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Vapor Pressure Measurements of 4,4'-Dimethoxyazoxybenzene
Vapor Pressure Measurements of 4,4'-Dimethoxyazoxybenzene J. F. Solsky and Eli Grushka* Department of Chemistry, State Universityof New York at Buffalo, Buffalo, New York, 74274 (Received August 8, 7973)
The liquid crystal 4,4'-dimethoxyazoxybenzene (p-azoxyanisole, PAA) has long been used as a stationary phase in chromatography. During some recent studies on the affect of chromatographic support material on the properties of that stationary phase, the PAA was found to strip from the support. Subsequent measurements of the vapor pressure of PAA using an isoteniscope in the 122-145" temperature range yielded significant values ranging from 0.48 to 2.05 mm. The implications of these measurements on chromatographic data are discussed in this paper. Other preliminary results indicate that liquid crystals similar in structure to PAA (different only by the length of the alkyl side chains) also have appreciable vapor pressures.
Liquid crystals are popular stationary phases in chromatography, especially in the separation of ortho, meta, and para isomers. The liquid crystal 4,4'-dimethoxyazoxybenzene (p-azoxyanisole, PAA) has long been used. as such a stationary phase.1-10 In addition, chromatographic systems are used to measure thermodynamic parameters of liquid crystals in general and of PAA1.2,6J1J2 in particular. Due to some recent studies i n our laboratory on the behavior of PAA as a stationary phase in capillary columns,1° it was decided to investigate the affect of the support material on the properties of PAA as a stationary phase. A study of the behavior of PAA coated on glass beads in one case and on Chromosorb W in another was attempted. In both cases, reproducible results could not be obtained. When the chromatographic columns were unpacked it was observed that the packing (Le., glass beads or Chromasorb W) closer to the column inlet was stripped of the PAA. After the columns were unpacked, their inner walls were washed with CHzClz. From the color of the solution, it was obvious that some of the PAA adhered and coated the tubing wall. It became obvious then that PAA had an appreciable vapor pressure when in its nematic and isotropic range. A literature survey revealed that in general no work had been done on the measurement of vapor pressure of liquid crystals with the exception of one paper published in 1939 by Neumann.13 This is somewhat surprising since the vapor pressure can be related to intermolecular forces. If indeed PAA has an appreciable vapor pressure, then some of its thermodynamic properties which were obtained from gas chromatographic studies can be in doubt. This communication describes the measurement and the magnitude of the vapor pressure of PAA. Experimental Section Instrumentation. Vapor pressure measurements were done with a modified isoteniscope shown in Figure 1.The U tube (A) of the isoteniscope was filled with Hg. The PAA was placed in an evacuated sample bulb B. The isoteniscope was placed in an oil bath and heated with an immersion heater controlled with a 110-V ac variac. The oil bath was stirred with an air-driven stirrer. Readings of the difference in the mercury level in the isoteniscope were done with a telescope attached to a graduated meter pole.
Reagents. The PAA was purchased from Eastman Kodak. It was purified by three consecutive recrystallizations from hot 95% ethanol. The dried crystals were then sublimed in uucuo onto a cold finger. Procedure. For the vapor pressure measurements, approximately 0.10 g of the sublimed PAA was put into the sample bulb which was then sealed. Mercury was introduced through the stopcock into its reservoir C. The assembly was connected to a high vacuum line, equipped with a mercury diffusion pump, and evacuated for 1 hr. At the same time the mercury was heated with a heat gun to degas it completely. The stopcock was then closed and the mercury poured from its reservoir into the U-tube of the isoteniscope. The isoteniscope was then lowered into the oil bath. An equilibration period of at least 30 min was allowed between runs at two different temperatures. After the 30 min the vapor pressure was obtained every 15 min until it remained constant. The actual reading of the vapor pressure was obtained from the difference in the mercury levels in the isoteniscope's U-tube.
Results and Discussion Vapor Pressure Measurements. Initially we introduced into the measuring device PAA which was recrystallized from hot 95% ethanol. The vapor pressures obtained were much higher than expected. In addition, when the isoteniscope was lowered back to room temperature, some final pressure was obtained. This indicated that perhaps the PAA was not completely pure and some other compound exerted its own pressure in addition to that of PAA. It was thought that perhaps some ethanol molecules were hydrogen bonded to the azoxy part of PAA. However, an ir spectrum of the recrystallized material failed to reveal any ethanol molecules (at least not to a large extent). Reevacuating the system still failed to yield reproducible results, although it was noticed that the pressures obtained a t this point were lower than in the previous experiment. It was decided to sublime the PAA under vacuum a t 150". The PAA thusly purified gave consistent readings. Also after heating the PAA from room temperature to 148" and cooling back down to ambient, no residual vapor pressure could be noticed, It should be mentioned perhaps that purifying PAA is not a trivial problem and recrystalThe Journal of Physical Chemistry, Voi. 78, No. 3, 7974
J. F. Solsky and Eli Grushka
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Figure 1. The isoteniscope arrangement TABLE I: PAA Vapor Pressure as a Function of the Temoerature Temp. 'C 199 n "
124.4 126.1 126.9 129.0 130.6
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n - , d~ 0.75 0.80 0.85 0.95 1.15
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134.2 136.6 138.4 140.4 144.5
1.25 1.40 1.50 1.65 1.80 2.05
lization alone is not sufficient. Hsu and Johnson," for example, purified PAA with 30 passes of a zone refiner. The data obtained are iiven in Table I. The vapor pressure values were rounded up to the nearest 0.05 mm. Below 122" the vapor pressure increased slightly and insignificantly. We did not include these values in Table I due to the large error associated with the measurement at that range of pressure. Between 122 and 124' the pressure increased drastically. The vapor pressure values given in Table I are accurate to within 5%. This is due partially to errors in reading the telescope (SzO.025 mm) and partially to errors inherent in the method of measurement itself as recently discussed by Carruth and Kobayashi.15 We did not correct the values obtained for changes in the density of the mercury. Figure 2 shows a plot of In (vapor pressure) us. l/TK for all the points except 'that a t 122". The straight line was least-squares fitted to the data. The coefficients of the equation relating the vapor pressure to the temperature were found to be In P = 22.0 - (8870/T) (1) The slope of the line is -8870. The absolute value of the correlation coefficient is 0.9894 which indicates a 99% confidence level of a linear relation between the two parameters plotted. It must be stressed that we did not use a weighted least-squares scheme as we did not know the measurements' error dependence on the temperature. From the slope of the line the heat of vaporization, AH,!. The Journalof PhySlCal Chemistry. VOl. 78. NO. 3. 1974
Figure 3. The behavior of PAA in the glass U-tube. of PAA was estimated to be 17.8 kcal/mol. This value might, at first, seem rather high. If, however, one uses the "Hildebrand rule"'B the heat of vaporization of model compounds such as azobenzene is shout 16.8 kcal/mol, which is not far from the value for PAA. The Hildebrand relationship cannot be used directly for PAA since this molecule is polar and since its boiling .point is not reported (most likely it decomposes). Nonetheless, the value of 17.8 kcal/mol seems reasonable for the heat of vaporization of PAA. Around the clarification point the slope of the line should change and the difference between the two AHv should give the heat of transition which is about 0.17 kcal/mol for PAA.8 Such a small change in the slope of the line would not be easily detectable. In Figure 2 one could fit a straight line to the first six points (up to 132.2') and to the last five points. However, the difference in the slopes yields a transition heat of several kcals. We, consequently, do not feel that the data represented in Figure 2 can be broken up into two groups. In order to obtain the heat of transition a much more accurate and sophisti-
Vapor Pressure Measurements of 4,4'-Dimethoxyazoxybenzene
cated method of measurement of vapor pressure should be used. Such a method is currently not within our capabilities. Our vapor pressure data are significant mostly because they are the only values available to date. Chromatographic Behavior of PAA. To visually observe the stripping of PAA of the chromatographic packing, one arm of a glass U-tube was filled with Chromosorb W coated with 15% w/w PAA. The U-tube was placed in an oil bath maintained at 129 f 1". One arm of the U-tube was connected to a helium tank. The flow rate of the He was about 35 cc/min. After 24 hr, about 0.5 in. of the Chromosorb next to the helium inlet was completely white (the coated support had a light yellow color). PAA crystals were condensing on the glass wall of the other arm of the U-tube immediately above the surface of the oil. This region of PAA accumulation is clearly visible in Figure 3 which is the photograph of the U-tube. Unfortunately, the color contrast of the pure white and the yellowish support is not very distinguishable in the black and white photograph. From a chromatographic point of view this observation has an important implication. Columns with PAA as the stationary phase on solid support could bleed and thus would, as we observed, cause the column to deteriorate with time. In a recent study we investigated PAA as a stationary phase in stainless steel capillary columns.10 In that case, however, we did not have any difficulties with data reproduction. Perhaps the stainless steel walls with their high-energy surface can "anchor" the PAA molecule more firmly than Chromasorb W or glass beads. This point should be investigated further. We recently repeated the U-tube experiment with 44'bis(hexy1oxy)azoxybenzene (BHAB). Again, we observed a noticeable stripping of this mesophase. This indicates that the vapor pressure of the BHAB is also high. We did not, as of yet, measure that vapor pressure.
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In summary, PAA has an appreciable vapor pressure in the nematic and isotropic range. Unlike the results of Neumann13 for p-azoxyethylbenzoate, the vapor pressure of PAA is not constant through the mesophase region. This liquid crystal is not a viable stationary phase in chromatography and some of the measurements done on it should be reexamined. Indications are that other liquid crystals of the same family also have high vapor pressures.
Acknowledgment. We would like to thank 0. T. Beachley, Jr., for the use of his vacuum line and related equipment and the valuable discussions with him regarding the measurement of vapor pressures. References a n d Notes H. Kelker, Ber. Bunsenges. Phys. Chern., 67, 698 (1963). H. Kelker, Z.Anal. Chem., 198, 254 (1963). M. J. S. Dewar and J. P. Schroeder, J. Amer. Chem. SOC.,86, 5235 (1964). H. Kelker, B. Scheurle, and H. Winterscheidt, Anal. Chirn. Acta, 38, 17 (1967). M. J. S. Dewar, J. P. Schroeder, and D. C. Schroeder, J. Org. Chem., 32, 1692 (1967). H. Kelker and A. Verhelst, J. Chromatogr. Sci., 7, 79 (1969). J. P. Schroeder, D. C. Schroeder, and M. Katsikas in "Liquid Crystals and Ordered Fluids," J. F. Johnson and R. S. Porter, Ed., Plenum Press, New York, N. y., 1970, p 169. L. C. Chowand D. E. Martire, J. Phys. Chern., 73, 1127 (1969). M. A. Andrews, D. C. Schroeder, and J. P. Schroeder, J. Chrornatogf., 71, 233 (1972). E. Grushkaand J. F. Solsky, Anal. Chem., 45, 1836 (1973). L. C. Chow and D. E. Martire, J. Phys. Chem., 75, 2005 (1971). L. C. Chow and D. E. Martire, Mol. Cryst. Liquid Cryst., 14, 293 (1971). K. Neumann, 2. Elektrochern., 45, 202 (1939). E. C.-H. Hsu and J. F. Johnson, Mol. Cryst. Liquid Crysf., 20, 177 (1973). G. F. Carruth and R . Kobayashi, J. Chem. Eng. Data, 18, 115 (1973). J. H. Hildebrand and R. L. Scott, "The Solubility of Non-electrolytes," Dover Publication, New York, N. Y., 1964, p 426.
The Journal of Physicai Chemisfry, Voi. 78, No. 3, 1974
