T H E MICROTOME METHOD O F T H E DETERMINATION O F T H E ABSOLUTE AMOUNT O F ADSORPTIOS BY JAMES w. MCBAIN ASD c. w. HUMPHREYS This work has been concerned with the creation of a microtome method for the determination of the absolute amount of adsorption a t statzc air-water interfaces, for which no previous method existed, and with the development of this method to the point where it gives sufficiently accurate and reliable results to serve as a crucial test of the Gibbs adsorption theorem. The validity of the Gibbs equation has never been demonstrated experimentally. In fact, as pointed out by McBain and DuBois,' practically all previous experimental work, where there was not an inherent error in the method used, has given observed values of the adsorption which are considerably in excess of those calculated by means of that thermodynamic equation, either in its strict form or in the approximate form commonly used. The work of McBain, Davies2 and DuRois, corroborated by the single measurement of Harkins and G a n ~shows , ~ quite definitely that moving surfaces carry several times more solute than is compatible with the Gibbs equation. However, these moving surfaces, which were used in all previous measurements, may not have represented perfect equilibrium nor have met all of the conditions of the Gibbs equation. Because of the wide use of that equation, its fundamental nature, and the lack of experimental proof of it, measurements of the absolute amount of adsorption at .static air-water interfaces are plainly needed. The method which has been developed in this work may be outlined briefly as follows. The solution being studied is kept at rest for any desired length of time in a shallow trough of pure silver surrounded by a saturated atmosphere. By paraffining the ends of the trough the solution is made to bulge up above them without overflowing. h uniform layer 0.05 to 0.1mm. thick is cut off from a known area (310 sq. cm.) of the surface by a small microtome blade traveling at a speed of about 35 ft. per second. This thin layer of solution is collected in a small silver-lined cylinder on which the microtome blade is mounted. The solution so obtained is weighed and its concentration is compared with that of the bulk of the solution in the trough by means of a Zeiss interferometer. From the observed difference in concentration, the absolute amount of adsorption at the surface of the solution can be calculated.
The Trough, Tracks and Microtome The main features of the apparatus designed for this work are shown in detail in the accompanying drawings and photographs. 1
J. W. McBain and R. DuBois: J. Am. Chem.
Sac., 51, 3534 (1929). McBain and G. P. Davies: J. Am. Chem. Soc., 49, 2230 (1927). W. D. Harkin8 and D. M. Gans: Colloid Symposium Monograph, 6,26 (1929).
* J. W.
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' FIG1 CROSS SECTION THROUGH CENTER OF INNER ENCLOSURE (THROUGH A - n ' FIG 3a)
20 CY
FIG 2b SIDE CLEVATDM
THREE VIEWS OF A SECTION OF THE TPACKS YIOwffi
FIG 2
T R 4 M AN0 m K E IN DETUL
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The trough B, Fig. I, which contains the solution being used is made of pure silver 0.5 millimeters thick. It is approximately 7.5 centimeters wide, 85 centimeters long and the ends and sides are respectively three and eight millimeters high. It is supported between two heavy steel rails, R, as shown in Fig. I . These rails are thirty-two feet long and it is upon them that the microtome carriage, which supports the microtome blade, slides. The tracks are fastened securely to steel ties a t intervals of two feet as shown in Fig. 2 . Xear the outer edge of each tie is a set screw, C, which rests upon the concrete floor and allows adjusting of the height and leveling of the tracks. The machined upper surface of the rails was sufficiently true for the major part of the tracks. For the part adjoining the trough a much finer adjustment was necessary and was obtained by lapping this portion true so that the variation in level over the whole length of the trough was less than 0.015 mm. The upper part of both sides of the tracks are also machined. The microtome carriage which slides along the tracks and carries the microtome blade which does the cutting is shown in detail in Fig. 6. It consists essentially of a frame, B, upon which is mounted the cylinder, C, which holds the blade. The cylinder consists of a section of one inch brass tubing, HI which is soldered at the ends to two brass disks. These disks are made with bearings, K, which fit closely through the angle pieces, MI thus allowing the cylinder to turn. The angle pieces are fastened to the carriage so that the cylinder is held firmly in place. A cover, J, made of one-half millimeter sheet silver is fastened over the remaining open part of the cylinder and curves around inside of it, thus acting as a baffle plate to keep the collected liquid from flowing out again. The microtome blade, XI is similar to a rigid safety razor blade. It fits into a small slot milled in the cylinder, as shown in Fig. 6d, and is soldered rigidly in place. The cylinder is lined with pure sheet silver and has a thin coating of paraffin so that the solution collected can be poured out more readily. The solution is removed by pouring it through one side of the same opening through which it enters. The cylinder is held in the proper position for cutting by the small steel pin, E, Fig. 6a, which rests upon the stop, S, and is held down by a thin strip of spring steel, F. The microtome blade is given the speed necessary for cutting such a thin layer from the surface of the solution by shooting the carnage along the tracks by means of a slingshot arrangement made of rubber tubing. Since it is traveling quite rapidly (at the rate of 2 5 miles per hour) the carriage must be stopped rather rapidly after passing the trough. To retain the collected liquid the cylinder is turned through about 120' so that the blade and opening are pointing directly upward as soon as the trough is passed. It is turned by the steel pin, P, striking the stationary device, V, Figs. 3a and 3c, and is held in this second position by a small braking device, D and E, Fig. 6c,which has a groove in which the steel pin, P, fits. The microtome carriage is stopped quickly but without any sudden shock by the brake shown in detail in Fig. 2 . This consists of two steel strips, ten feet long, fastened to the outer sides of the rails. The braking action is exerted upon the two sides of the microtome carriage, 0, Fig. 6c, which slide in the
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groove between the steel strips a d the rails. The desired braking action is obtained by adjusting the pressure on the sides by turning the nuts on the ends of the bolts, B, which press upon the springs, F, and by regulating the width of the groove with the set screws D. The device, E, Fig. ga, for stirring the solution in the trough consists of a long strip of silver, one-half millimeter thick and about two millimeters wide, just above the level of the sides of the trough, to which are fastened four cross pieces of the same material which rest upon the bottom of the trough. This long strip extends out beyond the back door and the solution is stirred by merely sliding it back and forth. The stirrer does not break the surface. d
The Saturated Vapor surrounding the Trough Since any evaporation from the solution in the trough must take place through the surface, its effect would be enormously magnified in the thin layer cut off in an experiment. Therefore it is necessary to prevent any such evaporation from taking place. The apparatus for doing this is shown in cross section in Figs. I and 5 and in general layout in Fig. 3. The silver trough is in an inner enclosure which is just large enough in cross section to allow the microtome carriage t o pass through and extends about ten centimeters beyond each end of the trough. Around this is a much larger, plate glass, outer enclosure as shown in Figs. 3, 4 and 5 , and Photographs 7 and 8. Both enclosures are nearly air-tight. The inner one is sealed off underneath the trough by the sheet of 0.001inch thick silver foil, V, Fig. I, which is clamped against the rails by the monel metal strips C and extends down across and upon the plate glass D, underneath the glass strips which separate the glass supports A and D. At the ends, the silver foil comes up and extends out over the ends of the silver-plated brass plates, K, Figs. 3a and 3b, upon which the doors at the ends rest. The sides of the inner enclosure are silver plated brass pieces, J, bolted to the tracks and milled out so that the microtome
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J. W. McBAIN AND C. W. HUMPHRETS
carriage can pass through as shown in Fig. I . The cover, N, is of plate glass. Vaseline is placed between the side pieces and the cover to give an air-tight fit. The sides, top and bottom of the outer enclosure are of plate glass and the ends are of silver plated brass. All exposed brass parts in either enclosure are
CROSS SECTION THROUGH TRACKS AND BOTH ENCLOSURES
(THROUGH
B-B’ FIG.
a)
FIG. 5
silver plated to limit corrosion; such suffaces come into contact only with vapor. The solution being studied comes into contact only with pure silver or, for brief periods, with glass. To help saturate the air space in the inner enclosure are two side trays, S, Fig. I , filled with the solution being used. In addition, the space above the silver foil between the glass supports A and D is filled with the same solution as shown in Fig. I . A stream of air or nitrogen saturated with respect to the
solution being used, by means of a rocking Washburn' saturator, may be passed into the inner enclosure also. I n the large outer enclosure are a large number of crystallizing dishes, as shown in Figs. 7 and 8, which contain solution t o saturate this outer enclosed air space. There are holes in the covers of both enclosures so that the whole apparatus can be set up and the solution then be put in. These holes are closed with cover glasses. The Automatic Doors Since the microtome carriage must pass through both enclosures in making a run, there is a close-fitting door in each end of each enclosure. These doors are automatically opened and closed by electrical contacts which are operated by the carriage itself when it is shot along the tracks. The front door to the outer enclosure is opened first and it is then closed as the other three open so there can be no rush of outside air through the apparatus to cause evaporation. A drawing of one of the doors is shown in Fig. 4 and the photograph in Fig. 8 shows quite well how they are made. A hole is cut through each of the brass
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FIG.7 1
E. JV. Washburn and E. D. Heuse: J. Am. Chern. SOC, 37, 309 (191j)
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JAMES W. McBAIN AND C. W. HUMPHREYS
end pieces large enough to let the car through as indicated by the dotted line I< in Fig. 4. The frame for the door is very light, being made of a piece of 1 / 3 2 inch duralumin A fastened to a piece of 1/16 inch duralumin B by small set screws. The frame is large enough so that the microtome carriage can pasx t,hrough without touching it. I t fits as closely 3.5 possible against. the end piece nnd still moves freely. Clamped between these two parts of tire framc is 3. piece of silver foil o.002 inch dhick which is cut out so that it, fits closely around thr iracks and down upon the brass block H, thus closing the opening
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8
in t.he end piece. With the doors t o the inner enclosure, the silver foil fits down upon the brass plates, I
