ON T H E THEORY O F PEPTISATIOK BY K. C. SEN
Several papers have been published which intend to give an outline of the general theory of peptisation of a substance. In general it is assumed that adsorption of the peptising agent precedes the formation of a colloidal solution. If we have a precipitated substance, then in order to peptise it, it is necessary that the cohesive force which exists between the individual particles of the agglomerated substance for one another should be made less. Thus according to Bancroftl, “peptisation consists in the disintegration of particles so that they form a colloidal solution. We get a permanent colloidal solution2 whenever the particles are small enough to be kept in suspension by the Brownian movements and i n some way are prevented from coalescing. Freundlich3 has postulated that all adsorption is accompanied by a lowering of the surface tension of the adsorbing phase. The theoretical deduction is unsound because the Gibbs theorem applies explicitly to true solutions and not to suspensions. On the other hand, Freundlich’s assumption seems to be true experimentally in all cases which have been studied from this point of view. If we accept Freundlich’s generalization as true empirically, a theory of peptisation follows a t once. Any substance which is adsorbed by a second will lower the surface tension of the second substance and will therefore tend to disintegrate it, in other words, to peptise it. If every adsorbed substance tends to peptise the adsorbing substance, we may expect to get peptisation by a solvent; peptisation by a dissolved non-electrolyte; peptisation by an ion; peptisation by a salt; peptisation by a colloid”. There is no doubt that Baqcroft’s conclusions are perfectly general if we assume lhat adsorption always lowers the surface tension be tween the adsorbing substance and the medium. The less the interfacial tension, the easier it should be to disintegrate the substance in the particular medium. Hence normally there should be some connection between peptisation and the amount of adsorption, though Bancroft considers it to be not necessary. “Since the adsorption depends on the surface and since peptisation involves breaking down the cohesion of particles, there is no necessary connection between the amount of adsorption and the ease of peptisation”. This reasoning may well apply in the case of porous substances such as charcoal, but it is difficult to follow in the case of, say, precipitated hydroxides like iron hydroxides or albuminium hydroxide. Thus in these cases it is no doubt true that “the colloidal mass which has only just agglomerated, can often be peptised without difficulty. If the coalescing surfaces are allowed to set or if the substance is heated so that sintering takes place, peptisation becomes extremely difficult”, ~~
J. Phys. Chem. 20, 8 j (1916). J. Phys. Chrm. 18, 552 (1914). “Kapillarchemie” 52, 154 (1909); Patrick: Z. physik. Chern. 86, 545 (1914).
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but along with this fact it should be observed that the amount of adsorption of the peptising substance is also diminished. This will be illustrated with an example from the results obtained with hydrated ferric oxide and arsenious acid, TABLE I S a t u i e of t h r hydroxide
Amoiint of :idsorption
Freshly precipitated dark brown
80%
Same sample after four months under water
40%
Air dried Sample
10%
Remarks
Peptisation extremely easy With low concentration of the acid, there is no peptisation Can be slightly peptised with high concentrations of the acid
It will be observed that with the change in the physical nature of the substance, peptisation becomes difficult and along with this, the amount, of adsorption also decreases. It is well known that increasing the concentration of the solute means an increased adsorption by the adsorbent. It is therefore expected that with higher concentrations of the arsenious acid, more of the hydroxide would be peptised. This view is confirmed by the results given in Table 11. TABLE I1 Adsorbent-freshly Conc. of Xs203 in grms. per IOO (
0.09468 0.12624
0.15780 0.18936
~
8
precipitated ferric hydroxide. wt. amount of adsorption in grams 0 ' 07448 0.08748 0.09228 0.09846
= 0.264 j
gr.
Remarks
No colloid No colloid Supernatant solution slightly turbid SuFernatant solution very turbid.
These results therefore show that there is some connection between Lhe amount of adsorption and ease of peptisation. It is also well known thai with the addition of greater and greater amounts of the peptising agents the colloid reaches a maximum point in stability, after which the stability actually diminishes in some cases'. Hence peptisation of a substance with increase in the addition of the peptising agent would also reach a limit in cases where Sen: J. Phys. Chem. 28, 1029 (1924); Sen and Mehrotra: Z. anorg. Chem. 142, 315 ( 1925 ) .
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there is no soluble salt formation a t higher concentrations of the peptising agent. This effect is due to the ion charged oppositely to the stabilising ion. This phenomenon has also been realised in the peptisation of ferric hydroxide by arsenious acid. Hence when ion peptisation is taking place, the amount of the substance which can be peptised is limited owing to the presence of oppositely charged ions in the solution, and to the possibility of the formation of a soluble salt. Thus in the peptisation of alumina by acetic acid, Bentley and Rose1 state that, “equal portions of a specimen of the hydrated oxide containing about four molecules of water, were treated with equal volumes of acetic acid varying by tens from 99.8 percent acid down to 40 percent and by ones down to one percent. It mas found that nbove 40 percent acid scarcely any hydrosol was formed, since hydrochloric acid did not produce coagulation. Normal aluminium acetate was the product formed. From 40 percent down, the amount of hydrosol steadily increased as did the ease with which the material dissolved until a 4 percent acid was reached. Below 4 percent the material dissolved only partly, the remainder swelling up to an almost transparent mass, which after prolonged treatment, formed an opalescent and most easily coagulated s o h tion. For practical purposes, we consider about 8 percent the most favorable strength”. It will appear therefore that when the same substance is used for peptisation, favourable conditions are a high degree of adsorption and a suitable concentration of the peptising agent. When however different substances are used with the same peptising agents, a marked specificity is observed. Table I11 the results obtained experimentally on the peptisation of different hydroxides by various acids and some salts are summarised.
TABLE 111 Peptising agent Hydrochloric acid Acetic acid Propionic acid Butyric acid Benzoic acid Amido benzoic acids Citric acid Tartaric acid Oxalic acid Sulphuric acid Arsenious acid Sodium arsenite Sodium phosphate Sodium citrate J. Am. Chcm. Hoc. 35, 149:) (1913)
Substance A1(OH)3
Cr(OH),
43
-
+-
__
++ ++ + -t++
+__
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K. C. SEN
I n Table 111 -Lt means very readily peptisable; -k means that the peptisation takes place only with high concentrations of the peptising agent whereas - means that no peptisation takes place under ordinary conditions1. The hydroxides used were all freshly prepared and from the results given in another paper it will be found that all these acids are greatly adsorbed by the hydroxides. The main point which has come out from these peptisation experiments is that though adsorption always precedes colloid formation, the peptising powers of diffwent acids are not necessarily proporiional to the amounts of their adsorption as measured by the decrease in the hydrogen ion concentration of the solutions. Also the same acids behave differently in the case of different hydroxides. Thus chromium hydroxide could not be peptised by the majority of the acids used though this hydroxide has the greatest adsorbing power. Aluminium hydroxide could not be well peptised by arsenious acid, though ferric hydroxide is peptised very readily by arsenious acid. From equivalent Concentrations, acids like oxalic, sulphuric, etc. are more adsorbed than acetic, benzoic or hydrochloric acids, but hydrochloric or acetic acid peptises the hydroxides of iron and aluminium while the former acids can hardly peptise them. On the other hand, benzoic acid does not peptise ferric hydroxide, but not only benzoic acid, but other derived benzoic acids such as oxybenzoic acid, o-m-p amido benzoic acids etc. can peptise aluminium hydroxide very readily. In the case of monovalent acids, the rule is approximately followed-namely, the greater the adsorption the greater is the peptising power. The exceptional behaviour of the dibasic and tribasic acids is due to the presence of divalent and trivalent negative ions which prevent the formation of a positively charged colloid. There are other differences manifested when salts are used. Thus sodium arsenite, sodium phosphate or sodium citrate can stabilise a suspension of ferric hydroxide with a negative charge; but they have no action on either aluminium hydroxide or chromium hydroxide, though chromium hydroxide adsorbs sodium arsenite to a much greater extent than ferric hydroxide does. Ib appears therefore that we have to recognise some selective tendency in colloid formation. Simply amount of adsorption is not a measure of the peptising power of an electrolyte, and the nature of the adsorbing surface is also important, The general conclusions of Bancroft have already been given. It will be noted that according to the theory, peptisation mrzy be brought about by any substance which is adsorbed by an adsorbing substnnce. Thus the peptising power of water on glass at higher temperature, that of a mixture of alcohol and water on zein etc. may be cited as instances of peptisation by the medium. Cases of ion peptisation and peptisation by means of a colloid are well known. Sugar and several such non-electrolytes prevent the precipitation of ferric hydroxide in solution, and Bancroft considers it to be a case of peptisation by non-electrolyte. Though no case of peptisation by a dissolved undissociated
* Compare however Sen and Dhar: Kolloid-Z. 33, 193 (1923); J. Phys. Chem. 27, 376 (1923).
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salb has been studied, he considers that such cases will undoubtedly be found. In this paper I shall only deal with the peptisation of substances by ions, by colloids and by non-electrolytes. Since the experiments have been mainly done in aqueous solutions, I shall limit the study to colloids which are dispersed in water, and as such the cases of organic liquids will not be considered. In the first page I have italicised a few words from the quotation taken from Bancroft’s paper. Since by peptisation we mean that a stable colloidal solution is obtained, it is obvious that disintegration of a precipitate must be followed by the stabilisation of the particles against gravitational force and the surface forces on the particles. This stabilisation after disintegration is a t least as important as the process of disintegration itself, and no study of peptisation is complete without a concomitant study of the stability relations of the peptised substance. A scrutiny of the sources of stability of these peptised substances reveal certain interesting facts which seem to the author to change some of our conceptions about the phenomenon of peptisation by different substances.
It is well known that the existence of a Brownian movement alone does not make a colloid stable and a suitable surface film’ is necessary to prevent the coalescence of the particles. I n the case of pure hydrosols, this film is almost entirely of electrical origin. This fact has been recognised by almost all the colloid chemists. Thus Hatscheli2observes: “It follows from Perrin’s investigation that particles in Brownian movement-like molecules of a gas or a dissolved substance-tend to fill the space in which they are contained according to definite laws. The movement must therefore be considered as one of the factors which keep a sol stable, but is not by itself, sufficient to account for its stability, as particles showing moderate Brownian movement may still settle with comparative rapidity. The stability is intimately connected with the electric charge, to which reference has already been made. It may be said generally that any substance in contact with water and many other liquids, assumes an electrical charge, the origin of which is not definitely explained. Most substances become negatively charged in contact with water. The charge can be varied and even reversed by the additon of electrolytes and may become zero at suitable concentrations. In this condition, as shown by Burton and by Hardy, sols are particularly unstable, and tend to precipitate. “It need be hardly mentioned that the electric charge is not confined to submicroscopic particles, but is found equally on the particles of a coarse suspension. It has also been known for a considerable time that the speed of settling in many suspensions -which settle in any event-can be increased by the addition of electrolytes. The greater sensitiveness of the highly dispersed systems must be ascribed to the very much greater charge due to the enormous increase in surface. At the same time, while the existence and thestabilisBancroft: J. Phys. Chem. 18, 552 (1914). ‘:An Introduction to the Study 01the Physics and Chemistry of Colloids”, pp. 27-29 (1913 I.
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K . C. SEN
ing influence of the caharge is fully established’, it must be said that the origin of the charge and the mechanism of its action is still rather obscure”. It is thus evident that the existence of electric charge which, among others, is the source of stability of a colloidal solution and is equally important as the Brownian movement of the particles must be accounted for in any theory of peptisation. Since electrical charge is necessary for the stabilisation of the sol, it is evident ihat the peptisation is not complete where simply the disintegrating effect of the peptising substance has taken place. To make the disintegrated particles stable as a suspension in the medium, the particles must get $he necessary amount of surface charge. Two processes are therefore involved in the phenomenon as pictured above: ( I ) The decrease in the interfacial tension between the particles and the medium owing to an adsorption of the peptising substance by the adsorbing mass whereby the cohesive force between the individual particles are diminished and ( 2 ) the formation of an electrical film on the surface of the particles which counteracts the effect of surface attractions of small particles to form big ones. Since it is now generally assumed that the charge on colloidal particles and surfaces is due to a preferential adsorption of some ions from the medium, it is easy t o account for the electric charge on the particles once they have been disintegrated through the adsorption of the peptising substance. If these considerations are applied to the case of peptisation of a precipitate by ions, the phenomegon becomes simple and the two processes become actually identical. If every adsorbed substance lowers the surface tension of the adsorbing siibstance, then when ions are adsorbed preferentially, both the disintegration of the mas? and the electrificaiion of che surface of the particles become probablr. The same reasoning would apply in the case of peptisation by colloids. In the case of peptisation by non-electrolytes and by undissociated neutral salts the explanation is however difficulh and we have to assume that disintegration and electrification of the surfaces are two distinct and separate processes. Since non-electrolytes would disintegrate the precipitate and preferential adsorption of ions from the medium would stabilise the suspension, it is diEcult to say which is really the peptising agent. We may however assume in the first instance that only ions are effective in peptisation. By peptisation is meant the final stage at which a precipitate or a precipitating substance is obtained as a stable colloidal solution. Assuming this as a working hypothesis, I shall deal with the effect of solvenl, of non-electrolytes, of ions and salts on the stabilisation of %I colloid. Cases of ion peptisation are well known in the literature. When one ion of an electrolyte is adsorbed more than the other, it will tend to peptise the adsorbing material and to give rise to a colloidal solution containing positively or negatively charged particles according to the ion adsorbed preferentially. It is obvious however that there may bc non-electrical suspensions, e. g . !Yo. Pauli’s electrolyte-free albumin, where the st,ability is probably due to t,he high solvation of the micellae. The conditioris of stability of these suspensions are rather obscure, and in the articlc only thaw colloids whiCh are usually clectrically charged have been considered.
THEORY O F PEPTISATION
,
I539
Since usually the higher the valence of an ion, the greater is its tendency to become adsorbed, it follows that higher valent ions will have a greater peptising power than ions of lower valence. There are however exceptions to this, namely in some cases, monovalent ions are more adsorbed than higher valent ions. Since hydroxides of metals are known to be good adsorbents of acids and alkalis, it is expected that ionic peptisation would be easy in these cases. Several instances of peptisation of metallic hydroxides by acids are already known in the literature. A Muller1 has prepared colloidal solutions of aluminium, iron, cobalt, thorium and yttrium oxides by peptisation with dilute hydrochloric acid. Bentley and Rose2 peptised alumina with acetic acid. In this paper I have shown that both ferric and aluminium hydroxides can be peptised by suitable acids, and the nature of the charge on the particles depends on the nature of the ion adsorbed preferentially3. Among peptisation by means of salts, Muller’s4experiments on the stabilisation of thorium hydroxide by thorium nitrate and zirconium hydroxide in zirconium nitrate solution may be mentioned. Szilardj peptised a number of rare earth hydroxides by means of the chlorides or nitrzles of the same rnetAs. Both chromic chloride and ferric chloride appear to dissolve a certain quantity of the respective hydroxides, and we know that these apparently clear sohtions contain peptised hydroxides. In a previous paper6 it has been assumed that the free acid is the real peptising agent, but it appears that it is almost impossible to decide experimentally whether the metal ions are also active or not7. I t is probable that both the hydrogen ion and the metal ion are simultaneously adsorbed by the hydroxide in some cases. In the case of peptisation by acids, it is usually considered that the hydrogen ion is the peptising agent. There is no reason therefore why in the peptisation of hydroxides by alkalies, hydroxyl ionq would not be considered to be the peptising agents. Both chromic oxide8 and copper oxideg are peptised by caustic alkali. HantzschlO considers that beryllium oxide is peptised by caustic potash and apparently so is cobalt 0xide.l’ An alkaline solution of zinc hydroxide may be colloidal,’2partly colloidallJormay be a definite zincate.14 Svedberg: “Die Methoden ziir Herstellung kolloider Losungen anorganischer Stoffe”, 400 (1909). .J. Am. Chem. Soc. 35, 1490 (1913). Sen and Dhar: Kolloid-Z. 33, 193 (1923); J. Phys. Chem. 27, 376 (1923). Ber. 39, 28j7 (1906); Z. anorg. Chem. 52,316 (1907). J. Chim. Phys. 5, 488, 636 (1907). Kolloid-Z. (1925). Compare TTeiser: J. Phys. Chem. 24, 310 (1920). Nagel: J. Phys. Chem. 19, 331, 569 (1915): Fischer and Herz. Z. anorg. Chem. 31, 3j2 (1902). Loew: Z. anal. Chem. 8 , 463 (1870); Fischer: Z. anorg. Chem. 40,39 (1904). Z.anorg. Chem. 30,289 (1902). l1 Tubandt: Z. anorg. Chem. 45,368 (19oj). l 2 Hantzsch: Z. anorg. Chem. 30, 289 (1902). l 3 Fischer and Hwz: Z. anorg. Chem. 31, 352 (1902). Klein: 2. anorg. Chem 74, 157 (1912).
’
Rancroft' considers that freshly precipitated zinc hydroxide is peptised by alkali; but the solution is very unstable, the zinc hydroxide often coagulating inside of half an hour. The relatively small amount of zinc remaining in solution is present chiefly or entirely as sodium zincate. Freshly precipitated alumina niay be partly peptised2 but the alkaline solutions cont'ain some sodium aluminate3. Alkaline copper hydroxide solutions present some peculiar features. That these solutions are more or less unstable is evident from the fact that light of suitable wave-length will decompose Fehling's solution4wit8hthe precipitation of cuprous oxide. When an ammoniacal solution of copper sulphate is exposed to strong sun-light, decomposition also t'alies place. Much more interesting is the fact that all alkaline solut'ions of copper hydroxide are blue in colour. Reference t'o this behaviour has already been drawn in a previous paper6. It is generally believed that alkaline solutions of tartrates dissolve cupric hydroxide forming a complex negative ion and the colour of these solutions is blue6. The same sort, of blue colour is obtained when cupric hydroxide is dissolved in ammonia, or caustic soda, as also in the case of peptisation of cupric hydroxide by means of alkali in presence of glycerol or sugars. I t is very difficult to explain the production of the same colour on the view of complex formation, as the same colour is developed by so many different' reagents. Since, however, the prod.uction of this blue colour is dependht upon an excess of hydroxyl ions, it' appears probable that the colour is due to the negatively charged colloid,al copper hydroxide. As a matter of fact, Grimaux7thought long ago that in ammoniacal copper oxide sollidions part of t'he copper oxide is colloidal and part dissolved. The peptisat,ion of met'allic sulphides by means of hydrogen sulphide is an inst'ance where the negative ion is more adsorbed than t'he easily adsorbable hydrogen ion, thus giving a negative charge to the particles. Thus colloidal copper sulphide8,cadmium sulphideQ,zinc sulphide'O etc. can be easily prepared by suspending the freshly precipitated and well washed sulphid.es in water and passing a current of sulphuretted hydrogen t'hrough them. The excess of hydrogen sulphide ma.y then be removed by a current of hydrogen, but it is not possible to free the colloids from a,dsorbed hydrogen sulphide". J . P h y . Chem. 20: 99 (1916). Mahin, Ingraharn and St,ewart,: J. Am. Chem. Soc. 35, 30 (1913). 3Herz: Z. anorg. Chem. 25, 155 (1900): Hantesch: 30,289 (1902); Ritbenbnuer: 30,331 (1902); Fischer and Herz: 31, 355 (1902); Slade: J. Chem. SOC.93, 421 (1908); Z.anorg. Chem. 77, 457 (1912); Trans. Faraday Soc. 10: 150 (1914); Rlum: J. Am. Chrm. 80c. 35, '499 (1913). Bennett: J , Phys. Chem. 16, 782 (1912); Lcighton: 17, 2 0 5 (1913): Ryk: Z.physik Chem. 49, 659, 679 ( 1 9 ~ 4 ) . Dhar and Sen: lor. cit. c. Masson and Steele: J. Chem. SOC.75, 7 2 5 (1897). Compt. rend. 98, 1434. Spring: Rer. 16, 1142 (1883). Prost: J. Chem. Soc., 54,653 (1888). I o Winssinger: Bull. (3) 49, 452 (1888). l1 Linder and Pict,on: J. Chem. Soc. 61, 116 (1892).
THEORY O F PEI'TIBATIOK
1541
Since from an acid solution either the positive or the negative ion may be adsorbed preferentially a t certain concentrations of the acid, it is evident that in studying adsorption we must take note of the simultaneous adsorption of both the ions from solution. The preferential adsorption of one ion stabilises the colloid, but with the increase in the concentration of the added electrolyte, the stability usually reaches a meximuin owing to the increased adsorption of the other ion of the added electrolyte. 'iS'iLh the further addition of the elertrolyte the colloid mill coagulate, and a t the coagulation point the stabilising ions adsorbed by the colloid are just neutralised by the adsorption of the oppositelv charged ions of the coagulating electrolyte. This being the usual conception of the coagulation process, it is evident that the adsorbed salt does not function as a peptising agent. Hence it becomes difficult to follow Bancroft's views as to the importance of the adsorbed salt in the phenomenon of peptisation. As the facts however have an important bearing on the theory, I will quote from Rancroft's paper to some extent. "The possibility of peptisation by an adsorbed salt seems to have been pretty generally overlooked in the books on colloid chemistry, presumably because an increase in the concentration of a peptising salt is npt to cause coagulation. Theoretically the matter is quite simple. lye start with an ion peptisation because one icn is adsorbed more than the other. TTith increasing salt concentration we reach the point where the adsorption of the first ion varies but slightly with the concentration. The adsorption of the second ion continues to increase relatively to the first ion until we get what has been called neutralisation of the adsorbed ion', and consequently coaguiation. At the same time the adsorbed salt is tending to peptise the substance; but if its peptising action is relatively small, there may be quite a large range of concentrations over which the ion peptisation has ceased to be effective and the salt peptisation has not begun to be effective. With still greater salt concentration, we should expect to get salt peptisation; but a number of disturbing factors may come in. The salt may not be sufficiently soluble a t the temperature of the experiment or it may react with the substance to be peptised. If we increase the hydrochloric acid concentration with the oxides of aluminium, iron, cobalt, etc., me finally get the chlorides of these metals in true solution. If we increase the caustic soda concentration with silicic acid, we consider that me get sodium silicate in true solution, If we increase the potassium bromide concentration with silver bromide we say that we get a complex salt in true solution. It may be that we are wrong in this and that we are getting peptisation in some of these cases. For years we thought that the so-called basic chlorides were definite compounds forming true solutions, whereas now we know that many of them are not. With silver iodide and a concentrated silver nitrate solu tionZwe apparently get a definite compound, zAgN03 , BgI; with silver bromide also a definite compound AgN03. AgBr; but il, is not 'Bancroft: J. Phys. Chem. 19, 363 (1915) Risse: A n n . 111, 39 (1859).
1542
K.
c.
SEN
probable that there is any such compound as 18AgN03.ilgC1, so it may be that here we have a case of peptisation by an undissociated electrolyte. “It is possible that von Weimam’s peptisation of cellulose1 by salt solutions may come under this head. Three grams of cellulose were heated with about IOO cc concentrated salb solution and were thereby peptised, the mixture usually cooling to a jelly. With XaI, CaI2, SrI2, CaBrz, Ca(SCN)2and Ba(SCN)2 peptisation took place at atmospheric pressure. With KaCl a temperature of 170’ snd a pressure of 8 atm. were necessary, and incipient decomposition seemed to take place. Deming2 peptised cellulose with salts dissolved in acid solutions. In all these cases of possible peptisation by undissociated salts, there may be an ion peptisation and a water peptisation superposed which, of course, complicates matters considerably. Oxides of mercury or less noble metals are adsorbed by mercury. I do not know any conditions under which they will peptise mercury unassisted, though it is very possible that this might take place a t higher temperatures. If we disintegrate the mercury mechanically, it is possible to obtain a colloidal solution and here we unquestionably have an undissociated salt”. This would be undoubtedly true if we can prove it, but unfortunately no case exists in which peptisation by undissociated salt has been obtained. The assumption that an undissociated salt may disintegrate a substance follows from the theory of Freundlich, but the adsorption of undissociated salt cannot stabilise a suspension because it does not confer any electrical charge nor will it usually form a suitable surface film on the particles. As such, theoretically no case would be found where a precipitate has been peptised by the adsorption of undissociated salt. When metals are disintegrated by means of high tension electric current, we ought to get a colloidal solution very easily. As a matter of fact, a number of precautions are due to be taken otherwise the particles would soon settle. A trace of alkali in bhe medium facilitates the formation of stable colloidal solutions of copper, etc. Beans and Eastlack’ conclude that the stability of the colloidal solutions of gold is due to the adsorption of OH’ ions from water. It is certainly true that with the increase in the concentration of the peptising agent, say hydrochloric acid in the case of alumina, we get a true salt in solution; that is chemical action takes place. Silicic acid can be peptised both by hydrogen as well as by hydroxyl ions. When caustic soda is added in excess, there is every probability of the formation of sodium silicate. There is nothing abnormal in these phenomena, The difficulty of explaining these lies in the way in which we have sharply different;ated the phenomenon adsorption from chemical reactions. I n another paper I have shown of that there is practicelly no difference in the mechanism of selective adsorption and chemical reaction. When dilute hydrochloric acid acts on alumina, considerable adsorption of the acid takes place. With increasing concentration of the acid, the adsorption also increases, hut after 1 2
Kolloid-2. 11, 41 (1912). J. Am. Chem. SOC. 33, 1j15 (1911). J. Am. Chem. SOC. 37, 2667 (1915).
THEORY O F PEPTISATION
I543
some time we get aluminium chloride in solution. Since adsorption gradually merges into a definite chemical change, we may consider that adsorption is preliminary to chemical reaction. If this is conceded, and there are numerous experiments to support it, there is no difficulty in understanding why in some cases we get true salt formation and not peptisation with the increased concentration of the electrolyte. Since peptisation is an intermediate condition between a precipitate and molecular solution, we can get all three states according to the amount and nature of adsorption. Thus if we have a positively charged silicic acid sol, we can coagulate it by the addition of alkali due to the adsorption of OH’ ions. On increasing the concentration of alkali, the silicic acid will be peptised and in this condition it mill be associated with a varying quantity of alkali. On further increasing the concentration of the alkali, we get a molecular solution of sodium silicate. The third stage may or may not be obtained according to the nature of the final product of the chemical change. If the final product is soluble in water, we may get it in true solution. Lottermoserl showed that a stable colloid solution of silver halides can be only obtained when either silver nitrate or the halide of potassium is in excess in the solution. The nature of the charge was different and it indicated that the stabilisation was due to the preferential adsorption of either silver or the halide ions. If me increase the amount of one reagent, we may get or may not get a definite salt formation and this does not affect the theory in any way. For the explanation of the peptisation, the sufficient condition would be to have the necessary amount of adsorption for the disintegration and the stabilisation of the particles. The stabilisation is naturally due to the adsorption of ions and as such there can be nothing gained by postulating that undissociated salts have any effect on these systems. Simple ion adsorption would explain the stability of these suspensions. From this point of view I would consider the peptisation of cellulose by salt solutions as cases of ion peptisation. Since cellulose appears to be a good adsorbing agent2, it is probable that preferential adsorption of some ions takes place when it is heated with salt solutions. This ion peptisation would be marked in an acid or alkaline solution. This was found by Deminga mho observed that working in an acid solution facilitated peptisation. It is of course not possible to say whether any preferential adsorption takes place from the medium when only salts are used. The H’ or OH’ ions of water may have something to do with it. When glass is peptised by water at high temperature, we have undoubtedly a case of ion peptisation since the resulting sol is electrically charged. Gelatine can be liquefied by the addition of potassium iodide. Here the peptisation is due to the adsorption of iodide ions. The peptisation by means of another colloid admits of the same explanation as in the case of ion peptisation. Since a colloid is usually charged, it can J. prakt. Chem. ( 2 ) 68, 341 (1903); 72, 39 (190j); 73, 374 (1906); Z. physik Chem. 62, 371 ( 1 9 ~ 8 ) . Leighton: J. Phys. Chem. 20, 188 (1916). J. Am. Chem. SOC. 33, 1515 (1911).
= 544
K . C . SEN
stabilise a suspension of another substance provided it is adsorbed in suitable amounts, though in many cases the stabilisation seems to bedue toion peptisation. Thus hydrous chromium oxide is an alkali-soluble colloid1. Hydrous chromium oxide adsorbs the hydrous oxides of iron, nickel, cobalt, manganese, and copper; and hence peptises them to a certain extent. When chromium salt is present in large excess relatively to the iron salt, no iron oxide is precipitated; when the iron salt is present in excess, the chromium oxide remains in the water phase, the latter becoming colourless. The usual explanation is that colloidal chromium hydroxide adsorbs the iron hydroxide, and the resulting complex is stable or unstable according to whether the chromium salt concentration is very high or not. There is no objection to this, but it complicates matters because chromium hydroxide is itself stabilised by a third substance, namely hydroxyl ions. One wonders why the same substance, namely the hydroxyl ions, is not given the credit of peptising the small quantity of ferric hydroxide. That alkali does not peptise pure ferric hydroxide easily is no difficulty. I have found experimentally that chromium hydroxide adsorbs more alkali than hydrous ferric oxide. We can therefore suppose that the ease of peptisation with hydrous chromic oxide is due to the fact that greater amounts of hydroxyl ions are adsorbed by it, and hence though no peptisation occurs with hydrous ferric oxide under the same conditions, the latter can be peptised if hydroxyl ion adsorption can be increased. When alkali is added to a mixture of iron and chromium salt, both the hydroxides will tend to precipitate, and in doing so, will adsorb each other. The resulting complex will have different adsorption value from that of the individual substances. If the iron salt is much less, then the complex will have an adsorption value almost equal to that of the pure chromium hydroxide of same weight, and so the amount of adsorption of hydroxyl ions by the complex will mean an adsorption far in excess to that which could be adsorbed by the ferric hydroxide if it was present alone. The alkali will therefore peptise both the hydroxides, In case the iron is in excess, the resulting complex adsorbs as if the whole substance is pure iron oxide, and hence alkali does not stabilise the precipitates. As a matter of fact, the amount of ferric hydroxide which can be kept in suspension by colloidal chromium hydroxide is relatively very small. It is also reported that colloidal copper oxide peptised by ammonia causes the peptisation of hydrous chromic oxide by ammonia2. A similar explanation will apply to this case also. The peptisation of substances by means of the so-called water-soluble colloids may be explained in the same way. It is probable that these colloids are really cases of ion peptisation and not water peptisation. There is no question about casein, which is insoluble in water and is only peptised by hydrogen and hydroxyl ions. Bancroft3 prefers to consider albumin as a case of ion peptisation. There is no reason therefore why gelatine or tannin should not be considered also as cases of ion peptisation, the only assumption being that the
3
Kagel: J. Phys. Chem. 19, 331, 569 (1915). Prnd'homme: J . Chem. 8oc. 25, 672 (1872). J. Phys. Chem. 19, 349 (1915).
THEORY O F PEPTISATION
154.5
range of instability is very small. That gelatine is functioning as an ion-peptised colloid at, least when it stabilises other suspensions seems to be probable from some results of Loeb’, who found that at the isoelectric point of gelatine, it will not stabilise a suspension of collodion particles, but will do so only in presence of salts. When positively charged gelatine is mixed with negative tannin, precipitation is more marked, and here we are considering the effect of the mutual adsorption of two ion-peptised colloids. When ammoniacal gelatine is added to a ferric hydroxide colloid, precipitation occurs, whereas there is no precipitation if the gelatine and ferric hydroxide sols are mixed before the ammonia is added. The explanation is not difficult. Ammoniacal gelatine is negatively charged and hence by the addition of this negative colloid to the positive ferric hydroxide sol, we get charge neutralisation due to the mutual adsorption of the two sols. ll‘hen however simply gelatine is added to the ferric chloride solution, in presence of acid, it becomes positively charged. By the addition of small quantity of ammonia a positively charged colloid complex of ferric hydroxide and gelatine is formed and when excess of ammonia is added, the charge on the colloid is reversed. The whole phenomenon seems nothing but a case of charge reversal of colloidal ferric hydroxide. An exactly analogous case will be given. IYhen alkali is added to a mixture of ferric chloride and glycerol or sugar, no peptisation is obtained. If a mixture of sugar and alkali is however added to a ferric chloride solution immediate precipitation of the hydroxide takes place. TTe know definitely that in presence of glycerol or sugar, hydrous ferric oxide can remain peptised either as a positively charged or a negatively charged colloid, depending upon the concentration of the added alkali. The only difference between the two cases is that in one we have sugar which is a non-electrolyte, and in the other gelatine which js either an uncharged substance or an ion peptised colloid. It is also well hnomn that gelatine behaves as a positively charged colloid in acid solution and as a negatively charged one in presence of hydroxyl ions. Thus Billitzer2 finds that gelatine precipitates such nega tire colloids as antimony ,sulphide and arsenic sulphide in acid or neutral solution, but does not precipitate positively charged sols such as hydrous ferric oxide. Positively charged sols are only precipitated when an alkaline gelatine is added. Bismark brown, which is a positive colloid, is precipitated by alkaline gelatine solution while eosine is precipitated by an acidified gelatine solution. When gelatine is mixed with the so-called soluble silicic acid, Graham3 finds that “silicate of gelatin falls as a flaky, white and opaque substance, when the solution of silicic acid is added gradually to a solution of gelatine in excess. The precipitate is insoluble in water and is not decomposed by washing”. This js apparently a coagulation of two oppositely charged colloids. When gelatine solution is poured on a freshly precipitated silver bromide, the latter is peptised4. The effect is more marked in presence of a slight J. Gen. Physiol. 5 , 379-504 (1923).
Z. phpsik. Chem. 51, 1 4 j (ISO,~). J. Chem. SOC.1 5 , 236 (1862).
Eder’s Handbuch der Photographie, 31, 28 (1992); Luppo-Cramer: Phot Correspondenz, 44, j 7 S (1907)
I546
X . C. SEN
excess of potassium bromide or silver nitrate. Freshly precipitated red silver chromate can be dispersed by gelatine into a yellow sol. In all these cases it is probable that the peptisation is due to the water-soluble colloid, but ion peptisation is also possible. It would be interesting to investigate whether isoelectric gelatine can emulsify pure silver bromide. The nature of peptisation of certain hydroxides in presence of some nonelectrolytes is interesting when we consider that these peptised solutions are only stable in presence of an excess of acid or alkali. Thus ferric hydroxide is stable both in acid and alkaline medium whereas copper hydroxide can only be peptised when the solution is alkaline. When alkali is added gradually to a mixture of ferric chloride and sugar, there is no formation of a precipitate but a t the same time no test of free alkali is obtainable in the solution. What happens is that at first a positively charged colloid is formed. Owing to the presence of undecomposed ferric chloride, the stability is quite high, but with the gradual addition of alkali,‘the charge diminishes and the colloid ultimately coagulates. If the hydroxyl ion concentration is still more increased, the coagulum dissolves, forming a negatively charged sol. This coagulation and stabilisation into either positively charged or negatively charged sol can be brought about as many times as desired by simply adding suitable quantities of either acid or alkali. Since this is so, the question arises, what is the function of sugar or glycerol in this case of peptisation? That the non-electrolyte has some action is evident from the fact that we do not usually get a negatively charged ferric hydroxide with caustic soda unless precautions are observed1. On the other hand, sugar does not stabilise the colloid in the absence of an excess of H‘ or OH’ ions. I consider that this simple experiment shows definitely that non-electrolytes like sugar or glycerol do not peptise a precipitate. Their disintegrating action, if any, is small. But the mechanism of their action lies in the fact that when they are present in the solution, they are adsorbed by the precipiting particles and thus prevent the growth of crystals2. I n other words, due to this surface film, coalescence is to a certain extent prevented and the precipitating substance is a t this moment stabilised by the preferential adsorption of some ions present in the solution. I t is also probable that the presence of these non-electrolytes increases the amount of ion adsorption. If no ion is preferentiadly adsorbed, the precipitate settles down because the surface film of the non-electrolyte is not sufficient to counteract the effect of the surface forces in the absence of electrical films over the particles. Thus the nonelectrolytes help the ion peptisation of the substance indirectly. This explanation of the protective action of the non-electrolytes seems likely from the fact that their adsorption is usually small, and in presence of electrolytes the stabilised colloid shows changes in its migration velocity under an electrical field almost proportional i o the concentration of the added electrolyte. 2
Powis: J. Chem. Soc. 107, 818 (1915). Kancroft: “Applied Colloid Chemistry”, 165 (1921).
T H E O R Y OF PEPTISATION
I547
\Then we powder a substance, we call it disintegration. \\‘hen dilute hydrochloric acid acts on alumina, we say that the alumina is disintegrated and stabilised into a colloid solution. This is peptisation. When gelatine acts on silver bromide we are getting peptisation. When non-electrolytes are present in the soIution before precipitation, they prevent the growth of crystals. In presence of gelatine, the crystals of barium sulphate are very small. Here me have got some protective action, but no peptisation. There may not be any real difference between disintegration and protective action because both are due to the adsorption of the disintegrating or the protective substance. But these disintegrated or protected substances must be stabilised by the preferential adsorption of some ions, which make the colloid stable. Here we actually get peptisation. This is true at least in the case of suspension colloids stable in aqueous solution, and may also be true in the case of p d e emulsions. From this point of view I think that the terms protective action, disintegration, and peptisation should be carefully defined. The general results of this paper are: I. In the peptisation of a substance, a high degree of adsorption and a suitable concentration of the electrolyte is necessary. When the same electrolyte and the same peptisable substance are 2. used, peptisation depends upon the smount of adsorption to a certain extent. 3 . The agglomeration of a precipitate decreases its power of adsorption and consequently its peptisability. 4. When different peptisable substances are used, peptisation is specific and depends both upon the nature of the adsorbent and also on the nature of the peptising agent. 5 . With different acids and the same adsorbent, there may not be any connection between the amount of adsorption and the ease of peptisation. Peptisation is markedly retarded by the presence of bivalent or trivalent negative ions, though these acids are usually the highest adsorbed. With monovalent acids, the rule is approximately followed-namely the greater the adsorption the greater is the peptising power. 6. It has been shown that almost all the known cases of colloid formation in aqueous solution can be explained as cases of ion peptjsation or peptisation by means of an ion-peptised colloid. It is extremely doubtful whether nonelectrolytes can peptise a substance. It has been shown that peptisation by an undissociated salt is not probable. 7 . The stability of hydrosols is dependent on the formation of a suitable surface film and in :he majority of cases it is electrical in origin. Hence in any theory of peptisation, the existence of these films must be explained. 8. The peptisation of some substances in organic solvents, such as pyroxylin in amyl acetate or ether-alcohol mixture, or the peptisation of vulcanised rubber has not been discussed because the conditions of stability in these cases are obscure. Departmmt of Chemistry, Allahabad Umserszty, Allahabnd. Indza.
