H. T. PETERSON, D. E. MARTIRE,AND W. LINDNER
596
dicated the presence of conformational changes during the binding of cyclopropane by sperm whale myoglobin. The results of the present study indicating a 1:1 stoichiometric interaction between Nz and legoglobin exclude the mechanism proposed by Bauer and Abel for nitrogen fixation.15-17 The Nz molecule appears to be bonded to one molecule of ferrolegoglobin and not to a dimer formed by two ferrolegoglobin molecules as suggested by Abe1.l' Also, our attempts to find evidence for a second point of attachment were futile. The present study indicates that the affinity of legoglobin for N2 is very low, while that for oxygen is known to be very high. I n fact, the affinity of ferrolegoglobin for 02 is the highest mea-
sured for any hemoprotein. Applcby has determined spectrophotometrically equilibrium constants ranging from 15 to 25 mm-l for the half-oxygenation of different ferroleghemoglobin components a t The value obtained for half-nitrogenation of fcrrolegoglobin a t 25" in the present study was 1.03 X mm-l. This large difference in the affinity for the two gases suggests that legoglobin is probably more concerned with the transport of 0 2 than with N2 and that the new mechanism for nitrogen fixation proposed by Bergersenzois probably correct. We suggest, however, that leghemoglobin may be an ancillary nitrogen carrier. (41) C. A. Appleby, B i o c h k . Biophys. Acta, 60, 226 (1962).
Activity Coefficient of n-Heptane in 4,4'-Dihexyloxyazoxybenzene Liquid Crystal by Henry T. Peterson, Daniel E. Martire,* and Wolfgang Lindner Chemistry Department, Georgetown University, Washington, D . C. 80007 (Received Auguat 23, 1971) Publication costs assisted by Georgetown University
A McBain-Bakr apparatus, utilizing a Cahn RG electrobalance, has been constructed for the static measurement of activity coefficients. The activity coefficient of n-heptane in the nematic phase (90.1') of 4,4'-dihexyloxyazoxybenzene over a mole fraction range from 0.008 to 0.080 is reported. Good agreement is found between the extrapolated infinite dilution activity coefficient and that obtained by gas-liquid chromatography.
Recently gas-liquid chromatography (glc) was used to study the infinite dilution thermodynamic solution properties of nonmesomorphic solutes in nematogenic liquid crystals. Earlier experiments2 had strongly supported the concept of a two phase (bulk gaseous phase and bulk liquid crystal phase) glc partitioning process, and, hence, of negligible surface effects at both the carrier gas-liquid crystal and liquid crystal-solid support interfaces, provided that a liquid crystal film thickness of greater than about 1000 A was present. This finding produced initial confidence that the thermodynamic solution quantities measured by glc would reflect true bulk liquid crystal behavior. Nevertheless, no independent measurements were available for comparison, Since glc is a particularly advantageous and rapid method for such thermodynamic studies11r3 it was deemed important to confirm the glc activity coefficient results through comparison with values measured by a well-defined static method on bulk liquid crystal. The Journal of Physical Chemistry, Vol. 7 6 , No. .& 1972
Accordingly, a vacuum apparatus, based on the XcBain-Bakr approach4for obtaining absorption isotherms of volatile solute-nonvolatile solvent systems, and utilizing a Cahn RG electrobalance as the weighing device15was used to determine the activity coefficients of n-heptane over a limited concentration range in the nematogenic liquid crystal 4,4'-dihexyloxyazoxybenzene (DHAB). Measurements were carried out at 90.1" which is in the nematic region of DHAB.z This system was selected from those previously studied by glc in this laboratory for the following reasons. First, the large vapor pressure of heptane at the maximum allowable temperature of the mercury manom(1) L. C. Chow and D. E. Martire, J . Phys. Chem., 75, 2005 (1971). (2) L. C. Chow and D. E. Martire, ibid.,73, 1127 (1969). (3) D. E. Martire, P. A. Blasoo, P. F. Carone, L. C. Chow, and H. Vicini, ibid., 72, 3489 (1968). (4) J. W. McBain and A. M . Bakr, J . Amer. Chem. SOC.,48, 690 (1926). ( 5 ) R. L. Pecsok and B. H. Gump, 1.Phys. Chem., 71, 2202 (1967).
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ACTIVITY COEFFICIENT OF ?-&-HEPTANE eter, 50°, enables accurate measurement of the absolute pressure of heptane over a reasonable pressure range. Second, at temperatures greater than 115’, the “mechanical noise’’ due to aerodynamic effects increased rapidly with increasing temperature. Thus, the high temperature of the nematic-isotropic liquid transition (128.2’) precluded accurate studies in the isotropic liquid state. The DHAB, obtained from Frinton Laboratories, was purified by oxidation with hydrogen peroxide followed by three recrystallizations from ethanol.6 Differential scanning calorimetry measurements utilizing a Perkin-Elmer DSC-1B indicated a minimum purity of 99.4%.2 The n-heptane (Phillips Petroleum Co., Research Grade, 99.78y0 pure) was distilled twice under high vacuum to remove any dissolved air and was stored over sodium. The Cahn electrobalance was placed in a sealed glass vessel which was thermostated to prevent condensation of the heptane. The liquid crystal solvent was placed in a sample pan and suspended from the balance arm into a jacketed tube heated to 90.1” by a Haake NBE circulating bath. The temperature was measured to f0.03’ with a, calibrated Anschuetz thermometer suspended with its bulb 1 in. from the sample pan. The mole fraction of heptane in solution was determined from the initial weight of the DHAB (210 mg) and the observed weight pickup (measurable to 1 2 pg) a t the equilibrium vapor pressure. The equilibrium time per datum point was 3-4 hr. The vapor pressure of the system was measured to =k0.03 Torr with a thermostated mercury manometer and a cathetometer (Gaertner Scientific Co.). The usual precautions were taken to minimize systematic errors. Solute activity coefficients corrected for the nonideality of the vapor phase were calculated from the well known equation
where p is the equilibrium vapor pressure of the system, z2 is the solution mole fraction of heptane, p2O is the
saturated vapor pressure of heptane, Bzz is the second virial coefficient of heptane calculated from the McGlashan-Potter’ corresponding states equation, and Vzo is the molar volume of heptane det.ermined from the density given by the law of rectilinear diameters. All of the physical properties of heptane needed for the above calculations were taken from Dreisbach.8 A least-squares fit of t,he experimental results to the empirical equation log yz =
Ax12
+
23x13
(2)
I
0.00
0.02
0.04
I
0.06
0.08
0 J
Figure 1. Logarithm of the activity coefficient (log 7 2 ) us. solution mole fraction ( 2 2 ) for n-heptane in DHAB at 90.1’. Key: 0, static measurements from this study; X, gas-liquid chromatography result (ref I); -, least-squares fit to log yz = Ax12 Bxla, with A = 0.8995 and R = -0.3572.
+
which satisfies the Gibbs-Duhem relation, where x1 is the solvent mole fraction, yielded the results: A = 0.8995 and B = -0.3572. Figure 1 clearly illustrates the experimental results and the goodness of fit t o eq 2. The average standard deviation of log y2 from the curve is 0.0016. For comparison with the glc results, which are infinite dilution values, we let x1 3 1 and find that yzm = 3.49, with an estimated probable error of k0.02. A glc value of 7%“ = 3.54 was found,’ with a quoted probable error of 1.5% or f0.05. Hence, the values agree within the combined error of the two different measurements. It is important to note that discrepancy between the static and glc values cannot be attributed to a systematic error due to solute adsorption at the carrier gas-liquid crystal interface in the glc experiment, because the observed difference is in the wrong d i r e c t i ~ n . ~Therefore, all evidence being considered, it appears that the assumptions made in the glc measurement of thermodynamic solution properties in nematogenic liquid crystals are valid. Acknowledgment. This work was supported by the
U. S. Army Research Office, Durham, N. C. (6) M. J. S. Dewar and R. S. Goldberg, Tetrahedron Lett., 24, 2717 (1966). (7) M . L. McGlashan and D. J. B. Potter, Proc. Roy. SOC.,Ser. A , 267, 478 (1962). (8) R. R. Dreisbach, Advan. Chem. Ser., 22, 24 (1959). (9) D. E. Martire, “Progress in Gas Chromatography,” J. H. Purnell, Ed., Interscience, New York, N. Y., 1968, pp 93-120.
The Journal of Physical Chemistry, Vol. 76, No. .4.? 1979
