April, 1961
ABSOLUTEADSORPTION ISOTHERMS O F S I T R O G E S , BENZENE A S D n-HEXASE
which also accords with Kozlov’s results. The usefulness of such a complex expression as (15) is questionable, of course. Acknowledgment.-The authors are pleased to
60 1
note that they have benefited from discussions of this work wi.th Dr. W. H. Avery, and from the able assistance of Mr. S. Favin in the numerical calculations.
THE ABSOLUTE ADSORPTION ISOTHERMS OF VAPORS OF NITROGEN, BENZENE AND n-HEXANE, AND THE HEATS OF ADSORPTION OF BENZENE AND n-HEXANE ON GRAPHITIZED CARBON BLACKS. I. GRAPHITIZED THERMAL BLACKS BYA. A. ISIRIKYAN AND A. V. KISELEV Adsorption Laborai‘ory, Department of Chemistry, M.V . Lomonosov Moscow State University, Surface Chemistry Group, Institute of Physical Chemistry, U.S.S.R.Academy of Sciences, Moscow,Russia Rcceined August I, 1860
Unified absolute adsorption isotherms for nitrogen a t --195”,and n-hexane and benzene a t 20” have been measured using a series of ‘‘graphitized’ medium thermal carbon blacks as adsorbents. Differential heats of adsorption as a function of
surface coverage mere also measured. The initial portion of the nitrogen isotherms is slightly convex and well described in terms of localized ,adsorption, taking into account interaction of adsorbate molecules. The adsorption isotherm for benzene, on the other hand, is initially concave, and the heat of adsorption is found to be practically constant over the entire monolayer coverage. In the case of *hexane, the heat of adsorption rises by about 15% during monolayer coverage, and the initial portion of the isotherm, like nitrogen, is slightly convex. The newly-measured heats of adsorption are in good agreement with earlier calculated values of the potential energy of adsorption.
Introduction In studies of a,dsorption on solid bodies, the measured quantities are usually relative. This is due to the indeterininate geometrical and chemical structure of most adsorbents. For this reason, there is an unfortunate gulf between experiment and theories of adsorption forces and adsorption equilibria, which, in quantitative form, refer specifically to homogeneous surfaces. l v 2 Even such a basic quantity as specific surface of the adsorbent frequently remains indeterminate, because a t the basis of the BET method8 lies (among other limitations) the assumption of a more homogeneous surface. It is important, therefore, to employ adsorbents of homogeneous surface which will provide reproducible results i n all laboratories. I n this connection, adsorbents having their surface composed principally of planes of a single index, and of sufficient area for precise measurements, are most desired. One of the simplest adsorbents of this type is “graphitized” carbon black. 1-4-16 Thermal (1) A. V. Kiselev, Vestnik Akademti Nauk SSSR, No. 10,43 (1957): N. N. Avgul, A. A. Isirikyan, A. V. Kiselev, I. A. Lygina and D. P. Poshkus, Izvestia Akademii Nauk SSSR, Dept. Chem. Sci., 1314 (1957); N. N. Avgul, A. V. Kiselev, I. A. Lygina and D. P. Poshkus, ibid.. 1196 (1959). (2) A. V. Kiselev, D. P. Poshkus, Dokl. Akad. Nauk S S S R , 132, 148 (1960). (3) S. Brunauer, P. H. E m m e t t and E. Teller, J . A m . Chem. Soc., 6 0 , 309 (1938). (4) R. A. Beebe, J. Biscoe, W. R. Smith and C. B. Wendell, J . A m . Chsm. 8%. 69, 95 (1947); C. Pierce and R. N. Smith, ibid., 76, 846 (1953): R. A. Beebe and D. M. Young, J. Phys. Chem., 58, 93 (1954). (5) W.D.Sohaeffer, W. R. Smith a n d M. H. Polley, Ind. Eng. Chem., 46, 1721 (1953). (6) S. Ross and W. Winkler, J. CoZZ. Sei., 10, 319, 330 (1955); S. Rosa and W. W. Pultz, ibid., 13, 397 (1958). (7) A. V. Kisdev and E. V. Khrapova, KoZZoidn. shurn., 2 3 , 2 (1961). (8) A. A. Isirikyan and A. V. Kiselev, ibid., in presa. (9) R. A. Beebe and J. M. Holmes, J . Phya. Chsm., 61, 1684 (1957).
carbon blacks heated in absence of air to 3000’ have an especially homogeneous s u r f a ~ e . ~ - ~ J 1 , ~ ~ - ~ ~ As a result of heat treatment, the initially spherical particles become polyhedra,6 with faces consisting mainly of homogeneous basal planes of single crystals of graphite. These single crystals are connected along the edges of the polyhedron. While these boundaries give rise to some residual heterogeneity, it is so small it does not influence adsorbate-adsorbent interaction. A comparison of adsorption isotherms and heats of adsorption measurements carried out on such carbon blacks in many laboratories demonstrates the excellent reproducibility of both the measurements and the surface uniformity. Using a number of graphitized thermal blacks, we have investigated the adsorption of nitrogen a t -195’: first, because reliable data are already available in the region of small6 and largeg coverages and, second, because nitrogen usually is used in determining specific surface by the BET method. Isotherms and differential heats of adsorption of benzene and n-hexane were also obtained. Al(10) A. V. Kiselev, KoZloidn. zhurn., 20, 338 (1958); A. V. Kiselev, N. V. Kovaleva, V. -4. Sinitsyn and E. V. Khrapova, ibid., 20, 444 (1958). (11) R. A. Beebe, J. M. Holmes, A. V. Kiselevand N. V. Kovaleva, J. Phys. Chem., in press. (12) N. iV. Avgul, A. V. Kiselev, A. S a . Korolev and I. A. Lygina, KoZZoidn. #hum., 20, 298 (1958). (13) N. N. Avgul, G. I. Berezin, A. V. Kiselev and 1. A. Lygina, Zhum. Ae. Khimzi, S O , 2106 (1956); Izv. Akad. Nauk S S S R , Dpt. Chem. Set., 1304 (1956); 1021 (1957); 787 (1959); A. V. Kiselev, “Proc. Second Internat. Congress on Surface Activity,” Vol. 2, London, 1957, p. 168. (14) W.B. Spencer, C. H. Amberg and R. A. Beehe, J. Phys. Chem.. 6 2 , 719 (1958). (15) N. N. Avgul, G. I. Berezin, A. V. K i e l e v and I. A. Lygina. Izv. Akad. Nauk S S S R , Dept. Chem. Scz., in press. (16) M. H. Polley, 5%’. D. Schaeffer and W. R. Smith, J . Phys. Chsm. 67, 469 (1953).
A. A. ISIRIKYAS A X D -4.T'. I~ISELEV
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0.1 0.15 0.2 0 0.2 0.4 0.6 0.8 I PIPS. P/P,. Fig. 1.-Absolute adsorption isotherm of nitrogen vapor a t -195' on graphitized thermal blacks: (1) MT-1 (3100"); (2) FT (2800"); (3) T-1 (3000'); (4) T-3(3200'); (5) MT (3100') data from (9); ( 6 ) FT (2800') data from (6).
though these molecules contain the same number of carbon atoms, they are radically different in structure. Benzene, though non-polar, has an uneven density distribution of 9-electrons. For this reason, adsorption of benzene is sensitive to presence of surface hydroxyl groups as are all molecules with large quadrupole moments. 17-21 n-Hexane is practically insensitive to this factor. Benzene molecules are planar and, in the case of adsorption on a non-polar adsorbent, the weak electrokinetic (dispersion) attraction between them is, apparently, balanced by the weak electrostatic repulsion of the CH dipoles and quadrupoles formed by n-electron clouds, so that the heat of adsorption remains nearly constant during filling of the monolayer.'* On the other hand, molecules of n-hexane are strongly attracted to each other and the heat of adsorption passes through a m a ~ i r n u m . ~ J ~ Experimental Adsorbents.-The adsorbents used in the present study were thermal carbon blacks graphitized at 3000 i: 300" in the absence of air. The thermal blacks T-1 and T-3 were prepared in this Laboratory and subsequently graphitized by us. The graphitized samples Sterling FT (2800') and Sterling hlT (3100') were obtained from Dr. W. R.Smith of Cabot Corporation, and were from the same lot as the (17) A. V. Kiselev, Dokl. A k a d . Nauk S S S R , 106, 1046 (1956); A. A. Isirikyan and A. V. Kiselev, ibid., 116, 343 (1957); A. V. Kiselev and L. D. Belyakova, ibid., 119, 298 (1958); "The Structure and Properties of Porous Materials," Ed. D. H. Everett and E . Stone, London. 1958, p. 195. (18) A. V. Kiselev and D. P. Poshkus, Dokl. A k a d . Nauk S S S R , i20, 834 (1958). (19) L. E. Drain, Trans. Faradail Soc., 49, 650 (1953): L. E. Drain and J. A. Morrison, ibid., 49, 664 (1953). (20) T. Hayakawa, Bull. Chem. SOC.Japan, 30, 236 (1957). (21) G . J. C. Frohrsdorff and G. L. Kington, Trans. Faraday Soc., 66, 1173 (1959).
samples previously supplied Dr. Ross and Dr. Beebe.689 The Sterling MT-1 (3100") was a freshly prepared sample obtained from Dr. Smith a t a somewhat later date. The samples T-1 and T-3 were graphitized in our laboratory a t 3000" for 45 and 10 minutes, respectively. The Sterling MT and FT samples supplied by Dr. Smith had been heat treated for two hours. Earlier studies have shown that graphitization at 3000" is very rapid and it can be assumed that all the above samples are at about the same degree of graphitization. Prior to the experiments, the samples were evacuated in the system at 450' for 6-10 hours.22 Absorbates.-Nitrogen was obtained first from sodium azide and then by purification of tank nitrogen. The benzene and n-hexane were the same as used in earlier work.*J2 Adsorption of nitrogen was measured on two devices with accurately calibrated gas burets, first directly in the calorimetric artr ridge,^,^',^^ and then ~ e p a r a t e l y . ~ In~ the low pressure region, corrections were introduced in accordance with Chu Liang.24 The saturation pressure, P., was measured with a nitrogen thermometer, and ranged from 780 to 810 mm. Measurements of the adsorption and heats of adsorption of benzene and n-hexane were carried out with the vacuum capillary microburet for liquids, and a calorimeter with constant heat exchange as described earlier .22
Results and Discussion Determination of Specific Surfaces.-Data from Ross6 and Beebeg for the adsorption of nitrogen on similar samples of graphitized thermal blacks were included with our data in preparing BET plots. All the data provided linear plots over relative pressures ranging from 0.0003 to 0.09. Values for the monolayer capacity a,, in micromoles per ( 2 2 ) -4. A. Isirikyan and A. V. Kiselev, Zhurn. fLz. khzm., 31, 2127 (1957); 32, 679 (1958); A. A. Isirikyan, A. V. Kiselev and G. G. Muttik, "Proc. Second Intern. Congress on Surface .4ctivity," Vol. 2, London, 1957. p. 214. (23) B. G. Aristor, A. P. Karnaukhov and A. V. Kiselev, Rolloidn. zhurn (in press). (24) S. Chu Liang, J. A p p l . Phys., 22, 148 (1951).
April, 1961
ABSOLUTEADSORPTIOU ISOTHERXS OF XITROGEU, BENZENE ASD 12-HEXASE
603
I . . c , H , ~
3 -
4
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0 0.2 0.4 0.6 0.8 1 PlP,. Fig. 2.-Absolute adsorption isotherm of benzene vapor at 20" on graphitized thermal blacks: (1) MT-1 (3100'); (2) FT (2800'); (3) T-1 (3000'); (4)T-3 (3200'). 0.02 0.04 0.06 0.08 0.1
to those obtained with nitrogen on the same adsorbents. Since it is difficult to make an independent determination of urn for n-hexane with a high degree of accuracy, this value was determined from the TABLE I values of specific surface found with nitrogen, and CHARACTERISTICS OF SAMPLES OF GRAPHITIZED CARBON the value wm as determined from the BET plot for BLACKS USED the adsorption of n-hexane. The values of for ----Results of applying BET method--n-hexane thus evaluated are given in column 7 of Nitrogen Benzene urn = 16.2 k.2 w,, = 40 k.1 %-Hexane Table I. They are in good agzeement, and proam am a,, vide an average value of 51.0 A.2 This value is s, &mole/ s, &mole/ emole/ rn.z/g. g. m.z/g. g. A.2 Adsorbent g. close to the 54 A.*value estimated in earlier work,13.25 T-1 (3000") 298.5 29.1 122.5 29.5 94.8 51.0 from the density of the liquid assuming T = 4 8. ... .. T-3 (3200") 6s 0 6 . 6 3 27.6 6.65 and the van der Waals dimensions of n-hexane exSterling FT tended along the surface. Thus, using the BET (2800') 125.3 12.22 50.8 12.23 39.9 50.9 method, it is feasible to determine the specific surSterling MT face from the adsorption of n-hexane a t 20°, em(3100') 66.8 6.51 . . . ... ... . . ploying urn = 51.0 8.as the cross-sectional area of Sterling RIT-1 the adsorbate. (3100") 78.4 7.65 31.9 7.68 25.06 50.8 The Absolute Adsorption Isotherm of Nitro1 gives the isotherm, at -195", of gen.-Figure The BET plots for benzene and n-hexane are linear over relative pressures from 0.04 to 0.12 for the absolute values of the adsorption of nitrogen benzene, and 0.002 to 0.12 for n-hexane. The corre- (per unit area), cy = u / s , * ~and the extent of sursponding values of a, are given in columns 4 and face coverage, 0 = alam. I n the case of Sterling G of Table I. Assuming, for a plane benzene mole- FT (2S0Oo), the value of 12.22 m.2/g. that we cule, am = V m / N T where om is the molar volume of determined coincides with that reported by liquid benzene, N Avogadro's number, and T, the Beebe, et aZ.,'* which indicates that the samples van der Waals thickn$ss of the molecule, as 3.70 A., were identical. This value was taken for recalcuwe obtain urn = 40.0 A.2 The corresponding values (25) A. A. Isirikyan and A V. Kiselev, Z h u m . Pz. khim. (in press). of specific axes, s, given in column 5 , are very close (26) a is the measured value of adsorption per gram of adsorbent.
gram, and specifc areas, s, using 16.2 for nitrogen cross-sectional area, were computed from these plots and are reported in columns 2 and 3 of Table I.
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A. il. ISIRIKYAN AND A. V. KISELEV
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CY, fi mole/m.*. differential heat of adsorption of benzene vapor at 20" as a function of the surface coverage of graphitbed thermal blacks: (1) MT-1 (3100'); (2) FT (2800"); (3) T-1 (3000"); (4) T-3 (3200').
lation of the data of Ross and co-workersRin terms of absolute v a l ~ e s . ~ For ' our sample of Sterling MI' (3100°), we obtained an area of 7.65 m.2/g., ~~ the value 6.3 m.*/g. while Beebe, el C Z ~ . ,report for their sample. Thus, apparently these were different though very similar samples of graphitized thermal black. Therefore, in the determinations of absolute values from our data, we used s = 7.65 m.2/g. We find that the data of Beebe, et aZ.,14 can be brought into agreement with our data on the basis of an area of 6.51 m.2/g., obtained from a BET plot in the same p / p 8 interval, rather than their 6.3 value. li'rom Fig. 1 it is seen that for all thermal blacks, heated t o about 3000°, the absolute values of adsorption, determined from both the measurements of Ross and Beebe,Gv8 as well as from our own, lie on one and the same isotherm.28 The absolute isotherm thus obtained may be attributed to the adsorption of nitrogen on the basal plane of graphite, To facilitate reproduction of this isotherm as a standard, the interpolated values of a and p / p s are provided in Table 11. A complete theoretical description of the isotherm for mono-molecular and, particularly, multi-molecular adsorption, requires a statistical-thermodynamic treatment, with account taken of the effect on the distribution function of the energies of adsorbate-adsorbent and adsorbate-adsorbate interactions and their variation with surface coverage. Since this has not been done as yet, use should be made of approximate equations, in which only the shape of the isotherm has been derived theoretically,
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(27) In Paper 6 the reported value of 8 11.7 m.*/g., for this sample, was eatimated from "point B." (28) T h e m m e conclusion, though somewhat less precise, waa made earlier, 7 ref., v i t h the help of data from references 6 and 16.
TABLE I1 ABSOLUTEVALUES OF ADSORPTIONOF NITROGENON THE BAs.kL FACE O F GRAPHITE AT - 195' The surface is defined according t o BET at urn = 16.2 A.2 (see column 3 of Table I) a,
ff,
p/p8
pmole/m.*
p/p.
pmole/m.l
0.00003 .00005
0.50 0.90 1.45 1.90 2.55 3.50 4.13 4.72 5.35 5.80 6.33 6.70 7 08 7.37 7.75 8.07 8.42
0.0024 ,0037
8.93 3.18 9.45 9.70 10.00 10.35 10.59 10.72 10.91 11.05 11.27 11.60 11.95 12.46 13.15 14.36 16.00
.00008
.00010 .00013 ,00017 .00020 ,00023 .00027 .00031 .00037 .00043 .00051 .00060 .00075 ,00095 .0013
.OO%
.0075 ,0035 .014 .025 .040 .06 .OS -10 .13 .16 .19 .22 .26 .30
p / ~ .
0.35 .40 .45 .50 .55 .60 .65 .70 .75 .80 .85 .90
.94 .96 .97 .98
pmole/ m.2
17.6 19.0 20.2 21.2 22.2 23.3 24.i 26.7 29.2 31.6 34.5 40.0 47.5 58.0 66.0 78.0
while the numerical values of the constants are found from experiment. The convex initial portion of the nitrogen isotherm on graphitized carbon black shows that in a more detailed description of the isotherm one cannot neglect adsorbate-adsorbate interactions as compared with adsorbate-adthe sorbent interactions. In the work of ROSS,~ adsorption isotherm of nitrogen in the region of monolayer coverage was described by three equations: the Henry equation, the Hill equation for non-localized monomolecular adsorption, which
April, 1961
ABSOLCTE ADSORPTION ISOTHERMS OF NITROGEN, BENZENEAND WHEXANE
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Fig. I.-Absolute
0.02 0.04 0.06 0.08 0.1
0
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PlPm ;adsorption isotherm of n-hexane vapor at 20' on graphitized thermal blacks: (1) MT-1 (3100'); (2) FT (2800"); (3) T-1 (3000').
accounts for adsorbate-adsorbate i n t e r a c t i ~ n , ~poorer ~ * ~ ~ results in this instance. Thus, nitrogen and the Langmuir equation for localized adsorption adsorption a t - 195' on the basal face of graphite is without interaction. I n papers7vl0the entire re- predominantly localized. Adsorption and the Heat of Adsorption of Bengion of monomolecular coverage was described by 2 presents an isotherm of the the following single equation, 1, which took into zene.-Figure consideration the adsorbate--adsorbate interactions absolute values of adsorption of benzene vapor on in the case of monomolecular localized adsorptionlo the graphitized thermal blacks MT-1 (3100°), FT (2800'), T-2 (3200') and T-1 (3100'). The data for 0 (1) these adsorbents lie close to a single general curve. P ~ P ' K ~ ( I- e)(l Kne) Table I11 contains the appropriate interpolated where 6 = a/aIn,and K1and Kn are equilibrium values of a and p / p s for the construction of this constants of adfsorbate-adsorbent and adsorbate- isotherm. In the region of monolayer adsorption, adsorbate interae tions. the isotherm is concme. Up to a = 9 pmole,/m.2 the isotherm in Fig. 1 is best described by equation 1 for 0 = a/9.23 (i.e., TABLE I11 for wm = 18 A.2), Iil = 900 and K, = 8) hence AESOLUTEVALUESOF ADSORPTIONOF BENZEKEO N THE
+
a/9.23
= 900( 1 - a/9.23)(1
+ 8a/9.23)
(2)
Equation 2 may be used as a good interpolation formula, because the isotherm calculated from it coincides with the experimental isotherm (up to a = 9 pmole/m.2 and relative pressure of 0.0025). I n the region of lowest coverage, the experimental points lie somewhat higher. This may be attributed to enhanced adsorption at sites of residual inhomogeneity of the surface of the adsorbent.a1 Hill's equation of non-localized adsorption yields (29) T. 1,. Hill, J. Chcm. PAYS., 14, 441 (1946).
(30) J. H. de Boer, "The Dynamicnl Character of Adsorption," Oxford 1953. (31) When applying equation 2, one must bear in mind t h w the values of 8 used for constructing the absolute isotherm of Fig. 1 were determined froin expermental isotherms by the B E T method using om 16.2 A.? T h e BET equation does not describe B wave-like isotherm; for one thing, it does not describe the convex portion of its Erst wave.
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BASALFACE OF GRAPHITS AT 20' The surface is determined from nitrk7gen a t -195' by the BET method, using urn = 16.2 A.?; p s = 75.4 mm. a,
+mole/
a,
m.2
P/P.
0.0005 .001 .002
0.15 .27 .50
,003
.il
,004 ,005 .006 .007 ,008 ,009 ,010 ,015
.90 1.09 1.28 1.45 1.60 1.74 1.87 2.40 2.74 3.02
0.030 .035 ,040 ,050 .06 .07 .OS .09 .10 .15 .20
P/P.
,020
.025
.25
.30 .35
+mole/ m.2
3.24 3.40 3.51 3.71 3.8i
4.00 4.11 4.19 4.27 4.61 4.83 5.OG 5.30 5.56
a.
p/p.
0.40 .43 .50 .55 .60
+mole/ m. 2
.70 .75 .80 .825 .850
5.86 6.25 6.77 7.33 8.08 8.88 9.80 10.80 11.90 12.50 13.40
.E375
14.40
.YO0 ,925
16.20 20.0
.B5
A. A. ISIRIKYAN AND A. V. KISELEV
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6 pmole/m.2. differential heat of adsorption of n-hexane vapor a t 20' as a function of surface coverage of graphitized thermal blacks: (1) NIT-1 (3100"); (2) FT (2800O); (3) T-1 (3000').
4
CY,
Fig. 5.-The
Figure 3 gives the differential heat of adsorption of benzene as a function of the surface coverage on the graphitized thermal blacks. All samples exhibit an initial region of enhanced heats, connected perhaps with penetration of the benzene molecules into grain boundaries of the single crystals of graphite. However, this region is small. Beginning with 8 = 0.1, the heat of adsorption is nearly constant to 8 = 0.9, after which it falls sharply in accordance with the transition to predominantly multilayer adsorption. The interpolated numerical data for the construction of this curve are given in Table IV. Extra olating to zero coverage, we find 10.15 kcal. mole for &&. In a previous publication,12 we have reported 10.2 and 10.3 kcal. for 8 = 0 and 8 = 0.5. These experimental values for the differential heats are in agreement with the value 10.3 kcal./mole calculated from theory.' It is observed from Fig. 3 that the net heat of adsorption is nearly constant over the monolayer, being 2.15 kcal./mole. It is further of interest that the net heat in the second layer is virtually constant a t a value of 0.15 kcal./mole. Thus the adsorption of benzene at 20' on the basal face of "graphite" in the region of first and second layer coverage provides an example of a constant net heat, consistent with a basic assumption of the BET theory. Adsorption and Heat of Adsorption of n-Hexane Vapor.-Figure 4 gives the absolute adsorption isotherms of hexane. The points for the three adsorbents MT-1 (3100°), FT (2800") and T-1 (3000') lie on nearly a single curve, although it may be noted that for the sample with the most homogeneous surface, Sterling MT (3100') the points in the initial region lie somewhat lower,
,P
TABLE IV THE VALUESOF DIFFERENTIAL HEATOF ADSORPTIONOF BENZENE AT 20" FOR VARIOUSCOVERAGES OF THE BASAL FACEOF GRAPHITE a, a, Q,. a, Q. Q9.
fiLmole/ m.2
0.2 0.6 1.0 1.4 2.0 2.6
koal./ mole
(10.3) 10.20 10.22 10.25 10.28 10.33
pmole/ m.2
3.2 3.6 4.0 4.4
4.8 5.2
kcal./ mole
10.38 10.30 10.00 9.55 9.05 8.70
pmole/ m.2
5.5 6.0 7.0 8.0 9.0 10.0
koal:/ mole
8.50 8.35 8.30 8.30 8.30 8.30
while in the region of sharp inflection of the isotherm, about 8 = 0.7-0.9, they lie somewhat above the points for Sterling FT (2SOO0) and the somewhat less homogeneous sample T-1 (3000'). The data for Sterling M T (3100') are closest to the absolute isotherm. Similar conclusions have been arrived a t for adsorption of ammonia on this a d s ~ r b e n t . For ~ this reason, these data were given preference in construction of the absolute isotherm. The interpolated data for construction of this absolute isotherm are given in Table V. The region of initial decline of the heats of adsorption, as shown in Fig. 5, extends to B = 0.1. Following this decline, they rise perceptibly in an approximately linear fashion and attain a maximum a t 8 = 0.9. They then fall off sharply in accord with a transition to predominantly multilayer adsorption. The numerical data for constructing this curve are given in Table VI.
April, 1961
ABSOLUTE QDSORPTION ISOTHERMS
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NITROGEN, BENZENE AND HEXANE
607
monolayer, ie., from a coverage of 0.1 to 0.9, the heat of adsorption increases by only 15%. The large value of the net heat and relatively small increase in the heat of adsorption during filling of the monolayer, coupled with a comparatively small a. net heat after completion of the monolayer, is re@mole/ @mPdle/ @mole/ sponsible for the satisfactory manner in which the PIP. m.2 PIP. m.% P / P ~ m.2 isotherms are described by the BET equation. 0.0001 0.12 0.004 2.63 0.35 1.79 However, the heat of adsorption does increase to a ,005 2.73 .40 5.18 ,0002 0.23 significant degree during monolayer coverage due to .0004 0.48 .01 2.98 .45 5.61 adsorbate-adsorbate interaction, and this is re.02 3.17 .50 G.06 ,0006 0.79 sponsible for t'he fact that the adsorption isotherm .03 3.26 .55 6.58 .0008 1.10 is initially convex32 (see data for M T (3100') in .04 3.33 .60 7.00 .0010 1.38 Fig. 4). Since the heat of adsorption of n-hexane .06 3.44 .65 7.51 ,0012 1.62 is large and the CH3 and CH2 groups in the mole.OS 3.52 .70 8.09 ,0014 1.82 cide can locate over the sites of highest energy in .10 3.60 .75 8.77 ,0016 1.97 the graphite planes (ie., mid-points of the carbon .15 3.77 .80 9.63 ,0018 2.10 hexagons), the adsorption of n-hexane may be pre.20 3.97 .85 11.0 ,0023 2.31 dominantly localized. .25 4.21 .90 13.5 .0028 2.43 Specific Surface from Absolute Isotherms..30 4.46 ,0034 2.54 From the absolute isotherms provided by the dat,a TABLE VI of Tables 11, I11 and V, it is possible to obtain the VALUES OF DIFFERENTIAL HEATOF ADSORPTIONOF n- specific surface of a carbon black adsorbent by measuring only a single adsorption point, a, at HEXANE A T 20" AT 'cTARIOUS C O V E R A G E S O F THE B h s A L some convenient relative pressure. The correFACE OF GRAPHITE sponding value of CY may be read from the absolute The value in parentheses has been extrapolated a, Q., isotherm and the specific area, s, obtained from the @mole/ Q. ,%e/ Q~, @mole/ lical./ relation s = CY.^^,^^ Since the adsorption of nm.2 kcal./kole m.2 kcal./mole m.2 mole hexane is insensitive to the degree of oxidation (11.40) 2.6 13.8 4.8 7.90 0.1 of the surface, the procedure should apply to all 2.8 13.10 5 4 8.00 11.35 .2 blacks as well as t'o the graphitized samples we have 0.0 8.12 3.0 12.70 11.50 .4 studied. This method would not, of course, be 6.6 8.14 3.2 11.50 11.70 .6 expect'ed t'o apply to carbon blacks possessing any 7.2 8.00 12.08 3.4 10.10 1.o considerable degree of porosity. The use of n7.8 7.80 3.6 9.00 12.40 1.4 hexane as an adsorbate provides a method not re12.75 7.80 8.15 8.4 3.8 1.8 quiring a supply of liquid nitrogen. 7.80 7.80 9.2 13.03 4.0 2.2 7.80 Acknowledgment.-The authors express their 7.80 10.0 13.13 4.4 2.4 gratitude to Dr. W. R. Smith of the Cabot CorThe pertinent da,ta from Fig. 5 may be shown as poration for the samples of graphitized carbon Coverage Heat of adsorption Net heat black, FT (2800') and MT-1 (3100'), and for his 0 Q. &a - L assistance in preparing our manuscript for publica0.1 11.3kcal. 3.7 hion. We also wish to t'hank Professor R. A. Beebe, .5 12.6 5.0 Dr. J. 34. Holmes and Dr. S. Ross for dat'a on the .9 13.2 5.0 adsorption of nitrogen on these carbon blacks. We 1.5 7.9 0.3 also t'hank Dr. K. Ti. Chmutov for support of this At a coverage of 0.5, the heat of adsorption for 1%- work. hexane is 12.6 koal., in good agreement with the (32) The concave nature of the initial portion of the isotherm, as value 12.4 kcal. calculated from the0ry.l At this published earlier,8?'3 is nom attributed t o the lower degree of homoof the adsorhents then in use. coverage, the heat of adsorption exceeds the heat geneity (33) -4. V. Kiselev, Collection of papers "Methods of Investigating of liquefaction by 65%, while a t 1.5 coverage it is the Structure of Highly Dispersive a n d Porous Bodies," U.S.S.R. only 4% higher. However, during filling of the Acad. of Sciences Press, Moscow, 1983, P. 86.
TABLE V ABSOLUTEV A L U E S O F ADSORPTIONO F n - H E X A N E O N THE BASAL FACE OF GRAPHITE AT 20" The surface is determined from nitrogen at 6 1 9 5 ' by the BET method, using urn = 16.2 A.2 01,
01,
