612
M. L. CORRIN
tions. The thermodynamic treatment of adsorption requires surface concentrations, expressed preferably in terms of moles per square centimeter. This letter is a plea that all adsorption data be reported, where possible, in terms of surface concentrations. Since monolayers have surface concentrations of the order of 1 to 10 X 10-10 moles per square centimeter, we propose a new unit of 10-lo moles per square centimeter, to be called "the Gibbs," in honor of Willard Gibbs, and to be expressed by the symbol G. The symbol for surface concentration is universally r, following Gibbs. To avoid confusion, the units of concentration should be expressed by a different symbol. There should be no danger of confusing this symbol with the gravitational constant or the Gibbs free energy. The following formula expresses the relation between surface concentration, r, and area per molecule, A , expressed in b.l
r
=
166.0 G I A
A tightly packed fatty acid monolayer has a limiting r of 6.3 G ; a monolayer of nitrogen occupying 16.2 per molecule has a of 10.25 G. ROBERTB. DEAN. Department of Chemistry University of Oregon Eugene, Oregon December 11, 1950
A COMPARISON OF T H E SPECIFIC SCRFACE AREAS OF SOLIDS AS CALCULATED BY T H E BRUNAUER-EMMETT-TELLER AiVD THE HVTTIG EQUATIONS I n view of the recent interest in the application of the Huttig equation to the determination of the specific surface area of solids (Huttig: Monatsh. 78, 177 (1948); Ross: J. Phys. & Colloid Chem. 63, 383 (1949); Fergusson and Barrer: Trans. Faraday SOC.46, 400 (1950); Hill: J. Am. Chem. SOC.72, 5347 (193)) it seems desirable to report some results obtained with this equation. The adsorption isotherms of nitrogen at - 195.8"C., of argon at - 195.8"C., of pentane at 20"C., and of 1-pentene at 20°C. were determined on eight carbon blacks and on rouge and rutile; these data will be discussed more fully elsewhere. The area of the solids as determined by the B.E.T. method (Brunauer, Emmett, and Teller: J. Am. Chem. SOC.60, 309 (1938)) ranged from 24.8 to 216 sq.m. per gram. The area occupied per molecule in the monolayer was calculated by means of the B.E.T. and Huttig equations from the isotherms on anatase, whose area had been measured by the calorimetric method of Harkins and Jura (J. Am. Chem. SOC.66, 1362 (1944)). These molecular areas are given in table 1. The isotherm data were treated by an increment ratio analysis to determine
613
SPECIFIC SURFACE AREAS OF SOLIDS
the linear portion of the B.E.T. and Huttig plots. From the forty isotherms measured thirty-nine B.E.T. and twenty-six Huttig lines were obtained. The results are presented in table 2. I t will be noted that in general the B.E.T. results are more self-consistent for different gases on the same solid. These results also indicate that, as the specific surface area of the solid increases, (a) the number of isotherms yielding a Huttig straight line increases, ( b ) the Huttig results become more self-consistent, and ( c ) the agreement between the B.E.T. and the Huttig areas improves. TABLE 1 Areas occupicd per molecule in the monolayer as calculated by the B . E . T . and Hiitlig equations
I
GAS
r (B.E.T.)
~
Iiitrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Argon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pentane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-Pentene . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D
(Huttig)
-1
A. 16.2 16.6 52.3 49.3
A. 14.9 15.3 48.i
TABLE 2 Surface areas as calculated with the B . E . T . and Hiittiy equations B.E.T. SOLID
Number of straight lines
Rouge.. . . . . . . . . . . . . . . . . . . . . . . . . . . Rutile.. . . . . . . . . . . . . . . . . . . . . . . . . . . Acetylene black. . . . . . . . . . . . . . . . . Philblack 0 . . . . . . . . . . . . . . . . . . . . . . Statex K . . . . . . . . . . . . . . . . . . . . . . . . . Graphon (P) . . . . . . . . . . . . . . . . . . . . . . Graphon L2237. . . . . . . . . . . . . . . . . . Kosmobile 7 7 E P C . . . . . . . . . . . . . . . . Spheron 6 . . . . . . . . . . . . . . . . . . . . . . . . Vulcan 2 . . . . . . . . . . . . . . . . . . . . . . . . .
4 4 4 4 4 4 3 4 4 4
z
Average
jeei;cg;
__ 24.8 66.7 64.3 81.0 81.4 85.8 95.1 111 120 216
-
5.0 1.0
1.1 0.8 1.6 1.0 0.7 1.3 1.3 1.3
,
1I ,
I
1
Htittig h-umber of straight liner
1 1
2 4 2 2 2 4 44
z
Average
__
f2iZ
28.8 74.6 71.1 85.7 85.0 86.5 99.0 112 122 214
2.3 5.3 4.0 0.6 7.2 2.5 1.9 2.5
I n only ten instances was the p / p o range for the Huttig line greater than that for the B.E.T.; eight of these instances were for Spheron 6 and Vulcan 2. The maximum range of the Huttig straight line was from 0.02 to 0.7 in p / p o for argon on Vulcan 2; for pentane on the same solid the range \vas from 0.05 to 0.55. In all other instances the Huttig straight line did not extend over a range greater than 0.3 in p / p o . hl. L. CORRIN. Department of Chemistry University of Chicago Chicago 37, Illinois December 9, 1950
