914
CHARLES
B. HURDAND STEPHEN H. LANINU
Vol. 58
STUDIES ON SILICIC ACID GELS. XVIII. THE ACTION OF SODIUM HYDROXIDE ON THE GEL BY CHARLES B. HURDAND STEPHEN H. LANING Chemical Laboratory, Union College, Schenectady, New York Received February 84, 1964
A study of the attack of sodium hydroxide on thin films of silicic acid gel is reported. The clear film of gel turns milkywhite about one second after the 1 M NaOH attacks it. This is followed by the appearance of a network of cracks all over the surface. These grow larger and deepen with tlme. Some shrinkage of the film occurs between the cracks. The film dissppears eventually. A series of photographs illustrates the phenomena. The time required for the NaOH to develop visible cracks on the surface of the film is approximately proportional t o the age of the gel when the NaOH is applied. We are undoubtedly here observing a reversal of the normal condensation reactions in simple silicic acid, which result in the formation of silicic acid gels. e
Introduction Although it has been known for many years that solutions of strong alkalies will dissolve silicic acid gel, cf., Monier,’ the actual process has attracted little attention. Silicic acid gels result when solutions of sodium silicate and acids are mixed, providing the concentration of SiOz is high enough and providing the pH of the mixture is less than 11. Another may of decreasing the pH has been employed by Treadwell and his c o - \ ~ o r k e r s , ~who -~ removed the NaOH from a solution of sodium silicate by electrolysis in a special cell. As the p H decreased, the solution formed a gel. The action of NaOH in dissolving a silicic acid gel evidently reverses the process involved in gel formation. Chance observations in this Laboratory of the appearance of,the gel surface when under attack by strong alkali led to the investigation reported here. Experimental
erably. They were acid gels, containing excess acetic acid. The pH and concentrations were not critical, although all pH values were below 7 . A few drops of the mixture were placed immediately on several microscope slides, which were placed flat in a desiccator containing water in the bottom. The gels set in from 10 to 15 minutes a t room temperature, 21 to 23 . The temperature was not critical. The age of the gel film was noted when 1 M NaOH solution was placed on it. This age, namely, the time elapsed between time of set of t h e gel film and the start of the attack by 1 M NaOH, was important. The action of the NaOH was followed visually and by photographs, with illumination from abovo. The main gel mixture was also kept and the amount of syneresis determined as a function of the age of the gel.
Results The gel films were clear, were from 0.1 to 0.4 mm. thick and appeared black in the photograph, before attack by the NaOH solution. No cracks were visible. The 1 M NaOH when applied caused the film t o turn white, both to the eye and in the photograph. About 1 second was required for a fresh Silicic acid gel mixtures were prepared by mixing solutions gel. of sodium silicate and acetic acid. The method has been The second phenomenon was the appearance of a reported many times.6 The silicate was prepared by di- maze of fine cracks on the gel surface. The time luting “E” brand silicate, made by the Philadelphia Quartz Company. The gels prepared contained about 0.6 g. mole required for these cracks to appear depended on the SiOz per liter, although the amount may be varied consid- age of the gel when the NaOH was applied. The cracks spread rapidly and the gel dissolved away from the edges, appearing to shrink and disappear. Results on a number of specimens showed the same relation, namely, that the time required for so the NaOH to show the first cracks in the gel surface is practically proportional to the age of the gel when the NaOH is applied. This is shown in Fig. 1. This gel set in 12 minutes. The curve shown is 0 .* practically linear. 2 40 Figure 2 shows three series of photographs to illustrate the behavior of the gel film under attack by M 1 M NaOH. The magnification in the photograph 3 20 is between 1 X and 2 X . The first series is for a gel film 3 hours old when the NaOH was applied. The photographs were taken 55, GO, G5 and 70 seconds after the NaOH was applied. The second series 0 is for a gel film 7 days old wheii the NaOH was ap0 200 -100 (io0 800 1000 1200 plied. The photographs were takcii 510, 550, 590 Age of gel in hours. and 750 seconds after NaOH \vas applied. The Fig. 1.-Etching time for fiodium hydroxide on silicic acid third series is for a gel film 20 days old when the gel as a function of age of the gel. NaOH was applied. The photographs were taken 1860, 1920,2010 and 2370 seconds after NaOH was ( I ) E. Monier, Comp rerid., 86, 1053 (1877). applied. (2) W. D. Treadwell, T r a n s . Fnraday Soc., 81, 297 (1935). (3) W. D. Treadwell and W. Konig, Helu. Chim.Acta, 16, 54 (1933). Discussion (4) W. D. Treadwell and W. Wieland, i b i d . , 13, 842 (1930). The films of silicic acid gel, having been kept in a ( 5 ) C. B . Hurd and P. S. Miller, THIS JOURNAL, 36, 2194 (19321, desiccator over water, could not have undergone and other papers of this series.
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.e
ACTIONOF SODIUM HYDROXIDE ON SILICA GEL
Oct., 1951
915
I
I1
111
Fig. 2.-Films
0
of silicic acid gel under attack by solutions of sodium hydroxide.
any drying by evaporation, since they were in an atmosphere saturated with water vapor. They were transparent before the NaOH was applied. The NaOH caused the films to become white, almost a t once. Because of the very short time required, one or two seconds, this must be a surface effect. Further attack by NaOH resulted in the appearance of cracks. The time required for the first recognizable cracks to appear is approximately proportional to the age of the gel when the NaOH is applied (Fig. 1). This must mean that the gel becomes harder with age. The cracks would appear to be due to strains set up in the gel due to shrinkage. This gradual shrinkage also results in syneresis and is probably due to continued condensation of the silicic acids. Attack by NaOH also causes additional shrinkage as shown by the latest photographs in each series, especially series 2 and 3. The honeycomb or cell-like structure shown in the photographs may have been present in the gel before attack, but it is probably the result of strains either in the gel or developed by the NaOH. This honeycomb structure can certainly not be taken as any proof of the Honeycomb Theory of Gel Structure proposed by
Butschli,6since the dimensions of the sections show11 between the cracks are surely of a much larger order of magnitude than suggested by Butschli. We are dealing here with a reversal of the reactions of condensation normally occurring in the formation of silicic acid gels, namely (H0)J3i-OIH[-Si(OH)a
+ ( HO)sSi-O-Si(
OH),
+ HzO
to illustrate only the first step in a long series of condensations. The NaOH neutralizes the simpler silicic acids, which are stronger acids than are the polysilicic acids. This neutralization, by removing the simplest species, upsets the equilibria present in a gel and causes the more complex condensation products to hydrate and form the simpler acids. This causes solution of the gel. A more complete discussion of this equilibrium has appeared recently’ in connection with a discussion of experiments on extended dialyses of silicic acid sols and gels. (6) 0. Biitschli, “Untersuchungen iiber Strukturen,” Leiprig, 1808. (7) C . B . Hurd, M. D. Smith, I?. Witael and A. C. Glamm, Jr., T ~ i s JOURNAL, 67, 678 (1953).
