J. Chem. l%ermodynamics 1974,6, 1191-1194
Thermodynamic properties of organic compounds IV. Miscibility gaps in mixtures of o-terphenyl + aliphatic dicarboxylic acid a AUGUST0 CINGOLANI, DANTE LEONESI Istituto
Chimico della Universit& degli Studi-
(Received 14 December 1973; in revised form 12 April
Miscibility gaps, critical solution temperatures, and critical compositions Cg to Cs aliphatic dicarboxylic acids with o-terphenyl are reported.
for mixtures of
1. Introduction Although miscibility gaps in molten organic mixtures have not been extensively studied, a recent work presents such a phenomena for stearic acid + glutaric acid and stearic acid + succinic acid. w Miscibility gaps in mixtures of o-terphenyl and aliphatic dicarboxylic acids are presented herein. 2. Experimental The freezing temperatures of the molten mixtures were taken as the temperatures when the first crystals were formed. The measurements were carried out by a visual method described previously. (2Y3, The melts were cooled at a rate of about 0.25 K min-‘, and only values within ) 0.25 K of the mean were accepted for the freezingtemperature determination. This temperature was measured by a chromel-to-alumel thermocouple, connected to a Leeds and Northrup type K-3 potentiometer, and checked by comparison with an NBS-certified platinum resistence thermometer. An accuracy in temperature within rtO.05 K was possible by this method of calibration of the thermocouple. The furnace for melting the mixtures was contained in a Pyrex cryostat and its temperature was controlled by a second thermocouple connected with a Leeds and Northrup CAT control unit. To avoid readjusting the automatic control of the furnace temperature, an auxiliary heater was used for a quick remelting of a mixture which had been partially frozen in a previous run. Schuchardt o-terphenyl, recrystallized from methanol, melted at 328.95 K in agreement with the literature (328.65 K, f4) 329.15 Kc5)); Fluka glutaric and azelaic acids, Schuchardt pimelic and suberic acids, and C. Erba RP adipic and sebacic a Work carried out with financial aid from CNR (Rome).
acids were also recrystallized from ethanol, when their original purity was less than 99 moles per cent. The melting temperatures of these dicarboxylic acid samples are reported in table 1.
3. Results and discussion In mixtures of o-terphenyl + glutaric acid, + adipic acid, + pimelic acid, + suberic acid, and + azelaic acid, miscibility gaps are evident; o-terphenyl + sebacic acid shows an S-shaped curve. The experimental results are given in table 1. TABLE 1. Experimental results for liquidus curves; Tf is the melting temperature of the pure acid, T, is the monotectic temperature, To is the critical solution temperature, x;l and x2 are the mole fractions of the acid in the two phases at T,,,, and x> is the mole fraction of acid in the critical mixture
o-terphenyl + glutaric acid adipic acid pimelic acid suberic acid azelaic acid sebacic acid
370.65 425.15 377.15 415.15 379.15 403.65
368.15 420.65 371.15 404.15 370.15
553.15 503.15 448.15 413.15 380.15
185.0 82.5 77.0 5.0 10.0
0.010 0.035 0.025 0.180 0.070
0.945 0.845 0.720 0.520 0.380
0.935 0.810 0.695 0.340 0.310
0.565 0.470 0.375 0.280 0.200
In none of the mixtures investigated was it possible to detect the eutectic point (evidently situated at mole fraction of acid < 0.05). Mixtures of o-terphenyl + succinic acid could not be studied because the succinic acid began to decompose at the temperatures required. The experimental and the ideal liquidus curves of these mixtures (calculated from T = AHf,[email protected]
% - R In xd))@) are shown in figures 1 and 2. Values of AHf, A and AS,, A of the dicarboxylic acid A were reported in a previous work.“’ All the mixtures show positive deviations from ideality as might be anticipated from solute-solvent interactions rather than from mixed-crystal [email protected]
’ In these mixtures the partial miscibility decreases with increase in the number of carbon atoms of the dicarboxylic acid chain. That azelaic acid (C,) presents a miscibility gap greater than that of suberic acid (C,) is to be attributed to the different behaviour of the dicarboxylic acid with odd and even numbers of carbon atoms in the chain. Since the miscibility gap is situated on the liquidus curve of the component with the higher melting temperature, the greater miscibility gap of azelaic acid is explicable. Odd dicarboxylic acids present lower melting temperatures than even ones. The beginning of the miscibility gap is immediate at very high mole fractions of o-terphenyl, while on the side rich in dicarboxylic acid, the beginning of the miscibility gap tends to be forced to lower mole fraction of acid with increasing number of carbon atoms in the acid chain. This is to be attributed probably to two types
GAPS IN o-TERPHENYL
1. Liquiduscurvesof the mixtures:0, o-terphenyl+ glutaricacid; A, o-ttmhwl -iadiplcacid; 0, o-terphenyl+ pimelicacid.The dashedlinesrepresentthe idealcurves. FIGURE
FIGURE 2. Liquiduscurvesof the mixtures: 0, o-terphenyl+ subericacid; A, o-terphenyl+ azelaicacid; 0, o-terphenyl+ sebacicacid.The dashedlinesrepresent the idealcurves. of interaction between the o-terphenyl and the dicarboxylic acid. These interactions could be o-terphenyl to -CH,chain and o-terphenyl to -CO,H groups. While the first interaction should cause a lowering of the excesschemical potential ,uz of the acid thus increasing the solubility, the second should causean increase of ,ui and thus have the opposite effect. From the observed behaviour, one may deduce that at high mole fractions of o-terphenyl the second interaction prevails, while at high mole fractions of acid the first interaction is predominant. The extension of the
miscibility gap should be attributed principally to the second interaction. In fact, o-terphenyl + glutaric acid has a very large miscibility gap. The shorter chain of the glutaric acid makes more evident the second interaction with respect to the first one. It is noteworthy that in this family of binary mixtures there is a linear relation between the critical solution temperature and the number of carbon atoms in the dicarboxylic acid chain. Similar behaviour was shown by a previous study on binary mixtures containing hydrocarbons. (‘-11) In the present work the increase in the L 300
FIGURE 3. The dependence of the critical solution temperature T, (0) and of the critical composition xi (0) with the number n, of C atoms of the acid chain.
number of carbon atoms in the acid chain depresses the critical solution temperature, as shown in figure 3. Moreover, the dependence between the critical composition and the number of C atoms of the acid chain is evident, as shown also in figure 3. Here increase in the chain length forces the critical composition to higher mole fractions of o-terphenyl. REFERENCES 1. Berchiesi, G.; Cingolani, A.; Leonesi, D. J. Thermal Anal. 1974, 6, 91. 2. Braghetti, M. ; Leonesi, D.; Franzosini, P. Ric. Sci. 1968, 38, 116. 3. Cingolani, A.; Berchiesi, G. ; Piantoni, G. J. Chem. Eng. Data 1971, 16, 464. 4. Greet, R. J.; Turnbull, D. J. Chem. Phys. 1967, 47, 2185. 5. Friz, G. ; Boullet-Krayer, E.; Nehren, R. European Atomic Energy Community-EURATOM, EUR-2223, d. 1965. 6. Sinistri, C.; Franzosini, P. Ric. Sci. 1963, 33, 419. 7. Cmgolani, A.; Berchiesi, G. J. Thermal Anal. 1974, 6, 57. 8. Ubbelohde, A. R. Melting and Crystal Structure. Ciarendon Press: Oxford. 1965. 9. Riccardi, R. ; Franzosini, P. ; Rolla, M. 2. Naturforsch. 1968, 23a, 1816. 10. Ferloni, P.; Geangu-Moisin, A.; Franzosini, P.; Rolla, M. 2. Natarforsch. 1971, 26a, 1973. 11. Ferloni, P.; Telea, C.; Franzosini, P. ; Rolla, M. Rev. Roumain. Chim. 1972, 17, 145.