April, 1962
THEIZbfODYKAMIC S T U D Y OF
THE S Y S T E M l-HYDRO-~-PEl~PLUOI~OHEPTAR.E-~CETONE621
A THERMODYNAMIC STUDY OF THE SYSTEM 1-HYDRO-n-PERFLUOROHEPTANE + ACETONE’ BY D. L. ANDERSEN,~ R. A. SMITH, D. B. MYERS,S.K. ALLEY,A. G. WILLIAMSON, AND R. L. SCOTT Department of Chemistry, University of California, LQSAngeles 24, California Recmwd October 16, 1961
The excess Gibbs free energy a t 0” and volume changes of mixing a t 20” and heats of mixing a t 0 and 35” have bcen determined for 1-hydro-n-perfluorohe tane acetone. The free energy, determined from total vapor pressures a t O”, is positive and sharply skewed towarxacetone (BEmax = 90 wl. at zacetans = 0.75). Volume changes are less positive than those observed for fluorocarbon hydrocarbon systems ( = 1.83 ~ ma t. 5 2~ = 0.70). Heats of mixing are Sshaped with minima near ze = 0.30 and maxima near z2 = 0.90. Nuclear magnetic resonance (n.m.r.) data yielded equilibrium constants at -20, 30, and 80’ for the formation of a hydrogen-bonded complex GF16H * * OCMee with a heat of formation of -2.5 f 0.5 kcal./mole for the h drogen bond. Symmetric exotherniic heats of complexing calculated from the n.m.r. results were subtracted from the oxserved heats, giving a skewed “physical” endothermic curve which agrees well with observed heats of mixing for fluorocarbon hydrocarbon s stems. The additivity of chemical and “physical” hetts of mixing was confirmed by measuring the heat of mixing perfluoroieptane and acetone outside the two-phase region at 35
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Introduction Hydrogen bonding long has been known to be a major cause of non-ideality in aqueous and alcoholic solutions. Hydrogen bonds involving hydrogen attached to carbon(CH..-B, where B is a Lewis base) also can cause deviations from ideality in non-electrolyte solutions. Pimentel and McClellan3 document the existence of such bonds and conclude that halogen-activated C-H, acetylenic C-13, aiid S-E should be included in the usual list (OH, ISH, and 1114’) of groups capable of hydrogen bonding to Lewis bases. Chloroform acetone is perhaps the classic example of C-€I participation in such bonding. The large deviations from ideality observed for this system are characteristic of associated systems: GE, gEl TS“ and VE are -133, -437, -304 cal., and -0.2 ~ m . respectively, ~, at 35’ and x2 = ‘iZ. Fluorocarbon hydrocarbon interactions are not nearly so well understood. Scott4 discussed the anomalous behavior of these solutions and concluded (1958) that more work, experimental and theoretical, was needed before the problem could be clearly understood; this situation still is true. Perfluoro-n-heptane isoiictane (2,2,4-trimethylpentane) provides a good illustration of the unexpectedly large excess functions to be explained : GE, RE,TSE, und PE are 326, 508, 182 cal., and There is an upper 4.8 cm.a a t 30’ and x2 = critical point at t, = 22.7’ and xo = 0.63. As part of our continuing program of research on fluorocarbon solutions, we have studied the thermodynamic properties of a system in which hydrogen bonding competes with “fluorocarbon hydrocarbon” interactions: I-hydro-n-perfluoroheptane acetone (C7FLbH OCMe2). Strong association does not occur in either of the pure components, so the observed properties of mixing are directly related to the competition between the hydrogen bonding (exothermic) and the “fluorocarbon +hydrocarbori” (endothermic) interactions. We will
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(1) This work was supported in part b y the U. S. Atomic Energy Commission. in part by the American Chenlicat Society Petroleum Research Fund, and in part by the National Scicmcr Voundation Undergraduate Research Program. (2) Participant in the National Science Foundation Undergraduate Summer Research Program. (3) G. C. Pimentel and A. L. h1cClellan. “The Hydrogen Bond.” W. 12. Freeman and Company, San Francisco. Calif., 1960, Chapter 8. (4) R. L. Scott, J . l’hys. Chem., 62, 138 (1958).
refer to the former as chemical, the latter as “physical” interactions. Experimental Materials.-(‘Baker
Analyzed” reagent grade acetone
wae dried before using by distillin from a Linde Molecular
Sieve followiniz the Drocedure of aoward and Pike.’ Derisity at 25”, 6.7834‘ g . / ~ m .(Howard ~ arid Pike reported 0.7842), 72% 1.3558. 1-Hydroperfluoroheptane was prepared by decarboxylating the sodium salt of perfluorooctanoic acid (original sample given to us by the Minnesota Mining and Manufacturing Company; subsequent samples purchased from Mat.heson Coleman & Bell) in ethylene glycol. Final dist,illat,ion from a Linde Molecular Sieve through a glasspacked column gave 70-80% yields of C,IIlsH, boiling range 94.7-95.0”, density a t 25’, 1.7278 g./cm.3, n% 1.2i02. The original perfluorooctanoic acid contained about 30% branched isomers7 and the hydrofluorocarbon used also wae thought to contain about 30% branched isomers. Nuclear magnetic resonance (n.m.r.) spectra* indicate that most of the branching in these impurities occurs a t the end of the molecule away from the hydrogen. Such branching probably ha3 a negligible effect on the thermodynamic behavior of hydroperfluoroheptane acetone. We prepared n-perfluorooctanoic acid by fractional recrystallization7 and verified this assumption for the heats of m i ~ i n g . ~1--Hydro-n-perfluoroheptane prepared from recrystallized acid distilled a t 94 .go and had a lower refractive index than the impure material, n U D 1.26m. Perfluoroheptane ( GFl8), obtained from Union Carbide and Carbon Chemicals, was distilled from a Linde Molecular Sieve and a cut taken between 82.3 and 82.8”; density a t 25”, 1.721 g./cmSa,n% 1.2593. This product is known to contain branched isomers (Blumkin, et aE.,l0 report 20% of branohed isomers in a sample with a boiling range of Go), but the presence of a small amount of these should have a negligible effect upon the thermodynamic properties we have measured. Vapor Pressures.-Crude total vapor pressureliquid composition data were obtained by a simple procedure that required less than 20 of hydroperfluoroheptane. GFlaFI and OCMez were weighed into a small bulb (about 5 0171.3) containing a stirring bar and attached to a closed-end mercury manometer. Before evacuating the bulb-manometer system to eliminate air, the misture w a frozen ~ in liquid Nz. Pressure readings were made with a meter stick after equili-
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