ADSORPTIONOF CARBON DIOXIDE BY GLASS
Nov., 1952
part in some of these reactions, but a t present there is no experimental evidence concerning them. The considerations which have been presented in this paper offer a possibility of extending the remarks of Rollefson and Boazla to account for the extreme specificity so often found in the study of the quenching of fluorescence. In their discussion it was pointed out that a high degree of specificity could be accounted for if the quenching consisted of a transfer of energy between the reacting molecules only when the queticher molecule has an energy state separated from the lowest state by approxi(13) C . K. Rollefson and H. Boaz. THISJOURXAL, 62, 518 (1948).
879
mately the same amount as the excited state of the fluorescer differs from a lower state of the molecule. Under such conditions the transfer can occur if the molecules merely come close to each other; ie., the entropy of activation would be about - 10. If this condition is not fulfilled, the queiicher can still remove energy from the fluorescer, but in order to do so they will have to essentially form an addition compound and thus fulfill the condition for which the entropy change is -30. Such a change mill reduce the quenching constant by a factor of lo4 and thus put it below the range which can be studied by the usual type of experiment.
ADSORPTION OF CARBON DIOXIDE BY GLASS BY JOHN B. THOMPSON,’ E. ROGER WASHBURN BND L. A. GUILDNER~ Avery Laboratory, University of Nebraska, Lincoln, Nebraska Received November 16, 2961
The continued interest in the adsorptive propekes of glass has given rise to the present study of low temperature coi adsorption on some commercially available kinds of glass, The present report includes Pyrex wool and two kinds of “Scotchlite” brand glass beads, In the course of this study, an extreme case of capillary condensation was encountered, the effect of etching the glass wits studied, and some data on the effect of water on COZadsorption was obtained.
Experimental The small surface areas of the adsorbents required an apparatus with which accurate measurement.s of small adsorptions could be obtained a t pressures up t80 atmospheric. The apparatus, which is shown in Fig. 1, is essentially a U tube, one arin of which is one met,er long and the other arm about 40 cm. The two arms form an absolute pressure manometer, a vacuum being maintained in the long arm by Germann’s inethod.3 The short arm, in a water jacket, is a gas buret of variable volume. The advantage of using a uniform tube instcad of the conventional series of bulbs is that the pressure in the t.ube is continuously variable. The mercury levels were read to t,he nearest 0.05 inm. with a 1meter cathetometer. The short tube was volumetrically calibrated in ternis of cathetometer readings from the weights of discharged mercury. The apparatus is operated in the conventional way: after the adsorbent is degassed, stopcock A is closed, gas IS admhted to the gas buret,, and stopcock B is closed. Three or four readings of the mercury levels are then made with t,he gas in the short tube at various pressures. Stopcock A is then opened and the adsorpt,ion measurements are made in the usual way. Dead space values were determined with helium. ’ With the present) apparatus, good accuracy may be obtained even when the ratio of gas adsorbed to gas in the dead space is as low as 0.1, as was the case with the unetched type 520 glass beads. Relative adsorption may be measured when the ratio is as low as 0.005, the absolute accuracy in this case being limited by the experimental error in dead space determination. The results of the adsorption measurements were calculated using the equation of state P V ( 1 Z P ) = nR7’ where P , V , 11, R and 7’ have their usual meanings and 2 is a function of temperature. For COI a t 25’, 2 = 0.00000774 when P is exprcssed in mm. of mercury, and a t -78”, Z = 0.0000329. These values of 2 were calculated from density data of COZ. For helium, Z was assigned t,he value zero.
+
(1) E. I. du P o n t de Nemours and Coml)any. Research Fellow, 19501951. Premnt address: E. I. du Pont d e Neinours and Company. Wilmington, Delaware. ( 2 ) Franklin E . a n d Orinda R f . .Jullnson Fellow, 1Y48-1919. Prese n t address: Massachusett3 .Institute of Technology. Caiiibridge. Mass. (3) A. F. 0. Cerninnn, J . .4m. Chem. Sor., 36, 245G (1914).
The COP was generated from Dry Ice and dried with Anhydrone or Drierite, from which other gases had been removed by repeated evacuation and flushing with COP. Adsorbents. 1 .-“Scotchlite” brand glass beads, type 520: This material is a high density glass supplied in the form of microspheres by the Minnesota Mining and Manufacturing Company, According to the manufacturer, thew spheres have fire polished surfaces and have not been subjected to the etching action of water. It may therefore be expected that the surface of the material is smooth enou.gh that the geometric area would be in good agreement wlth the area determined by gas adsorption. This expectation was fulfilled as discussed below. 2 .--“Scotchlite” brand glass beads type 120: This material is a lime-soda glass in the form of fire polished microspheres. 3 .-Pyrex wool: Pyrex brand glass wool (Owens-Corning Fiberglass Corp.) Catalog Number 800 was used.
Results 1. Unetched Adsorbents. Type 520 Beads.These beads gave S-shaped COz adsorption isotherms at temperatures from - 77.0 to - 77.6’. Experimental error obscures the temperature dependence of the isotherms in this range. The results of five isotherm determinations are shown in Fig. 2. The simple (two constant) Brunauer, Emmett and TelIer equation4 applied to this adsorbent gives a linear plot for relative pressures less than about 0.2. The specific surface area calculated by the BET method is 00.158 square meter per gram using the value 17.0 A.2 per molecule for C O Z . ~It is interesting to note that the surface area calculated from Emmett and Brunauer’s poin;t: B6 is 0.16 square meter per gram, a value which IS in reasonable agreement with the BET value. The specific surface area calculated from the size distril)ution of the beads and their density is 0.151 square meter per gram. The size distribution was determined from samples totaling 392 beads with .the results shown in Fig. 2. While this small a (4) 9. Brunauer, P. H. E m m e t t and E. Teller, ibid., 6 0 , 300 (1938). ( 6 ) P. H. Einniett a n d S. Brunauer, ibid., 59, 1559 (1937).
980
JOHN B. THOMPSON, E. ROGER WASHBURN AND L. A. GUILDNER
Vol. 56
I00 80 Liz'
60
w m
40
I 2 z
20
GAUGE
Bo 0
0 0
0
0
so
O0
C O Q
c
ADS OR PTlON CELL
WATER JACKET
i
Fig. l.--Adsorption
3 TO PUMP AND TRAPS n -0
ADS0 RBATE
I
I
1
I
0.2
0.4
0.6
0.8
1.0
RELATIVE PRESSURE.
SUPPLY
Fig. 2.-Type
520 glass heads; size distribution and COX adsorpt,ion isotherm.
glass was heated in the adsorption cell with the vacuum pump shut off. Curve A is the GOz isotherm at -78" obtained after degassing the adsorbent 1 hour at 70" and 17 hours a t 25" a t lo-' mm. pressure. The surface water remaining after this treatment results in a relatively large COZ adsorption. Curve B is the
.24
-
apparatus (scheitiatic diagram).
DIPMETER, microns.
..20 -
count of the beads cannot give a highly accurate value for the geometric surface, we feel that it is nevertheless sufficient to establish the fact that the geometric surface and the surface determined from adsorption data are equal within the limitations of the BET method. I n other words, the adsorbent is approximately smooth on the molecular scale, a t least with respect to COz molecules. The geometric surface was calculated by the equation
E
2
0 L
-
-
:.I6 c a n 3
where S is the specific surface area and )L is the number of beads having a mean diameter d. The adsorbent was degassed a t 150 to 180" for 18 hours initially and between determinations. Some gas was still being evolved a t the end of these a I I I I degassing periods while the adsorbent was hot, but 0' 0.4 0.6 0.8 1.0 0 0.2 the evolution ceased when the adsorbent was cooled below about 60". RELATIVE PRESSURE. Type 120 Beads.-A further study of degassing Fig. 3.-Type 120 glass beads: size distribution and GO1 conditions was made on the Type 120 glass beads. isotherms showing the effect of water on COZadsorption and This adsorbent contains considerable water, which themeffect of water diffusing to the surface of superficially was observed to condense in the gas buret when the dried glass.
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9
i
Nov., 1952
ADSORPTION OF CARBON. DIOXIDE BY GLASS
COz isotherm obtained after further degassing at 40 to 50" for 18 hours. The smaller adsorption after this treatment we ascribe to the partial depletion of the surface water under these degassing conditions. Curve C is the COz isotherm.obtained after evacuating the sample to 10-4 mm. at 25" and allowing it to stand without further pumping overnight. The increase in adsorption over the previous amount is due to an increase in the amount of surface water as a result of outward diffusion of water dissolved in the glass. Curve D is the COzisotherm obtained after degassing 18 hours a t 170-180". This treatment removes nearly all the surface water and the COz adsorption is only 1.7 times that calculated for the geometric surface of the sample. This factor of 1.7 may be due either to roughness of the surface or to water not removed by the 180" degassing. The loss in weight due to the degassing amounted to about 0.22 wt. yo. The fact that the middle portions of the isotherms are of the same shape, differing only in vertical displacement, indicates that a given amount of surface water takes up a constant amount of. COz above relative pressures of about 0.2 or less. The process by which this COz is taken up may be regarded as a chemical combination of COz and the surface water phase or as a case of chemisorption of COZ by water. The remainder of the COz is held by physical adsorption which is approximately equal for dry glass and for the water-Cop surface presented by wet glass. I t is more than possible that the rate of diffusion of water through a glass membrane may be measured by the influence of water on the adsorption of CO,. Pyrex Wool.-This adsorbent gives a COz isotherm at -78" which is everywhere convex to the pressure axis, the familiar Type 111 isotherm. However, during determinations of the isotherm, it was frequently observed that extremely large amounts of COz disappeared from the gas phase, only to reappear after a short time. The small adsorption, referred to hereafter as normal adsorption, was always less than about '/3 of a molecular layer (based on geometric surface) up to relative pressures
98 1
of 0.82, while the large adsorption, hereafter called abnormal adsorption, occasionally was equivalent to 30 molecular layers a t relative pressures as low as 0.62. The abnormal adsorption could not be made to take place in a reproducible manner, but started to occur as the relative pressure was increased to values, different for each determination, greater than 0.6. Normal adsorption could often be obtained a t relative pressures less than 0.8, but attempts to further increase the pressure while normal adsorption was being observed always resulted in abnormal adsorption. The abnormal adsorption is thought to be not true adsorption but rather capillary condensation of COz in the interfibrillar spaces of the Pyrex wool. The Kelvin equation for capillary condensation does not, however, account for the amount of COz condensed, giving values of only about l/loo of that observed even assuming the fibers were hexagonally close packed so as to give the maximum capillary space, which they were not. Etched Adsorbenk-Samples of both types of glass beads were allowed to stand with occasional stirring in carefully purified water a t room temperature for 33 days. The samples were then filtered, washed and air dried a t room temperature for 8 days. COZ isotherms a t -78" were then determined for the samples, which were degassed 18 hours at 150-170" initially and between determinations. The initially determined (Type 11) isotherms agreed with those made after further degassing, indicating that the increased adsorption observed was not due to incomplete removal of water. The adsorption by etched Type 520 beads was 21 times that of an equal weight of untreated beads. The adsorption by etched Type 120 beads was 12 times that of the untreated beads. The increased adsorption by the etched beads was taken to indicate an increase in surface area as the result of etching by water. Analysis of the filtrates from the etched samples and microscopic examination of the samples indicated that no important increase in specific surface resulted from a decrease in size of the spheres by the solvent action of water.
