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PERMANENCE AND SCANNING OF
THE HYSTERESIS LOOP. SILICAGEL-WATERSYSTEM
KITTUR SUBBA RAO Department of Chemistry, Central College, University of Mysore, Bangalore, India Received November 16, 1 9 B
With a view to elucidation of the exact cause of the vexing problem of hysteresis in sorption, a systematic investigation of the hysteresis effect was undertaken. The extraordinary permanence and the scanning of the hysteresis loop obtained in the sorption of water on titania gel have been reported in the previous communications (8, 9). A study of the hysteresis loop in the sorption of water on silica gel, carried out on the same lines, is presented in this paper. The hysteresis in the sorption of water on silica gel in the presence of air (4) has been found by Patrick to disappear (5) when air was removed by drastic evacuation. Lambert (2) and Foster (1) have obtained hysteresis loops, even in the absence of air, in the sorption of water, methyl alcohol, and ethyl alcohol on silica gel. PERMANENCE O F THE HYSTERESIS LOOP
Silica gel Silica gel of the glassy variety was prepared by the method described by Rao and Doss (7). It was washed free of chloride and dried a t 100°C. The dried gel was activated by passing a current of dry air a t 450°C. for 4 hr. The activated gel was found to contain 2.7 per cent of water. Making use of the quartz-fiber spring technique (€9,a series of sorptions and desorptions of water vapor on silica gel a t 30°C. was conducted; the results are shown in figure 1. At the end of the first desorption, the gel retained irreversibly a t zero pressure 2.4 per cent of water in the same way as titania gel (8). On continuing the evacuation for another 10 hr., it was reduced t o 2.2 per cent. This “bound water” is probably locked in the interstices of the gel (8) or held as hydroxyl groups on the surface of silica (6). From the second cycle of sorption and desorption, the hysteresis loop remains remarkably steady without any tendency toward “drift.” The third and the nineteenth hysteresis loops are identical with the second. h period of two months had elapsed from the commencement of the first sorption to the completion of the nineteenth desorption. The facts tend to show that there is no doubt about the reality of the hysteresis effect, and that a hysteresis loop which is perfectly reproducible a t the nineteenth sorption and desorption is satisfactorily explainable only on the basis of the concept of cavities with constricted ends (3, 8, 9).
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SCANNING O F THE HYSTERESIS LOOP
In exactly the same way as in the case of titania gel (9), the hysteresis loop obtained in the silica gel-water system has been scanned. Starting
Pressure in m m
FIQ.1. Hysteresis loop in the sorption of water by silica gel. A, first sorption ( 0 ) and desorption ( 0 ) ;B, second sorption ( 0 ) and desorption ( 0 ) ;C, third sorption ( 0 ) and desorption (0);D, nineteenth sorption ( 0 ) and desorption (0).
FIG.2. Scanning of the hysteresis loop in the sorption of water on silica gel
from any intermediate point on the sorption curve enclosing the hysteresis loop,-if desorption is tried, the loop is crossed until the main desorption curve is reached (figure 2). Starting from any intermediate point on the
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desorption curve, however,-if sorption is tried, the main sorption curve is not reached, but a separate curve is traced until the peak of the hysteresis loop is reached (figure 3). A detailed discussion of exactly similar results obtained in the titania gel-water system has been presented in the previous Paper (9). Figure 4 illustrates the interesting observations made on the scanning of the hysteresis loop in a modified way. A point on the main desorption curve has been reached separately in two ways: firstly, from the peak of the hysteresis loop and, secondly, from a point on the main sorption curve. After reaching this point, sorptions were tried. The curves followed are
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FIG.3. Scanning of the hysteresis loop in the sorption of water on silica gel
not coincident, but they are displaced. The curve obtained according to the first method follows an independent course until the peak of the hysteresis loop is reached, Le., in accordance with the general observations indicated in figure 3. But the course followed according to the second method has been displaced towards the pressure axis and finally it reaches the main sorption curve. Two such experiments, starting from two different points on the desorption curve, were conducted and have been illustrated in the figure. The “inset” of figure 4 shows clearly the distribution of the different curves in that part of the loop in which there is a crowding of the lines. At 2 in figure 4 there are some larger cavities in porous silica gel, not completely filled with water. On trying desorption, point X on the main
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desorption curve is reached. If sorption is tried again a t X, the curve reaches the main sorption curve a t R. In another experiment X i s reached, starting from the peak Y of the hysteresis loop at which all the cavities are completely filled up. On trying sorption a t X, owing to the presence of some wide cavities which were filled with water at or very near the saturation pressure, a separate curve is traced in juxtaposition to the main sorption curve until Y is reached. The observations on the scanning of the hysteresis loops in the titania gel-water (9) and silica gel-water systems have revealed certain common
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FIG.4. Scanning of the hysteresis loop in the sorption of water on silica gel
characteristics which tend to focus on the cavity concept w a general cause of hysteresis. COMPARISON OF THE CHARACTERISTICS O F THE HYSTERESIS LOOPS. SILICA GEL-WATER AND TITANIA GEL-WATER SYSTEMS
The hysteresis loop in the silica gel-water system shows no tendency to “drift,” unlike the hysteresis loop in the titania gel-water system. The tail-end of the loop terminates at a relative humidity of 0.35, and this corresponds to a capillary radius of 10 A. Beyond a rzlative humidity of about 0.6, corresponding to the capillary radius of 20 A., the hysteresis loop tapers. I n titania gel, however, the hysteresis loop is within the limits of relative humidities of 0.35 and 0.94, and these correspond to
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capillary radii of 10 A. and 160 A. Titania and silica gels behave alike with regard to the “bound” water which is irreversibly held at zero pressure. SUMMARY
In the sorption of water a t 30°C.,silica gel has exhibited a hysteresis loop which remains permanent and is perfectly reproducible, the loop having been traced a t the nineteenth sorption and desorption. The hysteresis loop has been scanned by traversing it from various points on the sorption and the desorption curves. The scanning of the loop has revealed certain characteristics which form a convincing proof of the concept of cavities with constricted ends as a general cause of hysteresis. A comparison of the hysteresis loops of the silica gel-water and titania gel-water systems has been made. The author is grateful to Prof. B. Sanjiva Rao for his kind encouragement in this work. REFERENCES (1) FOSTER.: Proc. Roy. SOC.(London) A l a , 129 (1934). (2) LAMBERT AND FOSTER: Proc. Roy. SOC. (London) AlS4, 246 (1931). (3) MCBAIN:J. Am. Chem. SOC.67,699 (1935). A N D PATRICK: J. Am. Chem. SOC.42, 946 (1920). (4) MCGAVACK (5) PATRICK:Colloid Symposium Monograph 7, 129 (1930). (6) RIDEAL:Trans. Faraday SOC.32, 4 (1936). (7)RAOA N D Doss: J. Phys. Chem. 36, 3486 (1931). (8) RAO:J. Phys. Chem. 46,500 (1941); Paper I. (9) RAO: J. Phys. Chern. 46, 506 (1941); Paper 11.
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PERMANENCE AND SCANNING OF THE HYSTERESIS LOOP. SILICAGEL-CARBONTETRACHLORIDE SYSTEM KITTUR SUBBA RAO
Department of Chemistry, Central College, University of Mysore, Bangalore, India Received May 3, 1940
The permanence and the scanning of the hysteresis loop in the sorption of water on gels of titania (3, 4, 5 ) and silica (6) have been indicated in the previous papers. It is obvious that the general characteristics, observed on scanning, are not connected in any way with the nature of the adsorbent or of the adsorbate, but are dependent solely upon the size and the shape of the cavities in the porous adsorbent. This generalization
