Department of Chemistry, Poniona College, Claremont, Colzfornia Received Febritary 6 , 1948
Although a considerable amount of work has been done on heats of adsorption (2), most of it is concerned with porous adsorbents for which the number of layers is limited by the filling of capillaries. Recently Harkins and Jura (7) have determined the heats of adsorption of polymolecular films of water on titanium dioxide by measurement of the heats of immersion of powders containing varying amounts of adsorbed water. They find that the heat of adsorption decreases as the thickness of the water film increases but that a t the thickest films that could be measured the heat of adsorption is still greater than the heat of liquefaction of water. Beebe, Biscoe, Smith, and Wendell (1) have measured the heats of adsorption of gases on carbon blacks, which are non-porous, by direct calorimetry. These measurements show that the heat of adsorption falls off rapidly after the first layer is completed, but they were extended only to a value of V / V , near 1.5. In later work (la) it is found that the heats of adsorption of hydrocarbons tend to be, even in the second layer, considerably in excess of EL. The isosteric method, which has often been used for measurement of the heat of adsorption, provides a more sensitive tool than calorimetry for investigation of the heat effects associated with the formation of polymolecular films. In calorimetric measurements the relative error increases as the heat effects become smaller, whereas the isosteric method does not depend upon measurements of small heat effects but rather upon relative pressure measurements, which are quite accurate up to the regions of high adsorption. &\ny shift of the volume us. relative pressure curve with temperature shows that the heat of adsorption is different from the heat of liquefaction, as pointed out by Coolidge (4), who has carefully examined the validity of applying the Clausius-Clapeyron equation to the equilibrium between vapor and an adsorbed phase. EXPERIMESTAL
A sample of graphite provided by the Kational Carbon Company' was used as adsorbent in this study. It has an ash content below 0.001 per cent and was prepared so as to be essentially free of oxygen complex. Before use it was heated at 900°C. for 3 hr. in vucuo. The surface area is 4 sq. m. per gram.3 We believe that the sample is quite similar to the one used by Harkins, Jura, and Loeser (S), which was also provided by the Kational Carbon Company. 1 This paper consists of a report of work done under contract with the Technical Command, Chemical Corps, United States Army. 2 We wish t o thank D r . L. L. Winter of the Sational Carbon Company Research Laboratory for preparation of the sample and for the analytical data. a The authors wish t o thank Professor R. A. Beebe and Miss R I . H. Polley of Arnherst College for determination of the nitrogen isotherni and coniputation of the area. 1111
.h
11 12
Ethyl chloride was used adsorbate because of its availability in high purity and its suitable vapor pressure range. Eastman “Tvhite label” grade ivas purified hy treatment with sulfuric acid and repeated distillation in the vacuum line. The final product had vapor preshures of 471 mm. at 0°C. and 3.44 mm. a t -78°C. The average heat of liquefaction, computed by the ClausiusClapeyron eqiiation from these vapor pressures, is 6700 cnl. per mole in the temperature range 0°C. to -78°C. Ice-water mistures and dry ice were respectively used for the two constanttemperature baths. The dry ice gave some supercooling when first added t o the Dewar flask but returned to a constant value within an hour. It was found
FIG.1. Isotherms for ethyl chloride on graphite. Upper curve a t -78”C., lower a t 0°C. The dashed curve is for desorption a t 0°C. The 0°C. curve gave a n adsorption of 11.9 cc. a t p / p o = 0.993.
best not to add acetone or other liquid to the dry ice bath. .Is used, the bath kept pure ethyl chloride within a vapor pressure range of ~2 per cent. All the isotherm points were determined gravimetrically, except those at relative pressures belon 0.05. The sample bulb as attached by a standard-taper joint, so that after each addition it could be removed and weighed. -411 pressures below 5 mm. were read from a McLeod gage. To minimize error due to unadsorbed gas in the sample tube we employed an 8-g. sample in a tube whose free volume (helium) a t 0°C. \vas only 8.5 ml. Isotherms for 0°C. and -78°C. are shown in figure 1. Each curve shows all experimental points for two or more separate determinations. The - ’78°C.
HEATS O F .IDSORI'TIOS.
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curve \vas not carried to higher relative pressures because of the A2 per cerit uncertainty in the pressure measurement, due t o temperature fluctuations in the dry ice bath. It is not possible to determine experimentally n-hether the curves come together at relative pressures belon- 1 .O or not, h u t there is no indication that this occurs. Desorption points, determined for the isotherm at O"C., are indicated hy the dashed curve of figure 1. Desorption n a s not determined for the -78°C. isotherm because of the large temperature change each time the tube was removed from the bath for neighing. A liy~tere4sloop, such as ihoirn in the 0°C. isotherm, is usually interpreted as due to capillaries 11 hich fill at the high relative pressures. Khether these are crevices in the graphite particles or voids betn een the particles cannot be determined, but in any event the small amount of capillarity doe, not affect the conclusion that most of the adsorption is in polymolecular films of considerable thickness, if capillaries are present, they are bo uide that they d o not fill iintil the film on each ~vallis yeveral molecules in thicliness. DI8CUdt.IOS
Thc Claii~iu~-Clape!-roiiequation
has h e n used to compute the differential heat of adsorption. Computed values are shown in figure 2, plotted as a function of the amount adsorbed. The basc line is E , , the heat of liquefaction of ethyl chloride, computed from the 110 T-alues a t the tn-o temperatures. -It all me:ismable pressures the heat of adsorption exceeds the heat of liquefaction. (This is .holm in the isotherms for the t n o temperature.. They \rould coincide if E = EL.) The upper scale of figure 2 shons heat of adsorption a, :I function of the number of statistical layers. The value of T',, \vas computed by B.E.T. plots for the tn-o isotherms as 0.62 cc. per gram a t -78°C. and 0.57 cc. per gram a t 0°C. In figure 2 we used an average value for 1', of 0.60 cc. per gram, since the isosteric heats represent the heat of adsorption at the mean temperature. I n plotting E us. number of layers we do not imply that a given value of E corresponds to covering the surface with a uniform layer 72 molecules in depth. Rather, we believe that for a given number of statistical layers part of the surface is covered by a greater number of layers and part by a lesser number, with some portions holding only a monolayer. Our isosteric heats cannot be compared with the calorimetric values of Ueebc and associates (1, l a ) , since our method lacks precision in the region of small zdsorption and the calorimetric method is not very exact beyond about 1.5 :o 2 The falling off in E with volume is much like that observed by Harkins and Jura (7) for Tvater on titanium dioxide. Y e have searched for other published sotherms of non-porous adsorbents, to see n-hether the effect is general, but hnd
vn,.
1114
PIERCE AND H. SELSOK SMITH
COS\\'VBY
only one other case where isotherms have been determined a t two temperatures and extended to high relative pressures. This is for the adsorption of butane a t 0°C. and -'78"C. by 200-mesh glass spheres, reported by Davis, DeWitt, and Emmett (6). Their results arc much like ours; the isotherm a t the lower temperature is shifted to the left of the one at the higher temperature, a t all pressures. Thus it is shown that E > E , a t all measurable pressures. There are NUMBER
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STATISTICAL L A Y E R S 5 6 7 8 9
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FIG.2. Isosteric lieat of adsorption for ethyl chloride on graphite. T h e base line a t 6700 cal. per mole is the lieat of liquefaction, computed from P O values a t the two temperatures.
a number of isotherms for porous adsorbents that shov a shift to more adsorption as the temperature is decreased, thus indicating that E > E L , but these usually reach saturation a t low relative pressure, owing to filling of capillaries, and one cannot determine from them n-hether E becomes equal to EL a t high relative pressures. The isotherms of Coolidge (G) for water on charcoal, and of Reyerson and Cameron (9) for iodine on silica gel, have been interpreted by Brunauer (2) as
.
HE.\TS O F 1DSOEPTIOK.
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evidence that E = EL for these systems which give Type 111 isotherms. Aside from these we know of no case in which it has been proved that adsorption will occur when E = EL. Even here there is some question as to whether the experimental precision is sufficient to detect the small shifts of isotherms with temperature that would result were the heat of adcjorption only slightly greater than the heat of liquefaction. In view of our data for a typical Type I1 isotherm and the observation that the butane isotherms of Davis, DeWitt, and Emmett (6) also show that E > EL up to multilayers many molecules in thickness, we feel that the B.E.T. assumption (3) that Ez = E, = . . . E , is unsound. After perhaps the third layer it is a fair approximation, but for smaller adsorptions E may exceed EL by as much as several hundred calories per mole. SUMRLkRP
Isost'eric measurements have been made of the heats of adsorption of ethyl chloride on graphite a t temperatures of 0°C. and -78°C. It' is found that the differential heat of adsorption is much higher than the heat of liquefaction during the adsorption of the first and second layers, that EAdrops to only slightly above the heat of liquefaction when the adsorbed film becomes several molecules in thickness, and that Ea never becomes equal to ELwithin the measurable pressure range. REFERESCES (1) BEEBE,BISCOE,SJfITH, A N D WESDELL:J. Am. Chem. Soc. 69, 95 (1947). SMITH,. ~ X DWESDELL:J. .km. Chem. soc. 69, 2291 (1947j. ( l a ) BEEBE,POLLEY, The Adsorption of Gases and I'npors, Chap. VIII. Princeton University (2) BRUNAVER: Press, Princeton, S e w Jersey (1943). EMMETT, . ~ S D TELLER: J . -km. Chem. soc. 60, 309 (1938). (3) BRUNAUER, J. .4m. Chem. SOC.48, 1795 (1926). (4) COOLIDGE: J. -4m. Chem. SOC.49, 708 (1927). (5) COOLIDGE: DEWITT,. ~ X DEMMETT: J. Phys. Colloid. Chem. 61, 1231) (1947). (6) DAVIS, A K D JURA: J. Am. Chem. Soc. 66, 919 (1944). (7) HARKIKS ASD LOESER:J. Am. Chem. SOC.68, 554 (1946,. (8) HARKISS,JTRA, .4ND C.%?JEROK:J . Phys. (?hem. 39, 181 (1935). (9) REYERSOS
HE-iTS O F ADSORPTION. 11' 11. SCLSOX SMITH
AND
C O X V A T PIERCE
I k p c i t / v ) r t z t of C h e m i s t r y , Ponzona College, CZnremont, Calzjo, t i i n ReceZled A p r i l 27, 1948
In the derivation of the Brunnuer, Emmett, Teller (B.E.T.) equation for multilayer adsorption (4) it is necessary to make the assumption that the heats of This paper consists of a report of work done under contract with the Technical Command, Chemical Corps, United States Army
