T H E EFFECT O F GELATIK AND SALTS O N COXGO RED* BY HERBERT L. DAIVS AND JOHN W. ACKERMAN
I n the practical application of colloid chemistry no property is more often used than that shown by a number of usually lyophilic colloidal substances by virtue of which they are able to stabilize other colloids, which are for the most part lyophobic. Many examples of this protective action are known and have been studied; but the student of these phenomena can not proceed very far before coming to the point beyond which our knowledge has not gone. This paper will present a contribution to this problem by showing that in certain systems containing gelatin and Congo red in which we should expect a protective action, the phenomena are for the most part to be interpreted on the assumption of an independent existence of these two substances in the mixed sols and only under special conditions does anything like protective action appear and then in a form not usually associated with these phenomena. A definition of protective action has been given by Thomas:l “When a solution of a hydrophilic colloid is added to a less stable dispersion, or suspension, generally there is no change in appearance of the system and the less stable dispersion is found to have become more stable, Le., it is no longer so sensitive toward coagulation by either the addition of electrolytes or by evaporation to dryness. The less stable dispersion is said to have been ‘protected, by the hydrophilic colloid; hence the term ‘protective colloid,’ which is commonly applied to the hydrophilic colloids, such as gelatin, gum arabic, albumin, etc.” But the addition of gelatin to anot,her colloid is often a much more complex phenomenon than would appear from this simple definition and the literature shows several cases of sensitizations such as are discussed by Bancroft .* “Since a colloid peptized by water may be charged positively or negatively, there is no reason why it should not precipitate another colloid under suitable conditions. We usually consider the colloids peptized by water solely as protecting colloids, but this is clearly an inadequate view, as is shown by the experimental data. Years ago Schulee* pointed out that small amounts of gelatin solutions were as effective as lime or alum in causing the rapid sedimentation of clay, and that addition of minute quantities of gelatin to barium sulphate simplified the question of washing and filtration very much. He, of course, gave no adequate explanation of the phenomenon and * This work is part of the programme now being carried out a t Cornell University under a grant to Professor Bancroft from the Heckscher Foundation for the Advancement of Research established by August Heckscher at Cornell University. * Bogue: “The Theory and Application of Colloidal Behavior,” 1, 346 (1924). * “Applied Colloid Chemistry,” 309 (1926). Pogg. Ann., 129, 369 (1866).
E F F E C T O F GELATIN AND SALTS ON CONGO RED
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the real explanation was given by Billitzer,’ who called attention to the fact that, while gelatin, agar-agar, etc., ordinarily check the precipitation of colloidal solutions, by electrolytes, small amounts of these same substances may have a precipitating action. This can be detected even when the gelatin produces no precipitation itself. Billitzer cites the experiments of Keisser and Friedemann in which it was found that a mastic emulsion containing a trace c?f gelatin was precipitated more readily by sodium chloride than when no gelatin is present.2 Billitzer finds that gelatin precipitated such negative colloids as antimony sulphide and arsenious sulphide in acid or neutral solution, but does not precipitate positively charged sols such as hydrous ferric oxide, Gelatin in ammoniacal solution precipitates hydrous ferric oxide, though no precipitation occurs if ammonia is added to a mixture of gelatin and ferric oxide. Bismarck brown, which is a positive colloid, is precipitated by an alkaline gelatin solution, while eosine is precipitated by an acidified gelatin solution.” A little later, p. 3 I I , Bancroft returns to the more usual protective action and the measure of it. “Coming back to the general problem, if the ratio of gelatin to the other colloid is increased sufficiently, we shall pass through the precipitation range into the range where the colloidal solution is stabilized by gelatin and then behaves more like a water-soluble colloid. Colloidal gold and colloidal silver solutions, when stabilized by gelatin, can be eraporated and redissolved, because the gelatin prevents the irreversible agglomeration. When less gelatin is used it may retard, though not prevent, the change of red colloidal gold to blue. Zsigmondy3 defines as the gold number the number of milligrams of a protecting colloid which just prevents the color change in a I O cc red gold solution, containing 0.0053-0.0058percent gold, when one cubic centimeter of a ten percent sodium chloride solution is added. Of course, a strongly adsorbed non-electrolyte, such as sugar, will act similarly to gelatin; but, in most cases, the adsorption is so much less that these substances are only interesting theoretically as stabilizers.” It is quite generally agreed that the protective action is due to the formation of an envelope or sheath of the protective colloid about the particles of the protected dispersion. As a result of this it is found that the conduct of the coated particles is identical with that of a solution of the protective colloid. This was shown by Loeb4 who found that collodion suspensions to which gelatin and other protein sols were added lost the properties of the collodion and behaved as sols of the protein alone toward salts, cataphoresis, isoelectric point, etc. I n certain cases there was poor protective action and in others there was slight deviation of the properties of the mixed sols from that of the protector, such as in the case of albumin where Loeb believed adsorption on the collodion particles produced an effect analogous to the so-called denaturation of the albumin. But as a general rule a substance which is to act as a
‘
Z. physik. Chem., 51, 145 (1905). Cf. Walpole: J. Physiol., 47, xiv ( 1 9 1 3 ) ;Biochem. J., 8, 1 7 0 (1914). Z. anal. Chem., 40, 697 (1901). J. Geo. Physiol., 5 , 479 (1922-3).
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HERBERT L. DAVIS Ah7D JOHN W. ACKERhlAN
protective colloid must be strongly adsorbed by the dispersion to be protected and will impart to the mixture properties of the protecting sol. Contrary to expectation gelatin is but weakly adsorbed by Congo red and the properties of the mixed sols are those of both of its components. Especially prominent among the latter are the properties of dyeing cloth by the mixed sols in the presence of various salts. The first direct evidence of this state of affairs was the behavior of gelatin swelling in dilute solutions of Congo red. When one gram of powdered gelatin is added to fifty cc of 0.006% solution of Congo red, the swelling gelatin a t room temperature is found to contain but little more dye on or within its particles than does the clear supernatant solution which is in turn only slightly lighter than a blank solution to which no gelatin was added. This indicates very clearly that little or no protective action should be expected.
The Effect of Salts on Congo Red Solutions I n spite of the very slight adsorption shown by the Congo red and gelatin there are some phenomena which would ordinarily be interpreted as showing protection. Before proceeding to them it will be well to show some of the properties of the Congo red itself, since these properties indicate Congo red to be somewhat intermediate between the lyophobe and lyophile classifications of colloidal substances. In the first place it can be precipitated by suitable concentrations of sodium and potassium chlorides and iodides. I n all these experiments a stock solution containing two grams of Congo red in a liter of solution was employed, ugually by adding five cc of the stock solution to sufficient of the other reagents to make z j cc. The dye solution as diluted finally, then, was 0 . 0 4 7 ~ ;samples of 0.01and 0.02 mol of the halides were dissolved in 20 cc of solution and added to five cc of the stock Congo red. I n these low concentrations of salts the precipitation was slow so that the final observations were made after 4 j hours. I n the case of the solutions which were 0.8 M with respect to the salts the differences were less than in the case of those 0.4 M, for in the more concentrated solution precipitation of the Congo red was nearly complete. Comparison of the precipitates and the supernatant solutions showed, however, that in both dilute and concentrated solutions the effect was in order K I > K C l > S a I > XaCl, the potassium iodide having the greatest precipitating effect and the sodium chloride the least effect. This shows that the iodides are more effective than the corresponding chlorides and that the potassium salts are more effective than the sodium salts. The fact that potassium chloride has greater precipitating power for Congo red than sodium iodide has shows that the difference between the metallic ions is greater than that between the chloride and iodide ions. This suggests a Hofmeister series for the coagulation of the negatively charged Congo red micelle' and the experiment showed that 0.4 M BaCL, when added as above, precipitates Congo red almost a t once, leaving a clear, colorless, supernatant liquid after ten minutes. In respect to its behavior Weiser and Radcliffe: J. Phps. Chem., 32, 1 8 7 j (1928).
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with salts Congo red acts like a typical lyophobe colloid; but it is readily peptized by water alone and this is a property more often found in lyophile colloids. The experiments with sodium chloride were continued to greater concentrations and it appears that these systems show a phenomenon akin to the “irregular series.” Systems of z j cc each were made up, each containing five cc of the stock Congo red and sufficient jN NaCl to make them one, two, three, and four normal with respect to the salt. Blthough the 4N KaC1 system grew quite turbid a t once and was soon completely coagulated, the Z X KaC1 produced finally much less precipitation of the Congo red than did either the I N or the 3N systems. Further experiments confirmed this and showed that there is a broad minimum in the coagulating power of sodium chloride when the salt concentration is 1.4-1.6E;systems 1.4-z.oN are not greatly coagulated on standing for several days while systems containing more or less salt are practically completely coagulated and precipitated. This appears to be a case of irregular series as discussed by Kruyt.’ “The phenomena observed when a sol is flocculated by means of an electrolyte such as AlC1, agree well with the changes in boundary potential Khen increasing amounts of AlC13 are added to a negatively charged sol of mastic, we notice that small concentrations cause flocculation, somewhat higher concentrations produce another stable sol of opposite sign, i.e., positively charged, while still higher concentrations bring about another flocculation. . . . ‘‘We have, therefore, first a non-flocculation zone; but as soon as the negative critical potential is reached, flocculation begins. This primary flocculation zone ends when, after reversal of the charge, the positive critical potential is attained. From that concentration on, the AlCl,, instead of causing flocculation exerts a recharging action. Hence the mastic sol is positively charged in this second zone of non-flocculation. But, positive boundary layers are discharged by anions and the positive potential, after going through a maximum value, is reduced by the C1 ions until the critical potential is reached. At that point, a second flocculation zone begins. “This phenomenon is designated as an irregular serzes. It will always occur when the potential-lowering effect of the cation is far in excess of the potential-raising effect of the anion. This lowering effect may be due either to a high valence of the ion or to a high degree of adsorbability. Polyvalent cations give, therefore, irregular series when they are combined with monovalent anions. But monovalent organic cations act in the same way. For instance, strychnine nitrate, as well as new fuchsin, yields an irregular series with As2S3 sol, and AgN03 with the sol of HgS, because in each case the cation is strongly adsorbed. “A converse reasoning applies to positively charged sols. Here the anion discharges, while the cation may raise the charge. Whenever there is a great contrast between the two ions, an irregular series will occur; hence we readily “Colloids,” 89 (1930).
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HERBERT L. DAVIS AKD JOHN W. ACKERMAN
understand why the positively charged sol of iron oxide forms an irregular series with sodium phosphate (monovalent cation us. trivalent anion), and with NaOH (monovalent ions, but anion strongly adsorbable).’’ The Congo red sol does not exactly fit this picture, for cataphoresis on the system that was 1.5” NaCl showed that in this zone of non-coagulation the sol is negatively charged as it is in the salt-free condition. I n addition to this, small additions of BaCh solutions caused immediate turbidity and rapid flocculation and precipitation of the dye in the I . ~ N NaCl; systems that were I.~N NaCl and O.INBaC12 were coagulated somewhat less rapidly but not less completely than systems containing only dye and O . I N BaC12. On the other hand, additions of sodium sulphate had no effect on the sodium chlorideCongo red systems except to clear up a slight turbidity produced by the sodium chloride on the dye. If the sol in the non-flocculation zone were positively charged, cataphoresis should have shown it and the sol should have been coagulated on the addition of sodium sulphate instead of barium chloride. We have here, apparently, an irregular series without the reversal of charge on the colloid. This must be due to a higher adsorbability of the sodium ions on the Congo red, the difference between the adsorbabilities of the sodium and chloride ions being too small to permit a reversal of the charge. The first addition of sodium chloride results in preferential adsorption of positive sodium ions and the reduction of the charge on the Congo red micelles below the critical value. Higher salt concentrations give also some adsorption of chloride ions sufficient to raise the potential above the critical value; and finally sufficient excess of sodium ions are adsorbed to reduce the potential again below the critical value and produce precipitation. It may be added that sodium sulphate also appears to give with Congo red such an irregular series as is shown by sodium chloride. Molar sodium eulphate produces markedly less coagulation and precipitation than do systems 0.5 or 0.75 M with sodium sulphate. It has been shown that Congo red may be precipitated by various salts in the proper concentrations. The addition of gelatin to such systems results in a great decrease in the flocculating power of the salts. Systems containing the regular amount of five cc of the stock Congo red solution, sufficient sodium chloride to make them 3.1 and 4.8N respectively, and without or with the addition of five cc of two percent gelatin solution gave the following results in two hours : Without gelatin With gelatin
3.1N NaCl greatly coagulated quite clear
4.8N NaCl completely coagulated slightly turbid
Solutions of gelatin protected also against precipitation by barium chloride, for whereas O.IN BaClz precipitates the dye rapidly and completely a t room temperature, o.8Y BaClz produced no observable effect on Congo red in presence of gelatin, even at 95’ for an hour. Another experiment with
97 7
EFFECT OF GELATIN AND SALTS ON CONGO RED
potassium iodide showed that in systems containing up to N K I there was increasing precipitation of the dye while in similar systems containing gelatin there was no coagulation. These results are typical of the behavior of the dye in the presence of gelatin and one would be inclined to interpret them on the basis of protective action by the gelatin. But if there be little adsorptive attraction between the gelatin and the dye, there can be little protective action and this has been shown to be the case in the absence of salts. I n the presence of salts however there does appear to be adsorption of the dye by the gelatin which will explain these phenomena. To demonstrate this, six mixtures were made up, each containing 50 cc of 0.0067~Congo red and to this was added the powdered gelatin and the sodium sulphate as shown below.
A Powdered gelatin Mols of NanSOl
B
o one gram 0
0
C
D one gram
E
o
o
F one gram
0.01
0.01
0.02
0.02
Sodium sulphate was chosen because it had been found very effective in salting the dye out of solution, and also because the gelatin would swell less in its solution than in the presence of the halides. The powdered gelatin did swell in all the solutions and took up dye from the solution, comparison of A and B showing that. The addition of salt in C gave a lighter solution than the blank A and for the moment it will be assumed that this change of color is associated with an increase in the size of the colloidal micelles of the Congo red since, of course, there can be no adsorption to decrease the concentration of the dye in the solution and it has already been shown that the addition of more sodium sulphate completes the agglomeration of the dye into particles so large that they are precipitated. The supernatant solution in E is also lighter than that in A but no precipitation of the dye takes place even on long standing. The swelling gelatin in D and F is increasingly darker with the adsorbed dye than is the gelatin in B, and the remaining solution in D and F is increasingly lighter than that in A showing that the gelatin in the presence of increasing amounts of sodium sulphate is adsorbing increasing amounts of the Congo red, or that the salt in concentrations that would otherwise not precipitate the dye can nevertheless cause it to be adsorbed on the gelatin. In the next section it will be shown that the addition of salts causes the dye to be adsorbed more strongly on cloths also. It is not possible to coagulate the dye alone from a system containing 0.04% Congo red and 0.4% gelatin by means of sodium sulphate, for any concentration of this salt that will have any effect is more than the half-saturated solution a t which gelatin alone is coagulated and thus carries down the dye with it. The case under discussion is, however, not a case of the carrying down of the dye by the swelling gelatin for there is a definite difference between the systems containing salt and those without the sodium sulphate, the same amount of gelatin being present in each. Further than that, if i t were merely a case of filter action, continued shaking should remove all the dye from solution whereas
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HERBERT L. nAvrs AND JOHN
w.ACKERMAN
this is not the case. There is a rapid adsorption of the dye on the gelatin which then reaches an adsorption equilibrium leaving a large proportion of the dye still in solution. The final condition is independent of the order of mixing the dye, gelatin, and salt solution. The assumption introduced above, Le., that the change of color of the Congo red solution on adding sodium sulphate solution and the increased adsorptive attraction between the altered dye sol and the gelatin are accompanied by B decrease in the dispersion of the dye is not without support in the literature although it is not generally expressed so explicitly. Bancroft’ discusses the problem in these terms: “With colloids stabilized by an electric charge, we find that the amount of salt necessary to produce precipitation varies with the way in which it is added, more being necessary if the salt solution is added slowly.? The precipitation value obtained by adding the electrolyte all a t once is such a concentration of the precipitating ion that sufficient adsorption to cause neutralization can result in a definite time. Weiser3 considers that if the same amount of electrolyte is added very slowly, there results a gradual increase in the size of the particles due to partiaI neutralization by adsorption. After the addition of enough electrolyte, partial agglomeration takes place. These coagulated particles have adsorbed not only enough to effect their complete neutralization; but the neutralized particles have carried down an additional amount during agglomeration. This adsorption of electrically neutral particles during the fractional precipitation accompanying slow addition of the electrolyte causes such a decrease in the ionic concentration that a greater amount must be added to effect complete neutralization by this fractional process. Rapid addition furnishes a t once all the critical concentration of precipitating ion necessary for neutralization by adsorption.” In the experiments reported above, the addition of the precipitating salts has been made all at once, so that question does not enter here. It is suggested however that the addition of the sodium sulphate makes the Congo red particles more instable and therefore more easily adsorbed on the gelatin, which if it were peptized would tend to prevent bhe precipitation of the dye and thus give the appearance of protection without the existence of any gelatin envelope before the addition of the salt. This problem has been further discussed by Bancroft, p. 299. “Making a colloidal solution instable will increase the amount of adsorption by a solid adsorbent until the agglomeration of the colloidal particles becomes too great, when the large particles will not be held firmly by the adsorbent. This principle is made use of in dyeing with substantive dyes.i Substantive dyes dye cotton direct without a mordant. They are usually sodium salts of color acids and are always in colloidal solution. They are taken up as salts and not as color acids by the fibers. Sodium chloride,
’ Bancroft: “Applied Colloid Chemistry,” 296 (1926).
* Freundlirh: 2. physik. Chem., 44,
143 (1903). Weiser: ,J. Phys. Chem., 25, 404 (1921). Briggs: J. Phys. Chem., 28, 368 (1924).
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979
sodium sulphate, 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 sulphate, therefore, decreases the adsorption of an acid dye by wool and increases the adsorption of a substantive dye by wool or cotton. I n both cases 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.” Considering the first sentence of the above quotation, it appears obvious that the increase in particle size does not suddenly begin and all take place just a t the moment of coagulation or a t the concentration a t which the particles become too large to be held firmly by the adsorbent. Increase in particle size must begin with the first addition of the precipitating agent and be responsible for the observed increase in adsorbability. I n other words the slowly coagulating particle shows an enhanced readiness to adsorb nearly anything such as colloidal substances which would tend to keep it suspended, or, failing that, solid adsorbents such as cloths to be dyed. The drowning man grasps a t the straw which would have proved no attraction whatever while he was well able to keep himself afloat. This may be the explanation for the slow precipitations so often observed. Some particles are neutralized by the electrolyte and in turn adsorb other relatively unaffected and stable particles. The particle size thus increases while the charge ratio diminishes until particles grow large enough to be seen and finally settle out. The theory of dyeing with substantive dyes was formulated and summarized by Briggs in the paper referred to above. I n his discussion of the properties of aqueous solutions of these dyes (p. 370)~Briggs says: “The dialyzed solutions, like those of the soaps, are excellent conductors; they exert in a collodion osmometer a surprisingly high apparent osmotic pressure; in the absence of a membrane they possess an easily measurable power of diffusion; and they invariably contain amicrons or ultramicrons when viewed in the ultramicroscope. It is possible to. bring about an increase in the size of the ultramicrons and a corresponding decrease in the degree of dispersion, without causing any visible or actual flocculation of the suspended dye-a property of the substantive dye solutions which is of the utmost significance in the theory of substantive dyeing. Increase in the size of aggregates is observed as the concentration of the dye is increased, or as the temperature is lowered; a t low temperatures concentrated solutions of the substantive dyes set t o a jelly. Decreased dispersion is also caused by destabilizing electrolytes, while electrolytes in excess bring about floccu1ation.l The growth of aggregates is opposed by stabilizing protective colloids such as gelatin.2 Practically all of the very complete evidence now available supports the conclusion that in neutral or alkaline dyebaths the substantive dyes are present as electronegative colloids capable of a very high degree of dispersion in the absence of destabilizing agents. ‘Cf. Biltz and von Vegesark: Z. physik. Chem., 73, 481 (1910);Wo.Ostwald: Kolloidchem. Beihefte, 10, 197 (1919);.kuprbach: Kolloid-Z., 31, 37 (1922). 2Cf. Bayliss: Kolloid-Z., 6,23 (1910);also Biochem. J., 1, 17j (1906).
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HERBERT L . DAVIS AND JOHN W. ACKERMAN
“While a flocculating agent will tend to force the suspended colloid into the interface, it should not be present in amounts large enough to produce actual coagulation. If the latter occurs, the agglomerated flocks may be too large to be retained in the interface and will therefore not be adsorbed, a t least strongly. Since flocculating agents in amounts insufficient to produce actual coagulation are known’ to destabilize sols by decreasing their dispersion, such agents will aid in the adsorption of a colloid up to the point of flocculation but beyond this point will act in an opposite direction. The amount of adsorbed colloid in the interface will therefore pass through a maxinzum as the concentration of the flocculating substance is increased beyond its coagulating value. Such adsorption maxima have been observed and they show, in reality, that a dispersed colloid is most strongly adsorbed when its dispersion has some intermediate value, as Wo. Ostwald2 has suggested. “Flocculating agents are not the only cause of decreased stability and correspondingly increased adsorption within limits, for the stability of a sol may change with the temperature, with age, and with its previous history. The stability and the dispersion decrease with increase in the concentration of the sol, which may have something to do with the fact, which has puzzled some of us, that more is adsorbed from a concentrated sol than from a dilute one, and the adsorption curve looks like the typical isotherm obtained with a dissolved adsorbed substance: p. 3 7 2 . “In terms of the present theory, an acid, base, or salt which acts 89 an assistant in the substantive dyebath does so because it makes the particles of dye more strongly interfacial between bath and fiber. This in turn it accomplishes by decreasing the dispersion and stability of the dye in the suspension or sol which constitutes the bath. We have seen that electrolytes are known to produce this effect in the case of various organic colloids in suspension and we should expect the same effect with colloidal dyes. Many instances are indeed known, especially among the experiments of Dreaper and his coworkersP Baylisss found that the addition of two percent of sodium chloride to a weak suspension of Congo red trebled the adsorption of this dye by filter paper. Concerning the effect of this salt, he wrote as follows: ‘Notwithstanding the fact that no actual precipitation takes place in these experiments, the addition of electrolytes to Congo red, for example, causes an increase in the size of the colloidal particles. . . . so that the solution is on its way to precipitation even when this does not actually occur. The specimen of Congo red used in all my experiments showed the Tyndall phenomenon very faintly in solution in distilled water; but when sodium chloride was added 1 Note Linder and Picton’s work on the “degradation” of arsenious sulphide sols by sodium chloride. J. Chem. SOC.,67, 73 (1895). “Grundriss der Kolloidchemie,” 417 (1910). Cf. Trauhe and Shikata: Kolloid-Z., 32, 316 (1923). 3Cf. Briggs: J. Phys. Chem., 19, 2 1 0 (1915). Dreaper: “Chemistry and Physics of Dyeing,” 2 5 1 et seq (1906). 5 Biochem. J., 1, 175 (1906).
‘
EFFECT OF GELATIN AND SALTS O N CONGO RED
..
981
the beam of light became much more distinct.’ . The agglomerating and destabilizing action of weak flocculating agents in solutions of the substantive dyes may be regarded as absolutely established a t the present time,” p. 376. Kraemer in his chapter’ on “Colloids” points out some cases of this; “In many cases a relatively slight aggregation occurs without continuing through the stages of complete coagulation, Le., an aggregate of primary particles may possess a normal degree of stability. The red-to-blue color chsnge in Faraday gold sols upon the addition of insufficient electrolyte to precipitate them is due to such a partial aggregation. Another is the spontaneous formation of ‘tactosols’ (aged sols of VZOS, FezOs or benzopurpurin united into loose swarms) already referred to. Varying degrees of partial aggregation also likely occur in the high-viscosity lyophilic dispersions. Rather direct evidence of such a condition is provided by the rigidity and elasticity, on a microscopic scale, of apparently fluid sols of gelatin or soap.” But the experimental evidence in favor of this view is not universally accepted and that on the gold sols may or may not be true. Therefore it is quite interesting that another method of investigation has brought substantiation to the views here advanced. These experiments have to do with the addition of aluminum sulphate, aluminum nitrate or magnesium sulphate to sols of gold, gum mastic or arsenic trisulphide. Burton and Annettsl say: “The original intention in the present experiments was to use the changes in scattered light to follow changes in samples of colloid to which extremely small amounts of various electrolytes had been added. These results led to the complementary experiment of testing the effects of the coagulation process on the light transmitted by a sample of colloid. The latter in turn led to the discovery of the existence of apparently permanent stages of partial coagulation which do not appear to have been accentuated before.” Items in the summary of this paper include: “(I) Experiments have been carried out on the measurement of scattered light and transmitted light from samples of colloidal solutions. These give indication of distinct changes in the colloid on the addition of very small traces of electrolyte even before coagulation sets in. ( 2 ) By adding successively very small traces of electrolyte to solutions of gold, mastic and arsenious sulphide, the existence of stages of partial coagulation has been demonstrated.” This idea has also appealed to Mullin3 for he says: “It is a heretofore unexpected fact, possibly not at all characteristic of dyeing acetate silk only, that certain compounds (dyestuffs) have a much greater affinity for acetate silk when in colloidal solution, possibly in even a rather coarse dispersion, than when they are in true solution. This is particularly the case where the compound is solubilized by means of chemical combination with some solubilizing reagent, such as the solubilization of certain bases by means of hydrochloric acid to form their hydrochlorides, or Taylor: “A Treatise on Physical Chemistry,” 2, 1695 (1931). Burton and Annette.: J. Phye. Chem., 35, 48 (1931). 8 Mullin: “Acetate S ilk and its Dyes,” 278 (1927).
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HERBERT L. DAVIS AND JOHN R . ACKERMAN
by sulfonation.” A little later we find: “If we neutralize an aqueous basehydrochloride solution with an alkali such as sodium carbonate, the free base is precipitated as a colloidal or even coarser dispersion in the bath and this free base is rapidly taken up by the acetate silk, as was mentioned in connection with the application of bases for the developed colors on acetate silk, and we get much deeper shades than where we depend only upon the hydrolysis of the dyestuff (base-hydrochloride) to dye the fiber.” Kow, obviously, the change in particle size is not the only condition that is changing here since, for instance, in the case of Mullin’s dye-hydrochloride adsorption, the addition of sodium carbonate will have a large effect in increasing the hydroxyl ion concentration of the solution and such an increase would, of course, increase greatly the adsorption of the free dye base. Likewise, the addition of salts may have a similar effect on the adsorption of Congo red on the gelatin or cloths. It will be impossible to discover whether increased particle size or the changed ionic environment is responsible for the increased adsorbability until some ready method can be evolved for changing the one of the factors without affecting the other. Boiling effects the coagulation of many sols and we know that, in general, adsorption, such as dyeing, takes place more rapidly in Farm solutions. Unless the stages of such coagulation are spontaneously reverfed on cooling, cloths should take up increasing amounts of dye from sols that have been boiled for increasing periods and then cooled to room temperature and this should hold nearly to the point of complete precipitation. Increased particle size appears to be the one factor common to increased adsorbability. Another instance of the protective power of gelatin for Congo red is found in acidifying the solution. Congo red is an indicator changing from red in alkali t o blue in acid solution (pH range 3.0-4.0). Addition of hydrochloric acid effects an immediate development of the blue color and a rapid and complete coagulation of the dye. Such a solution gives a t once a clear colorless filtrate. If sufficient gelatin be added to the dye to make the final solution 0 . 4 7the ~ ~ final dye concentration being 0.04:$, the addition of sufficient hydrochloric acid to make the final system o.16K produces as before the deep blue color but no precipitation and the blue sol is stable indefinitely. Boiling for ten minutes, however, coagulates the blue sol completely, and the clear, colorless filtrate appears to have lost permanently the power to protect Congo red from precipitation by acid. If this filtrate be cooled and neutralized, and to it are added new amounts of Congo red and hydrochloric acid immediate coagulation results. This coagulation cannot be due to the sodium chloride produced in neutralization for such a concentration would have no effect on a normally protected sol and did not produce any effect before the addition of the acid. It seems scarcely likely that the dye precipitated in the first instance should have carried down all or practically all the gelatin present.
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Dyeing with Congo Red In recent attempts to dye with certain water-insoluble dyes formed in and protected from precipitation by gelatin, the difficulty was encountered that such sols would not dye satisfactorily, the dye being taken up too little or washing off too easily. This could have but two possible explanations; either the dye sols were dispersions so coarse that sufficient penetration into the fiber was impossible, or the gelatin was being adsorbed with the dye, cutting down the adsorption of the dye itself and also facilitating its removal during the washing process. The latter seemed the more probable explanation and to test this the same procedure was applied to a substantive dye for cotton and one which was therefore known to be fine enough to penetrate properly and to be adsorbed strongly. Gelatin should diminish the adsorption and the fastness to washing of such a dye and methods which might be found to destroy the effects of the gelatin on the ordinary substantive dye should be effective also in making possible satisfactory dyeing with the insoluble dye protected by gelatin. This procedure had a flaw in its premise and it appears worth while to report the discovery of that flaw while the solution of the main problem is still being sought. The flaw is that at least in the case of Congo red, the substantive dye selected, gelatin does not form a protective film about the dye particles but the two stable sols exist in the mixture as independent colloidal particles, and only under special conditions such as on the addition of salts does any phenomenon such as protection make its appearance. It is found that gelatin swelling in a Congo red solution has but little power to adsorb the dye but that this adsorption is increased somewhat by the addition of suitable salts. If, therefore, the gelatin and the Congo red exist independently in the solution, this is quite a different situation from that which prevails in the case of the para red, an insoluble developed dye which may be protected from precipitation by forming it in the presence of gelatin when the particles of the dye must surely be surrounded by an envelope of gelatin. The problem of breaking such a sheath in such a way as to cause the enclosed dye to be adsorbed on the cloth is quite different from that involved in bringing about the precipitation of dye rather than gelatin from a mixture of the sols. The Congo red used in the experiments was dissolved two grams in a liter to make the stock solution, and five cc of this added t o gelatin, salt, etc., to make 2 j cc total volume, the final dye concentration being 0.04%. The gelatin was a powder from Baker and was usually dissolved to make a two percent solution shortly before use; five cc of this solution in the z j cc gave a final concentration of 0.476 gelatin. The usual procedure was to immerse the cloths for one hour, dry, divide the cloth, and thoroughly soap one half and wash it in warm water. The first experiments showed that a t room temperature pieces of cloth immersed for one hour were dyed much more deeply in the absence of gelatin than in its presence. This is in agreement with the findings of Briggsl with Briggs: J. Phys. Chem., 28, 384 (1924).
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HERBERT L. DAVIS AND JOHN W. ACKERMAN
other substantive dyes. He showed that this action of the gelatin is opposite to the action of salt and made a rough measurement of the effect. I n connection with this it is interesting to find that in the absence of gelatin, the addition of sodium chloride resulted in increasing dyeing up to a concentration of 0 . 2 5 M. Increased salt concentration caused decreased dyeing until a t 4 M approximately half the maximum amount of dye was adsorbed. The dye used in this experiment was Erie Red 4B. This decrease in dyeing from the 0.470 gelatin-dye sols did not appear when the cloths were dyed a t 70'. In the hot solutions the cloths were dyed two to three times more deeply than a t room temperature but the difference due to the gelatin was very slight. This effect of the gelatin in the cold solutions is t o be ascribed to a blocking effect whereby the gelatin rather than the dye is adsorbed on the cloth. In the hot solutions the penetration of the dye is increased and its stability is decreased with consequent increase in the amount adsorbed by the cloth. Likewise, in the hot solutions the gelatin tends to be peptized, and can thus offer less blocking effect to prevent the dye adsorption. If this is the explanation, the blocking effect might be observable even a t higher temperatures if larger quantities of gelatin were present. This was found to be true, for when cotton was dyed a t 70' in solutions containing the same amount of dye dissolved in water only or in the presence of 107~ gelatin the cloth dyeing in the gelatin system adsorbed much less dye than the one in water only. The Effect of Salts on Congo Red Dyeing I n the attempt to destroy the effect of gelatin on the dyeing with Congo red, the addition of salts was the first expedient. The effect of salts should be a dual one, on the dye and on the gelatin. As has been shown above, several salts are able to bring about the more or less complete coagulation of Congo red and, as Briggsl showed, substances thus effective may be used for the partial destabilization of the sols of such substantive dyes, producing decreasing dispersion and increasing adsorption of the dye by the cloth. The salts employed may be classified roughly into those which exert a liquefying or peptizing action on the gelatin, such as the halides and alkalies, and those which raise the gelation point, such as sodium sulphate. All of the salts employed of either class have a definite action in forcing the Congo red on cotton. But the portion of dye thus forced on by the salt, either with or without the addition of gelatin, does not penetrate SO well nor adhere so firmly as the dye put on from the aqueous sol alone. This is shown by the fact that cloths dyed in the presence of salts are found to be made up of threads which are alternately light (or white) and dark, the light places being those areas of the thread to which the dye did not penetrate merely because of the overlying thread. Threads of cloth dyed in the aqueous sol are much more uniformly dyed even though the intensity of dyeing be less than that found on the surface of the cloths in the sols containing salts. In addition to this, in many cases the extra dye forced on by salt is lost on vigorous soaping Briggs: J. Phys. Chem., 28, 368 (1924).
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in warm water so that the washed samples may show little effect of the presence of the salt while the unwashed samples show large differences. This is especially true of the liquefying salts a t room temperature, and to a lesser extent of sodium sulphate, but it is not true of the cloths dyed in the hot solutions of either type of salts. Apparently in the hot solutions the penetration into the fibers is more complete and the adsorption more irreversible. There may be also some heat coagulation of the adsorbed dye. As was shown by Briggs for other substantive dyes, there is in the case of Congo red also an optimum concentration of salt: increasing concentrations up to this point produce increase dye adsorption by the cloth, while greater concentrations of salt produce decreased dyeing, the dye being flocculated within the solution instead of on the fiber. This agrees with the work of Auerbachl who showed that the best dyeing resulted from that concentration of dye which just produced turbidity in the dye solution during the hour of immersion. Potassium iodide, which has such a well-known peptizing effect on gelatin was one of the first salts used and its effects are typical of the liquefying group of salts. In the absence of gelatin the first addition of potassium iodide (one gram in the 2 5 cc system) produced a marked increase in the amount of dye put on the cloth in one hour of immersion a t room temperature. Two, three, or four grams of the salt gave about the same increased dyeing as shown by one gram; but, while the one gram sample showed only some turbidity, the remaining systems showed increasing coagulation and precipitation of the dye in the solutions during the hour a t room temperature. Sols made as those just mentioned but containing five cc of the 2 % gelatin in each all show less dyeing than the corresponding sols without gelatin and the difference produced by the first addition of potassium iodide is less than in the absence of the gelatin. The addition of the gelatin is sufficient to prevent the precipitation of any dye in the second set. In both series, soaping thoroughly in warm water removed practically all the excess dye forced on by the salt and while the soaped gelatin-sol dyed cloths are all lighter than the corresponding cloths dyed in the absence of gelatin, the soaped cloths in each series whose dyeing was aided by potassium iodide are of about the same shade as the blank (no salt) in each series. The explanation of these phenomena is that potassium iodide destabilizes the Congo red making it more readily adsorbed on the surface of the cloth, The dye thus forced on is too coarse to have penetrated far or to be as strongly adsorbed as dye taken up in the absence of salt and is, therefore, less fast to washing. Obviously, the dye adsorbed on the surface will be more effective in making the cloth appear well dyed than that which has penetrated further but the latter which is adsorbed within the fiber will be better protected from removal by washing or rubbing. There is also seen the effect of the gelatin in being adsorbed on the surface and thereby covering it or portions of i t so that the dye can not be there adsorbed and must diffuse through to and within the fiber. The potassium iodide therefore, has two effects. It tends to force Kolloid-Z., 30, 166 (1922)
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HERBERT L. DAVIS AND JOHN W. ACKERMAN
the dye out of solution and in the absence of gelatin on to the cloth. It also tends to liquefy the gelatin or to send it into the solution phase as against its being adsorbed on the dye or on the cloth. The fact that the cloths dyed in the gelatin sols are all lighter and that the remaining solutions in that series are clear and without precipitated dye shows that the gelatin is not all peptized from the cloth but still exerts some blocking effect and that the destabilized dye is forced out not only on the cloth but also on the peptized gelatin which is still able to keep it suspended and to prevent its adsorption on the cloth. Sodium iodide was found to haye a similar effect to that shown by potassium iodide in dyeing from the gelat'in-dye s o h A comparison was made between sodium iodide and sodium sulphate as these are representatives of the two classes of salts. This comparison shows that sodium iodide is more effective than the equivalent concentrations of sodium sulphate in forcing dye on cotton from gelatin-dye sols because the iodide tends to precipitate the dye but peptize the gelatin while the sulphate tends to precipitate both dye and gelatin on the cloth and thus accentuate the competition between them. Final systems were made up to contain 0.8 and 1.6 S IiaI as well as 0.4, o.8,and 1.6 N Pr'a2SOa,eachsol being also 0 . 4 7 ~gelatin and 0 , 0 4 7 ~Congo red. The cloths dyed in the presence of sodium iodide were darker (both original and washed samples) than any of those dyed in the presence of the iSazSOcsystem was the best sodium sulphate. The cloth dyed in the 0.4 I in that set and was not much inferior to that dyed in the 0.8 S Na1,'while the latter was markedly better than the cloth dyed in 0.8N SazSOa. The 1.6 N NazS04is distinctly beyond the optimum concentrat.ion, producing almost complete flocculation of the dye in solution and dyeing only a little better than the blank containing dye and gelatin only. Both the sodium iodide sols grew turbid during the hour while only the most concentrated sulphate sol did so. It appears, therefore, that the sodium sulphate tends rather to force the adsorption of the destabilized dye on the gelatin rather than on the cloth and that the dye-gelatin adsorption complex is not strongly adsorbed by the cloth for the washing carries off very little dye from the cloths dyed in the presence of the sodium sulphate. I n other words the effect of the sodium sulphate is to decrease the dispersion and to increase the adsorptive power of both dye and gelatin for concentrations below that which causes coagulation. On the other hand the effect of the sodium iodide is to decrease the dispersion of the dye and cause it to become more adsorptive but at the same time the peptizing action on the gelatin is one of increasing dispersion and greater stability in the liquid phase and, therefore less adsorptive power. The result is that the Congo red micelles adsorb each other and precipitate instead of adsorbing gelatin and remaining suspended. This is shown by the fact that the sodium iodide samples (in common with those of the other liquefying salts a t room temperature) lose considerable dye on washing. It is thus seen that a very important factor in the washing fastness of dyes is the size of the particles of dye on the cloth. The peptized gelatin being little adsorbed permits growth of the dye particles to such size that they are readily
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washed off, while the greater adsorptive power of the gelatin in the presence of sodium sulphate prevents the development of such large aggregates and the loss on washing is less. The difference here can not be ascribed to a difference in the amount of adsorption of the gelatin on the cloth which might then carry off dye with it on washing for in that case the sulphate samples should have more gelatin on the cloth and wash off worse; actually they wash off less than the iodide samples. Therefore, if this factor enters a t all, it is secondary to the factor of particle size and adsorbability.
Conclusions I. When sols of gelatin and of Congo red are mixed the two sols appear to continue to exist independently. Gelatin adsorbs Congo red so slightly from solution as to render im2 possible any protection in the ordinary sense. 3 . Congo red, in the absence of gelatin, behaves as a sol intermediate between the lyophile and the lyophobe types. It is precipitated by sodium and potassium halides and by various other salts. 4. I n the presence of gelatin this precipitation by salts is greatly diminished or entirely prevented. This apparent protection by the gelatin is shown to be due to a decreased stability and decreased dispersion of the dye by virtue of which it is more strongly adsorbed by the gelatin. 5. Salts which liquefy gelatin and salts which precipitate gelatin have different effects on the dyeing of cotton cloth from the mixed sols of gelatin and Congo red. 6. Both types of salts increase the adsorption of dye from the mixed sols, the liquefying salts being more effective because of their action in peptizing the gelatin.
Cornell Uniuersity.
