THIRTY YEARS OF COLLOID CHEMISTRY’ WILDER D . BANCROFT 7 East Avenue, Ithuea, New York
Received July 14, le41
I thank Professor Weiser and the other colloid chemists for the great compliment that they have paid me in making me the guest of honor a t this meeting. I expect to cover thirty years in about thirty minutes; but I shall do it in my own way. I shall not try to give an exhaustive or an impartial discussion. I am going to talk about what happens to interest me especially, regardless of its real importance. If anybody misses anything that he think should have been included, he will know that it was my fault and that it was probably done deliberately. There can be no question about where I stand or why I stand there. I have been fortunate enough to have foreseen developments occasionally before they were clear to the majority of the chemists; but I have not had the ability to make people see things aa I saw them. Very early in the game I recognized the importance of Rooseboom’s work on the phase rule, and I wrote a book to set forth my views. It was not well written, and Findlay has shown how such a book should be written. After I had read Freundlich’s book on colloid chemistry I began to see some of the possibilities in this field. About 1912 I started lecturing upon the subject and I have never lost interest in it. While it seems to me that I wrote a pretty good book on colloid chemistry, many people did not agree with me. There are now many books on the subject which teachers prefer to mine. While it is not colloid chemistry, it seems to me desirable to recall that G. N. Lewis pointed out, many years ago, that there waa practically no volume change or heat effect on mixing pairs of what he called ideal liquids, such as toluene and benzene. I did not see at the time that this meant no change in the dissolved substances from liquids to gases. Lumsden showed that for non-electrolytes the dissolved substance was always present as a liquid. In 1935 I stated that there was no reason for Lumsden’s having confined his conclusions to nonelectrolytes, and that a liquid solution consists always of a mixture of liquids. Since dissolved sugar is a liquid, possibly hydrated, dissolved salt must also be a liquid, possibly hydrated. This way of looking a t things makes it clear that alcohol precipitates sugar or sodium chloride from aqueous solutions because we are then dealing with two practically non-miscible liquids. The reason that gases become less soluble with rising temperature is that the heat of liquefaction 1 Presented at the Eighteenth Colloid Symposium, which waa held at Cornel1 University, Ithacs, New York, June 19-21,1941. 1
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usually determines the sign of the heat effect. With solids, if the heat of fusion determines the sign of the heat effect, a solid will be more soluble with rising temperature. Other factors may come in. Anhydrous sodium sulfate shows decreasing solubility with rising temperature around 33°C. We do not yet know whether there is a formation of some hydrated salt in solution or whether there is a displacement of water equilibrium. Sodium nitrate increases the solubility of potassium nitrate, and there is no evidence of the formation of a double or complex salt. People have postulated double or complex salts because they could not think of any other way of accounting for the observed facts. When one precipitates a binary salt by adding another with an ion in common, it is not justifiable theoretrically to ignore the third ion; but this is always done. Much of the thermodynamics of solubilities is sloppy, because the author thinks that he is dealing with a gas, and not with a liquid which may have special solvent action. Over forty years ago, Lash Miller and Kenrick showed that adding a suitable amount of sodium chloride to a suitable solution of aqueous alcohol caused a lowering of the boiling point, because the increase in the partial vapor pressure of the alcohol is greater than the decrease in the partial pressure of the water vapor. Nobody understood that a t the time and few do now. I hope, but do not expect, that today there will begin a trend towards more accurate thermodynamics. Emulsions are colloidal systems, and a t Cornel1 University we showed the conditions determining the formation of an oil-in-water emulsion or a water-in-oil one. Briggs showed how to recognize the type under the microscope. The conclusions reached have now been pretty generally accepted, but for several years people tried to believe that the valence of the metal in an emulsifying soap was the decisive factor. Louis Fuertes called my attention to the fact that there is no blue pigment in the feather of any bird. With the collaboration of Mr. Mason for the microchemistry it was shown that this was a problem in colloid chemistry. What is needed is a suitable turbid medium with a dark background. In the feathers the turbidity is caused by air bubbles; but a similar effect can be produced in glass by the right amount of devitrification. The white tip on the feather of a blue jay is due to the absence of black pigment. If one backs the feather with ink the tip becomes blue. The blue of the veins in one’s arms is a structural blue. There is no blue blood there. There are blue pigments in flowers and in animals, so there is no apparent reason why blue pigments should not have been produced in blue feathers or in blue eyes. There are a few feathers which are colored by a green pigment, but most green feathers owe their color to the combined effect of a structural blue and a pigment yellow. There are no bright-colored pigments in the tail and breast feathers of peacocks or in the throat feathers of humming birds. In fact, all metallic colors are a colloidal phenomenon, being due to thin films. What we call Newton’s are interference colors due to colloidal films. Newton recognized the bright colors in the feathers of peacocks as thin-film colors.
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The striking thing about oil films on pavements and about the iridescent pigments in feathers is that the colors are metallic, and here we get into the field of psychology. The sensation of metallic luster may be obtained with metals; with many sulfides, phosphides, silicides, selenides, tellurides, and arsenides of the heavy metals; with selective reflection, as in the case of solid magenta; with oil films and the iridescent feathers of birds; with exfoliated mica sheets; with total reflection; with moonlight on the water or the setting sun reflected from window panes; and with stereoscopic luster. These apparently different phenomena were included under one formula by Allen and myself in 1925. We postulated that one gets the sensation of metallic luster when there is sufficient reflection from practically one surface with suitable variations of intensity either in space or in time. Metals are very opaque, and consequently the light that is reflected to the eye comes practically from the surface. A crystalline or hammered surface is more metallic than a more nearly smooth one. The sulfides of the heavy metals are also very opaque and consequently look metallic. All solid substances which show selective reflection have metallic luster, magenta and methyl violet being typical cases. Since these substances show intensely strong absorption for the rays which they reflect selectively, these rays come practically from a single surface and therefore give the sensation of metallic luster. ks with metals, the metallic effect is increased by slight irregularities in the surface. In the case of films thin ehough to give interference colors, the thickness of the films is negligible so far as the eye is concerned and the light comes practically, though not theoretically, from a single surface. The contrast in color as well as in surface makes these films astonishingly metallic. A roll of cellophane may look exactly like silver, or, when properly tinted, exactly like sheet copper. As far as the chemist is concerned, he can be certain that crystals having metallic luster fall into one of three categories. They are very opaque like stibnite; they show selective reflection like magenta; or they crystallize in fine plates or needles, giving the effect of multiple films. This incursion of the colloid chemist into the field of the psychologist was not welcomed by the psychologists. In 1914 I discussed the colloid chemistry of dyeing with acid and basic dyes. hn acid dye has the color in the acid radical, will dye wool and silk directly, and will not dye cotton satisfactorily in the absence of a mordant. A basic dye has the color in the basic radical, will dye wool and silk directly, and will not dye cotton satisfactorily in the absence of a mordant. Under ordinary conditions acid and basic dyes are in true solution. At the same dye concentration, an acid dye will be adsorbed more strongly in an acid solution than in a neutral solution and will be taken up least in an alkaline solution. A readily adsorbed anion will decrease the amount of an acid dye taken up and a readily adsorbed cation will increase it. With a basic dye the conditions will be reversed. The dye will be adsorbed most readily in an alkaline solution, but may be taken up in a neutral or acid solution. A readily adsorbed cation will cut down the adsorption of a basic dye and a readily adsorbed anion will increase it. Sodium sulfate
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tends to force a basic dye on the cloth and to strip an acid dye from the cloth. This was entirely unintelliglble until considered as colloid chemistry. A few years later Brigga extended the discussion to cover the case of substantive dyes, which dye cotton directly without a mordant. They are usually sodium salts of color acids and are always in colloidal solution. Making a colloidal solution instable will increase the amount of adsorption by a solid adsorbent until the agglomeration of the solid particles becomes too great, when the large particles will not be held firmly by the adsorbent. The substantive dyes are taken up as salts and not as color acids by the fibers. Sodium chloride, sodium sulfate, and sodium citrate increase the amount of dye forced on the fiber when these salts are used in moderate concentrations. At higher concentrations they strip the dye. Sodium sulfate therefore decreases the adsorption of an acid dye by wool and increases the adsorption of a substantive dye by wool or cotton. In both caaes the color is in the acid radical; but the acid dye is in true solution and it is the color acid which is adsorbed, while the substantive dye is in colloidal solution and it is the salt which is adsorbed. The relative stability of aluminum kettles in water and in air is due to the presence of a coherent film of oxide or hydrous oxide which protects the underlying metal. One would expect zinc to corrode less readily than magnesium and more readily than iron; but the different properties of the colloidal films account for the differences in behavior. With amalgamated aluminum no protecting film forms and the metal corrodes readily. If the oxide film is removed, the electroplating of nickel on nickel is as simple as that of copper on copper. The properties of passive iron are due to the presence of FeOs stabilized by adsorption. The flowing of finely ground powders is due to the presence of a film of adsorbed air. In 1887 Ostwald worked out a method for measuring single potential differences by determining the voltage difference which gave the liquid electrode the maximum surface tension. The method failed because of adsorption from solution by the electrode, a thing that nobody considered in those days. In 1902 Billiter assumed that zero potential difference occurred when the sign of the electrical cataphoresis changed. This method gave values differing from the Ostwald values by about 0.8 volt. In 1936 Porter and I obtained a third set of values which were consistent among themselves, but nobody knows whether they are true values or not. The experiments of Pelet-Jolivet (1910) showed that wool, though a protein, does not form definite chemical compounds with some acids or bases. Since wool is not dissolved or peptized appreciably by acid solutions, there is no difficulty in distinguishing between adsorption and compound formation. With gelatin and many other proteins the problem is more difficult. By treating powdered gelatin with hydrogen chloride gas, Belden showed that a definite chemical compound was formed, containing about 105 mg. of hydrogen chloride per gram of gelatin. Barnett showed that certain other proteins,-zein, for instance,--do not form definite chemical compounds with hydrogen chloride gas but do adsorb it. These results have not been popular with organic chemists.
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They are willing to admit that charcoal does not form a compound with acetic acid, but they insist on compound formation with a substance containing socalled basic nitrogen. Similarly people wish to have sodium cellulosates, quite regardless of the facts. Since the living body is a colloidal system or a collection of colloidal systems, it seems reasonable to suppose that changing the degree of dispersion of any of the proteins in the body might readily show itself as involving a change in the health of the body as a whole or of some part of the body. Sleeplessness due to tired nerves can be checked by giving purified sodium thiocyanate, a peptizing agent. The kind of sleep thus obtained must differ from that produced by narcotics, which are agglomerating agents. Sciatica can be helped by sodium thiocyanate but lumbago cannot, apparently because lumbago is a trouble of the muscles and sciatica one of the nerves. Morphine addiction and alcoholism can both be helped by purified sodium thiocyanate. On the other hand, there is no evidence that either peptizing or agglomerating agents have any perceptible effect on cancer. Peptizing agents have a beneficial effect on some forms of mental trouble, such as manic depressive insanity. Dr. Gutsell obtained some surprising results by giving sodium thiocyanate to two patients in the early stages of poliomyelitis. Of course, two cases are of no value in a medical problem; but so little is known about poliomyelitis that it seemed to me that every possible clue should be followed up. I tried to get the Harvard Medical School and the Cornel1 Medical School to make experiments with monkeys, because suitable monkeys can be inoculated with poliomyelitis.* Neither medical school was interested in the subject and so we do not know whether a promising line of attack has been passed up or not. It is all right to give a patient bromide as a sedative but one must not give it to him as a peptizing agent, and it is anathema to give him purified sodium thiocyanate, which is a better peptizing agent than bromide. It is quite possible that chemists may make still better peptizing agents for proteins than sodium thiocyanate and obtain still better results. I am glad that I do not remember the name of the man who said in print that he got the same results that we did if he did the experiments as we did them; but that he got different results if he did them differently. That sounds like a harmless statement of facts; but then he drew the startling conclusion that therefore we were wrong. Too many medical men wish to keep medicine an art and to prevent its becoming a science. That cannot last forever, and some day we shall recognize medicine as part of colloid chemistry. Some years ago I defined colloid chemistry aa the chemistry of grains, drops, bubbles, filaments, and films; several people looked upon that as an ill-advised *Note added in prooj: Since this Colloid Symposium medical men have announced that white mice can be inoculated with poliomyelitis. There will therefore be plenty of anima!s available for research. In the Reader’s Digeesl for December, 1941, there is a reference to Sister Kenny’s new treatment. It is not yet known whether this does or does not involve peptization.
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attempt a t humor. Today I should like to make that definition broader and to say that colloid chemistry is the chemistry of life and inheritance; and of bubbles, drops, grains, filaments, and films. Some day I hope that we shall acclaim Darwin and Pasteur as two of the greatest colloid chemists that ever lived, even though it is probable that neither man knew that he was interested in colloid chemistry. When that day comes we shall have to distinguish more sharply than the chemist does now between inheritance phenomena and what are purely chemical phenomena. When white lilacs are grafted on the stocks of the ordinary lilac, the appearance of white flowers is due to the graft and not to soil or weather conditions. The protection of French grapevines from phylloxera by -grafting upon American stocks is not primarily a question of soil or weather, though soil and weather do affect the quality of the crop enormously. The colors of hydrangea flowers can be changed a t will by changing the soil conditions; but we do not yet know why this is of so limited applicability. The leaves of the Norway maple rarely turn red in the fall in Ithaca, and the leavps of the sugar maple almost always do. I have not yet been able to find out what happens to the Norway maple in America when it grows near the northern limit of its range. I rather suspect that it turns red more frequently. With the sugar maple, weather conditions have a great deal to do with the time a t which the red appears and with the degree of redness. We know that girdling a branch of a sugar maple will make the leaves turn red earlier and that this effect may last a year or two. We know that the autumn foliage in Vermont is usually more striking than in New York; but the problem has never been studied as one in colloid chemistry, so the field is open. The pigments in the red autumn leaves are known as anthocyanin pigments, and there is a wide difference of opinion as to the way they originate. The German chemist Willstatter believes that they are formed in the plant by reduction of flavones, while the English chemist Robinson believes that they may also be produced in the plant by hydrolysisand perhapsoxidation of what are known as leucoanthocyanins. The leucoanthocyanins were not known to Willstatter, so he did not discuss them. It is possible, and has proved to be the fact, that flavones and leucoanthocyanins may coexist in some plants. Whether in the formation of anthocyanins we are dealing with the reduction of flavones or with the hydrolysis of leucoanthocyanin, with or without accompanying oxidation, there must be another factor, because we do not yet know of any straight reducing agent which the plant could use which would reduce a flavone to an anthocyanin, and because it seems improbable that there would ordinarily be sufficient acidity to hydrolyze a leucoanthocyanin in a reasonable time. The presence of flavones or leucoanthocyanins is not sufficient to account for leaves turning red in the autumn. Flavones are present in the leaves of the ginkgo, but I have never seen any red either on stems or leaves. The green leaves of the Norway maple in Ithaca contain flavones and no leucoanthocyanin. The leaves very rarely turn red; but the stem of the leaves usually do. Miss
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Wheldale (1911) says that (‘there is little doubt but that the formation of anthocyanin does involve a series of progressive reactions, each of which is controlled by an enzyme.” The electrolytic reduction of flavones to anthocyanins calls for an overvoltage, and yet sunlight is able to do the reduction in some cases though not in all. The difficulty disappears if we follow Miss Wheldale and postulate the occurrence of a suitable enzyme. It then follows that the differences between the autumn foliage in this country and in Europe are due in part to the failure of the plant to produce the proper enzymes in Europe. In our present state of knowledge that is not very helpful; but it is a step forward to have the problem formulated clearly. Since the leaves of some of the American trees turn red after transplantation to Europe, the Euorpean climate is not destructive to the unknown enzymes. In a book on Canada John MacCormac (1940) says, “an Englishman’s reaction to the Canadian woods in autumn is that they are impossible and in any case are not in good taste.” Last winter the newspapers reported that a gardener in Albany had dug up a pink lilac bush in the winter and had brought it indoors. When the bush bloomed, presumably in inadequate light, the flowers were white. A similar phenomenon was reported independently for a Japanese quince. I have not yet found any botanist who would say whether this was possible, or who cared. The matter seems important to me. If it is true, we have a marked, though perhaps temporary, change in an inherited property. It also gives us a line for experimental attack on the problem of the autumn foliage in Europe. I hope that some chemist, perhaps an agricultural chemist, will clear up this matter. If one examines green leaves before they turn red, one can tell whether they contain practically only flavones, practically only leucoanthocyanins, or appreciable quantities of both. From these data one can make a good guess as to the probable precursors of the anthocyanins in many cases. Mr. Rutzler found that the leaves of the sumach, the barberry, and the flowering dogwood contain flavones and no, or practically no, leucoanthocyanins, though there may be a mere trace of leucoanthocyanin in the leaf of the dogwood. The leaves of the sugar maple and the Virginia creeper (Parthenocissus quinquefolia), and the skin of the Seckel pear contain leucoanthocyanins and no, or practically no, flavones, while the leaves of the Japanese creeper (Ampelopsis) contain both flavones and leucoanthocyanins. One kind of guava has been found to contain chiefly leucoanthocyanins. If this is converted into jelly with pectin and a minimum of heating, one gets an almost colorless guava jelly, which turns red if the juice is heated longer. After the red has pretty well disappeared from the young leaves of the copper beech in summer, leucoanthocyanins are found to be present. On this statement of facts, it seems probable that the red is developed from the leucoanthocyanins. Abbott (1909) has found that the leaves of the copper beech are green in the spring when grown in the dark, and turn red when exposed to light. He did not analyze these green leaves for flavones or leucoanthocyanins; consequently one step in the proof is missing.
e
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One might say that chlorophyll is not inherited, because light is required to produce it; but the poasibility of producing chlorophyll is inherited. The photochemical production of anthocyanins from flavones in the presence of catalytic agents is not an isolated phenomenon. Zinc oxide has been shown to act catalytically in causing photodecomposition of certain systems. This contradicts the statement commonly attributed to Einstein that the absorption of a sufficientlylarge quantum of light will produce chemical change. This does not occur with many solutions when exposed to ultraviolet light. Consequently Einstein did not prove thermodynamically that a change must occur under these conditions. When zinc oxide acts catalytically to cause photodecomposition the absorbed quantum must have been large enough; but it was not applied efficiently in the absence of the zinc oxide. I believe that Einstein never claimed that he had proved thermodynamically that absorption of one quantum of a sufficient size would produce a chemical change. What Einstein said was that this would happen if there were no disturbing factors, which is quite a different thing. There seems to be a feeling among scientific men that anything which is attributed to Einstein is necessarily right, regardless of whether he said it or not. It seems to me that a great deal of work will have to be done in measuring the chemical effectiveness of the lightquantum in different cases. I have not yet succeeded in impressing anybody with the accuracy of my point of view; but also nobody has ever tried to show me wherein I have erred. The subject seems to be taboo. It is a pity that Einstein has not discussed the question of disturbing factors. People are reluctant to believe that Einstein’s treatment is inadequate, unless he admits i t himself. Copper sulfate solution is not reduced to copper by ultraviolet light. It has been stated that mixtures of hydrogen and bromine are not photosensitive at ordinary temperatures, but this has not yet been tested satisfactorily. I have cited the photocatalytic action of zinc oxide in some systems as proof that the law of photoequivalence is not valid. It will now be well to say a few words on the chemical action of light, because photography always involves colloid chemistry though photochemistry does not necessarily do so. When light is absorbed by a chemical compound the absorbed light tends to increase the chemical potential of the molecule giving rise t o the absorption bands or lines under the condition of the experiment, thus making the compound more reactive and tending to eliminate it, in case there is no appreciable intramolecular transfer of energy. If the light absorbed per unit time and converted into chemical energy is sufficiently large, chemical action will take place; otherwise not, the absorbed energy being converted into heat, fluorescence, or some, thing similar. If the compound is not decomposed by light under the conditions of the experiment and if we can destabilize the molecule sufficiently without changing its absorption too much, the compound will become photosensitive. In other words, decreasing the stability tends to increase the photosensitivity. When exposed to light, a photosensitive substance will tend to change at the point in the molecular structure which is weakest with reference to light energy.
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If the change is one that can be produced by electrolysis, we shall probably say that light acts like an electrolytic current. Grotthuss said this and I followed him. If the change is one that can be produced by heating, we shall probably say that light acts like heat. Both statements may be right for the conditions under which they were made; but neither statement is the whole truth. Theoretically, it is possible to destabilize an electrolytic solution, say of copper sulfate, to any desired degree by subjecting it to an impressed voltage less than' the decomposition voltage. Consequently all electrolytic solutions can be made photosensitive under suitable conditions. With silver bromide it is not yet known whether the important decomposition voltage is the one that carries the bromide to silver or whether it is the one which carries the salt down to the developable stage, probably the latter. No data are available for the solarization stage. From the behavior of films in the photography of stars, Mees of the Eastman Kodak Company concluded that there was first a mysterious storing of energy in the molecule. That cannot be true. Light will either produce a chemical change or it will not. In the cases referred to by Mees the difficulty was undoubtedly in the inadequate method of analysis. At the Rice Institute Weiser has published three volumes of Inorganic CoZbid Chemistry. At the moment I do not see how to claim his early work on flame spectra as colloid chemistry; but that may be ignorance on my part. We do not yet know whether a catalytic agent can start a reaction or not. I am inclined to doubt that potassium chlorate would decompose in a million years; but one cannot test that experimentally. We do not know to what extent catalytic agents are necessary in the production of hard and soft coals or in the natural synthesis of oil deposits. The catalytic action of zinc oxide in certain photochemical reactions is apparently a case of a catalytic agent initiating a reaction. Ladies and gentlemen! I have tried to give you my present concept of colloid chemistry,-thirty years of colloid chemistry as it appeals to me; not necessarily as it interests others. I doubt whether anybody will agree with all that I have said, but I hope that you have been interested and that each one has heard something that will be helpful to him or her. I thank you again for giving me this opportunity of speaking t o you.
