1939
BENZENE-ISOMERIC XYLENESYSTEMS
Molar Excess Enthalpies and Volumes of Benzene-Isomeric Xylene Systems at 25"' by Jaswant Singh,2 H.D. Pflug,' and G.C. Benson Division of Pure Chemiatry, National Research Council of Canada, Ottawa, Canada (Received January 8, 1968)
The changes of enthalpy and volume on mixing benzene with 0-, m-, and p-xylene were measured at 25". Interpretation of t,he results in terms of the theories of Barker and of Flory was investigated. Both approaches indicate an increase in the aromatic-aliphatic interaction energy in the benzene solutions of 0- and m-xylene as compared to those of p-xylene. Introduction The benzene solutions of o-xylene, m-xylene, and p-xylene form an interesting series of systems involving quite similar molecular structures which interact only weakly. Reasonable interpretation of their excess properties offers a valuable test of relevant theories. In the present paper, measurements of the molar excess enthalpies and volumes of these solutions at 25" are reported. The data are analyzed according to Barker's quasi-lattice theory4 and the recent theory of Flory.6 Experimental Section The calorimeter6 and dilatometer' used for measuring heats of mixing and volumes of mixing have been described in previous papers from our laboratory. Reference should be made to these for details of the equipment and operational procedures. The benzene used (Philips Petroleum Co., research grade) was specified by the manufacturer to be 99.91 mole %. It was used without further purification since chromatographic analysis failed to show any significant impurities. o-Xylene and p-xylene (Eastman Organic Chemicals) and Spectroquality m-xylene (Matheson Coleman and Bell) were all purified chromatographically using a column packed with 20% PEG 1000 on Chromosorb W. Densities of the final products were 0.87590, 0.85988, and 0.85675 g cmV3 for the ortho, meta, and para isomers, respectively. Results The molar excess enthalpies (HE) of the systems benzene-0-xylene, benzene-m-xylene, and benzene-pxylene at 25" are given in Table I. Volumes of mixing (VE) for the same systems (also at 25") are summarized in Table 11. In all cases, z1 is the mole fraction of benzene. The excess enthalpy and volume results were fitted with the forms
H E = ZlZZ[C1"
+ CZH(Z2 - 21) + Ca"(Z2
- Z d 2 + CAH(Z2 - s)al (1)
and
VE
=
ZlZ21ClV
+ C2(Z2 - $1) +
c8v(x2
- zd2I
(2)
respectively. Values of the coefficients determined by the method of least squares are listed in Table 111; the last column of the table contains values of Q the standard deviation of the fit in each case. The experimental results for H E and VE are plotted in Figures 1 and 2. The magnitudes of the excess functions follow the order meta > ortho > para for the enthalpies and volumes of the three systems. Excess properties of benzene-xylene systems have also been studied recently by Rastogi, Nath, and Misra.*J' For comparison, values calculated from the equations given by these authors are shown as dotted curves in Figures 1 and 2. The curves for the excess volumes of benzene with o-xylene and m-xylene (from ref 9) pertain to 30"; all other results are for 25". Both the excess enthalpies and excess volumes given in ref 8 and 9 are smaller than our results. The discrepancies are particularly large between the two sets of enthalpy data. In the case of the benzene-p-xylene system, the value 172 J mol-' reported by Cheesman and Ladner'O for an equimolar solution at 24.4' is more in line with our data although the difference exceeds the combined estimates of the experimental errors and of the effect of difference in temperature, (1) Issued as NRCC No. 10144. (2) National Research Council of Canada Postdoctorate Fellow, 1965-1967. (3) National Research Council of Canada Postdoctorate Fellow, 1965-1967. (4) J. A. Barker, J . Chern. Phya., 20, 1526 (1952). (5) P.J. Flory, J . Amer. Chem. Soc., 87, 1833 (1965). (6)A. E. Pope, H. D. Pflug,B. Dacre, and G. C. Benson, Can. J . Chem., 45,2666 (1967). (7)H.D.Pflug and G. C. Benson, ibid., 46, 287 (1968). (8) R. P. Rastogi, J. Nath, and J. Misra, J . Phys. Chem., 71, 1277 (1967). (9) R. P. Rastogi, J. Nath, and J. Misra, ibid., 71, 2524 (1967). (10) G.H.Cheesman and W. R. Ladner, PTOC. Roy. SOC.(London), A229, 387 (1955). Volume 78, Number 6 J w e 1068
J. SINGH, H. D. PFLUG, AND G. C.BENSON
1940 Table I: Experimental Values of the Molar Excess Enthalpy HE (J mol-') of Beneene-Xylene Systems a t 25"'
250
l
,
,
I
,
1
,
,
,
SYSTEMS AT 25OC.
BENZENE-XYLENE
21
0.0332 0.0869 0.1542 0.2363 0.3181 0 3940 0.4662 0.5377 0,5859 0.6381 0.6441 0.7043 0.7625 0.8171 0.8687 0.9152 0.9498 0.9753 I
25.4 63.9 106.4 149.0 181.4 202.0 212.9 214.9 214.7 207.7 205.2 188.7 166.4 139.1 107.7 73.9 46.0 23.2
0.0447 0.0999 0.1772 0.2678 0.3559 0.4342 0.4856 0.5065 0,5445 0.5716 0.6039 0.6288 0.6677 0.6794 0,7279 0.7879 0.8474 0.9011 0.9418 0.9701
34.8 74.2 121.8 166.1 198.3 216.6 219.7 224.7 222.6 224.4 218.5 217.7 207.3 206.5 189.2 162.8 129.1 90.7 56.8 30.3
0 0537 0.1150 0.1793 0.2596 0.3367 0.4188 0.4862 0.4983 0,5376 0.5571 0.5938 0.6158 0.6506 0.6736 0.7095 0.7886 0.8419 0.8941 0.9380 0.9693
29.3 60.3 88.9 118.8 140.5 156.4 162.9 163.1 165.2 164.7 163.8 160.5 157.8 152.0 146.5 122.4 99.8 72.8 45.7 23.7
zl, mole fraction of benzene.
a
.."
O I r : ' ' 0.0 0.2
'
I
0.4
'
1
0.6
: 0.8
I '.-d I.o
X, ,MOLE FRACTION OF BENZENE
Table I1 : Experimental Values of the Molar Excess Volumes V E (cma mol-') of Benzene-Xylene Systems a t 250a -CsHso-CaH4 (CHa)T-. cCeHsm-CeH4 (CHs)z21
0.0293 0.0861 0.1578 0 2348 0,3082 0.3765 0.4416 0.4997 0.5485 0.5899 0 6044 0.6255 0.6500 0.6999 0.7527 0,8044 0.8572 0.9081 0.9513 0.9814 I
I
a
VE
0,0253 0.0720 0.1207 0.1677 0.2026 0.2273 0,2422 0.2489 0.2492 0.2457 0.2445 0.2399 0.2373 0.2215 0.1987 0.1699 0.1337 0.0925 0.0515 0.0202
21
0,0258 0.0666 0.1174 0.1909 0.2625 0.3308 0.4047 0.4648 0.5173 0.5320 0.5629 0.5685 0.6026 0.6097 0.6365 0.6548 0.7027 0.7537 0.8040 0.8521 0.8974 0.9357 0.9630 0.9853
VE
0.0185 0.0581 0.1045 0.1622 0.2094 0.2439 0.2717 0.2861 0,2919 0.2978 0,2921 0.2966 0.2882 0.2915 0.2837 0.2809 0.2640 0.2391 0.2065 0.1682 0.1249 0,0826 0.0485 0.0194
-CaHsp-CoHd (CHa)r21
0.0386 0.0915 0.1572 0.2320 0.3041 0.3718 0.4375 0.4937 0.5432 0,5846 0.6177 0.6615 0.7092 0.7612 0.8124 0.8619 0.9111 0.9515 0.9807
HE
0,0270 0.0613 0.0995 0.1362 0.1647 0.1855 0.1999 0.2067 0.2089 0.2075 0.2005 0.1972 0.1819 0.1643 0.1412 0.1120 0.0781 0.0455 0.0191
a!, mole fraction of benzene.
Barker Theory. Estimates of the excess properties of a mixture calculated according to the method described by Barker4 refer to a mixing process in which the volume for each molecule remains constant. However, The Journal of Physical Chemistry
Figure 1. Excess enthalpy of benzene-xylene systems; experimental results of this research: 0, benzene-o-xylene; 0, beneene-m-xylene ; 0, benzene-p-xylene. Single result of beneene-pxylene (W) due to Cheesman and Ladner (see ref 10). Dotted curves based on results of Rastogi, Nath, and Misra (see ref 8 and 9); broken curves calculated from Flory theory using X I Zvalues from Table V.
it is usual t o compare the theoretical values with experimental results measured essentially at constant pressure and corrected t o constant total volume. This course was adopted in the present work, and the relation
UEV = HE - T(aIP)VE
(3)
was used to calculate the molar excess energy at constant volume and temperature ( T = 298.15"K) from the smoothed excess enthalpy and excess volume results (at constant pressure). Values of CY and p (the coefficients of thermal expansion and isothermal compressibility) for the mixtures were obtained from those of the pure components, as listed in Table IV, assuming additivity on a volume fraction basis. Results for the molar excess energies are shown in Figure 3 as points at a mole fraction interval of 0.05. It is interesting to note that the relative order of the UEv values for the three xylene systems is ortho > meta > para and differs from that found earlier for HE and VE. The new order is the same as that of the boiling points of the isomeric xylenes and apparently reflects the more direct dependence
