J. Chem. Soc., Faraday Trans. I, 1984,80, 1517-1524 Binary Systems of 1, I ,2,2-Tetrachloroethane with Benzene, Toluene, p-Xylene, Acetone and Cyclohexane Part 2.-Dielectric Properties at 308.15 K BY JAGAN NATH* AND A. D. TRIPATHI Chemistry Department, Gorakhpur University, Gorakhpur 273001, India Received 12th September, 1983 Measurements of dielectric constants, E, have been made for binary liquid mixtures of 1,1,2,2-tetrachloroethane (TCE) with benzene, toluene, p-xylene, cyclohexane and acetone at 308.15 K. Measurements of refractive indices, n, have also been made for binary liquid mixtures of CHCl,CHCl, with acetone at 308.15 K. The values of the quantity A&, which refers to the deviations of the dielectric constants of these mixtures from values for the ideal case, have been calculated.The positive values of A& obtained for TCE+acetone mixtures are attributed to the formation of a molecular complex between acetone and CHCl,CHCl,, whereas the negative values of A& for the other systems may be explained as being due to a decrease in the degree of alignment of the molecular dipoles with changing composition of the mixture. The values of the equilibrium constant, K,, for the formation of a 1 : 1 intermolecular complex between acetone and TCE have been calculated, using the dielectric-constant data. In addition, values of the apparent dipole moments, papp, of TCE at various mole fractions in the solvents cyclohexane, benzene and p-xylene have also been calculated. The values of pipp show that the molecules of TCE are self-associated, and this self-association is more favoured in cyclohexane solution than in solutions of benzene andp-xylene; this is attributed to the existence of specific interactions between TCE and the aromatic hydrocarbons.The values of pipp also show that the strength of specific interactions of TCE with p-xylene is greater than in the case of benzene. Binary systems of 1,l72,2-tetrachloroethane (TCE) with aromatic hydrocarbons, acetone and cyclohexane are of interest because there exists a donor-acceptor interaction between the components. This is caused by the presence of four C1 atoms and two H atoms in TCE, which can thus act as a a-acceptor toward, and be involved in the formation of hydrogen bonds with, aromatics and acetone. The latter act as n-donors and an n-donor, respectively.The system cyclohexane + TCE, in which only dispersion, dipolar and induction forces are believed to be present between the components, can be used as a reference system. Extensive studies of the interactions between components in such systems have not been made. However, recently' measurements of ultrasonic velocities, adiabatic compressibilities and excess volumes in the case of binary liquid mixtures of TCE with benzene, toluene, p-xylene, acetone and c-C,H,, were carried out at 298.15 and 308.15 K; from these studies there appears to exist a specific interaction between TCE and the other component. Dielectric- constant measurements of binary mixt~res~-~ have also shed light on the existence of a specific interaction between the components. Furthermore, the values of the apparent dipole moments, pap?, of the various components in ~ o l u t i o n ~ - ~ have also provided evidence concerning interactions between the components.The values of paPp can be obtained from measurements of the dielectric constants of binary liquid mixtures. Hence, in order to obtain conclusive evidence of the existance of a specific 15171518 DIELECTRIC PROPERTIES OF BINARY SYSTEMS interaction between TCE and aromatics or acetone we have made measurements of the dielectric constants for binary liquid mixtures of TCE with benzene, toluene, p-xylene, acetone and c-C,H12 at 308.15 K. Since the refractive indices of acetone, TCE and binary mixtures of TCE with acetone were needed to calculate both papp values of TCE in various solvents and the molar polarisations for mixtures of TCE with acetone, measurements of refractive indices have also been made at 308.15 K.The results of the present measurements are interpreted in this paper. EXPERIMENTAL MATERIALS Benzene, toluene,p-xylene, acetone, cyclohexane and 1,1,2,2-tetrachloroethane were purified, and their purity was checked as described earlier.' METHODS The dielectric-constant ( E ) measurements were made at 308.15 K and at a frequency of 1.8 MHz with a dekameter (type DK,,, Wissenschaftlich-Technische, Werkstatten, Germany), using one cell (MFL 1/S, no. 2078) for mixtures of TCE with benzene, toluene, p-xylene and cyclohexane, and another (MFL 2/S, no. 2084) for mixtures of TCE with acetone. The cells were thermostatted through the outer jacket using a water bath whose temperature was controlled to within kO.01 K.The two cells were first calibrated using liquids of known dielectric constants,a and dielectric-constant measurements were then made for the pure liquids and binary mixtures studied in the present programme. The precision of the dielectric-constant measurements was better than 0.0004 unit for dilute solutions of TCE in benzene, p-xylene and cyclohexane, and ca. 0.001 unit for mixtures of TCE with toluene and acetone and for mixtures having higher concentrations of TCE in benzene, p-xylene and cyclohexane. Refractive-index (n) measurements accurate to within f 0.0002 were carried out using a thermostatted Abbe refractometer at 308.15 K. The values of n were obtained for sodium-D light.RESULTS AND DISCUSSION The experimental values of e for the pure liquids benzene, toluene, p-xylene, acetone, cyclohexane and TCE, and for binary mixtures of TCE with benzene, toluene, p-xylene, acetone and cyclohexane at 308.15 K are given in table 1, where xA refers to the mole fraction of TCE. The values (see table 1) of E obtained for benzene, toluene, p-xylene, cyclohexane and acetone are 2.2540, 2.3552, 2.2454, 1.9992 and 19.749, respectively, in excellent agreement with the literature values8 of 2.2540, 2.3547, 2.2460, 1.9990 and 19.745, respectively. The value of E for TCE was found to be 7.096, which is in good agreement with the value of 7.090 which is based on data available in the literat~re.~ The present experimental values of the refractive indices, n, for acetone and TCE are 1.3520 and 1.4865, respectively, which can be compared with literature valueslO of 1.351 57 and 1.48658, respectively.The values of the refractive indices, n12, for TCE+acetone mixtures have been fitted by the method of least squares to the equation (1) where xA refers to the mole fraction of TCE. Values of Ae, the deviation in the dielectric constants of the mixtures from values obtained from the ideal volume-fraction-mixture law, have been calculated from the relation where el, is the dielectric constant of the mixture and el and E , are the dielectric constants of the pure liquids 1 and 2, for which volume fractions in the mixture are n12 = 1.35 1 60 + 0.195 26x, - 0.060 62xi (2) A& = & 1 2 - 4 1 & 1 - 4 2 & 2J.NATH AND A. D. TRIPATHI 1519 Table 1. Dielectric constants for TCE in various mixtures at 308.15 K TCE + benzene TCE + toluene TCE +p-xylene & & X A X A X A & 0.0000 0.0028 0.0035 0.0052 0.0071 0.0 164 0.0350 0.0454 0.05 19 0.0662 0.0759 0.0886 0.2087 0.3252 0.5357 0.6 150 0.6559 0.6593 0.7016 0.7599 0.97 17 1 .oooo 2.2540 2.2636 2.2658 2.2718 2.2786 2.3106 2.378 2.410 2.436 2.493 2.525 2.563 2.994 3.436 4.332 4.696 4.893 4.897 5.109 5.432 6.89 1 7.096 0.0000 0.0982 0.2067 0.2459 0.3894 0.4528 0.5239 0.5696 0.6434 0.657 1 0.6636 0.7093 0.8106 0.9524 1.0000 2.3552 2.638 2.959 3.109 3.593 3.821 4.152 4.352 4.667 4.770 4.790 4.980 5.618 6.628 7.096 0.0000 0.00 13 0.0030 0.0060 0.0074 0.0084 0.0275 0.0437 0.0542 0.0626 0.0730 0.0808 0.1967 0.2074 0.3393 0.4539 0.5880 0.6002 0.8807 0.9592 1 .oooo 2.2454 2.2487 2.2528 2.2598 2.2642 2.2659 2.314 2.359 2.386 2.416 2.447 2.467 2.780 2.8 16 3.203 3.663 4.190 4.23 1 5.966 6.638 7.096 TCE + cyclohexane TCE +acetone X A X A & & 0.0000 0.0005 0.0009 0.0025 0.005 1 0.0082 0.0096 0.0372 0.0460 0.0470 0.0760 0.0862 0.1 123 0.1650 0.2225 0.497 1 0.5178 0.6952 0.7053 0.921 1 1 .oooo 1.9992 2.0008 2.0020 2.0067 2.0 150 2.0249 2.0285 2.1 12 2.138 2.139 2.209 2.239 2.306 2.491 2.708 3.804 3.910 4.886 4.938 6.476 7.096 0.0000 0.0778 0.1016 0.2004 0.3019 0.4049 0.4904 0.5532 0.5996 0.67 14 0.7002 0.7795 0.8793 1 .oooo 19.749 18.817 18.601 18.102 17.161 16.540 15.474 14.42 1 13.741 12.634 12.21 1 1 1.032 9.340 7.0961520 DIELECTRIC PROPERTIES OF BINARY SYSTEMS m m *.+ 2.0 ' 8 8 w +'*O* 0.0 0.0 - 0.4 1 % a "$ A I d -0.8t I ! I , 0.0 0.2 0.4 0.6 0.8 I 0 Fig. 1. Plot of A& against the mole fraction of TCE, xA, at 308.15 K: a, TCE+benzene; V, TCE + toluene; A, TCE +p-xylene; 0, TCE + cyclohexane; ., TCE + acetone. and 42, respectively. It has been pointed out that dielectric constants of polar mixtures can be represented as linear functions of the volume fraction." Fig. 1 , however, shows that the values of A& are negative for the systems TCE+benzene, TCE + toluene, TCE +p-xylene and TCE + cyclohexane, and positive for TCE + acetone. Comparison on a volume-fraction basis largely compensates for the ' dipole- dilution' effect, as it has been termed by Franks and Ives.12 The negative values of A&, which are of about the same order of magnitude (see fig.1) for the systems TCE + benzene, TCE + toluene, TCE +p-xylene and TCE + cyclohexane, can be attributed to a decrease in the degree of alignment of the molecular dipoles with changing composition of the mixture. A& is found to be positive for systems where strong specific interaction is believed to be present between the components.2 Measurements of excess volumes'. l3 indicate the existence of a specific interaction leading to the formation of complexes between TCE and the aromatic hydrocarbons. The negative values of A& for the systems involving TCE and the aromatic hydrocarbons may be explained by the predominance of contributions to A& from dipolar forces over those from specific interactions. The highly positive values (see fig.I ) of A& for the system TCE + acetone show that acetone forms a strong molecular complex with TCE in the liquid state; this is also true in the case of the pure binary system pyridine + chloroform, in which case a 1 : 1 complex is believed to be formed2 because of hydrogen bonding between chloroform and pyridine. In order to obtain further evidence to support the fact that acetone forms complexes with TCE we have calculated values of the total molar polarisation, P, for acetone, TCE and binary mixtures of TCE with acetone at 308.15 K, using the Kirkwood-Frohlich equation :14 (3) ( E - n2) (2e + n2) V 9& P =J. NATH AND A. D. TRIPATHI 1521 620 540 - I - E 460- E e 1 420- 380 - 340 - I I I I I 1 0.0 0.2 0.4 0.6 0.8 .1.0 *D Fig. 2. Plot of the apparent molar polarisation, PD, against mole fraction, xD, of acetone for the system TCE + acetone at 308.15 K. where Vdenotes the molar volume. The values of n used to calculate P for the mixtures were obtained from eqn (1). The molar volumes of acetone and TCE were obtained from the available density data,lo whereas those for the mixtures were estimated from the molar volumes of the pure liquids and measurements of the excess volumes.1 The total molar polarisations of the mixtures were then used to calculate values of the apparent molar polarisation, PD, of acetone at different compositions of its mixtures with TCE, in a manner similar to that described by Rastogi and Nath.15 The values of PD so obtained are plotted as a function of mole fraction of acetone, xD, in fig.2, which shows that the value of P,, increases sharply with a decrease in the mole fraction of acetone. This confirms that there exists a strong specific interaction between acetone and TCE, leading to the formation of molecular complexes between the two species in the liquid state. Similarly, values of the apparent molar polarisation, PA, of TCE at different compositions of its mixtures with acetone were calculated. Considering a 1 : 1 complex DA to be formed between acetone (D) and TCE (A), the value of the total molar polarisation, PDA, of the complex formed was obtained in a manner similar to that described by Earp and Glasstone.ls The value of PDA was found to be 733.4 cm3 mol-l. Furthermore, the values of the equilibrium constant, K,, for the formation of the complex DA, were also calculated as described by Earp and Glasstone.16 The results show a significant variation in Kf with the composition of the mixture.Rivail and Thiebaut2 have also observed that in the case of the system pyridine +chloroform, the values of Kf estimated from dielectric-constant data exhibit a significant variation with the composition of the liquid. As pointed out by Rivail and Thiebaut,2 a theory1' based upon electrostatic interactions of the solute with the liquid predicts a linear variation of the logarithm of Kf with the quantity1522 0.4 C 0 .I Y z 0.0- i? e, - v 1 -0.4 - M - -0.8 DIELECTRIC PROPERTIES OF BINARY SYSTEMS - - - I I I I I 0.12 0.13 0.14 0.15 0.16 f (€1 Fig. 3. Plot of log [&/(mole fraction)-’] againstflc) for the system TCE + acetone at 308.15 K.where E, refers to the infinite-frequency dielectric constant of the mixture. For calculations offTE) we have taken E, = n2. In fig. 3 are plotted values of log Kf as a function off(&). There is a linear variation of log Kf withf(E) for TCE+acetone, thus suggesting that the values of Kf calculated using the simple approach of Earp and Glasstonels are in accord with the theory2. l7 based upon the electrostatic interactions of the solute with the liquid. McClellan and Nicksicl* have shown that the molecules of TCE are self-associated through hydrogen bonding, and Campbell et aL6 and Stokes and Marsh5 have shown that the values of the apparent dipole moment, papp, of polar solutes in non-polar solvents furnish useful information about both self-association of the solute molecules and association of the solute molecules with the molecules of the solvent.Hence, to determine whether the present study gives evidence concerning the self-association of TCE molecules, and to find out if a specific interaction exists between TCE and the aromatic hydrocarbons, we have calculated the values of the apparent dipole moment, papp, of TCE at various mole fractions in the non-polar solvents benzene, p-xylene and cyclohexane using the equation5* l9 where xA is the mole fraction of the polar solute, E is the dielectric constant of the solution, E~ is the dielectric constant of the non-polar solvent (benzene, p-xylene or cyclohexane) and E; is the internal dielectric constant of the solute.Vm, 4 and & are the molar volumes of the solution, the non-polar solvent and the polar solute, respectively. N is Avogadro’s constant and k is Boltzmann’s constant, p s , o is the moment of the isolated polar molecule and g is the Kirkwood correlation parameter.2o As mentioned by Stokes and M a r ~ h , ~ we have taken (gp$-,)i to be equal to the apparent dipole moment, paPp, of the solute. The values of 4, V, and Vm used to calculate (gp:, ,); from eqn (5) were ascertained as described earlier in this paper. The value of E;, which was obtained from the refractive index of TCE as described by Stokes and M a r ~ h , ~ was 2.387. Fig. 4 shows the concentration dependence of pipp for TCE in (i) cyclohexane, (ii) p-xylene and (iii) benzene, with a logarithmic scale along the abscissa.The dielectric behaviour of TCE in benzene is similar to that in p-xylene, but different from that in cyclohexane. The concentration dependence of pipp for TCE in benzene and p-xyleneJ. NATH AND A. D. TRIPATHI 1523 I I I I I I - 3.0 -2 .o -1 .o 0 .o I .o log (C/mol dm-3) Fig. 4. Plot of pgPp against the logarithm of solute concentration for TCE in (i) cyclohexane, (ii) p-xylene and (iii) benzene. solutions is similar to that of octanols in benzene s~lution,~ whereas the concentration dependence of pipp for TCE in cyclohexane solution is similar to that of octanols in cyclohexane solution.' The concentration dependence of &pp in fig. 4 gives information about the association behaviour of TCE.The initial rise in p:pp is indicative of the formation of the first, high-dipole-moment species. Fig. 4 therefore shows that the formation of this species takes place at lower concentrations in cyclohexane solution and that it is delayed in benzene and p-xylene solutions. This shows that the TCE monomer is stabilised little by interaction with the non-polarisable solvent (cyclo- hexane), but is stabilised more by association with the solvents benzene and p-xylene. The self-association of TCE is therefore inhibited by solute-solvent interactions in the solvents benzene and p-xylene. Woolley and Hepler,21 using thermodynamic data, have found similarly that phenol is more self-associated in cyclohexane than in benzene. The association of TCE with benzene and p-xylene can be attributed to the existence of a specific interaction between TCE and the aromatic hydrocarbons, which may be due to the formation of a weak hydrogen bond through the interaction of the hydrogen of TCE with the n-electrons of the aromatic ring.However, there is also a possibility that TCE is involved in the formation of a charge-transfer complex with the aromatic hydrocarbon through the interaction of the chlorine atoms with the aromatic n-electrons. Fig. 4 also shows that a maximum in pipp occurs at low concentrations of TCE in benzene and that this maximum is more pronounced in p-xylene, whereas in cyclohexane such a maximum does not appear. This observation, that the maximum inp:,, at low concentrations of TCE is more pronounced inp-xylene than in benzene, shows that the strength of the specific interaction of TCE withp-xylene is greater than that with benzene.This can be attributed to the fact that the n-electron density of the aromatic ring is increased in p-xylene due to the presence of two CH, groups. CONCLUSIONS In conclusion, we note that the dielectric-constant data show that molecules of TCE are self-associated, and that this self-association is more favoured in cyclohexane than in benzene and p-xylene, thus indicating that the TCE monomer is stabilized little by1524 DIELECTRIC PROPERTIES OF BINARY SYSTEMS interaction with cyclohexane in comparison with benzene and p-xylene. This shows that the self-association of TCE is inhibited by solute-solvent interactions in the solvents benzene and p-xylene, a fact which confirms the existence of a specific interaction between TCE and the aromatic hydrocarbons.The plots of the values of ptpp in fig. 4 show that the specific interaction of TCE is stronger with p-xylene than with benzene, a fact which has been attributed to the increased n-electron density of the aromatic ring of p-xylene. The dielectric-constant data also show that acetone forms strong complexes with TCE in the liquid state. The presence of a specific interaction between TCE and the aromatic hydrocarbons can be explained as due to the formation of a weak hydrogen bond between the hydrogen atoms of TCE and the n-electrons of the aromatic ring. There is, however, also a possibility that TCE may form a charge-transfer complex with the aromatic hydrocarbons, via chlorine- atom-n-electron interactions.On the other hand, the complexation between acetone and TCE can be attributed to the formation of strong hydrogen bonds between the hydrogen atom of TCE and the lone-pair electrons on the oxygen atom of acetone. We are grateful to Prof. R. P. Rastogi, Head of the Chemistry Department, Gorakhpur University, Gorakhpur, for his encouragement during the course of these investigations. Thanks are also due to the C.S.I.R. and U.G.C, New Delhi, for financial support. J. Nath and A. D. Tripathi, J. Chem. Eng. Data, 1983, 28, 263. J. L. Rivail and J. M. Thiebaut, J. Chem. Soc., Faraday Trans. 2, 1974,70, 430. J. Nath and S. N. Dubey, J. Phys. Chem., 1980,84, 2166. J. Nath and S . S . Das, Indian J. Pure Appl. Phys., 1981, 19, 343. R. H. Stokes and K. N. Marsh, J. Chem. Thermodyn., 1976, 8, 709. C. Campbell, G. Brink and L. Glasser, J. 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