SILICIC ACID GELS.
XI11
1263
(7) GROXWALL A X D L AMER: J. Phys. Cliem. 31, 393 (1027). (8) HOFMEISTER: Arch. exptl. Path. Pharmnkol. 24, 1, 248 (1889); 26, 1 (1889): 27, 395 (1800): 28, 210 (1891). (9) H r s : Compt. rend. 202, 1779 (1936j. (10) ISGHARI: J. Chem. Soc. 1930, 5-12. (11) Internntionnl Critical Tables. 1-01,111, p. 91. lIcGmw-Hill Book Company, Inc., S e w York (1928). (12) JABLCZYSSKI: Roczniki Chrm. 13, 167 (1933). (13) .JOSER ASD R.AY:J . Am. Chern. SOC. 69, I87 (1937). SD C o s v \ r r ~ I'c~rliandcl. : .4kad. Tt'ctcnscliappcn Amstcrntuiirkundt~36, 812 (10263. (15) LEWISASD I t . t s D . A L L : ThernrodyiLamics nntl f h c I,'rte E n t r g T i!f C'hcmical Sube t n n c t s , p . 3 5 . NcGraiv-Hill Book ('ompnny. I n c . , l e i v l-orl; (1023). (16) M . m r w n . r JIHRSTORFER, ASD ZEPTER: Z.anorg. allgem. Cl~cni.141, 45 (1924). (17) MASSOS:Phil. Mag. 171 8, 218 (1029). (IS) PAVLI: Beitr. Chem. Physiol. P a t h . 3, 225 (1902). (19) PEARCE .ASD ECKBTROM: .J. -1m. Cheni. Soc. 69,2689 (1937). (20) REDLICHASD ROSESFELD: Z . physik. Chem. A166, 65 (1031). (21) R O O T :J. :hi. Chem. SOC.66, 850 (1933). (22) SCOTT: J. Phys. Chem. 36, 2315 (1931). (23) SPIRO:Eeitr. Chem. Physiol. Path. 6, 276 (1904). (24) URB.AX: ,J. Phys. Chem. 36, 1108 (1932). (25) \VADE: J. Chcm. Soc. 76, 254 (1809). (26) KEITZASD STOXM:Ber. 61B, 1114 (1925). (27) WIRTH: J. Am. Chem. SOC.69, 2549 (1937).
STUDIES O S SILICIC ACID GELS. XI11 SOME EXAMPLES O F RE-GELATION CHARLES B. HURD
AND
LOUIS W. T H O M P S O S , JR.'
Department of Chemzstry, Cnaon College, Scheneclady, S e w York Recezved October 1 7 , lQ@ INTRODUCTIOh-
Several years ago, while studying the p H of silicic acid gels, Hurd and Griffeth (6) noted that a silicic acid gel which had been beaten into very fine particles in a n equal volume of distilled mater apparently possessed the property of re-gelation. The finely divided, milky material settled, and this settled material became mushy and finally firm. TVe thought then that the phenomenon was related to the behavior of a silicic acid gel misture, which, if stirred just bel'ore it sets, will still set to form a firm gel. Certain gels are known, consisting of fine, intermeshed crystals, which 1
Present address: E. I. du Pont de Xemours & Company, Waynesboro, Virginia.
1264
CHARLES B. HURD AND LOUIS W. THOMPSON, JR.
show thixotropy as defined by Freundlich (2). They are liquefied on shaking, but, upon standing, they suddenly set again. This change may be repeated. Pauli and Valk6 (10) reported that silicic acid gels containing from 1t o 2 per cent of silica showed thixotropy. Ultrasonic waves failed t o liquefy silicic acid gels (4), except in the case of dilute gels of pH 8.5 to 9.5 (3). The gels studied in the present work were of higher silica concentration and could not be liquefied by shaking or by stirring. They were easily broken up by being beaten in an equal volume of water. EXPERIMENTAL
Gels were made by mixing solutions of “E” brand sodium silicate (Philadelphia Quartz Company) diluted to 1.19 AT sodium hydroxide equivalent and solutions of 0.98 N acetic acid. When set, the gels were broken into lumps and beaten in distilled water, using a motor-driven stirrer, 3600 R.P.M., with stainless steel blades. Gravity settling was very slow, but the same results were obtained with a centrifuge in 2 per cent of the time. A quinhydrone potentiometer (6) was used for pH measurements in acidic mixtures, while colorimetric methods were used in basic mixtures. On the original gel, the pH of the whole sample was taken, while on centrifuged samples the pH of the supernatant liquid was taken. To prevent the drying out of the gels, beakers with gels were kept in desiccators over water. It was found that a glass rod, tilted a t 2OoC., would stand temporarily in a mass which had just been centrifuged and was obviously not set. Therefore, a test similar to that of F l e m i n g ’ s (1) tilted test tube was used to determine when the residues from the centrifuge had set. This method, though not perfect, is certainly comparative. The results are given in table 1. The first column gives the identifying number of the mixture, the second column the pH of the original gel, and the columns thereafter show the pH of the wash water after each beating. An underline signifies that the mass failed that time to re-gel, and the number underlined gives the pH of the whole beaten mixture. I n each gel, the concentration of silica was 0.615 g.-moles per liter and that of sodium was 0.372 g.-ions per liter. The acetic acid concentration varied. DISCUSSION
Several observations are worth noting. After the first beating and centrifugal settling, gels 1 through 6 appeared to have about 30 per cent greater volume than the original gel. These, after the second beating, and gels 7 through 11 after the first, showed about the same volume as the original gel. On the other hand, gels 12 through 17 showed a smaller volume of residue after beating and centrifuging than the original gel
SILICIC ACID QELS.
1265
XI11
volume. The material appeared less firm. It should be noted that these last did not re-gel. This apparent volume and the ability to re-gel are obviously a function of the pH of the original gel. When the centrifuged masses were drained and allowed to stand in order t o re-gel, slight syneresis occurred in gels 8 , 9 , and 10, but here only during the first re-gelation. It did not occur in the other gels. The observations of Kandelaky (8) on hydrated ferric oxide could not be repeated for the silicic acid, owing to our procedure. However, a microscopic examination of the semi-solid produced by re-gelation showed fairly large aggregates within which were smaller and much smaller particles. TABLE 1 Number of times of re-gelation and the p H of each midure QEL YIXRTBE 1 0 .
pk?
OF W A S H W A W B Al'mB
i
a
____
1lUQIXAL
QEL
i
j
2
l
1 2 3 4 5 6
4
__ 6 __ 5.2 5.2 5.3 5.5 5.7
DdCE BIATIXQ _ .
7
6.3 5.3 5.3 5.4 5.3 5.4 5.5 5.6 5_7 7-5
8
.
0
__ 5.5 5_5 5.4 5A5 4 & 5 5.6
k6
7
8 9
10 11 12 13 14 15 16 17
10.9
10.8 11.1
11.3
11,1
11.2 11.3 11.4 11.5
Did Did Did Did Did Did
not not not not not not
re-gel re-gel re-gel re-gel re-gel re-gel
This was very different from the bmooth, slightly hazy appearance of the real silicic acid gel, as examined before beating. Three characteristic properties of the material produced by this beating with water, separation with a centrifuge, and what we have here called regelation, are the rigidity, e h t i c i t y , and viscosity. The original gel showed rigidity in excesa of that required to support the rod or to support itself in the tilted tube. It was elastic, as has often been noted. It did not show viscous flow under ordinary conditions. The material produced by re-gelation did not show true elasticity, but was more like a plastk or a thick viscous fluid. Immediately after ceiitnfugal separation, it showed little resistance t o viscous flow but, upon standing, reached a condition where it would support itself for borne time in the inclined tube. Such
1266
CHARLES B. KURD AND LOUIS W. THOXPSON, JR.
mixtures we have considered to have undergone re-gelation. With long standing they undergo plastic flow, which a true silicic acid gel will not do under its own weight. When a silicic acid gel is disturbed before it has set, by stirring during the viscous stage, the condensations are still proceeding sufficiently vigorously so that the whole mass knits itself together and really sets. After the gel has set, however, beating it up with water into fine particles and allowing it to settle, either slo~vlyby gravity or more rapidly by centrifugation, produces a loose type of coagulation, giving a viscous mass. The large number of connections produced in the original condensation are not present here, but a certain degree of connection is formed during standing. This decreases after successive beatings, although there is very little loss of the material itself, until, for each mixture, a time comes when the material will not re-gel after beating. The p H is very important in this connection, as table 1 shows. The most acidic mixture, p H = 5.1, re-gelled the largest number of times,namely, eight. With gels of higher pH, the ability to re-gel decreased, until with mixtures more basic than pH = 10.5, no re-gelation was observed. This observation suggests that a n excess of hydroxyl ions causes a weakening of the secondary forces which cause re-gelation. So one who has seen the product of this re-gelation, especially under the microscope, would confuse the result nith an ordinary silicic acid gel. During these experiments we have, of course, been removing the electrolytes, since each time the gel has been beaten with an equal volume of water and then settled n i t h the centrifuge. If we assume a uniform distribution of electrolyte, this process would account for a loss of about half of the elertrolyte each time. If a portion of the ions are adsorbed on the colloidal silicic acid, the loss, while considerable, would be less than half. Mixture KO. 1 is well buffered, and eight extractions raised the p H from 5.1 to 5.5. The acidic niistures showed a tendency for the p H t o drop from 0.1 to 0.3 unit after the first beating and to rise thereafter. It is important to note that the final pH is not the Same for all. Mixtures 8 and 9 nere close to the neutral point. Both re-gelled twice, but their final pH values are x-erj-clifferent,--0.7 and 9.2. Of mixtures 10 through 17, all basic, the two least basic ones (numbers 10 and 11) were able t o re-gel once, while all of the rest failed t o re-gel a t all. It would appear from this that, while the original gelation occurs in mixtures with pH up to at least 11.3, >!hen the gel is broken u p regelation does not occur with p H over 10.5. This phenomenon probably is explained by the fact that silicic acid gels set relatively easily over a pH range from 5 0 to 9.0. Above a p H of 9.0 they set very ~ l o d y . This was first shown by Holmes (5), w-ho gave a
SILICIC ACID GELS. XI11
1267
curve showing the relation between time of set and the acidity of the mixture. A similar curve for the relation of time of set and p H has just been published from this laboratory ( 7 ) . This curve shows that, while acidic mixtures set relatively rapidly in the p H range 5.0 t o 7.0, basic mixtures with p H = 11.0 set very slowly. The amount of silicic acid formed in a mixture of this p H is probably barely sufficient to cause the gel to set the first time, and it is certainly insufficient to cause re-gelation. The fineness of subdivision is probably important. Experiments of this kind should be carried out with a colloid mill. SUMMARY
Silicic acid gels (containing 3.5 per cent of silica) after setting were beaten into fine particles in an equal volume of distilled water. The settled material showed a type of re-gelation. Acidic mixtures (pH = 5.1) re-set eight times, while basic mixtures did not re-gel a t all. These gels did not show true thixotropy. The condensations which apparently cause the setting of the original geI mixture proceed only to a limited extent in these sedimented residues. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) '10)
FLEMMING, W.: 2. physik. Chem. 41, 427 (1902). H.: Kolloid-2. 46, 289 (1928). FREUNDLICH, FREUNDLICH, H . , AND GILLINGS, D. W.: J. Chem. SOC. 1938, 546. FREUNDLICH, H . , ROGOWSKI, F., AND SOLLNER,K . : Kolloid-Beihefte 37, 223 (1933). HOLMES,H. N . : J. Phys. Chem. 22, 510 (1918). HURD,C. B . , AND GRIFFETH, R. L . : J. Phys. Chem. 39, 1155 (1935). A. J.: J. Am. Chem. SOC. 62, 2767 (1940). HURD,C. B . , AND MAROTTA, KANDELAKY, B. S.: Kolloid-Z. 74, 200 (1936). S . , AND SADOWSKY, C.: Kolloid-Z. 6, 292 (1910). PAPPADA, PAULI,W., AND VALK6, E . : Kolloid-2. 38, 289 (1926).
