T H E PERIODIC COAGULATION OF G O I D SOT, I N THE PRESENCE OF COLLOIDAL SILICA WILLIAhf URE
AND
J. NoIyroN W I L s O s
Department of Chemistry, The University of British Columbia, Vancouver, Canada Received June 9.9, 1997
The Liesegang phenomenon of periodic precipitation in gels lias iiot as yet, received a general explanation, and the four principal theories advanced have been applied with partial success only to specific systems ( 1 , 2, 7, 8). I n the earlier treatment attention was focussed upoii olcctrolyte concentration in relation to conditions of solubility and supcrsaturat,ion. I t was later shown, however, that in many cascs ring systems could bc readily formed by coagulation of substances originally diepcrsed as colloids in gels ( 5 ) , and it appears likely that even when electrolytes ititcrilct thc compound formed may exist primarily as a colloid, protect'ed to some exbent by the material of the gel. The theory proposed by Dhar and'chatterji explains the ring forniation as due to adsorption of colloidal particles by the coagulum which f o r m t h e ring (2). As a consequence a portion of t h e gel succeeding the ring is depleted more or less completely of colloidal material arid the diffuaing rlectrolyte must proceed further before coagulation again ensues. 7'hc chief funct'ion of the gel is then to pcptize or protect the colloidal dispersion, as well of course as to provide structural support to the system. In support of these ideas it is shown that freshly formed precipitates may adsorb their peptized sols, and that there is a parallel bct\.vcen the protective action of gels and their power to cause rhythmic formations (3). Whether or not this explanation applies in all cases of ring forniatioii it is evident that there must exist some sort of concentration limit with regard to the diffusing ions which may be a metastable supersaturation limit or a cboagulation limit, but which determines the agglorncration of the precipitate. There are also cases where the colloid theory would secm to have no application,-in the periodic dcpositiori of sodium chloride by salting out in capillaries, and in the reactions of gases diffusing in narrow tubes. It is important that further information be collected as to the behavior of simple colloi(la1 systems in whir11 ring structures are produce.tl 021 coagulation. The present work deals with a study of colloidal gold prepared iri ttir prcscnco of silicn sol, tlir system setting to R gcl eithrr in ttip cotirse 151 THE J O U R N A L OF PHYSICAL CHEMIBTXY, V O L . 42, N O . 2
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WILLIAM URE AND J . NORTOK WILSON
of time or through the influence of diffusing electrolytes. Stable red gold sols may be obtained in colloidal silica under carefully controlled conditions, the p H of the system being of prime importance. When setting was allowed t o take place before introduction of electrolytes ring formation did not occur, the diffusing salts producing merely a change of color from red to bluc. Apparently the protective action of the gel is sufficient to prevent extensive coagulation of the gold particles. It is possible, however, to adjust the p H of the gold-silica mixture to a point at which no gelation takes place on standing for a long period of time, and with these fluid mixtures the gold coagulated to form sharply defined rings when various electrolytes were allowed to diffuse in. Following the ring formation, setting to a gel took place under the influence of the advancing electrolyte and thus the structure was preserved. A study has been made of the conditions under which this interesting phenomenon takes place, and some conclusions have been drawn as to the bearing of these experiments on the theories of periodic structures. EXPERIMENTAL
Materials A commercial water-glass was diluted with three volumes of water and filtered through fuller’s earth. A t,ypical solution analyzed as follows: SiOz, 12.75 per cent; Na20, 6.11 per cent. The nuclear gold sol was prepared by using 240 i d . of double-distilled water, 3.7 ml. of 0.25 per cent gold chloride solution, 4 ml. of 0.18 N potassium carbonate, and 0.8 ml. of a saturated solution of phosphorus in ether; this gave a deep red and very stable sol. Dialysis was carried out in collodion sacs, while for diffusion membranes du Pont Cellophane No. 300 was cut into small squares and allowed to soak in water for at least two days before use. Preparation of gold-silica sols Experiments with various concentrations of water-glass and acid showed that with equal volumes of 1.16 sp. gr. water glass and 3 N hydrochloric acid a clear sol was obtained which would set to a gel in three or four days. Before introduction of the gold this sol was dialyzed until no test for chloride was obtained. Dialysis was found to decrease the rate of setting both by removal of electrolyte and through actual loss of silica. In one experiment 34 per cent of the silica was thus removed. To reduce this loss sols were allowed to age for some eight haws before dialysis, which was then carried out for about, twenty hours. Colloidal gold was prepared by Zsigmondy’s method of reduction of gold chloride by formaldehyde at 100°C. in t h e presence of potassium carbonfitc.. Reduction was carried out directly in t h e silica sol, and in
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most of the experiments 2 ml. of nuclear sol was added to produce uniform particle size. Following reduction the mixture was dialyzed for some forty-eight hours before the diffusion experiments were started. It is important that the system be adjusted to a suitable pH before the gold is reduced. If the solution is too acid, reduction will be slow arid incomplete, resulting in a blue or purple sol. On the other hand, the pH must not be too high if the silica is to set eventually. The influencc of pH is shown in table 1, where 30-ml. samples of silica sol were adjusted to various p H values by addition of potassium carbonate and gold chloride, and the gold reduced by formaldehyde (without addition of nuclear sol). The results are somewhat variable but indicate that a p H in the neighborhood of 6.0 will produce a stable red gold sol which will set to a gel in a few days, whereas above this value stable silica sols may be maintained. TABLE 1 InfEuence o j p H u p o n set and stability of sols KiCOa
PH
(0.5 N )
4.70 5.60 5.90 5.90 5.95 6.00 6.00 6.05 6.20 6.30 6.40
0.3 0.4 0.45 0.6 0.60
mi.
0.6 0.7 0.55 0.75 0.8 I .25
AuClr 1.7 PER CENT
SET
COMR
Yes days days days days 3 days 5 days 4 days 9 days No set No set
Blue Red Red Red Red Red Red Red Red Red Red
STABILITY (AFTER 2 MONTHS)
ml.
2 2
2 2.5 2.5 2 2.5 2.5 2.5
2 2
3 5 4 4
Faded Blue-purple Good Murky Blue Purplish-red Good
pH values were obtained with the quinhydrone electrode, which is not entirely satisfactory in the presence of gold salts. It is possible to obtain the carbonate requirement by adding potassium carbonate to the silica sol before introduction of gold chloride and thus adjusting the pH, the buffer action of the carbonate being sufficient to prevent any large change. Some results from such procedure are shown in table 2 for a series of samples. All of these sols eventually set. The preparation of a satisfactory sol which would not set on standing, but which could be set by electrolytes, was carried out as follows: A silica so! was made from 80 ml. of water-glass (1:3), 20 ml. of water, and 100 ml. of 3 N hydrochloric acid. After standing for nine hours it was dialyzed for twenty-four hours. To 150 ml. of this sol were then added 10 ml. of gold chloride solution (0.25 per cent) and 3 ml. of potassium carbonate solution
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um
A N D J . NORTON WILBON
(0.5 N ) , bringing the pH to 6.2. Two ml. of nuclear sol was added, the mixture lieated to 90°C., and 5 nil. of 0.2 per cent formaldehyde stirred in rtftcr removal of the flame. Reduction to clear red was complctc in wvcral minutes. Dialysis then took place for forty-eight hours. T h r rebulting sol contained 1.79 per cent of SiOz.
Digusion experiments A seriea of test tubes was *prepared containing gold-silica 501 of pII about 6. Setting was allowed to take place, and the gels were overlaid with solutions of sodium chloride, sodium sulfate, potassium dihydrogen phosphate, magnesium chloride, and aluminum chloride a t concentrations from 1 M to 0.025 M . In every case a continuous change from red to blue uithout ring formation was observed. A sol of pH 6.5 was now placed TABLE 2 Carbonate requirement i n reduction o j gold TITRATION Ot SILICA
75 ML.
REDUCTION OF 5 ML. OF AuCls WITH 1.5 ML. O F XYUCI.EAR M)L IN 75 ML. WITH 5 ML. OF F O R M A L D E H Y D E
Extent of reduction
1 Timo of reduction
~
Color
ml.
0.0 0.4 0.5 0.6 0.8 1.o
5.90 6.50 6.65 6.72 6.85 6.97
Slight Slight
Hours Hours
Light blue Purple
111 a collodion sac immersed in a phosphate buffer solution of pH 6.24. As diffusion progressed the normal change from red to blue was accompanied by the formation of a series of dark rings, and setting of the so1 took place. For more convenient study of this process pieces of Pyrex tubing 4 in. in diameter and 6 to 8 in. long were capped with Cellophane membranes secured by rubber bands, and filled with about 30 ml. of the gold-silica sol. The tubes were then set into Erlenmeyer flasks containing the diffusing electrolyte, and the whole placed in an air thermostat which was kept between 25" and 30°C. to 0.2"C. by means of two lamps and a mercury thermoregulator. A small fan was used to prevent local heating. Some of the structures obtained are illustrated in figure 1 for various electrolytes. The reproducibility is surprisingly poor even under these controlled conditions, as is shown in figure I C for the same concentration of potassium chloride. Some difference in the perineability of the membranes is indicated. In some of the experiments three tubes with the sol ~ ~ callowed re
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to dip into a large beaker containing the electrolyte solution, which was continuously stirred and covered with a layer of paraffin wax to prevent evaporation. Usually under these conditions the rings formed were observed to be regularly spaced, but even here puzzling discrepancies appeared. I t appeared also t h a t the light of the lamps was having an effect on the character of the rings but was not the prime cause, since similar periodic structures were also produced in the dark. That there is some regularity in the spacing of the rings, a t least in a considerable number of cases, is shown by the curves of figure 2 , where the height of the ring as measured with a cathetometer is plotted against the ordinal number of t h e ring, starting with t h e lowest observable.
The effect of various electrolytes With these sols, ring formation has been observed under the influence of sodium sulfate, sodium chloride, potassium chloride, silver nitrate, aluminum nitrate, aluminum chloride, and several buffer solutions a t various concentrations. The behavior of the different electrolytes seems to be quite specific, and no direct effect of valence was noted, although some such effect is to be expected. Thus the same concentrations of potassium chloride and sodium chloride usually gave quite different results, and with the latter salt the results were much more uncertain than with the former, which could usually be relied upon to give regular systems. With barium chloride no definite rings were obtained, merely maxima and minima of density of coagulum. With 0.025 N aluminum chloride a system of very fine, closely spaced rings formed in the lower part of the tube and coarser rings in the upper part. Silver nitrate was found to produce effects like potassium chloride. If the electrolyte is too concentrated the rings are indistinct. With decreasing concentration the ring system increases in sharpness to the point at which the silica will no longer set and the flocculated gold merely settles to the bottom of the tube. Comparison of ring systems was made using potassium chloride a t a series of concentrations between 2 N and 0.015 N . Some of the results are shown in figure 3, and also in figure Id. I t is evident that decreasing the concentration of diffusing electrolyte decreases the number of rings, and increases the rate at which the distance between adjacent rings spreads out. I n some experiments the concentration of gold in the sol was varied by using amounts of gold chloride from 2.5 t o 15 ml. per 150 ml. of sol. With increasing concentrations the rings increased in density, while a t the highest concentration coagulation of the gold was incomplete, the spaces between the rings showing red gold colloid. The setting of the silica was followed in a number of cases by means of a thin capillary tube of about 0.5 mm. outside diameter, which was carefully
FIG.1, 8-d 156
PERIODIC COACUI..4TION OF GOLD SOL
157
Fm. 1 e, f FIG.1. Ring formation in gold-silica 601s in the presenre of various electrolytes. (a) Withphosphate buRer;pH 7.7. (b) With0.2M aluminum nitrate;nornernbrane. ( e ) Regular and irregular rings produred with potassium chloride. (d) EReet of varying electrolyte concentration (0.015, 0.020. and 0.025 N potamium ehloridc). The gel prepared with 0.015N potassium chloride is so weak that it will not stick in the tube. (e) DiRusion at various anglei. (f) Potassium chloride, 0.015 N; &odium chloride, 0.025 N .
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FIG.2. Height of the ring plotted against the ordinal number of the ring. C u r i e I, 0.025 2%- aluminum chloride: (a) fine rings in lower part of tube; (b) coarser rings in upper part. Curve 11, 0.026 N potassium chloride. Curve 111, 0.025 Ai sodium chloride.
n
FIG.3. Comparison of ring systems using potassium chloride in different roncentrations. Curve I, 0.025 N ; curve II,0.020 N ; curve 111, 0.015 A'.
thrust down the center of the tube as the rings were forming. The point a t which the flexible tube began to bend as it was moved from side to side was taken as the approximate limit of the setting process. This was found
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159
to be about 2 mm. behind the ring which was forming, but it is probable that thc silica coagnlates sufficiently t,o give support a t the ring itself. The gel first formed is somewhat fluid, as shown by experiments in which thc tubes were tiltcd from the vertical (figure le).
The function of the gel It. is npparent from t h e above t h s t gel formation is ne espcrimcnts for structural support. In a case where the electrolyte was iillowed to diffuse from the top of the tube the gold was completely coagulated aiid fell to the bottom. With a horizontal tube a layer of coaguluni was formed 011 the lower side of the tube. There exists the possibility that the periodic phenomenon is confined to thcl silica in its sett,ing, and that the coagulated gold merely serves to i i i d i ~ a f tlie c bowidary between sol and gel. Such a gelation process has not as far as we know been observed, and would distinguish the systems liere studied from those involving gels. 2, series of tubes containing silica sol without gold were set up wit11 0.025 N potassium chloride diffusing upwards. After some hours fine I)l:tck sand was dropped in from t'he top at the rate of a few grains every Iinlf-hour for about two days. No discontinuities were observed in the tlistribiition of the particles. Similar results were obtaincd using emcry powdcr and sulfur. When, however, the setting sol was overlaid wit11 R siispension of silver chloride, a series of rings were formed as the silvci. chloride particles settled down. The success of the last experiment may be due t o the smaller size of the particles used, but does not unequivocally indicate pcriodic gelation, since coagulation of t'he silver chloride particles under t,he influence of the upwardly diffusing electrolyte probably took place. Further tests of this idea were based on the following considerations. The diffusion rate of electrolytes is slightly less in gels than in sols. Cosgulatioii of the silica may be accompanied by adsorption of electrolyte. It is therefore possible that measurable discontinuities in the diffusion strrniii may occur. The diffusion of an electrolyte, silver nitrate, was therefore l'ollowrd pot,erit,iomctrically. Three diffusion tubes were set up containing in thc first 0.001 N socl_iim nitrate, in the second silica sol without gold, mtl in thc third a gold-silica sol. The tubes were dipped into a largt? h a k c r of 0.002 N silver nitrate, which was constantly stirred and protwtrd from evaporation. Silver electrodes were let iiito the sides of the t h e tiiheP ahout 4 cni. abovr the membrane, and a single silver elcctrodc n n n placed in thc large Iwaker. Measurements of potentials I)etwceii the rrfcrcnre elertrode and t)he rlrctrodes in the tubes were made simultanco~islyovcr a period of one hundred fifty hours during which tlie potentials tlrtrpped from about 175 to 20 niv. as the silver-ion concentration t,enilrtl t o tqiializ(1 l)ctn.ecn the clcrtrodes. Any changes in the rate of cliffu;iiull
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in the tubes containing silica should be observable by comparison with the tube containing sodium nitrate solution. The curves obtained by plotting millivolts against time are not shown, since the values have no absolute significance. They were displaced from one another in the three cases, but were found to be almost parallel throughout. Certain irregularities in slope were found to be almost duplicated in the three cases, and must, bc attributed to changes in the membranes and in other external conditions. I t is evident from this that no appreciable discontinuities of electrolyte diffusion are produced by the setting gel. During the course of the experiments a very regular ring system was formed in the tube containing the gold. DISCUSSION
In common with other examples of the Liesegang phenomenon there appears to be in the systems studied here a general regularity with regard t o the spacing of the rings. A number of expressions, mostly empirical, have been developed by various authors connecting the height, h, of the ring with its ordinal number, n. More fundamental treatment of periodic processes has been given by Morse and Pierce (6), and by Fricke (4). These authors, however, have confined their attention chiefly to the silvrr chromate rings in gelatin, and hare treated the process from the point of v i e r of opposing diffusion streams of electrolytes, with precipitation at supersaturation. In the present case we are dealing with the coagulation of a colloid by electrolytes, and it becomes necessary to consider a singlr diffusion stream and to replace the supersaturation limit of concentrations b y some limit necessary to coagulation. Using the same method of approach as that of Morse and Pierce, however, it is possible in our case to arrive a t an equation for the distribution of the rings, which is based on the following assumption. There is necessary for coagulation of the gold particles a definite concentration of electrolyte, but, once started, coagulation can continue throughout the sol until some definite lower concentration of electrolyte is reached. This means, in effect, that the coagulum formed a t the upper limit of concentration may remove particles by adsorption from a region between the higher and lower limits I t is also to be expectecE that the settling of the coagulated gold is rapid with respect to the advance of gelation, and that the latter corresponds to the advance of some definite concentration of electrolyte. For thc diffusion of the advancing electrolyte in the tube we haveI Rccortlina to Firk’s equation,
PERIODIC COAGULATION OF GOLD SOL
161
where a2 is the diffusion coefficient, and u the concentration of electrolyte a t the distance z from the starting point. It is assumed that the diffusion gradient is not affected by the formation of the coagulum, as the potentiometric measurements seem to show. The solution of the equation is of the form
where U ois the initial concentration of electrolyte and t is the time. Now if u is to be constant for the formation of each ring the lower limit of integration must be constant, and hence x / d must be constant. Let c1 represent the electrolyte concentration a t which the silica will set to a gel, c2 the concentration a t which the coagulation of the gold will take place, and c3 the lower limit of concentration necessary to removal of the gold to the ring. From the experiments it is probable that c1 and c2 are not far different. Let hl, h2, and h3 be the corresponding values of 2 a t which these concentrations are reached a t the time t. The gold in the region h2 to h( has coagulated and accumulated a t hl to form the ring. Now for coagulation to set in again the concentration a t hs must rise to the value c2. This will happen a t a M e r time t'. Meanwhile the gelation has advanced to a point h:. The gold from h3 to some higher point h4 will coagulate and fall to hi, forming the next ring. From the previous considerations we may set h2 =
Iczl/t
and
hB = k z d / t ' -
where kz is a constant, and a t the time t hs = k 3 4 T
where k3 is some other constant. Hence
Also we have, at the rings themselves Then h:/hl =
l/t'/t
= ks/kz = a constant
In general, for successive rings,
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WILLIAM URE AND J. XORTON WILSOIi
where n is the ordinal number of the ring. This equation has been proposed empirically by Schleussner (9). The final equation has been tested for these results by plotting the logarithm of h against n, since h, = Kh,-l = Kn-'hl log h,, = ( n - 1) log K + log hl
A straight line should be obtained. Some of the results obtained are shown in figure 4. While in a number of experiments little regularity was shown in the ring spacing, yet the more regular cases seemed to agree reasonably well with the equation given.
n FIG.4. Plot of log h against n. Curve I, 0.25 N potassium chloride; curve 11, 0.025 N sodium chloride; curve 111, 0.01 N potassium chloride; curve IV, 0.025 A; potassium chloride.
I n conclusion it should be noted that the mechanism outlined above is very probably not the only one which will lead to an equation for ring spacing of this type, and that similar results could be expected if the gel were setting in a discontinuous manner. The experiments described here may be interpreted to lend further support to the coagulation-adsorption theory as a reasonable explanation of a number of periodic processes. SU.MiMARY
1. Experiments are described in which periodic coagulation is produced in colloidal gold in the presence of silica sol by diffusion of various electrolytes.
PERIODIC COAGULATION OF GOLD SOL
163
2. The effect of different electrolytes seems to be highly specific, but in general increase of concentration produces more closely spaced rings. 3. There appears to be no discontinuity in the diffusion of the electrolyte during ring formation and setting of the gel. 4. The results agree moderately well with the assumption that the coagulation of the colloid is accompanied by adsorption of colloidal particles in the adjacent region. REFERENCES (1) BRADFORD: Biochem. J. 14,29,474 (1920); Kolloid-Z. 30,364 (1922).
(2) (3) (4) (5)
(6) (7) (8) (9)
DHARAND CHATTERII: Kolloid-Z. 31,15 (1922); 49,97 (1926). Kolloid-Z. 37,2 (1925). DHARAND CHATTERJI: FRICXE:Z. physik. Chem. 107,41 (1923). HEDQESAND HENLEY:J. Chem. Soo. 1938,2714. HEDQES:Kolloid-Z. 62,219 (1930). MORSEAND PIERCE:2. physik. Chem. 46,589 (1903). OSTWALD, WILHELM: Lehrbuch der allgemeinen Chemie, p. 778. Leipsig (1898). OSTWALD, Wo.: Kolloid-Z.S6,380 (1925). SCHLECSSNER: Kolloid-2. 34,338 (1924).
