678
C. B. HURD,nq. D.
SMITH,
F. WITZEL AND A. C. GLAMM, JR.
chemisorbed oxygen, rather than a stoichiometric oxide, was the cause of the phenomena described.
Vol. 57
However, to date this approach has not lent itself to formulation of a satisfactory explanation.
STUDIES ON SILICIC ACID GELS. XVII. DIALYSIS OF THE GEL MIXTURE AND OF THE GELS BY CHARLES B. HURD,MARVIND. SMITH,FRANK WITZELA N D ARTHURC. GLAMM, JR. Contribution from the Butlerfield Chemical Laboratory, Union College, Schenectady, N . Y . Received March 8, 10.59
Dialysis of hydrosols, made by mixing solutions of sodium silicate and acetic acid, shows that simpler silicic acids predominate a t first. As the sol ages, more complex materials form. The simpler molecules are able t o diffuse through collodion membranes. Withdrawal of simpler silicic acids b dialysis apparently disturbs the equilibrium, resulting in the formation of more of the simpler molecules by hydrolysis of tge com lex polysilicic acid. This is the reverse of the reaction which results in gel formation. Even in a gel or an old hydrosol, t i e existence of sim ler molecules in a type of equilibrium appears to have been demonstrated. A discussion of the theory of condensation of sitcic acids and of the reverse process is given.
Introduction Dialysis has been used to purify hydrosols of silicic acid or hydrous silica since the pioneer work of Graham.l Where hydrosols have been made by mixing solutions of sodium silicate and acid, dialysis has removed excess acid and sodium salts. Rate of dialysis and ampunt of silica in the dialyzate were studied by Z s i g m ~ n d yand ~ , ~by others. It has been concluded that sizable amounts of silica can dialyze from freshly prepared sols, but that this decreases with age of the hydrosol and that no silica can dialyze out as the sol approaches the setting time. This has been explained by particle size. Remembering that precipitates of hydrous silica and silicic acid gel are soluble in sodium hydroxide and that, furthermore, most chemical reactions are reversible, Hurd and Merz4 have studied the dialyzate with particular attention to low concentrations of silica. They found that the concentration of silica in the dialyzate, resulting from dialysis, for a fixed time, of samples drawn successively from a given hydrosol, decreased with age of the hydrosol. It never became zero but approached a constant value as hydrosols neared the setting point. The same was true if the sol set to a gel and the gel wa8s broken up and placed in the dialysis sack. In their work, they used sols of sufficiently short setting time so that the time used for dialysis permitted actual changes in the sol. We report here experiments with sols of such long setting time that changes in the sol samples during dialysis can be neglected. We also report dialysis experiments here involving such long periods that an equilibrium must be established inside and outside the dialysis sack. Experimental Hydrosols were prepared by pourin sodium silicate solutions into acid solutions and mixing tioroughly by pouring back and forth. Silicate solutions were prepared by dilution of E brand silicate (Philadelphia Quartz Company), analyzed by titration with standard HzS04and methyl orange. The Si02 (1) T. Graham, Ann. Physik, 190, 187 (1861); Ann. Chem. Pharm., lSi, 1 (1862). (2) R. Zsigmondy, KoEZoid-2.. 8, 55 (1911). (3) R. Zsigmondy and R. Heyer, 2. unorg. Chem., 68, 169 (1910). (4) C. B. Hurd and P. L. Merz, J . A m . Chem6 Bot., 68, 6 1 (1946)i
content was obtained from the NaOH equivalent and NazO: Si02 ratio, also gravimetrically and colorimetrically. The solutions used were approximately equivalent to 1.0 N NaOH. Silica in the dialysate was determined gravimetrically and, especially for low concentrations, colorimetrically. Essentially, Winkler’s6 method was used, forming the yellow silicomolybdate color, with KzCrOl solutions as standards. We found it reliable. I n the early part of the work (Smith), only gravimetric methods were used, but in the later parts (Witsel and Glamm) both methods were used. Dialysis sacks were made of collodion. Six-inch testtubes were filled with U.S.P. Collodion, Code No. 1611, General Chemical Company, drained and allowed to dry for 15 minutes. The tube was filled with distilled water for 10 minutes, the sacks removed and kept under distilled water. Only for Table IV was the drying time for the sacks varied. The silicate and acid solutions were measured out, enough for all of the samples to be withdrawn. They were thermostated to give correct temperature. The time of mixing was taken, called 0 time for age of the sol. Sols were kept covered in the thermostat. Long setting mixtures, 100 hours or more, were used. Measurements of pH were by the quinhydrone method as previously described .6 Samples of 10 ml. were withdrawn at definite time intervals after mixing, placed in collodion sacks just removed from distilled water. The sacks were tied and suspended in an 8-inch test-tube for 10 minutes in 50 ml. of the dialyzing solution. This had the same concentration of sodium acetate and acetic acid as the spl inside the sack. The only difference lay in the silica or silicic acid.
Results It was necessary to know whether use of a collodion sack
rendered it less permeable, in other words, did the pores become clogged as dialysis proceeded. Tests showed no tendency for clogging, as follows, with one example. Three samples of 10 ml. each, of a mixture with setting time 168 hours, were placed in identical collodion sacks at 21 hours. The sacks were hung in stoppered containers with 50 ml. of dialyzing solution. At 50 hours, 5 ml. of the first sample was withdrawn, placed in a fresh sack, in 50 ml. of fresh dialyzing solution. The original sack still containing 5 ml. and already used 29 hours, was placed in 50 ml. of fresh dialyzing solution. Both were run 43 hours more. The dialyzate for each was analyzed. This is sample 1 in Table I. Sample 2 ran for 308 hours, was divided and the two parts were dialyzed 76 hours more before analysis. Sample 3 ran 838 hours, was divided and ran 68 hours more before the two parts were analyzed. These are typical data. They show that the pores of the collodion sacks do not become clogged during dialys~sof the silicic acid sol. Solutions kept in glass containers dissolve very small amounts of sihca or silicates. Since we have used very (5) L. W. Winkler, 2. unoru. Chem., 27, 511 (1914). (6) C. B. Hurd and R. L. Griffeth, THISJOURNAL,SQ, 1155 (1935).
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SILICIC ACIDGELS : DIALYSIS
Oct., 1953
679
TABLE I TESTS SHOWING THAT COLLODION SACKS Do NOT CLOG Sample
Amount SiOn through original sack
$mount Si02 through new sack
1 2 3
0.0050 .0029 ,0029
0.0052 .0028 ,0025
sensitive methods, this appeared important. Table I1 shows the result of leaving the sodium acetate-acet,ic acid solution in cont,act wit,h glasRware for various lengths of time. TABLE I1 SILICAPRESENT IN SOLUTIONS WHICHHAVENOT BEENIN CONTACT WITH GEL MIXTURES Time of exposure
Si01 in p.p.m.
10 min. 1 hr. 70 hr. 170 hr.
154 154 152 157
This shows an average of 155 p.p.m. SiOz in the solutions which have been in contact with glassware. It may have been in the sodium acetate, acetic acid or water. Since it was essentially constant, as shown by Table 11, it was subtracted from all results for SiOz, giving figures which follow in this paper. Numerous runs were made to study dialysis of the silicic acid hydrosols, in terms of the age of the sol. A typical run gave the graph of Fig. 1. The mixture set in 288 hours. Temperature was 20". Samples of 10 ml. were withdrawn from the mixture at definite time intervals since the sol was produced. Time of dialysis through collodion sacks into 50 ml. of dialysis solution, containing the same concentration of sodium acetate and acetic acid as were present in the gel mixture, was 10 minutes. The abscissa is the age of the sol before the sample was withdrawn. As the graph shows, the amount of Si02 in whatever form, dialyzing through the collodion membrane in 10 minutes, decreased rapidly as the gel mixture aged before the sample was withdrawn for dialysis. This was ex ected. The curve reached a value of about 505 p.p.m. Si& at the time the mixture set, 288 hours from the time it was mixed. Up to this time, 10 samples, each of 10 ml., had been withdrawn from the original mixture for dialysis, each' in a fresh sack. All dialyses following were from sample 10 in the same sack since the gel had set. It was kept, between dialyses, in fresh samples of 50 ml. dialyzing solution. Intervals between the 10-minute dialyzing runs, in fresh 50 ml. of dialyzing solutions, were 20 hours or more. Analyses of the 50-ml. solutions for 10-minute dialyses showed the Si02 dropping as low as 325 p.p.m. a t 500 hours, but rising until a value of 875 p.p.m. was reached at 2800 hours, where the run was stopped. This is more than 100 days. The 50-ml. solutions in which the sack was kept between the 10-minute runs after the 428th hour were analyzed. The long duration of exposure should have made these reach equilibrium with the Si02 inside the sack. The final value appears to be 0.00426 g. of SiOzper ml. or 4260 p.p.m. Obviously this is the summation of all SiOzpresent. An interesting fact, noted in each run, was that the concentration of the Si02 in the dialyzate both for the 10-minute and the 20-hour or more dialysis, decreased to a minimum after the gel had set and then rose to a steady value. We know of no explanation as yet. Continuous dialysis, where the liquid outside the collodion membrane was drawn away slowly, being replaced steadily by fresh solution, was studied. During this time the contents of the collodion tube were not disturbed. If dialysis started soon after the mixture of acid and silicate was prepared, that is, within a few minutes, between 95 and 100% of the Si02 in the tube (0.190 g. SiO2) could be removed, but the total time of dialysis was 2000 hours. During the first hour of dialysis, 0.0717 g. of Si02 passed through the membrane. The mixture inside the tube did not set at all, too much Si02 having been removed. If an hour elapsed after the mixture was prepared, before dialysis started, only
Upper curve 50 30 40 20 Lower curve 2000 2500 1000 1500 0 500 Age of sol in hours. Fig. 1.-Weight of SiOz dialyzed in relation to age of sol. 0
10
0.0549 g. of SiOz dialyzed out in the total time. Table I11 shows the complete group of results. The collodion membranes used so far have all been made in the same way, with a drying time of 15 minutes. A series of experiments were made to show the effect of drying time of the membrane on the amount of Si02 dialyzed in 10 minutes. Every factor was the same except drying time. Results are shown in Table IV.
TABLE I11 TOTAL AMOUNTOF SILICADIALYZED OUT IN CONTINUOUS DIALYSIS IN RELATION TO AGE OF MIXTUREBEFORE START OF DIALYSIS Dialysis continued 2000 hours. Time of set of original gel mixture, 288 hours. Run
Time (hr.) before sample withdrawn
Wt. SiOr dialyzed, g.
Si02 dialyzed, %
2 3 4 5 6 7 8 9
0.00 1.00 21 .o 71 .O 93 118 142 168
0.1821 .0549 .0743 ,0827 ,0863 .0852 .0856 ,0896
96.9 29.2 39.4 43.9 45.9 45.1 45.5 47.6
TABLE IV EFFECTOF DRYING TIMEOF COLLODION MEMBRANE ON AMOUNT OF SILICADIALYZED IN 10 MINUTES The smaller amount of Si02 dialyzing through the sacks,
with longer drying time and smaller pore size, is evident. Sample
Drying time, min.
Amount dialyzed,Si02 g.
1 2 3 4 5 6 7
15 30 45 60 75 90 1180
0.0147 .0120 .0115 .0106 .0102 0074 ,0068 I
Discussion The results described here show that mixtures of solutions of sodium silicate and acetic acid contain silica in a form capable of dialyzing through collodion membranes. At first the solution probably contains simple silicic acid. This, being of small molecular size, dialyzes easily through collodion membranes, as shown by Fig. 1. As the sol ages, less silicic acid is able to diffuse through the membrane in a given time, indicating the presence of smaller numbers of small molecules.
C. B. HURD, M. D. SMITH,F. WITZELAND A. C. GLAMM, JR.
680
That the small molecules do not disappear completely is shown by the fact that some silicic acid is able to dialyze out, even if the original solution had formed a gel. That the smaller molecules are formed by a reaction which is the reverse of the condensation reaction is shown by the fact that more silicic acid can be dialyzed out each time the sack containing the gel mixture is placed in a fresh dialyzing receiving solution containing sodium acetate and acetic acid. Table I11 shows that practically all of the silicic acid (97%) may be removed by continuous dialysis from the original mixture, while a mixture run 168 hours, whose setting time was 288 hours, lost 48% during continuous dialysis of 2000 hours. Probably, if sufficient time were available, essentially all of the silicic acid here could be removed. Membranes with smaller pore sixe permitted less silicic acid t o diffuse in a given time, since the number of molecules of size sufficient to pass through these smaller pores was smaller. With hydrosols containing lower concentrations of Si02 it would appear that the smaller molecules of silicic acid are proportionally more plentiful. To express these results by chemical equations, we may write, since sodium silicate is not made up of single species (Na,O)a(SiO&
+ 2uCHsCOOH + (2b - a ) &0 bSi(0H)r + 2aCH1COONa ----f
The condensation of simple monosilicic acid has been represented’ (H0)sSi-0
1 H + HO I -Si(OH)s
(HO)sSi-O
--
-1-
$i-0-Si(OH)s, AH
(7) C. B. Hurd, Chem. Revs., 88, 403 (1938).
etc.
57
Once a trisilicic acid has been formed, there are two different kinds of OH groups present, the 6 OH groups on the end Si atoms and the 2 OH groups on the Si atom not at the end. As the chains become longer, the number of OH groups on end Si atoms remains 6, but the number of the other kind increases. Condensations involving only OH groups on end Si atoms give linear polymers, while the other types of OH groups gives cross linkages. This explains why the gel is formed suddenly, the increasing number of OH groups not on end Si atoms compensating for the greater activity of OH groups on Si atoms on the end. Table V shows by a few figures how this proportion changes. TABLE V RELATIVEPROPORTION OF OH GROUPSIN LINEARPOLYSILICIC ACIDSWHICHARE ATTACHED TO Si ATOMSAT THE ENDOF THE CHAIN
No. Si atoms in chain % O H groupsonend Siatoms
2 3 4 10 50 100 75 60 2 1 . 4 5 . 5
Our data illustrate the reversibility of the reaction of condensation which produces polysilicic acids from simple silicic acids. DISCUSSION IRVING CmPP.-Have gels in alcohol?
OH
1701.
you made any studies of silicic acid
CHARLES. B. HuRn.-About 95% of our attempts to form alcogels with silicic acid were unsuccessful. It is true, however, that we had some success using varied amounts of alcohol with tetraethyl orthosilicate, but only when water was present.
