Feb., 1962
ADSORPTION ISOTHERMS OF NITROGES ON CHANNEL BLACKS
205
ADSORPTION ISOTHERMS OF KITROGEN, BENZENE AXD n-I-IEXASE AND THE HEATS OF ADSORPTION OF BEKZENE AND n-HEXANE ON GRAPHITIZED CARBON BLACKS. 11. ADSORPTION ON GRAPHITIZED CHANNEL BLACKS BYA. A. ISIRIKYAN AND A. V. KISELEV Adsorption Laboratory, Chemical Department. Moscow State University, and Surface Chemistry Group Institute of Physical ~, Chemistry, U.S.S.R. Academy of Sciences, M o s c ~ U.S.S.R. Received May 88, 1061
The adsorption isotherms of nitrogen a t -195", n-hexane a t 20°, and the differential heats of adsorption of n-hexane on graphitized channel black have been measured. The shape of the adsorption isotherms and the heats and entropies of lidsorption on graphitized carbon blacks are dependent on the dispersion, particle packing, and mosaic character of the surface. When passing from gra hitised thermal blacks t o graphitized channel blacks, the adsorption isotherms of both nitrogen and n-hexane change &om concave t o convex in the initial region. Heats of adsorption of helium (measured by other authors) are sensitive indicators of surface inhomogeneity of graphite.
Introduction I n the strict sense of the word, an infinitely large continuous surface with identical arrangement of its constituent atoms, molecules or ions should be regardcd as a homogeneous surface of a solid body. For crystalline bodies, this is the surface of a plane of specific index. I n practice, we usually encounter substances that are, to one degree or another, dispersed. The adsorption properties of unit total surface of a dispersed substance should differ from the adsorption properties of unit surface of a definite face of the crystal. This difference should increase with increasing degree of dispersion due to greater contribution to the total surface of areas associated with edges and angles of crystals, or of faces of other index. This refers not only to the dispersion of single crystals, but also to the appearance of crystal faces on the surface of initially inhomogeneous bodies as a result of heat treatment. An example of the latter arises in the heat treatment, of carbon blacks a t temperatures close to 3000'. I n this ease, the initially spherical particles of carbon blacks rearrange to polyhedra,' the faces of which are formed by single crystals of graphite grown inside the particle. I h the case of thermal carbon blacks composed of large particles some 5000 A. in diameter, such treatment leads to the main surface of the polyhedral particles of carbon black being formed by homogeneous basal planes of graphite, and the influence of inhomogeneous sites a t edges and angles of polyhedra becomes extremely ~ m a 1 1 . In ~~~ this way, it is possible, from the adsorption isotherms and heats of adsorption obtained on such carbon blacks, to differentiate clearly the role of adsorbateadsorbent and adsorbate-adsorbate interactions. I n the case of channel blacks treated at close to 3000' (Graphon) with particles of considerably smaller size, i e . , 300 k.,that have partially grown (1) W. D. Schaefier, W. R. Smith a n d M. €1. Polley. I n d . B ~ QChem., . 45, 1721 (1953); hl. I%. Polley, W. D. Schaeff~rand W. R. Smith, J. Phye. Chem.. I T , 469 (1953); D. Graham a n d W.S. Kay, J . Collord Sci.. 16, 182 (1961). (2) R. A. Bccbe, C. 1%.Amberg and B. Spencer, Can. J . Chem., 33, 305 (1953). (3) A. A. Iairikyan a n d A. V. IGselev, J. Phye. Chsm.. 66, 001 (1961). (4) N. N. Avgul. A. V. Kiselw and I. A. Lygina, Kolloid. Zhur.. 23, N5 (1901); A V. Kiselev. Zhur. Fss. Khrm.. 86, 233 (1961).
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together, the role of the inhomogeneous surface is appreciably great.er. Here, the inhomogeneous adsorbate-adsorbent interactions interfere in a clear-cut manifestation of adsorbate-adsorbate interactions. Thus, whereas for graphitized thermal blacks of low surface area the adsorption isotherm of nitrogen is a t first convex to the prcssure a x e ~ , ~for J - ~the finer graphitized channel blacks it is concave in the same pressure range.6 Charact.eristic differences likewise are observed in tjhe case of adsorbates that strongly interact with onc another. Thus, the adsorption isotherms of alcohols and ammonia on graphitized channel black are satisfactorily described by equations that take into account adsorbate-adsorbate interactions for localized adsorption.'^* However, in the case of graphitized thermal blacks which possess the maximum homogeneous surface, the adsorption isotherms of alcohols9 and ammonia'o are better described by equations for non-localized adsorption. We also have shown that under pressure an increase in the number of coiitacts of particles of graphitized channel black increases the inhomogeneity of its surface." The present paper describes the isotherm and differential heats of adsorption of n-hexane vapor as well as the adsorption isotherm of nitrogen on Graphon.12 It also compares the adsorption isotherms of nitrogen, benzene and hexane, and the heats and entropies of adsorption of benzene, hexane and helium on both graphitized channel and thermal blacks. Possible causes of the inhomogeneity of the surface of graphitizcd channel black also are considered. ( 5 ) S. Ross and W. WinMor, 1. Colloid Sci., 10, 310 (1956): S. Ross and W. W. Puitr, ibid., 13, 397 (1968). (6) A. V. Kiselev and E. V. Khmpova, Kolloid. Zhur., 23, 163 (1961). (7) N. N. Avgul, G. I. Berrzin. A. V. Kiselev and I. A. Lygina, Imert. Aknd. Nauk S.S.S.B., Dept. Chem. Sei., 205 (1961). (8) A. V. Riselev, Kolloid. Zhur., 20, 338 (1958): A. V. IGsclev, N. V. I
