A NEW METHOD FOR OBTAINING DATA FOR THE SORPTION O F VAPORS BY SOLIDS J. L. PORTER Department of Chemistry, Stanford University, Stanford Univewity, California Received December 11, 19%
A technique for obtaining sorption data more simply and rapidly than those ordinarily used has been developed for the sorption of vapors by solids. It arose by following up an idea suggested by two sentences found in papers by Cude and Hulett (1) and by Tryhorn and Wyatt (2)) respectively. Cude and Hulett filled charcoal with water to get the pore space, not the sorption. Tryhorn and Wyatt noticed that the weight of benzene in charcoal when the visible liquid surface had just disappeared by evaporation was the same as that obtained by exposure to fully saturated vapor. However, the significance of their results is left uncertain, since in both cases capillary liquid might equally well have been present. The only test would be to compare the values obtained by wetting and superficial drying of so microporous a body as charcoal with values obtained for the sorption isotherm for distinctly unsaturated vapor. The same comment applies to any powder. EXPERIMENTAL METHOD
The solid was placed in a commercial microfilter tubel with a sintered glass filter from which the excess liquid could be removed by suction or by centrifuging. As the liquid must be introduced on to the evacuated solid, the filter, A in figure 1, was placed in a bulb B with a ground glass joint C to the stopcock D through which the liquid could be introduced. The bulb and tube with stopcock was joined to the vacuum line by a ground glass joint E. About a gram of the material to be used was placed in the filter and in certain cases washed with water, which was removed by suction and centrifuging. Weighings were taken of the bulb, tube, and filter separately and of the filter and the solid material. The solid was evacuated with heating by an electric furnace around the lower part of the bulb. The tube and bulb were detached a t E and weighed, giving the weight of the evacuated solid. The liquid to be introduced was placed in the upper part 1 Jena Gerate glass, size 12, porosity 4, 1 centimeter inside diameter, Fish-Schurman Corporation, New York City.
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of the tube and slowly admitted through the stopcock. In order to prevent the liquid from washing any stopcock grease down on to the solid, the lower bulb was cooled and the upper tube warmed so that the liquid vaporized into the bulb and condensed on the solid in the lower part of the bulb. When the solid was covered with liquid, the bulb was opened a t C, the filter removed, and the excess liquid centrifuged off. In certain cases suction was applied for a few seconds to remove liquid from the sintered glass. The filter with the solid and the sorbed liquid was weighed in a weighing bottle and from these values the amount sorbed a t saturation was calculated. An isotherm was determined by evacuating to a definite pressure and reweighing. The solid could be reevacuated and the experiment repeated with the same or other liquids. TABLE 1 Maximum amount of various liquids retained at 23°C.by dehydrated chabasite i n the centrifugal method
I SUBSTANCE
Water. .................................... Formamide ................................ Formic acid.. .............................. Methyl alcohol.. ........................... Ethylene oxide.. ........................... Methyl cyanide. ...........................
I
sim
CUBIC CENTIMETER0 PER GRAM
0.323
0.324
0.088*
0.473t 0.268# 0.056 0.048
0.386 0.339
MOLECULAR VOLUMB
18 33 37 40 49 51
* Mean of 0.078,0.099,and 0.087.
t Mean of 0.485 and 0.461.
$ Mean of 0.268,0.272,0.270,and 0.263. EXPERIMENTS WITH CHABASITE~
It was most interesting to begin with a crystalline solid containing no pores in the ordinary sense of the word. The zeolites (3) are hydrated calcium aluminum silicates which can give up their water of hydration on evacuation or heating without destroying the crystal form of structure. The spaces left by the water may be filled indifferently by any molecule small enough to enter. They then form, as McBain (4) pointed out and as is now generally accepted, almost perfect molecular sieves or semipermeable membranes of great regularity. Evans ( 5 ) quotes Taylor (6) to the effect that the water molecules in chabasite lie on non-intersecting triagonal axes of the cubic space group. No other atoms lie on these axes, so there are long channels passing through the structure wide enough to accommodate fairly large molecules and empty of everything but water molecules. 2 CaAlzSi~Ol~.6H~O subject to slight variation due to base exchange. Theory gives 21.3 per cent of water, but G. Friedel (Bull. SOC. franp. min6ral. 22, 5 (1889)) found 22.28 per cent.
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The chabasite crystals were washed with water before weighing, as in the routine described. It was found that evacuation for twenty-four hours cm. of mercury. at 350°C. brought the chabasite to constant weight at The average of nine values gave 24.4 per cent of water lost by the chabasite. Expressed as ratio of weight of water to weight of dehydrated chabasite, x / m = 0.323. The results with liquids are summarized in table 1. As is seen from table 1, the two substances with the largest molecular volume are taken up only to a small fraction of the extent of water, methyl alcohol, and formic acid. The small amount of formamide indicates that mere molecular volume is not the sole determining factor. The shape and composition of the molecule must also be of importance. Formic acid has a small molecular volume, but it also attacks the chabasite and a deposit is observed on the filter. A sample of chabasite, used first with formic acid, was used again with water, which it took up to the extent of 28.5 per cent as compared with the original 24.4 per cent. This enlargement of the sorption space is fully adequate to account for the higher value of formic acid sorbed. The heat of sorption of methyl alcohol by chabasite is considerable, the filter and glass bulb being observed to heat up to above 60"C., a much greater effect than was observed with charcoal and far too great to be explained as heat of compression. Rapid sorption of methyl alcohol vapor by chabasite disintegrates the crystals to a powder, owing to the sudden unequal heating and e~pansion.~ The heat on rapid sorption may decompose the alcohol and change the colorless crystals to yellowish-black. It was found impossible to desorb all t.he alcohol, and if the temperature were raised above 400°C. during evacuation, the crystals became yellow or black. EXPERIMENTS WITH CHARCOAL
The charcoal was highly active (82 per cent)' air and steam activated charcoal made from especially pure sugar. It was covered with two thicknesses of silver foil pierced with fine holes and was evacuated for several days at 500°C. and cm. of mercury. The volumes of liquid sorbed at 28°C. at saturation pressure or just below were 0.59 cc. of benzene, 0.6 cc. of water, and 0.61 cc. of acetic acid per gram of charcoal. ISOTHERMS
It is possible to obtain isotherms by this method, especially on desorption, as a manometer in the vacuum system gives the pressure after partial Compare the observation of M. G. Evans (Proc. Roy. SOC.London A134, 97 (1931)) on the disintegration by ammonia vapor. One gram of 200 mesh charcoal sorbs in three minutes from 50 cc. of a 0.2 N aqueous solution of iodine in 0.27 N potassium iodide, 82 per cent; method of N. IC. Chaney, A. B. Ray, and A. St. John (Ind. Eng. Chem. 16,1244 (1923)). ,
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-
0.2
METHYL ALCOHOL ON CHABASITE
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0 0.0
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0.4 m
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ACETIC ACID ON’CHARCOK
A BENZENE ON CHARCOAL
0.3 0.2
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1
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. 0.2
1
03
-
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0.4 0.5
0.6 0.7
0.8
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FIG.1
FIU.2
FIG.1. THEFILTER TUBEAPPARATUS FOR MEASURINQ SORPTION BY THE FILTER TUBEMETHOD FIG.2. ISOTHERMS
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evacuation, and a t any stage the stopcock in the tube may be closed and the bulb, tube, and filter removed and weighed without exposing the solid to the atmosphere. The results are shown in figure 2. Inspection of the isotherms for methyl alcohol and the crystalline chabasite reveals the ideal form of isotherms for It is even more rectangular than those obtained uniform with highly evacuated, active charcoal (7). Here, therefore, is most definite proof of persorption and not condensation of capillary liquid. The isotherms for acetic acid and benzene on charcoal likewise show that sorption is far advanced a t infinitesimal pressures. The isotherms for the sorption of water by charcoal are of a wholly different type, owing to formation of a two-dimensional liquid permeating the charcoal, the water molecules being held as much by mutual polarization as by att,achment to the charcoal (8). Comparison of these isotherms with those for organic vapors on the same charocal shows them to be due to sorption, or rather persorption, in both cases and that the isotherms with water cannot be interpreted as measuring the volume and distribution of the assumed pores of the charcoal. Inspection of the extreme right hand of each diagram in figure 2 shows the small additional amount of liquid held by capillary condensation in each case, an amount which is only a minute fraction of the total sorption. SUMMARY
A simple method for the rapid and moderately accurate study of sorption of liquids and vapors by solids is described. Sorption isotherms obtained with a molecular sieve, dehydrated chabasite, are nearly rectangular, even more so than those for organic vapors with highly activated charcoal where the sorption goes largely to completion a t infinitesimal pressures. The sorption of water, although of a different type, is likewise interpreted as persorption and definitely not as capillary condensation. I n conclusion my thanks are due to Professor J. W. McBain at whose suggestion this work was undertaken. REFERENCES
(1) CUDE,H. E., AND HULETT,G. A.: J. Am. Chem. SOC.42,398 (1920), footnote 1. (2) T R Y H O R F.~G, . , AND WYATT,W. F.: Trans. Faraday SOC.22, 137 (1926). (3) For more'complete references and discussion, see McBain, J. W.: The Sorption of Gases and Vapours by Solids, Chapter V, page 167. George Routledge and Sons, Ltd., London (1932). A. S. Coolidge (J. Am. Chem. SOC. 49, 712 (1927), figure 3) has published a very similar result, ascribed to J. C. Woodhouse, for the sorption of water by chabasite.
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(4) MCBAIN,,J. W.:Colloid Symposium Monograph 4, 1 (1926); Kolloid-Z. 40, 1 (1926). SCHMIDT, 0.:Z. physik. Chem. 133,280 (1928). (5) EVANS, M.G.:Proc. Roy. SOC.London A134,97 (1931). (6) TAYLOR, W.H.:Z. Krist. 74, 1 (1930). See also PAULINQ, L.: Proc. Nat. Acad. Sci. 16,453 (1930). (74 MCBAIN,J. W., LUCAS,H. P., AND CHAPMAN, P. F.: J. Am. Chem. SOC.62,2668 (1930). D. N., BAKR, A. M., AND SMITH, H. G . : J. Phys. Chem. MCBAIN,J. W.,JACKMAN# 34, 1439 (1930). MCBAIN,J. W., AND BRITTON, G . T.: J. Am. Chem. SOC.62,2198 (1930). (8) For much further evidence, see MCBAIN,J. W., PORTIR,J. L., AND SESSIONS, R. F. : Communicated to J. Am. Chem. SOC.;also reference 3, p. 143.
