Isentropic Compressibility for Binary Mixtures of Propylene Carbonate with Benzene and Substituted Benzene

Abstract

Ultrasonic velocities (u) for binary mixtures of propylene carbonate (PC) (1) with benzene and substituted benzenes (2) viz. benzene, ethylbenzene, o-xylene and p-xylene have been measured at 288.15-308.15 K over the entire range of composition. The experimental values of ultrasonic velocities (u) have been utilized to calculate isentropic compressibility ($K_s$), intermolecular free length ($L_f$), acoustic impedance (Z).

INTRODUCTION

Ultrasonic techniques have been applied for long years to get information concerning molecular interaction. The review of literature on acoustical studies of solutions reveals that ultrasonic measurements are used to estimate the different elastic properties of the molecule from which the type of molecular interactions can be very well understood. Ultrasonic velocity has proved to be useful in understanding the physico-chemical behaviour of the particular system. Ultrasonic velocities have been very widely used to study binary liquid mixtures.1

The present paper is a part of our ongoing research on thermodynamic properties of liquid-liquid mixtures.2-7 This paper includes ultrasonic velocities of binary mixtures of PC (1) with benzene, ethylbenzene, o-xylene and p-xylene at 288.15-308.15 K over the entire range of mole fractions. The experimental values of ultrasonic velocities along with densities6 are used to calculate the values of isentropic compressibility (Ks), intermolecular free length (Lf), acoustic impedance (Z). The variation of these parameters with mole fraction is used to interpret the intermolecular interactions present amongst the liquid components.

PC is an aprotic solvent and behaves as a normal polar liquid with dipole-dipole interactions, but with little or no specific associations.8 The aromatic hydrocarbons viz. benzene, ethylbenzene, and xylenes are non-polar compounds with no measurable dipole moments. Thus they involve weak intermolecular interactions. The choice of these solvents was done because of their opposite nature of polarity and their wide range of applicability.

 

EXPERIMENTAL

Chemicals

PC (Merck, >99%) was refluxed over anhydrous calcium carbonate and distilled at atmospheric pressure.9 Benzene, ethylbenzene, o-xylene and p-xylene (all of S. D. Fine chem., Pvt. Ltd.) were distilled at atmospheric pressure. All the liquids were double distilled. The middle fraction collected of all the liquids was stored over 4Å molecular sieves.

Procedure

The binary liquid mixtures were prepared by mixing known masses of pure liquids in airtight-stoppered bottles in order to minimize the evaporation losses. All measurements of mass were performed on a Mettler one pan balance (E-Mettler, Zurich) which can be read up to the fifth decimal place with an accuracy of ±0.05 mg.

The ultrasonic velocities were measured at 1 MHz with a single crystal variable path interferometer (F-81 Mittal Enterprises, New Delhi) and the accuracy of ultrasonic measurement was ±0.1%. For all the measurements, the temperature was controlled by using an ultra thermostat Julabo F-25 (made in Germany) which has an accuracy of ±0.02 ℃.

 

RESULTS AND DISCUSSION

The compressibility (Ks) is the ease with which a system can be compressed. Larger the compressibility value, larger the system can further be compressed, and also denotes that more space is available between the components. The values of isentropic compressibility are calculated using the equation,10

Where u is ultrasonic velocity and ρ is density of the mixture.

Table 1.Values of ultrasonic velocities (u), isentropic compressibilities (Ks), acoustic impedance (Z), and intermolecular free length (Lf) for the binary mixtures of PC (1)+benzene (2) at 288.15-308.15 K

Table 2.Values of ultrasonic velocities (u), isentropic compressibilities (Ks), acoustic impedance (Z), and intermolecular free length (Lf) for the binary mixtures of PC (1)+ethylbenzene (2) at 288.15-308.15 K

Table 3.Values of ultrasonic velocities (u), isentropic compressibilities (Ks), acoustic impedance (Z), and intermolecular free length (Lf) for the binary mixtures of PC (1) + o-xylene (2) at 288.15-308.15 K

Table 4.Values of ultrasonic velocities (u), isentropic compressibilities (Ks), acoustic impedance (Z), and intermolecular free length (Lf) for the binary mixtures of PC (1)+p-xylene (2) at 288.15-308.15 K

Free length (Lf), which is an intermolecular property defined as the distance between the surfaces of the molecules, are evaluated from the values of compressibility employing equation,

Where K' is the Jacobson’s constant, which is temperature dependent and equal to (93.875+0.375T)×10-8.

These two parameters basically depend on the interaction phenomenon and they represent the strength of the interaction present in the component molecules.11,12

Kinsler et al.13 have suggested that acoustical impedance (Z) is more significant parameter to describe the medium and the intermolecular interactions than the ultrasound velocity and density individually. The acoustical impedance is product of ultrasonic velocity and density and evaluated for all the binary mixtures using following equation,

Where u is ultrasonic velocity and ρ is density of the mixture.

The evaluated values of Ks, Lf and Z along with the experimentally determined values of u are presented in Tables 1-3 and 4 for the binary mixtures at 288.15-308.15 K. Figs. 1, 2, 3 and 4 show the graphical variation of u, Ks, Lf and Z with increasing mole fractions of PC for all the binary systems at 298.15 K.

As can be seen from Tables 1-4 for all the binary systems studied, the values of ultrasonic velocity increase with increasing mole fraction of PC and decrease with increasing temperatures.

From Fig. 1, it can be noted that the ultrasonic velocity (u) values follow the order benzene < p-xylene < ethylbenzene < o-xylene at lower mole fractions of PC (i.e. up to 0.2 mole fraction). But as the mole fraction of PC increases the values for ethylbenzene and benzene are observed superimposed upon each other along with the lowest values of p-xylene and highest that of o-xylene.

From Tables 1-4, the values of isentropic compressibility (Ks), decrease with increasing mole fraction of PC and increase with increasing temperatures for all the four binary systems studied.

Fig. 1.Variation of ultrasonic velocities, u for binary mixtures of PC (1) + benzene (◆), ethylbenzene (■), o-xylene (▲) and p-xylene (×) at 298.15 K.

Fig. 2.Variation of isentropic compressibilities, Ks for binary mixtures of PC (1) + benzene (◆), ethylbenzene (■ ), o-xylene (▲) and p-xylene (×) at 298.15 K.

From Fig. 2, it can be noted that the isentropic compressibility values for benzene, ethylbenzene and p-xylene are superimposed upon each other along with the lowest values of o-xylene (up to 0.5 mole fraction of PC), which are also getting superimposed on to the other three systems at higher mole fractions of PC.

The values of intermolecular free length (Lf) follow the same trend as isentropic compressibility. Thus the Lf values decrease with increasing mole fraction of PC and increase with increasing temperatures for all the four binary systems studied.

From Tables 1-4, the values of acoustic impedance (Z) increase with increasing mole fraction of PC and decrease with increasing temperatures for all the four binary systems studied.

From Fig. 4, it can be noted that the acoustic impedance values are superimposed on each other for all the binary systems studied. Up to 0.4 mole fractions the values for o-xylene are higher and can be distinguished but later on with higher mole fractions these values are also getting superimposed on other system values. Hence no definite trend could be explained.

Fig. 3.Variation of intermolecular free lengths, Lf for binary mixtures of PC (1) + benzene (◆), ethylbenzene (■ ), o-xylene (▲) and p-xylene (×) at 298.15 K.

Fig. 4.Plot of acoustic impedances Z for binary mixtures of propylene carbonate (1) + benzene (◆), ethylbenzene (■), o-xylene (▲) and p-xylene (×) at 298.15 K.

All the four hydrocarbons studied in the present investigation are not having much structural differences. Ethylbenzene, o- and p-xylene are in fact isomers of each other. Because of the less structural differences not much difference in the interactions is seen and the parameters studied are having superimposed values.

In the present investigation, the decrease in Ks and Lf values represent less possibility of compression of the system and less space is available between the components. PC is having less or no self association. As the mole fraction of PC increases there is no much interspaces available between two molecules.

The small interaction observed may be because of the interaction of carbonyl group in PC with the pi electron cloud of the hydrocarbon systems. The carbonyl group in PC is having n as well as pi electrons. In the present investigation the possibility of n-pi interactions between the carbonyl group of PC with the electron cloud of benzene can be predicted.

The molar volumes of the components studied at 298.15 K are 85.25 cm3 mol-1 for PC and 89.42, 123.92, 123.10, 121.23 cm3 mol-1 for benzene, p-xylene, ethylbenzene and o-xylene respectively. The interstitial accommodation because of differences between the molecular sizes of the mixing components might be another factor behind the interaction observed. The same thing is explained earlier for the same systems studying excess molar volumes and viscosity deviations by our group.6

The substitution of methyl or ethyl group increases the electron donor-acceptor interaction. With substitution of ethyl group in ethylbenzene, the inductive effect (electron donating) of ethyl groups might be increasing the electron density of the hydrocarbon and thus increasing the repulsive interaction between the pi-electrons of the carbonyl group with those of aromatic hydrocarbons, thus giving rise to weaker interactions in these systems. The same is the case in o-xylene mixtures also. The presence of two methyl groups ortho to each other have greater electron donating effect than that present in p-xylene in which the two methyl groups are in para position.

Thus electron donor-acceptor interaction, n-pi interaction and structural effects are observed for these systems but the magnitude of these effects is very small.