NOTES
3676 6. Micropore Volume of Carbolac 1. The abnormal shape of isotherms for certain carbon blacks, such as Carbolac 1, suggests that they contain narrow micropores that fill at low relative pressure. When the volume adsorbed in such pores is included in the V , measurement of the surface area computation, the result is to give an apparent area that is much too large. I n 1959 we used the ideal isotherm to analyze adsorp&ion3by Carbolac 1, finding that the area of surface not in micropores is about 460 m2/g instead of the apparent value of near 1000 m2/g. Walker and Kotlensky19 have reported a similar analysis for Carbolac 1. Recently de Boer and a s s o c i a t e ~have ~ ~ also reported micropore analyses for Carbolac 1, based on V-t plots, but apparently they were unaware of the previous works, since no reference is made to them. Here too we find that use of our n’s gives quite different results than those with Lippens’ d s . An analysis for de Boer’s Carbolac 1 isotherm, using both sets of n’s, is shown in Figure 2D. If we assume that micropores fill by capillary condensation at relative pressures lower than those normally required to reach V m on a free surface, as demonstrated in studies of charcoals with very narrow pores,20it follows that the slope of the straight line portion gives the V , value for free surface not in pores and the intercept of this line on the Ti axis gives the amount adsorbed in the filled pores. Using our n’s for the V-n plot gives V,, the volume adsorbed in the filled micropores, as 126 cc (STP) and V,, the monolayer volume on free surface not in pores, as 105 cc (STP). With Lippens’ n’s we find V c = 155 cc (STP) and V , = 53 cc (STP). de Boer’s own analysis gives V , = 197 cc (STP) and V , = 64 cc (STP), but he has (incorrectly we think) assumed that the t values may be used to describe adsorption on the walls of very narrow pores. As noted above, such micropores appear to fill by capillary condensation which occurs at relative pressures even lower than those normally required to complete the first layer on a free surface. Consequently the t values based on free surface adsorption are not applicable. This point has also been noted by Sing.21 These Carbolac results, standing by themselves, do not provide any basis for a choice between our n’s and Lippens’. However, since his n’s do not fit the other isotherms discussed, it seems safe to conclude that the results with our n’s are the correct ones. Also we note that when our n’s are used to construct V-n plots for de Boer’s isotherms of two other carbon blacks, Spheron 6 and Elf 5, we find no indications of micropores in either whereas his own analysis does show such pores. To summarize we find no evidence in the recent work of de Boer and associates to discredit either our former n values or the results obtained in analyses of experimental isotherms by these. I n particular we find no reason to doubt our conclusions regarding the surface The Journal of Physical Chemistry
areas of uniform surface graphites and we still recombe used for the mend that a cross section of 19.3 nitrogen molecule at completion of the first layer on such surfaces.
Az
Acknowledgment. This research was supported in part by a grant from the Petroleum Research Fund administered by the American Chemical Society. Grateful acknowledgment is made to the donors of this fund. (19) P. L. Walker, Jr., and W. V. Kotlensky, Can. J. Chem., 40, 184 (1962). (20) C. Pierce, J. W. W h y , and R. N. Smith, J. Phys. Colloid Chem., 53, 669 (1949). (21) K. S. W. Sing, Chem. I n d . (London), 829 (1967).
Dielectric Relaxation in Pure Chloroform1
by Thirumalai V. Gopalan and Prasad K. Kadaba Department of Electrical Engineering, University of Kentucky, Lexington, Kentucky 40606 (Received February 88, 1968)
There is considerable discrepancy in the values of the loss factor e” of pure chloroform as reported by different workers. Conner and Smyth,2 for example, reported a value of 0.37 at 25” in the 10-cm region. The critical wavelength according to their measurements is 1.4 cm. F i ~ c h e ron , ~ the other hand, obtained a much smaller value of 0.0058 at 25” in the 5-cm region, where, according to the data of Conner and Smyth, one should obtain a value higher than 0.37 at 5 cm unless two peaks exist in the loss factor-frequency curve, one near the 1-cm region and the other above 10 cm. I n the present investigation, a systematic study of the microwave absorption of chloroform has been undertaken to verify the earlier results.
Experimental Section The real part E’ and the imaginary part e’’ of the dielectric constant of chloroform have been measured at 30” in the microwave region from 1.2 mm to 10 cm. A slight modification4 of the method developed by Surber6 has been used for the centimeter region. The measurements at 6.1 and 3.05 mm were made using the method of Pine, Zoellner, and RohrbaughU6 The (1) This work was supported in part by a contract from the U. E;. Atomic Energy Commission arid in part by a grant from the National Science Foundation. (2) W. P. Conner and C. P. Smyth, J. A m e r . Chem. SOC.,65, 382 (1943). (3) E. Fischer, Z . Naturforsch., Sa, 168 (1953). (4) P. K. Kadaba, J. Phys. Chem., 62, 887 (1958). (5) W. H. Surber, Jr., J. App2. Phgs., 19, 514 (1948). (6) C. Pine, W. G. Zoellner, and J. H. Rohrbaugh, J. Opt. SOC.Amer., 49, 1202 (1959).
3677
NOTES
2
4
3
5 €'
the Cole-Cole plot' and the plot of loss factor us. frequency are obtained and are shown in Figures l a and b, respectively. The Cole-Cole plot yields a semicircle with the extrapolated infinite frequency dielectric constant em = 2.18 and the relaxation time 7 = 5.4 X 10-12 sec. These values are in good agreement with e, = 2.16 8 at 20" and 7 = 5.4 X 10-l2 sec at 30" reported in the literature. Antony and Smyth,l0however, report the same value for 7 at 20". The values of e, calculated from the experimental values of e' and e'' at different wavelengths agree with each other within the limit of experimental error. It is also seen that the loss factor-frequency curve shows only one peak, and the data on the low frequency side of the peak seem to be consistent with the data of Conner and Symth2 rather than that of F i ~ c h e r . ~Using the value of e,, PE+A, the sum of the electronic and atomic polarization, = (em - 1)M/ was calculated from the relation PE+A (e, 2)d, where M is the molecular weight and d is the density. The value of PE+Athus obtained was 22.79 cc. PE,the electronic polarization, was calculated to be 20.79 cc by using the Cauchy dispersion equation" PE = R[1 - (ho2/h2)]
+
I
1
5
IO
.
>
20
I
....
50 10
W X IO
IN
RADIANS
I
I
100
ZOO
SEC.-'
Figure 1. (a) Cole-Cole plot for chloroform a t 30'; (b) loss factor E'' us. angular frequency w for chloroform a t 30'. The points marked by X indicate the 2.2-mm measurement a t 20' of S. K. Garg and C. P. Smyth, J. Chem. Phys.,
42, 1397 (1965).
method involves the free-space analog of a shorted-line reflectometer. Electromagnetic energy at 3.05 mm was produced using a harmonic doubler and the 6.1-mm klystron source. The 1.2-mm measurement was made using a FS-520 Fourier spectrophotometer, manufactured by the Research Industrial Instrument Company, London. The accuracy in the determination of e' was 1% and in the determination of e'' was 3%, The static dielectric constants have been measured at a frequency of 50 kc using a 716-C General Radio capacitance bridge. The accuracy in the determination of the static constants was 0.5%. The refractive indices have been measured using an Abbe refractometer. The calculations of the dielectric constant and loss factor were made using an IBM 360 computer.
where R is the refraction at wavelength X and Xo is the characteristic wavelength. The value of PA,the atomic polarization, thus obtained was 2.01 cc. This value is slightly higher than the 1.57 cc12found in the literature.8 The present value, however, is low compared with the value of 4 cc obtained from the variation of the total polarization with temperature in the gaseous state.'* The value from gas measurements is uncertain, however, because of long extrapolation.
Acknowledgment. We express our thanks to Miss Lena Foley for typing the manuscript. (7) K. 8. Cole and R. H. Cole, J. Chem. Phys., 9, 341 (1941). (8) 5.X. Garg and C. P. Smyth, ibid., 42, 1397 (1965). (9) S. Mallikurjun and N. E. Hill, Trans. Faraday Soc., 16, 1389 (1965). (IO) A. A. Antony and C. P. Smyth, J. Amer. Chem. Soc., 86, 152 (1964). (11) C. P. Smyth, "Dielectric Behavior and Structure," MoGrawHill Book Co., Inc., New York, N. Y.,1955,p 405. (12) This value is erroneously reported as 1.77 in ref 8. (13) Reference 11, p 420.
Materials Chloroform was obtained from the Eastman Kodak Co. and was chromatoquality reagent grade.
Electron Affinities and the Electron-Capture Method for Aromatic Hydrocarbons
Results and Discussion The dielectric constant and loss of chloroform have been measured at 10.09, 3.21, 2.68, 2.17, and 1.29 cm and 6.1, 3.05, and 1.2 mm at 30". The measured static dielectric constant at 30" was 4.65 and the measured refractive index was 1.4454. Using these data,
by L. E. Lyons, G. C. Morris, and L. J. Warren Department of Chemistry, University of Queensland, Brisbane, Australia (Received April 22, 1968)
The electron-capture method' of Wentworth, Chen, and Lovelock yields an energy quantity, which we call Volume 73,Numbw 10 October 1068
