LOW PRESSCRE ADSORPTION O S -1JVASHED GL.1SS SURFACE* BY HENRY S. FRANK
The experiments herein described do not constitute as complete an investigation as it was planned to carry out. Although the work had to be discontinued, however, the results obtained are not devoid of interest and bearing upon other current work, and it is considered desirable t o report them. The apparatus also presents some interesting features. The adsorption of water-vapor upon a heavily acid-washed surface of Pyrex was measured a t pressures ranging from j . 2 X IO-^ to 8.; X IO-^ mm. of mercury. The method used was the familiar one of introducing a known amount of water into a space of known volume at a known temperature, and measuring the pressure produced. The amount by which the latter fell short of the value calculated from the gas laws represented the water removed from the gas phase by adsorption on the walls. Since the area of the walls v a s accurately known, this enabled the amount adsorbed per square centimeter to be calculated for each of a series of equilibrium pressures. The pressure gauge used was of the membrane, or diaphragm type, but differed from any of the others’ of which the writer has heard in various respects. It was an all-glass instrunient, and utilized a high-frequency “ultramicrometer” method for measuring the movement of the diaphragm.
The Manometer The instrument is shown in Fig. I . A is a thin-walled glass bulb, blown on the end of the tube F, and having its end flattened to form a diaphragm. This diaphragm has mounted on it the thin aluminum disc B, which makes no metallic contact with anything except the lead wire I . d brass collar C, clamped to the tube F, carries three brass rods which support the heavy brass ring R. The circular brass plate P is supported from R by three springs S which are held in tension by screws brazed in P and passing loosely through K . These are held below R by knurled nuts ?; (only two shown) which thus serve to make the position of P adjustable. A hole in the center of P allows €3 to hang below it, B and P thus forming a parallel-plate condenser, with adjustable plate-separation. Through X, R and C , P is in metallic connection with the lead 1’. The proportions are about as shown, and the overall diameter of the ring K is just under j . 7 j cm. The whole is surrounded by a * Contribution from the Chemical Laboratory of the 1-niveraity of California. ICE. for example Scheel and Heuse: I-erh. deutsch physik. Ges.. 1909. I ; Fry: Phil. Mag., 25. 494 (19131;Baume and Robert, Compt. rend., 168, 1199 : 1 9 1 g ’ ; Daniels and Bright: J. .Im. Chem. Soc., 42, 1131 (1920‘8; Iiarrer. Johnston and Wulf: J. Ind. Eng. Chem., 14, 1 1 0 j (1922); IYhiddington: Phil. Nag., 46, 60; (19231;Smith and Taylor: J. .im, Chem. Sac., 46, 1393 (19241; Klemenc: 47, 2 1 7 3 (19zj:; .Illsop: Safety in Mine? Research Board, Paper 10. 16 ( 1 9 2 j j .
LOW PRESSURE ADSORPTIOK O N .4 WASHED GLASS SURFACE
971
glass shell of internal diameter 5.75 cm., with a ring seal a t D and a side tube T of 2 . 2 cm. diameter. The latter is so located that before it was sealed off a t E the nuts N could be adjusted by means of a stiff wire. The tube F had a bore of I I mm., and the b y p a s s G, which carried the stop-cock H was of 5 mm. tubing. The tube communicated with the vacuum line a t K. The whole instrument was of Pyrex, and when in operation was immersed to the level of D in a thermostat. It is obvious that the greater the area of the plates and the smaller the distance separating them the greater will be the change in capacity for a given small displacement of the diaphragm. I n this instrument an upper limit is placed on the area by the exigencies of glass-blowing and a lower limit on the separation by the accuracy to which a thin aluminum plate can be made plane and the steadiness with which it can be supported by the glass diaphragm. The distance which was found practicable for the separation of B and P was of the order of tenths of a millimeter.
The Electrical System The ultramicrometer circuit used is FIG.I one given by Gunn.' Fig. z represents the arrangement used. 0 iq an oscillator operating a t about 1,500kilocycleq. R is Gunn's circuit, with S representing the plates of the pressure gauge. The sensitivity claimed by Gunn was not attained; but the sensithjty which was attained was satisfactory for the purpose a t hand, Due to fluctuations in the operation of batteries and tubes the following arrangement was also used: d two-way switch was inserted between C 4 and X which made connections alternatively with the manometer or with a fixed air condenser of about the same capacity. During the course of an experiment enough observations viere made with this condenser substituted for the manometer to enable a plot to be made of corrections to be applied to the readings made with the manometer. The results obtained were in the main satisfactory. The manometer was calibrated by comparison with a McLeod gauge, using air dried in a liquid-air trap. The calibration curve was a l w a y linear over the range employed, but itq slope was found to change somewhat over 1
Gunn: Phil. Mag., 48, 224 (1924).
972
H E S R T 9. FRBSK
a period of weeks. A fe\T calibrating observations were therefore included in each experiment. There was no hysteresis in the membrane within the limits of accuracy of observation. The probable error in a pressure measurement was about 5 X I O + mm.
R
0
FIG.z
Measurement of Adsorption The adsorpt'ion apparatus is shown in Fig. 3 . G is the manometer, and D is the by-pass (not shown) corresponding to GH of Fig. I . A is the adsorption chamber, consisting of a number of coaxial Pyrex tubes, bossed to keep their services apart. The innermost tube is sealed shut, enclosing a thermometer. Between marks on the upper and lower tubes A presents a n internal area of 8120 sq. em. and a volunie of 2 2 0 0 cc. The over-all length of A is about, j o em. and the overall dianieter about 8 em. B is a manifold giving entrance to several capsules E which serve to introduce the waterrapor. The trap K and the stop-cocks F and H are used to admit small charges of dry air for calibrat'ion purposes, the reading of the manometer being compared with the pressure shown by the 1IcLeod gauge (not shown). -4detail of the capsule is shonm in Fig. 1. It is first weight-calibrated for the volume from the tip of the capillary to the constriction. It is then sealed onto a vacuum line at P, and evacuated for I O hours or more, during which time it is several times heated just below the softening temperature. (The const'riction is act,ually softened and becomes quite flexible during the heating.) It is then allowed to stand overnight in communication with a cell containing carefully boiled-out water which is maintained at 0°C. Rapid sealing off a t the constriction thus gives a bulb of known volume, containing water-vapor a t a known pressure. This water is introduced into the adsorption vessel by breaking the capillary tip by means of the slug S. The adsorption chamber X after leaving the glass-blower was allowed t o stand for one week filled with chromic acid cleaning solution, which vias several times heated t o boiling. The vessel was then rinsed with distilled water, and finally washed seventeen times with conductivity water. It was then sealed onto the apparatus and dried under vacuum. Fig. 3 does not show a largp balloon flask abore which communicated with the main vacuum line through a \vide-bore stop-cock. It was evacuated
L O T PRESSURE ADSORPTIOS OX A WASHED GLASS SURFACE
973
before an experiment was begun and the stop-cock then closed. Opening the stop-cock when the system contained water-vapor suddenly lowered the pressure and enabled the adsorption equilibrium to be approached from the high-pressure side.
FIb
.+
I n carrying out a r u ~ ithe , whole syPtciii ( F i g 3 was first ryacuxted, stopcocks C and D being open, F and H closctl, ant1 liquid air wrrounding the traps I< and I,. :'cvcr.al wading-; ~ v e r cthen taken with tliv nianonir~tr.rto cJstalilish its zero. $top-cocks (.' :ind D iycrc' thPn c l o d , arid t h t , capsulcs 1.: discharged snccessi\cslg- into thc tcni. After m c h tip TWS broken sufficknt time was alloiveci to p a s b for L' lihriuiii to hr cbt:ihlishccl. At such tiini:I' was desired the l:i~,gcbs t o p c o c k n . a ~opc3ncd to adriiit y:ipor into thc I Y ~ -
974
HEKRY S. FRASK
serve balloon described above. When as many readings had been taken as were desired, the stop-cock C was opened and the system evacuated. Trap T was then closed, and stop-cocks F and H used to admit successive charges of dry air for calibrating the manometer. T and D ITere finally opened, terminating the run. I
I
t
e; 7 12
i
L*
60
30
90
FIG j
TABLE I Point I
n'
o.jj6
2
1.15
3 4
2.57
5
3.91 3.28
P
n"
n
A
L
j.2
0 .Oj2
0 .j O 4
0.50
0,186 0.656 I . 2 j
0.964 1,913 2.6;
0.945
2.33
9767 981; 9833 9939 9984
13 .4 46.2 87 . o 64..8
These data are represented graphically in Fig. 6. n' = Total amount of water in system, in mols X IO+. n" = Amount of mater in gas phase, in niols X 10-j. n = n' - n" = Xniount of water adsorbed, in niols X L = Thickness of adsorbed layer, in molecules. P = Observed pressure, in millimeters X IO-^. h = .Irea of surface in square centimeters.
0.9jz I 88 2.58
2.27
IO-;.
Fig. j shows graphically the results of such a run, in which the introduction of water vapor was begun within an hour of the time that a good vacuum (less than I x IO-5 nim-unreadable on the &Lead gauge) had been established. Here pressures observed are plotted against elapsed time during the experiment, The discontinuities in pressure show n-hen capsules were broken, or in the case of the one a t time 84 minutes, when the reserve
LOW PRESSURE ADSORPTIOS O S .1. WASHED GLASS SURFACE
975
cock was opened. The capsules were not all of the same size, so that the curve gives no indication on its face of the adsorption isotherm which it, represents. It will be noted that the increase in pressure-de-adsorption, that is, following the opening of the reserve cock-was still proceeding a t a considerable rate when the last capsule was broken, but that the amount of water-vapor introduced by that capsule was apparently just enough to p;oduce equilibrium. This is confirmed by the fact that on the adsorption i-0therm the point corresponding to this equilibrium, reached in so roudabout a way, falls, as nearly as can be ascertained, just in the line of the other equilibria.
FIG.6
The isotherm itself is shown in Fig. 6. Here the amount of water adsorbed per square centimeter of glass surface is plotted against the equilibrium pressure. The unit of amount adsorbed per square centimeter is so chosen that unity represents the amount of water which would produce a single layer one molecule deep on the surface, assumins the latter to be plane and the water molecule.to occupy it' on a square 4 Angstrom units on a side. This is the average dimension given by kinetic theory calculations and X-ray meaaurements on ice crystals. The data from which Fig. 6 is taken are summarized in Table I. The observed pressures were read from a large scale copy of Fig. jj the asymptotes shown being those chosen as representing the equilibrium pressures. Since the amount adsorbed is calculated from a difference which is large compared with the subtrahend, i.e. the observed pressure, any error in the latter produces only a much smaller one in L. Trial calculations show that an error of I X IO-^ mm. in the assumed asymptotes for the equilibrium pressure-iyhich is much too large to be admissible-would move point 4 by an amount about equal to the thickness of the line representing the curve.
976
HESRY S. F R A S K
The measurements represented by the square points on Fig. 6 were made in an earlier run before the necessity was apprehended of heating the capsules under vacuum before filling them. The capsules used in this run therefore undoubtedly contain some foreign gas. The effect of this is to make the equilibrium pressure too high, and the apparent adsorption (Le. difference between computed pressure and observed equilibrium pressure) too low. This would make the points lie below and to the right of the true curve, as in fact they do. Discussion Fig. j shows that the adsorption process involved here is s1oLv-i.e. a steady state is not', in general, reached in I j minutes. The good agreement of point j with the curve drawn through the other points in Fig. 6, however, indicates that the process is rtversible. There is apparently no critical importance attaching to a layer-thickness ( I f nile molecule. This, together with the slowness, constitutes evidence against the applicability of the Langmuir theory of adsorption to the process here in question.' Indeed this experiment can be regarded as lending strong circumstantial support to the conclusions of Frazer, Patrick, and Smith,? and of LathanP regarding the nature of acid-washed glass surfaces:. That ii. these results are entirely compatible with the view that such a surface is covered Tyith a !ayer of silica gel, and are difficult t o reconcile with the contrary view that such a surface is plane. The new experiments of Frazer,' giving optical measurements of adsorption of water vapor upon freshly fractured glass surfaces are further evidence that the surface here in question iF of a very different nature. The measurenients correspond to a temperature of 20'C. The manometer was thermostated a t 2 5 0 1 but its surface was less than 2 % of the total internal area exposed, and it is not felt that this temperature difference played any significant part in the result. The writer gratefully acknowledges the encouragement and guidance given this work by Professor G. S . Lewis. Summary .In all-glass iiiembrane manometer hits been constructed, suitable for measuring pressure differences up t o 0.1 niiii. to the thousandth of a millimeter of mercury. This manometer has been used to measure the adsorption of water vapor on a w:ished Pyres surface. The probable nature of the glass surface is discussed in thc light of the results obtained. 1 Since the Langliiuir t h e o r y a s i i i r n ~ s t h e c x i a t r n c r 01 a p l n n ~m r i a r e . ("ompare, howeyer. I,a~igrnuir: .J. Am. Chcni. S w . , 40, 1361 ' r g ~ b arid ~ ; V a r i e r : 45, h j i 1 9 2 y . Fmzer: Patrick anti Smith: J . l'hys. C'hrIn., 31, 697 ~ 1 9 2 7 ) .
3
Latliam: J. . h i . Chem Phys. K F V , 3 3 , 9; :1929:.
' Frucr:
