ALLOTROPIC SILVER AND ITS COLORS BY F.
E. GALLAGHER
A discussion of the cause of the colors of colloidal metals is given in a paper by Stoeckl and Vanino.’ Several possible explanations, which depended on the behavior of light rather than on a distinctive property of an allotropic form, were offered. The work of Christiansen’ on colors depending on the refractive indices of a transparent solid and a containing liquid, was revised and applied to the case of colloidal suspensions. In another division the possible relations of light absorption to the colors of colloidal suspensions were developed. Under this explanation the colors are supposed to be dependent on the thicknesses of the colloidal particles or filmsthe various thicknesses letting through light of greater or lesser wave-length. Some experiments of this kind seemed to be a step in the right direction, for it hardly seemed reasonable that the different colors were due to the existence of several allotropic forms of a metal as some previous experimenters were led to believe. Stoeckl and Vanino performed no direct experimental work to prove or disprove any of their various suggestions, so this work was undertaken with the hope of finding that the colors of colloidal silver were due to some physical phenomenon, and to discuss which one best explained the facts. Previous Work The first work of importance on colloidal silver was done by Carey Leal3 who studied the methods of preparation] and obtained allotropic silver of many colors. This work was later extended by Blake14who made a simpler classification _
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Stoeckl and Vanino : Zeit. phys. Chem., 30, 194 (1899). Christiansen : W i d . A n n . , 23, 298 (1884). M.Carey Lea: Phil. Mag., 31, 320, 238, 497 ; 32, 337. Blake: A m . Jour. Sci., 16, 282 (1903). Cf. Gutbier and Hofmeier: Zeit. anorg. C h e m . , 45, 77 (1905). 1
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F. E. Gallagher
of the facts that silver existed in three or possibly four allotropic forms: white, blue and red, with yellow as the possible fourth. The other colors, to which Lea had assigned separate forms, were supposed to be mixtures of these three or four classes. Blake also tried to find definite conditions for the formation of the different forms, but found that a t best they were somewhat uncertain. The conclusions of Lea and Blake will be discussed further on in this paper. The Christiansen Effect
Christiansenl has shown that color can be a refractive index phenomenon when a transparent solid is in a liquid medium. Ordinary glass has a refractive index of about 1.54, carbon tetrachloride of 1.46, benzene 1.51, and carbon bisulphide 1.64. Since the three liquids are miscible in all proportions, it is thus possible to have a liquid with a refractive index anywhere between 1.46 and 1.64. We can thus get a containing medium of the same refractive index as the glass for any one wave-length, since the above figures vary for the different spectrum lines. If we put a piece of glass into benzene and add carbon bisulphide, the glass practically disappears as the refractive index of the solution approaches that of the glass. At the same time the solution becomes colored. If the glass powder be finely ground, the color effects are brighter. There is of course no refraction of the particular wavelength for which the glass and the solution have the same index of refraction. If this were the only wave-length traasmitted , we should have R convenient method of obtaining monochromatic light of any desired wave-length. Although this method was reconimended by Christiansen, it does not seem a practical one for obtaining monochromatic light. If the layer of glass be thick, very little light is transmitted. If the layer be only moderately thick, so many of the other wave-lengths are transmitted by reflection from the glass surfaces that the effective color is due to them and not to the tinrefracted ray. Thus the addition of carbon bisulphide Wied. Ann., 23, 298 (1894).
AZZotvojic Sitvei. and I t s Colovs
703
with a high refractive index always displaced the apparent color toward the red end of the spectrum in spite of the fact that the refractive index of the glass is higher for blue light than for red light. The phenomenon is really one of relative dispersions. Although the transmitted light seemed to the eye to be rather pure, a spectroscopic examination showed that it corresponded to quite a broad band, containing relatively large amounts of the neighboring colors. As carbon bisulphide was added the apparent color changed toward the red, which change was accompanied by a shifting of the spectrum band in that direction. More and more of these other colors can be cut out by increasing the thickness of the layer, but this cuts down the amount of transmitted light so much that by the time the layer is thick enough to restrict the color to blue for instance, very little blue is trmsmitted. When repeating Christiansen’s experiments it was found unsatisfactory to examine the color against the sky as he suggests, because the color is dependent upon the position in which one holds the vessel in reference to the light and eye, and because the source of light is of variable intensity and not strong enough. By protecting the eyes so that only the transmitted light falls upon them, sunlight can be used; but very satisfactory and uniform results were obtained by using a lantern projection dish ( I cm X 5 cm X 7 cm) in an arc lantern which threw the color on a white screen. The experiments were first tried with pulverized glass. Instead of grinding in water and drying preparatory to using with benzene, etc., the glass was ground in benzene from which the fine portions were floated out and used. The finest ground glass gave the purest colors. A projection dish was about half filled with the glass and 2 0 cc of benzene added. The transmitted light was blue. When carbon tetrachloride was added 0.5 cc a t a time the blue became deeper, then violet and a t last almost black, very little light being transmitted when the refractive indices of the glass and liquid were greatly different. Now upon the addition of
704
F. E. Gallagher
Soltition number
6 --I
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Allotropic Silver nizd Its Colovs
*
705
I1 was poured into I. This gave solutions with a AgNO, concentration varying from 0.5 gram to 0.03 gram per liter. The solutions were prepared in test-tubes, placed near a window having a northern exposure, and allowed to develop. It was found that such a set of solutions would give a wide variety of colors. Solution No. I , the most concentrated, would reduce quickly and would color a yellowish brown. Within a few minutes No. 2 came down blue, but in the course of a half hour had changed through a green to a yellow or yellow-brown. The more dilute solutions first came down blue. At the end of several hours the colors were as follows: I and 2 , yellow-brown; 3 , green; 4, 5, 6, blues of falling intensity. This is not an exact program of what will happen every time, for it was found that uncontrollable and even unknown conditions influence the formation of the colloidal solution. On repetition it often happened that No. 2 would become yellow a t once, without first becoming blue. The general law, however, holds that the color tends to approach the red end of the spectrum with increasing concentration, and that the change with time is always in that direction. I n no case of simple reduction in water solution did a color change toward the violet end of the spectrum. After considerable time even the dilute solutions of set A became yellow, and from the yellow or yellow-brown solutions the silver separated as a black powder. Similar sets of solutions were prepared, which separately a large excess of reducing agent, and contained ",OH, KNO, for an electrolyte. There were no striking differences shown. In the ammonia solution the colors formed. more slowly; the electrolyte had but little effect, and the reducing agent the more quickly threw out the silver as a black powder. The conclusion drawn from the behavior shown by Set A of solutions was that the color of the colloidal silver solutions was due in some way to the coagulation or concentration of the colloid, the finer particles or the presence of small amounts of those particles giving colors toward the violet end. As the coagulation or concentration increases, the
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E: E. Galtagher
color approaches the red end. A further explanation of this will be given later. The production of so many colors in this way seemed to indicate that the color could hardly be due to different allotropic forms. The attempt to change these colors by various solvents was next tried. There is no convenient liquid having a refractive index lower than water, We used necessarily liquids with for which ,u=1.331 at 20'. higher refractive indices, such as acetone, p = I .36; ethyl alcohol, p = 1.36; and benzene, p = 1.j. Benzene was used in alcoholic solution so as to render it miscible with water. The addition of these liquids had a marked effect on the color. Acetone was added to a bluish green solution, which in a few moments became deeper blue. When acetone was added to a yellow-brown solution, the latter changed to a green. In these cases 2 cc of acetone were added to 2 cc of solution. Alcohol behaved in much the same way. When benzene was added to the alcoholic solution it seemed to carry the color even further toward the violet. Since we were supposing that this change of color was due to the change of refractive index, we ought to reverse the color change by again decreasing the refractive index. To solutions which had been turned violet by the addition of alcoholic benzene, and which had a refractive index greater than 1.36, large excesses of water and of alcohol were added. The excess of water caused the benzene to separate as a second liquid layer; the alcohol was simply a diluting agent. In neither case was there a marked color change, nothing that could be called a reversal. Other trials gave similar results. This point was against the refractive index idea. The effect of varying amounts of the alcohol, acetone, etc., on the colors of the solutions was next tried. To 2 cc of a yellow-brown solution, alcohol was added in amounts varying from I drop to 2 cc. At the end of one-half hour the colors varied from a yellow to a green, in order of amounts of alcohol used. The color effect with different solvents is not due then to traces of the substances, but depends upon the amount.
AZZotro$zc SiZver and Its CoZors
707
Solutions treated with alcohol, acetone, benzene, etc., do not change (toward the red end) on standing in the light, as was shown to be the case in Set A, which were simply in water solution. If a solution went blue on adding alcohol, it stayed blue until the black silver powder precipitated. We come to the conclusion therefore that the different solvents act in some way as decoagulators, displacing the color toward the violet end and preventing the change which in their absence causes the metallic silver to precipitate from yellow or red solutions.’ Light is essential to the various color changes. A series like Set A if made up in the dark and left there will remain colorless, except for a slight yellow coloration in the most concentrated solutions. If exposed to diffused daylight the color formation takes place slowly. If exposed to direct sunlight the change takes place very quickly. Light is necessary in producing the color changes caused by alcohol, acetone, etc. Four cc of alcohol were added to 2 cc of a yellow solution in the dark. No change occurred as long as it was in the dark, but when placed in the focus of an arc lamp, the solution quickly went greenish blue. A whole series of solutions containing alcohol, acetone and benzene were made up and left in the dark for a week; aside from some yellow colorations no change occurred. Upon being exposed to sunlight various colors quickly appeared. Many experiments on the effect of light were made, all of which showed that but very slight color changes occur without it. These facts would seem to dispose definitely of the possibility of the color being due to the Christiansen effect, for if the color were a phenomenon that depended upon a change of refractive index, then upon the addition of such a liquid as alcoholic benzene to the containing medium, the refractive index of that medium at least would have to change, even in the dark, thus giving some difference in color. Since no I t is possible that something of this sort was the cause of Faraday’s apparently contradictory results with colloidal gold. Phil. Trans., 147, 165 ( I857 ).
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F. E. Gnllaghev
change occurs in the dark, the refractive index cannot be responsible for the colors of colloidal silver. This brings us to what seems to be the most plausible idea, that the colors are due to a light absorption effect depending upon the thickness of layer, degree of coagulation, or concentration of colloidal silver particles in the solution, any of which would have about the same effect, by cutting down the total light transmitted. Gold and silver have been shown to be translucent in thin films, and to possess both strong reflective and strong selective absorption. By selective %absorption is meant that some particular wave-lengths are more strongly absorbed than others, and so the light transmitted is stronger for some wave-lengths. Quite a number of facts indicate that this is the case. Other i1,vestigators have reported that the color of silver films depended on the thickness. Foucault' says that in thin layers metallic silver is blue. Bothe2 reports the same. Christomanos3 found very thin layers of silver to be blue, thicker layers blue-green and still thicker ones yellow or yellowbrown. The purest colors obtained by us were a t the blue end. The yellows were brown-yellows, and the reds, brownreds. Brown is a mixture of red, yellow and blue, so its presence is well explained in this way. An examination of the light transmitted through different colored colloidal mirrors, always showed, when examined by a spectroscope, the presence of considerable quantities of blue. The blue mirrors also contained colors corresponding t o longer wavelengths, but not in quantities to compare with the blue light transmitted, while in the yellow and red mirrors the blue seemed to be quite an important constituent. This could not occur if the mirrors were optically homogeneous. A spectrophotometric study of the mirrors is much to be desired. If the colors of colloidal silver are to be explained by this theory4 it is necessary to show that increasing the depth even Foucault : Comptes rendus, 63, 413 (1866). Bothe : Jour. prakt. Chem., 92, 191 (1864). Christomanos : Zeit. Chemie, 1869, 310. Cf. Siedentopf and Zsigmondy : Drude's Ann.,
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(1903).
AZZotvojz’c SiZvev and I t s CoZovs
709
of a colloidal silver solution would cause the color to shift toward the red. Solutions of different colors corresponding to Set A, already described, were made up in 2 0 0 cc portions. In order to study the effect of varying the depth of layer a tube 2 cm in diameter and 50 cm long was ground off at one end, to which a piece of plate glass was cemented. The walls of the tube were then covered with black paper, to exclude light from the sides. The sunlight was reflected by means of a mirror directly up through the tube, thus providing a strong illumination and making it possible to obtain light through quite a depth of liquid. Without a strong light the liquid soon absorbs so much light that but very little variation of the thickness of the liquid is possible. By means of this arrangement it was shown that varying the thickness of a liquid layer of colloidal silver did shift the color toward the red end in a most remarkable way. A small amount of a green solution was placed in the tube. As more solution was poured in, the strength of light was greatly decreased and the solution took on a distinctly yellower color. Upon being removed from the tube the solution was still the original green color. The change to a yellow was therefore not due to the sunlight having coagulated the green solution. It was to prevent this that alcoholic solutions were used, for as has already been shown they are not changed toward the red end of the spectrum by sunlight. A brown-yellow solution which was studied in the same manner, appeared as a beautiful red. The most successful of these experiments was made on a greenish blue solution, which went brownish yellow and finally a brown-red on increasing the depth of the absorbing layer. After this surprising agreement with the theory it remained to show that the colloidal mirrors behaved in the same way. To do this it was necessary to prepare blue colloidal mirrors, for if we had the thinnest mirrors which would be the blues, we hoped to make the others by using several thicknesses of the blue ones. Good blue mirrors were very easily made by Carey Lea’s1 recipe for making “gold- and copperCarey Lea : Phil. Mag. [SI, 31, 320 (1891).
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F. E. Gallaghey
colored colloidal silver,” in which he uses 40 grams of NaOH and 40 grams of dextrine dissolved together in two liters of distilled water; 28 grams of AgNO, in solution are very slowly added and stirred in. In this way a reddish-brown, almost black solution of colloidal silver was obtained. Into this solution chemically clean glass plates were dipped, after which they were put in an oven and dried at 50°--600 C. Since the silver was in solution as colloidal silver, it was deposited as a colloidal film, blue by transmitted light, upon removing the solvent. The fact that blue colloidal silver is deposited from a reddish brown solution in this way would in itself seem to argue against red and blue colloidal silver as different allotropic forms. An attempt was made to show that severalblue mirrorsplaced together would give a green, yellow or red transmitted color, This did not succeed because too little light was transmitted, owing to the increased absorption in the several pieces of glass, and to the fact that a film on each piece of glass, which was coated on both sides, was presenting a fairly good reflecting surface. The mirrors were next deposited successively one on another from the same solution. As many thicknesses as desired could therefore be obtained on one piece of glass. Each coating was dried before the next was added, and the successive deposits were not carried quite to the height of the preceding one, thus giving on the finished plate a series of bands consisting of one, two, three, etc., thicknesses of the colloidal mirror. On one plate the effect of different thicknesses could be seen, and it did show that here as in the case of liquids the thickness of the layer determined the color. A specimen of a plate obtained is given in Fig. I . The different successive deposits are given by the numbers I , 2 , 3, etc. The first two were blue, the third showed a fairly good green, the lower ones showed red in increasing amounts, not even the fourth exhibiting a satisfactory yellow, although yellows were obtained on some other plates. The strips marked a,”“ b,” “c,” etc., were due to the fact that after dipping the dry plate into the solution the surface tension
AZZotropic SzZver aizd I t s CoZovs I
Blue
a
Y el 1ow
I
Blue Yellow i 3
4
Green
Yellow and Red
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Yellow-Brown and Red
le Red
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Fig.
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between the dry undipped portion and the liquid held a band of solution thicker than that just below it which drained better. So here on the edge of each new film we had a deposit from a deeper layer of liquid, which necessarily made a thicker mirror at that place, and in turn gave an absorption effect equivalent to several successively deposited thin films. Thus we had at “ a ” a fairly good yellow between two blues. This in itself was fortunate for it was possible that the character of the deposit might vary, depending on whether it were deposited on glass or on silver. In other words the silver might cause the formation of a different allotropic form (further coagulation or some other effect that would change the color of the second film). The fact that the second film remained blue even when deposited on silver and that an intermediate film of yellow was obtained rather disproved this possibility. As a further proof that the different colors obtained were really ‘‘ blue ” silver in different thicknesses and not due to further coagulation, allotropic forms or some other reason,
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F. E. Gallaghey
it remained to show that decreasing the thickness of the yellow, green, and red mirrors caused the transmitted color to again become blue. It was possible to do this because colloidal silver is soluble in water. The plate was allowed to stand edgewise in a large quantity of water, so that there was dissolved only the right-hand half of the deposits the left-hand half remaining in its original form for comparison. When the plate was removed from the water and dried, a beautiful blue was found to have replaced the former yellow, green, and red. When the plate was wet the yellow solution of dissolving silver obscured the real color of the plate. (It will be remembered that the mirrors were first deposited from a deep brown solution, which on dilution became yellow. The dissolving silver gave a similar solution.) If the yellow or red forms had been caused by coagulation of the blue form, we should expect them to be less soluble than the blue, which was in a finer state of division and consequently we should have expected the blue to have dissolved first, leaving the other colors, provided always that the mirrors are pervious to water. For the guidance of any one who should at any time repeat these experiments it would be well to explain that the color effects obtained in the experiments on varying the thickness of the colloidal layer, both in the liquids and mirrors, were not always distinct or brilliant. Some very conclusive results were obtained when working with liquid layers, but because of the great light absorption when working with increasing depth of liquid it is not possible to pass froma deep blue to a brilliant red by the use of one solution. Likewise. the colors obtained on the mirrors were sometimes dull, yet on the whole, conclusive, and this also is to be expected for throughout the work it has been found that the variables influencing the production of the different colors are many and hard to control, and would assert themselves as much in methods dealing with the successive production of the colors, as under simpler conditions.
Summary In this investigation it has been shown t h a t : ( I ) Colloidal silver solutions of all colors can be easily prepared, from one set of reagents, by varying simple conditions. Since the color changes can be brought about also by varying the thickness of the solution, the colors cannot be due to allotropic modifications. (2) Certain organic solvents have been found to act as '' decoagulators, preventing the light from producing the color changes in the colloidal suspension, which occur in solutions not so treated. Their action depends on the amount used. These solvents do not prevent the precipitation of metallic silver but do affect the color of the solution from which the silver separates. (3) The color changes in colloidal silver solutions are not satisfactorily explained by the Christiansen refractive index phenomenon. A color change produced by increasing the refractive index of the liquid was not reversed when the refractive index was again diminished. No change of color occurred when the refractive index was changed in the dark, which showed that the col'or changes depended on a direct action of light upon the colloidal suspension and not on a change of refractive index. (4) Light is essential to the changes of color, and the speed of these changes depends on the intensity of light. The light is supposed to cause the coagulation of the colloidal suspension, thus increasing the size of the particles, or to affect the concentration of the particles in solution by increasing the reduction of the silver nitrate to colloidal silver. (5) The color of colloidal silver is due to a selective light absorption. With increasing thickness of the absorbing layer or with increasing size of the particles the amount of blue light transmitted decreases relatively much more rapidly than does the amount of red light transmitted. This points to internal selective reflection. "
714
Allotropic Silver and I t s Colors
Conelusion There is only one form of colloidal silver, not several allotropic colloidal forms, and the various colors and properties such as electrical conductivity, density, action toward reagents, etc., concerning which much has been written, are to be explained by the state of aggregation or number of the particles of this colloidal form. This investigation was suggested by Professor Bancroft and has been carried on under his supervision. Cornell Unzversaty.
