Heats of mixing and diebieckie consta~nts of some partially miscible liquid pairs

Transcription

1 Heats of mixing and diebieckie consta~nts of some partially miscible liquid pairs A. N. CAMPBELL AND E. M. KARTZMARK Department of Chemistry, U~ziversitj~ of finitoba, Winnipeg, Manitoba Received May 2, 1968 The physical properties mentioned in the title have been determined for the six systems: (a) anilinehexane, (6) methanol~yclohexane, (c) methanol carbon disulfide, (d) acetic anhydride carbon disulfide, (e) acetic anhydride cyclohexane, and (f) triethylaminewater, over the complete range of composition. All six systems are partially nliscible, above or below a critical solution temperature (c.s.t.). From the experimental data, the partial molal heats of mixing have been calculated, using the Redlich and Kister equations. The enthalpy of hydrogen bonding in the triethylaminewater conlpound appears to be about 1.33 kcal per hydrogen bond. The orientation polarization, according to the Syrkin formula, appears always to exhibit negative deviation from ideality, at least over part of the concentration range. Canadian Journal of Chemist;?, 47, 619 (1969) Previous work horn this laboratory has dealt with congruent concentrations (I) of the six systems, (a) anilinehexane, (6) methanolcyclohexane, (c) meihano! carbon disulfide, (d) acetic anhydride carbon disu!fide, (e) acetic anhydride cyclohexane, and (f) triethylaminewater. Another paper (2) is confined to the system anilinehexane, and deals with excess volume, vapor pressure, surface tension, and viscosity. The present work deals with the same systems and studies the properties mentioned in the title. For those liquid systems which form two layers, the change of volume on mixing is usually positive but this is not always the case. That it should not be true of the system triethylaminewater, where both AV and AN are strongly negative, is not surprising since we have shown that here a compound is formed (31, which probably colztinues to exist, to some extent undissociated, even at room temperature. The density measurements, however, show that AV is negative over the whole concentration range for the system anilinehexane, and over a park of the range for methanol carbon disulfide, and there is no possibility of compound formation between aniline and hexaue. According to Bellemans (41, upper critical solution phenomena occur in endothermal mixtures but a lower critical point can only occur in exothermal mixtures. Experimental The method of determining heat of mixing has been described previously (5, 6). The purity of materials was the same as previously described (I). The dielectric constants were determined with a Sargent oscillometer, the sample being thermostated at a tempera ture high enough or low enough to ensure homogeneity over thecomplete concentration range (7). Determinations with this instrument cannot be made on substances having higher electrical conductances than the alcohols, although we found it possible to use conductance water as a calibration point. No mcasurernents could, however, be made on mixtures of triethylamine and water, containing considerable quantities of water, because of the high electric conductance. Dilute solutions of triethylamine in water gave apparent dielectric constants as high as 120, obviousiy due to the electric conductance. TABLE P Heats of Inlxkng Acetlc anhydride caroon drsulfide* Conlposition AH Composition AH *Composition in mole % acetic anhydride, A H in callmole mixture, 7 = 34.2 'C. Methanol carbon disulfide". Composition AH Composition AN *Composition in mole Oj, methanol, AH in cal/mole mixture, T = 36.5 "C. Drucker and Weissbach (8) have determined the heats of mixing of solutions of methanol and carbon disulfide outside of the gap, at 20 'C. Insofar as the results are comparable, they are in good agreement.

2 TABLE I (Corzcliided) Methanolcyclohexane* CANADIAN JOURNAL OF ( ZHEMISTRY. VOL. 47, 1969 Composition AH Composition AH TABLE I1 Dielectric constant (E), molar polarization (P in cc), AP (Po,,, PC,,,,), and Syrkin's orientation polarization (P,) for various mixtures Acetic anhydride carbon disulfide* Composition E P (cc) AP P, *Cornnosition in mole $/.,. methanol. Aff in callmole mixture. T = 50.5 =c. According to Monda~nMonial(9) the heat of mixing of methanolcyclohexane has a value of 4 cal/g, at the maximum. Th~s amounts to 253 cal/mole and is to be compared with our value of 300 cal at the maximum (40 mole % methanol). Wood (10) calci~lates a value of 800 cal/mole mixture, a value which is obviously absurd. Acetic anhydride cyclohexane* Composition AH Compos~tion AH *Composition In mole "/, acctic anhydride, AH in cal/inole mixture, 7 = 59.0 'C. *Composition in inole % hexane, AH in cal/mole mixture, T 66.5 "C. Triethylamineaater* Composition AH Composition AH "Composition in mole ';: trieth?lamine, AH 111 caljniole mixture T= lnr.", The results of Copp and Everett (11) are in fair agreement w~th ours, as regards thc shape of the curie and the ialue IfE at the maxim~~ni, but \%hereas they obtain a maxiinurn value of 595 cal,'niolc at a composition of 25 molc "/: triethqlamltie, our mariniiim value 1s 580 cal at 41 mole % tr~ethylarnine. Copp and Everett's curie is, houever, b~~?te fat at the top and a small error oil their part or ours mould qrtng the composit~ons into agreement (pure CS2) *Compos~tion in mole "/,acetic anhydride, T = 32 "C. Methanol carbon disulfide" Compos~t~on E P (cc) AP Pi *Composition in inole %methanol, T 40 "C... Methanolcyclohexane* Composition E P (cc) AP Pi.pp 0.00 (pure cyclohexane) 'Composition in niole O,:,methanol, T = 50.0 "C.

3 PA ~ CAMPBELL AND KARTZMARK: PROPERTIES OF PARTIALLY MISCIBLE LIQUID PAIRS 621 TABLE I1 (Concl~idcd) Acetic anhydride cyclohexane* Con~position E P(cc) AP Pt (cyclohexane) *Composition in mole %acetic anhydride, T = 58.3 'C. Hexaneaniline* pp~ I0 mo'e fioclon,herane Composition E P(cc) AP Pi. FIG. 1. Partial inolar excess enthalpies : aniline 0 (aniline) : 'lexane : *Composition in % hexane, T = 67.5 'C. Discussion The heats of mixing can be represented by the Redlich and Kister equation (12), viz. ffe = XI X2 [A + B(XI X2) + C(XI X2)'] where X, = nlole fractioil of the con~ponent first named. The constants for the above equation were determined from our data, using a least squares fit, on the computer. The program was essentially o o 8 10 that of Myers and Scott (13). The constants, mole frocton, methonal "getjler with the deviations of the FIG. 2. Partial lllolar constants, are given in Table 111. cyclohexane. excess entilalpies: methanol TABLE 111 Constants of the Redllch and Klster equation and their de\lat~ons System A ci B o C o Acetic anhydride carbon disulfide Methanol carbon disulfide 761.O MethanolLcyclohexane Acetic anhydride cyclohexane Hexaneaniline T~iethylaminewater

4 622 CANADIAN JOURNAL OF Using the above constants, the partial molar hears of mixing can be calculated from the equations HIE = (1 XI)'[(A f B(4X1 1) + C(2X1 1)(6X1 I)] H2E = XI2[A + B(4Xl 3) + C(2Xl 1)(6Xl 5)] The results are expressed graphically in Figs. 1 to 6. FIG. 3. Partial molar excess enthalpies: acetic anhydride carbon disulfide. All heats of mixing are positive except those of triethylaminewater, which are strongly negative1. The partial molar heats of mixing require no comment, except those of triethylaminewater. In view of the equilibrium existing in solution, viz. the limiting value of the partial molar heat of mixing of triethylamine in infinite excess of water may represent the enthalpy of formation of the compound. The calculation was therefore carried out for low mole fractions of triethylamine (0.011 and 0.05). The limiting value is 2650 cal. This gives a value of kcal for the hydrogen bond. When dielectric constant is plotted against mole fraction, negative deviation from straight line behavior is usually observed. The exception to this is the system acetic anhydride carbon disulfide, CHEMISTRY. VOL. 47,!969 7 mole fiact~on,ncef!c anhydride FIG. 4. Partial molar excess enthalpies: acetic anhydride cyclohexane. male fract#on,rnethonol FIG. 5. Partiai nlolar excess enthalpies: methanol carbon disulfide. which shows slightly positive deviation. We have not used our results for triethylaminewater, because we do not think these results are reliable in the presence of appreciable amounts of water. The molar polarization, 'This is in agreement with the predictions of Haase and Rehage (14) who state that an upper critical solution temperature is associated with a positive enthalpy of mixing and a lower critical solution temperature with a negative. as a function of mole fraction, shows strong positive deviation from linearity. Finally, we

5 FIG. 6. water. CAMPBELL AND KARTZMARK: PROPERTIES OF PARTIALLY MISCIBLE LIQUID PAIRS mole froct~on,triethylominc Partial molar excess enthalpies : triethylamine owe to Syrkin (16) a formula for orientation polarization, viz. where P is the molar polarization and R,, R, are the molar refractions of the pure components. When these results are plotted against mole fraction, negative deviation is always observed, at least over part of the concentration range, a behavior which, according to Syrkin, is attributable to dipole induced dipole interaction, dipole association, or a decrease in hydrogen bonding with dilution. It does not necessarily follow, however, that a mixture of polar and nonpolar liquids shows negative deviation from additivity of Syrkin polarization. Thus, the behavior of a benzenenitrobenzene mixture is ideal and this mixture does not show partial miscibility. On the other hand, mixtures of acetic acid with carbon tetrachloride, which are completely miscible, show marked negative deviation from additivity. We determined the refractive indices of all solutions at the temderatures of investigation of the other properties (above the c.s.t.). In every case, the molar refraction was so close to that calculated from the mixture rule that the slight differences might be attributed to experimen<al error. This is to be expected if, as the Clausius Mosotti theory teaches, the molar refraction represents the true volume of matter in a mole. 1. A. N. CAMPBELL and E. M. KARTZMARK. Can. J. Chem. 45, 2433 (1967). 2. A. N. CAMPBELL, E. M. KARTZMARK, S. C. ANAND, Y. CHENG, H. P. DZIKOWSKI, and S. M. SKRYNYK. Can. J. Chem. 46, 2399, E. M. KARTZMARK. Can. J. Chem. 45, 1089 (1967). 4. A. BELLEMANS. J. Chem. Phys. 21, 369 (1953). 5. A. N. CAMPBELL, E. M. KARTZMARK, and H. FRIESEN. Can. J. Chem. 39, 735 (1961). 6. A. N. CAMPBELL and J. M. T. M. GIESKES. Can. J. Chem. 43, lo04 (1964). 7. A. N. CAMPBELL and J. M. T. M. GIESKES. 42, 1379 (1964),.,. 8. C. DRUCKER and H. WEISSBACH. Z. Physik. Chem. 117, 209 (1925). 9. P. MOKDAINMON~AL. Compt. Rend. 183, 1104 (1926). 10. S. E. WOOD. J. Am. Chem. Soc (1946). 1 I. J. L. COPP and 93. H. EVERETT. Discussions Faraday Soc. 15, 174 (1953) REDLICH and A. T. KISTER. Ind. Eng. Chem. 40, 345 (1948). 13. D. B. MYERS and R. L. SCOTT. Ind. Eng. Chem. 55, 431 (1963). 14. R. HAASE and G. REHAGE. Z. Elektrochem. 59, 994 (1955) LAURANSON, P. PINEAU, and M. L. JOSIEN. Ann. Cllim. 9, 213, (1964). J. H. LADY and K. B. WHETSEL. J. Phys. Chem. 64, 1001 (1964). 16. Y. K. SYRKIN. Compt. Rend. U.R.S.S. 35,43 (1942).