benzoate, are listed in Table 2.
3. Results and Discussion
The experimental density () values of binary mixtures were used to calculate the excess molar volumes as where is the molar mass; subscripts 1 and 2 stand for pure components, ethyl benzoate and NNDMF, NNDMAc, and NNDMA, respectively. The uncertainty in is estimated to be within .
Assuming that ultrasonic absorption is negligible, adiabatic compressibility () can be obtained from the density and velocity of ultrasonic sound () using the relation Deviation in adiabatic compressibility was calculated using the relation where is the compressibility of the mixture.
Intermolecular free length () has been calculated using the relation where is Jacobson’s temperature-dependant constant and is equal to .
The excess intermolecular free length () at a given mole fraction is the difference between mean free length and the sum of the fractional contributions of the two liquids given by where and are the individual intermolecular free length values of pure liquids in the binary mixtures.
The experimental values of ultrasonic velocity () and values of density (), at the four temperatures, namely, K, 308.15 K, 313.15 K, and 318.15 K, along with the derived values of adiabatic compressibility (), intermolecular free length (), and molar volume () and their excess parameters are given in Tables 2, 3, and 4.
The excess properties were fitted to a Redlich, Kister-type  polynomial equation The optimum number of coefficients, , was ascertained from an examination of the variation of the standard deviation . The values of coefficients, , were evaluated by using the method of least squares, with all points weighted equally. The coefficients , , and along with standard deviations, of fit for all the mixtures are listed in Table 5.
In this investigation, the values of ultrasonic velocity decrease with increase in the concentration of ethyl benzoate and decrease with increase in temperature at any particular concentration for all the three systems (Figure 1). The ultrasonic velocity values decrease with increase of temperature due to the breaking of hetero-and homomolecular clusters at high temperatures . Lagemann and Duban  were the first to point out the ultrasonic velocity approach for qualitative estimation of the interaction in liquids.
The intermolecular free length is the distance between the surfaces of the neighboring molecules. The variation of ultrasonic velocity in a solution depends upon the increase or decrease of intermolecular free length after mixing the components. The interdependence of intermolecular free length and ultrasonic velocity was evolved from a model for sound propagation proposed by Kincaid and Eyring . The ultrasonic velocity should decrease if the intermolecular free length increases as a result of mixing of components. This fact is observed in the present investigation for ethyl benzoate + NNDMF, ethyl benzoate + NNDMAc, and ethyl benzoate + NNDMA systems. Figure 2 represents the variation of excess intermolecular free length with mole fraction of ethyl benzoate. The excess intermolecular free length values are negative, and the curves appear to reach negative peak value at about 0.45 mole fraction of ethyl benzoate. According to Ramamurthy and Sastry , the negative values indicate that sound wave has to travel a longer distance. This may be attributed to the dominant nature of interactions between unlike molecules.
The compressibility behavior of solutes, which is the second derivative of the Gibbs energy, is a very sensitive indicator of molecular interactions and can provide useful information about this phenomenon [26–28]. The structural change of molecules takes place due to the existence of electrostatic field between interacting molecules. The change in adiabatic compressibility value in liquids and liquid mixtures may be ascribed to the strength of intermolecular attraction. The effect of depolymerization increases the compressibility of the system . Electrostatic attraction and association decrease the compressibility. The compressibility of the mixtures is the result of these two effects depending upon the predominance . According to Jacobson [31, 32], the adiabatic compressibility can be studied more through intermolecular free length.
Figure 3 represents the deviation in adiabatic compressibility with the mole fraction of ethyl benzoate. In the present investigation, deviation in adiabatic compressibilities is found to be negative over the mole fraction of ethyl benzoate indicating the presence of strong interactions between the molecules. The negative deviation in adiabatic compressibility reaches a peak at about 0.45 mole fraction of ethyl benzoate in all the three systems chosen.
The deviation in adiabatic compressibility can be explained by taking into consideration the following factors.(a)Loss of dipolar association and difference in size and shape of component molecules which lead to decrease in velocity and increase in compressibility.(b)Dipole-dipole interaction or hydrogen-bonded complex formation between unlike molecules which lead to increase in sound velocity and decrease of compressibility.
The actual deviation depends on the resultant effect. The strength of the interaction between the components increases when excess values tend to become increasingly negative. This may be qualitatively interpreted in terms of closer approach of unlike molecules leading to reductions in compressibility and volume [33, 34]. This type of interactions for the binary mixtures has been already reported previously . The deviations in adiabatic compressibilities are found to increase with increasing temperature which is in agreement with the previously reported results . The and minima occur at the same concentrations further strengthen the occurrence of molecular associations .
The sign of of a system depends upon the relative magnitude of expansion and contraction of the two liquids due to mixing .
The negative arises due to dominance of the following factors.(a)Chemical interaction between constituent molecules such as heteromolecular association through the formation of H-bond, often termed as strong specific interaction.(b)Association through weaker physical forces such as dipolar force or any other forces of this kind.(c)Accommodation of molecules of one component into the interstitial positions of the structural network of molecules of the other component.(d)Geometry of the molecular structure that favors fitting of the component molecules with each other.
The values are negative over the entire mole fraction range and at all temperatures investigated for all binary systems under study (Figure 4). The observed negative values of for the three mixtures indicate the presence of specific interactions between ethyl benzoate and amide molecules. The negative values are attributed to strong dipole-dipole interactions between unlike molecules in the mixtures. The values are more negative for ethyl benzoate + DMAc than those for other two systems.
The electron density at oxygen atom of the carbonyl atom of DMAc is greater than that of DMF due to the presence of methyl group at carbon atom of carbonyl group in DMF resulting in stronger interaction in thesystem [39, 40]. The values of ethyl benzoate + DMA are in between these two. The strength of interactions between ethyl benzoate + DMA is stronger than ethyl benzoate + DMF due to the fact that negative charge on nitrogen in DMA is more than that of nitrogen in DMF due to conjugation with C=O group (this is also evident from the fact that the DMA is more basic than DMF).
Rao and Reddy  reported increased negative values of excess molar volumes with the increase in carbon chain length of the ester, in the binary mixtures of DMF, and aliphatic esters. The observed higher negative values of () for aromatic ester with DMF in the present investigation, when compared to that of ethyl acetate and DMF () at the same temperature (303.15 K), indicate much stronger interactions between the unlike molecules of components in this binary mixture due to the formation of not only dipole-dipole interactions but also of induced polar interaction like dipole-induced dipole and dipole interactions between aromatic ester and DMF. Similar interactions are reported in the mixtures of DMF and polycyclic aromatic hydrocarbons by Nikam and Kharat [42, 43] and by Ramadevi and Prabhakara Rao . Also, formation of induced polar interactions like dipole-induced dipole interactions between polycyclic aromatic hydrocarbons and NMP (acyclic amide) is reported by Sugiura and Ogawa . This further supports that amides react with aromatic compounds more strongly than the corresponding aliphatic counter parts.
The values decrease (become more negative) with increase in temperature for all the 3 systems. This is attributed due to the dissociation of self-associated amide molecules resulting in the more favorable fitting of the smaller molecules into large voids of bigger ethyl benzoate molecules leading to contraction in volume and hence resulting in more negative values with rise in temperature.
The adiabatic compressibility () and intermolecular free length () both have an inverse relationship with ultrasonic velocity (). Occurrence of maxima, , and minima at the same concentrations indicates the strong interaction through dipole-dipole interactions between the components. The negative values are attributed to strong dipole-dipole interactions between unlike molecules in the mixtures.
The authors sincerely thank the University Grants Commission, India, for funding the current research work under UGC Scholarship Assistance Program (SAP) in the Department of Physics, Andhra University.
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