Derived Thermodynamic Properties of Binary Mixtures of m-Xylene , o-Xylene

Experimental data on dynamic viscosities and densities of binary mixtures of N,Ndimethylformamide (DMF) and o-xylene, m-xylene and p-xylene were measured at 293.15, 303.15, 313.15, and 323.15 K. Computed data from experiment for deviations in viscosity, excess molar volume and excess Gibbs energy of activation of viscous flow were correlated with Redlish-Kister polynomial equation. Grundberg-Nissan interaction constant (d'), Hind, Frenkel and a modified Kendall-Monroe equation (Eηm) were determined quantitatively. Deviation in viscosity were both negative and positive while excess molar volume and excess Gibbs energy of activation of viscous flow were all positive. These results have been interpreted on the basis of intermolecular interactions between unlike molecules.


INTRODUCTION
Intermolecular interactions have continued to capture the interest of chemists because of its important role in many chemical and biological systems. 1 Solvent systems are frequently used as media for many chemical, industrial and biological processes in that they provide a wide range of desired properties. 2 Properties such as density and viscosity at several temperatures both for pure chemicals and their binary liquid mixtures over a whole composition range are useful for a full understanding of their thermodynamic and chemical engineering purposes. 3Experimental data and excess thermodynamic properties of liquid mixtures are necessary in drawing information on the structure and interactions of liquid mixtures. 42][3][4][5][6][7] In the pure state, N,N-dimethylformamide show association through dipole-dipole interaction. 7It finds applications in polymer science and pharmaceutical Industries and serves as a model compound of peptides to obtain information on protein systems. 6Formamides also find application in industrial processes that involve their use as synthetic intermediates in the preparation of fungicides and polymers such as polyacrylonitrile. 8lenes are frequently utilized as raw materials in many organic syntheses.High-purity xylenes are difficult to separate from common distillation operations, as it forms several binary azeotropes.N,N-dimethylformamide has been widely used as a selective extractant solvent for xylene separation from naphtha feed. 1 Measurements of densities, viscosities, speeds of sound and other thermodynamic properties of binary mixtures of o-, m-, p-xylene and isopropyl benzene and 2-butanone, 9 pyridine, 10 1-decanol, 11 N,Ndiethylformamide, 8 acetic acid, 12 methylformate 13 have been studied.
There are apparently no studies in the literature of thermodynamic studies of densities, viscosities and excess properties of the xylenes with N,N-dimethylformamide at the temperatures 293.15,  303.15,313.15 and 323.15 K. 1,7 We have studied o-, m-, p-xylene with pyridine at 293. 15, 303.15,313.15 and 323.15K to determine inter-and intra-molecular interactions that exits in their binary mixtures 10 In this study, Experimental viscosities and densities are reported at four temperatures 293.15, 303.15, 313.15 and 323.15K for binary mixtures of m-xylene, o-xylene and p-xylene with N,N-dimethylformamide.Deviation in viscosity (∆η), excess molar volume (V E ) and excess Gibbs free energy of activation of viscous flow (∆G *E ) have been calculated from the density (ρ), and viscosity (η), data.Modified Kendall-Monroe 10,14,15 equation with no parameters has been used in correlating viscosity data of the binary mixtures.Calculated deviation in viscosity and excess functions were fitted to the Redlich-Kister 16 polynomial equation and the results analyzed in terms of molecular interactions.

Mixture preparation
Binary mixtures were prepared by weighing appropriate amounts of N,N-dimethylformamide and xylene on an electronic balance.An AE Adam balance (Adam Equipment Inc. USA) model PW124 with a maximum capacity of 120 g, a readability range 0.0001 g and repeatability (S.D.) of 0.00015 g, linearity 0.0002 g, operating temperature +10 o C to 40 o C was used in all measurements.

Density measurement
Density measurements were carried out with an Anton Paar DMA-4500 M digital densitometer thermostatted at different temperatures.Two integrated Pt 100 platinum thermometers were provided for good precision in temperature control internally (T ± 0.01 K).The densitometer protocol includes an automatic correction for the viscosity of the sample.The calibration for temperature and pressure was made by the producer.The apparatus is precise to within 1.0x10 -5 g/cm 3 , and the uncertainty of the measurements was estimated to be better than ± 1.0x10 -4 g/cm 3 .Calibration of the densitometer was performed at atmospheric pressure using doubly distilled and degassed water.

Viscosity measurement
Viscosity measurements were carried out using Anton Paar SVM 3000 Stabinger Viscometer.The viscometer has a dynamic viscosity range of 0.2 to 20 000 mPa.s, a kinematic viscosity range of 0.2 to 20 000 mm 2 /s and a density range of 0.65 to 3 g/ cm 3 .The instrument is equipped with a maximum temperature range of +105 o C and a minimum of 20 o C below ambient.Instrument viscosity reproducibility is 0.35% of measured value and density reproducibility of 0.0005g/cm 3 .The values of V E , ∆η and ∆G* E were correlated by a Redlich-Kister 16 type polynomial, equation 5.

RESULTS AND DISCUSSION
... (5)  The values of the parameters A k , are obtained by fitting the equation to the experimental values with the least-squares method.The correlated results for excess molar volume, viscosity deviation and excess Gibbs free energy of activation of viscous flow are presented in Table 3.The standard deviation s(∆Y) is calculated from equation 6.
The correlating ability of equations 8, 9, 10 and 12 were tested by calculating the average percentage deviations (APD) between the experimental and the calculated viscosity as shown in equation 14.
... (13)  where η expt and η calc represent the viscosity of experimental and calculated data, N is the number of experimental data points.1(a-c).Viscosity deviations for N,N-dimethylformamide (DMF) with o-xylene and p-xylene are positive while that of N,Ndimethylformamide (DMF) and m-xylene were found to be both positive and negative.Deviation in viscosity are related to molecular interactions such as hydrogen bonding, charge transfer interactions and physical interactions of dispersion forces or weak dipole-dipole interactions between compounds in a mixture. 23The reason why a positive V E and a negative ∆η may result in a binary system is because of a disruption in associated molecules while a negative V E and positive ∆η is caused by an association or compound formation between components. 24In a system where positive values of Dh are observed, specific interactions cause complex formation.When negative values of ∆η are observed, dispersion forces are therefore dominant. 23In our system, positive deviations in Dh show a complex formation between N,Ndimethylformamideand o-xylene, m-xylene and p-xylene while negative deviations in ∆η, have resulted as a consequence of disruption in hydrogen bonding and increase in temperature as observed by other researchers. 24N,N-dimethylformamide has a lone pair of electron on the oxygen atom making it an electron donor while the xylenes on the other hand are electron acceptors.It is therefore possible for hydrogen bonding interactions between N,Ndimethylformamide and the xylenes to occur resulting in complex formation. 8e plots of excess molar volume (V E ) against mole fraction at 293. 15  2 (a-c).Excess parameters associated with a liquid mixture are a quantitative measure of deviation in the behavior of the liquid mixture from ideality. 10These functions are found to be sensitive towards the intermolecular forces and also on the difference in size and shape of the molecules.Excess volumes of liquid mixtures reflect the result of different contributions arising from structural changes undergone by the pure cosolvent.Positive contributions arise from breakup interactions between molecules namely, the rupture of hydrogen bonded chains and the loosening of dipole interactions. 25xcess molar volume were found to be both positive and negative for the binary system between N,Ndimethylformamide and o-xylene, figure 2 3(a-c).Excess properties provide information about the molecular interactions and macroscopic behavior of fluid mixtures which can be used to test and improve thermodynamic models for calculating and predicting fluid phase equilibria. 10The magnitude of ∆G* E represents the strength of interaction between unlike molecules. 26xcess Gibbs free energy of activation of viscous flow are positive at all experimental temperatures.A c o m p a r i s o n o f e x p e r i m e n t a l thermodynamic data of multicomponent mixtures with that calculated by means of various predictive methods is very useful from different points of view: (i) it suggests which model is more appropriate to the characteristics of the system, (ii) it may indicate which parameters should be improved when the model involves group contributions and (iii) it may allow the identification of some model as a convenient reference for the interpretation of the deviations observed. 10,27The viscosity data have been correlated with semi-empirical equations of modified Kendall and Monroe, Frenkel, Hind, and Grunberg-Nissan.The values of the Grunberg and Nissan constant (d¹) and modified Kendall-Monroe (Eh m ) for all systems under study are presented in table 2. Grunberg-Nissan interaction parameters are both positive and negative while the modified Kendall-Monroe viscosity correlation data are all positive.Plots for the modified Kendall-Monroe viscosity correlation are presented in Figure 4(ac).Plots of modified Kendall-Monroe viscosity correlation at different temperatures show a decrease in viscosity with increase in temperature.The value of Frenkel and Hind are presented in Table 4. Positive and negative Grunberg-Nissan parameters indicate the presence of both strong and weak interactions between unlike molecules. 28

CONCLUSION
and excess Gibbs free energy of activation of viscous flow ∆G* E , for (m-xylene + N,N-dimethylformamide), (o-xylene + N,N-dimethylformamide), and (p-xylene + N,N-dimethylformamide) as a function of mole fraction.To investigate the molecular interaction between N,N-dimethylformamide and m-xylene, o-xylene and p-xylene, viscosity deviation, ∆h, excess molar volumes V E and excess Gibbs free energy of activation of viscous flow, ∆G* E , have been evaluated from experimental density and viscosity using the equations 1 and 2 respectively....(1) ...(2) Wherex 1 and x 2 are the mole fractions calculated from mass fractions.M 1 and M 2 are molar masses, ρ 1 and ρ 2 are densities, η 1 and η 2 are viscosities of pure components 1 and 2 respectively.ρ m and η m are the density and viscosity of the mixture.The excess Gibbs free energy of activation of viscous flow were obtained from equation 3. ...(3) whereR is the universal constant of gases, T is the absolute temperature, V 1 and V 2 are the molar volumes of component 1 and 2, x 1 and x 2 represents the mole fraction of component 1 and 2. V m is obtained from equation 4. η 1 , η 2 and h m are the viscosity of component 1 and 2 and mixture respectively....(4)

Table 1 :
Kendall and Monroe 14 derived equation 7 for analyzing the viscosity of binary mixtures based on zero adjustable parameter....7) ...(8) where Eη m is a modified Kendall-Monroe 10,15 equation, 8.The predictive ability of some selected viscosity models such as the one parameter model of Frenkel 20 equation 9 and Hind 21 equation 10, apply to the studied binary mixtures....(9) Comparison of experimental densities (ρ) and viscosities (η) Deviations in viscosity (∆η) for the binary system N,N-dimethylformamide (DMF) and o-xylene, m-xylene and p-xylene at 293.15, 303.15, 313.15 and 323.15 K, plotted against mole fraction with the curve calculated from Redlich-Kister equation are presented in figure

Fig. 1 :Fig. 2 :Fig. 3 :Fig. 4 :
Fig. 1: Plots of deviation in viscosity (∆η) against mole fraction for the system (a) o-xylene (1) + N,N-dimethylformamide(2); (b) m-xylene (1) + N,N-dimethylformamide(2); (c) p-xylene (1) + N,Ndimethylformamide(2) at different temperatures: ®, 293.15 K; ¢, 303.15 K;p, 313.15 K; x, 323.15 K.The solid line represents the corresponding correlation by the Redlich-Kister equation (a).The binary system of N,N-dimethylformamide with m-xylene and p-xylene, figures 2(b-c), excess molar volumes are negative.For the binary systems, N,Ndimethylformamide with m-xylene and p-xylene, that form a negative deviation, the curves are not shifted in a regular manner with increase in temperature.For the binary system of N,N-dimethylformamide and o-xylene, positive deviation is observed at 293.15 K only.At the other temperatures, deviations are negative.Mixing N,N-dimethylformamide (DMF) with o-xylene, m-xylene and p-xylene may have resulted in complex formation leading to a volume contraction.Excess Gibbs free energy of activation of viscous flow (∆G* E ) against mole fraction at 293.15, 303.15, 313.15 and 323.15K for N,Ndimethylformamide (DMF) and o-xylene, m-xylene and p-xylene calculated from Redlich-Kister equation are presented in figure Density and viscosity for the binary mixtures of N,N-dimethylformamide with o-xylene, m-xylene and p-xylene are presented as a function of mixture composition at 293.15, 303.15, 313.15, and 323.15 K.These results have been employed to calculate deviations in viscosity, excess molar volume and excess Gibbs free energy of activation of viscous flow.The study has shown that there are intermolecular interactions between the components of the systemN,N-dimethylformamide (DMF) with o-xylene, m-xylene and p-xylene, leading to hydrogen bonds and complex formation.A comparison of experimental thermodynamic data with calculated data by means of various predictive methods are presented resulting in positive and negative correlations.

Table 1
presents a compar ison of experimental densities and viscosities of all liquids at T = 293.15,303.15, 313.15 and 323.15)K used in this work as well as accepted literature values.

table 2 .
The table also lists deviation in viscosity, (∆η), excess molar volume, V E

Table 4 : Fitting parameters with Average Percentage Deviations (APD) for binary mixtures at various temperatures
the molecular interactions leading to viscosity changes with one parameter to estimate the dynamic viscosity of binary liquid mixtures.
, 303.15, 313.15 and 323.15K for N,N-dimethylformamide (DMF) and o-xylene, m-xylene and p-xylene calculated from Redlich-Kister equation are presented in figure