ON T H E STABILITY O F COLLOIDAL SOLUTIONS PART I. ALUMINIUM HYDROXIDE SUSPENSION BY K.
C . SEN
I n a recent paper it was observed that an undialysed sol of ferric hydroxide required more of an electrolyte for coagulation than a dialysed sol, and the following results were obtained. The concentration of the sol, that is, the iron content was the same in both cases, while the chlorine content was obviously different. Electrolyte
KN03 Z N BaClz zN Mg SO4 N/ZOO Cd SO4 N/ZOO
cc. of electrolyte required for coagulating 5 cc. of dialysed sol
undialysed sol
3.0 3.2
6.I
0.8 0.6
1.6 1.8
7.2
It was remarked in t,hat paper that one possible explanation of this behaviour may be that small quantities of hydrochloric acid might have a peptising influence over ferric hydroxide, the removal of which by dialysis makes the sol less stable. Similar instances are also known in the case of arsenious sulphide where addition of H2S makes the sol more stable towards monovalent positive ions. It is well known that many colloids owe their stability to the presence of adsorbed electrolytes. Thus colloidal manganese dioxide' coagulates on dialysis. The investigations of Ruer2, and Hantzsch and Desch3show that well dialysed colloidal ferric hydroxide still contains some chloride. In fact it may be taken for granted that in all experiments on colloid precipitation, the colloids were never free from peptising electrolytes, the quantity of which has varied between wide limits in the samples of different investigators working on the same colloid. In 1917,Neidle4 showed in an interesting paper, that the stability of ferric hydroxide hydrosol was dependent not so much on the method of preparation as has so long been assumed, but to a greater extent on the purity and concentration of the sols employed. He prepared a number of perfectly clear hydrous oxide sols. I n each series the iron concentration was constant, while the chlorine content was varied by the addition of known amounts of hydrochloric acid. The sols were precipitated by potassium sulphate solution, and Ganguly and Dhar: J. Phys. Chem. 26, 703 (1922). Ruer: Z.anorg. Chem. 43, 85 (1905). Hantzsch and Desch: Ann. 323, 28 (1902);compare aha, Duclaux: J. chim. phys. 5, 2 9 (1907).
Neidle: J. Am. Chem. Soc. 39, 2334 (1917).
1030
K. C. SEN
the amount of d t required for the complete precipitation of the sol was taken R S a measure of its relative stability. The data showed that for a given iron Concentration the stability increasep with the chlorine content, while for sols of a given purity, Le. ratio equivalents Fe : equivalents C1, the stability decreases as the concentration increases, this being most pronounced for very pure sols. I n recent years the stability of colloidal solutions has been much examined and it has been shown that in all precipitation experiments, the influence of the concentration of the colloidal solution must be taken account of. No such attention has however been paid to another important cause affecting the stability of suspensoids, namely the amount of stabilising agent present in the system. Unless the amount of the stabilising agent is known and experiments are carried out at the same concentrations of the stabilising agents, it is obvious that the results obtained in the coagiilation experiments of two sols of the same substance will be different. I n other words, the stability of a sol is dependent not only on its concentration, but also on the amount of impurity it, contains. In considering the coagulation of ferric hydroxide sol, it is evident from the results of Neidle, that the stability increases with the increase in the amount of HC1 added to a certain extent and then decreases. At the higher concentration of the acid, a portion of the colloid is dissolved with the formation of ferric salt. It is interesting and theoretically important to enquire the cause of this increasing stability on the addition of slight quantities of hydrochloric acid. Neidle considers that, by the addition of slight amounts of acid, more stable oxychlorides are formed. According to him therefore we have t o assume a series of oxychlorides of varying composition with the progressive addition of hydrochloric acid. It will be shown later on that the phenomenon can be explained equally well on the assumption that greater quantities of hydrogen ion are adsorbed by the ferric hydroxide and this increases the stability of the colloid.
It is apparent from what has been already stated that in order to study completely the stability relations of a colloid, a systematic attempt should be made to study the coagulation of different colloids with varying degree of electrolyte content. In order to obtain some experimental results of this nature, some experiments were designed similar to that made by Neidle, by adding varying quantities of stabilising electrolyte to well dialysed sol and then determining the precipitation value of a fixed electrolyte with this sol. A difficulty was however soon experienced-namely an uncertainty as to whether true equilibrium occurs between the dialysed colloid and tthe added electrolyte. Unless an equilibrium is reached between the colloid and the electrolyte, whereby with increasing concentration of the electrolyte more of it is adsorbed by the colloid particles, the exact role of the stabilising agent is
STABILITY OF COLLOIDAL SOLUTIONS
103I
difficult to explain. For, it is generally assumed that only that portion which is adsorbed can be effective in the stabilisation of the sol. It is therefore important to investigate first whether any stable equilibrium point is reached or not. Another point of importance is that though dialysis is theoretically a reversible phenomenon, in actual practice often it is not so. Thus the last traces of chloride cannot be easily dialysed out from a ferric hydroxide sol. Hence we cannot start with a sol of zero impurity. In our experiments with ferric hydroxide, we have found that it2is very difficult to obtain an equilibrium point with hydrochloric acid. Considering all these facts it occurred to me that before beginning to work on a colloid prepared before hand, it would be better to obtain some comparative data on some substance which can be obtained in a pure state, and which can be easily peptised by means of a suitable peptising agent. In a previous paper1 I have studied the peptisation of hydroxides by different acids, and these hydroxides seemed to he suitable to start with in this investigation. It has been observed that freshly precipitated and well washed aluminium hydroxide ca,n be peptised very readily by mean? of benzoic, acetic, propionic, hydrochloric acids, etc. When very small quantities of acids are used, the suspensions of the hydroxide are turbid, pass through filter papers, and are stable for a t least 48 hours. With weak acids like acetic, benzoic, and propionic acids, the equilibrium point is reached quite readily; but with hydrochloric acid, the acid is continually adsorbed for some time by the hydroxide, and the conductance of the mixture hydroxide +HCI falls gradually with time. For this reason, hydrochloric acid has not been used in peptising the hydroxide, and in general 24 hours have been allowed for the equilibrium to set in. The stability of the suspension was measured against a standard potassium sulphate solution by coagulating a known volume of the suspension. Both the amount of the peptising acid as well as that of the concentration of the suspension have been varied. The determination of the exact coagulation point has been a matter of considerable difficulty, for the amount of electrolyte necessary to coagulate completely the suspension varied so much with the time employed, that no satisfactory data could be obtained. Hence it was decided to take that concentration of the electrolyte which coagulated the suspension immediately as the precipitating concentration of the electrolyte. By this method reprodiicihle values were obtained, but the results were only relative and an appreciable error was introduced to which attention will be called later on. The following results were obtained :1
Sen and Dhar: Kolloid-Z. 33, 193 (1923)
K. C. SEN
1032
Cone. of the acid in millimoles per litre
0.312
0.624 0.78 0,936 I .092
1.248 I.464 3.08 6.16 9.I4 12.32
15.4 18.48 2.4 4.8 7.2
9.6 12.0
(a) Stabilising Agent -Benzoic Acid Purity o! the ccs. of N/~ooo K2904 req'd to
suspensionratio millimole Al2O3 millimole acid
20.6 10.3 8.24 6.86 5.88
coagulate 5cc. of the
original suspension (6.43 millimole AllOs per litre) 2.5
4.5 5.0
suspension 1/2 diluted
suspension
1/4diluted
(3.215 millimole All03 per litre)
(:.Po75 millimole 81@ per litre)
2.4 4.1 4.4
3.9
5.5 5.0 6.0 5.3 5.15 6.5 5.4 4.58 6.5 5.5 (b) Stabilising Agent-Acetic Acid 2.087 3.0 2.6 3.5 2.9 1.043 0.696 4.0 3.3 0.521 4.6 3.8 0.417 5.1 4.4 0.348 5.6 5.0 (c) Stabilising Agent-Propionic Acid. 2.68 2.3 2. r 5 1.34 2.7 2.6 0.893 3.7 3 *6 0.67 4.5 4.35 0.446 5.5 5.35 0.383 6.5 6.3
2.1
4.0 4.55 4.6 4.6 4.65 2.3 2-75
3.0 3.7 4.2 4.9
1.9 2.5
3.4 4.2 5.1
6.I 14.4 From the above results it will be observed that with the same concentration of the suspension and increasing quantities of the stabilising agents, the suspension becomes more stable towards potassium sulphate. Also, with suspensions having the ratio AlzOs/acid equal, the stability decreases with the decrease in t,he concentration of the suspension. These results are very interesting from the fact, that the stability of a suspension or homogeneous colloidal solutions is dependent on the concentration as well as on the amount of the peptising agent contained in the system. A well dialysed sol is consequently less stable than an undialysed one and the anomalous results recorded by several investigators' on the coagulation of sulphide sols when sulphuretted hydrogen is present may be explained on the basis of these experiments. An examination of the results further show that though the concentration of benzoic acid is less than that of acetic or propionic acid, it acts as a better 1
(2)
Mukherjee and Sen: J. Chem. Soc. 115, 461 (1919); Lottermoser: J. prakt Chem.
75, 293 (1907).
I033
STABILITY O F COLLOIDAL SOLUTIONS
stabilising agent. This is in accord with the fact as has been observed by me that benzoic acid is much more adsorbed than either acetic or propionic acid. Propionic acid is itself slightly more adsorbed than acetic acid and this explains the fact that at equal concentrations propionic acid is a better stabilising agent than acetic acid. It is apparent therefore that adsorption is playing an important r81e here. It will be further observed that the curve, Fig. I , obtained by plotting the acid concentration against the amount of electrolyte nocessary for coagulation shows a well defined curvature in the case of benzoic acid. I n the case of propionic acid the curve, Fig. 2 , is a straight line and on being produced backwards, cuts the ordinate at a point well over zero. Similar is also the case with I
FIG.I
FIG.2
acetic acid. Since however a t the zero concentration of the acid the stability is necessarily zero, the curve should pass through the zero point. Neidle has also obtained similar results, but he considers that the colloid may be stable even at the negative side of the abscissa, but he does not discuss this “negative stability” of colloidal solutions. It appears to me that this fact is due to experimental error involved in the determination of the coagulation point of colloids and incidentally my results show the amount of error that is possible in these experiments. The dotted curve has been drawn parallel to the experimental curve and probably the theoretically correct values may be obtained from it. Another interesting fact that may be discussed here is what happens if the concentration of the acid is increased more and more. Will the colloid or suspension be stabilised more and more ad infinitum? With an increase in the amount of acid, say in the case of aluminium hydroxide suspension, two things may happen. Firstly, since the suspension in this case is positively charged, the increasing concentration of the negative ion of the acid will have a coagulating effect on it, and secondly some of the hydroxide may be actually diesolved in the acid forming a salt. It has also been found by me that in the case of these acids the amount of adsorption reaches a maximum with increase in the concentration of the acid. The curve with benzoic acid shows that the
1034
K. C. SEN
stability of the suspeneion with the addition of the acid reaches a maximum. Neidle also observed that with increasing concentration of HCl, the stability of a ferric hydroxide colloid reaches a maximum and then decreases. He found also that with higher amounts of HCI, a little quantity of the colloid dissolved in the acid forming salt which could be tested after coagulation of the colloid. I n my investigation, no salt formation WAS observed because the acid concentration was usually small. Theoretically however a point of maximum stability should occur with increasing concentration of the stabilising acid.
It is of interest here to discuss the constitution of hydroxide colloids. There is a prevalent view that the colloidal hydroxides are something like oxychlorides with a variable composition and Pauli and Matula' have proposed even a definite formula for the ferric hydroxide colloid. I have however repeatedly ohserved that if the concentration of the acid be small, and specially if the arid be weak like acetic acid, ferric hydroxide adsorbs the acid appreciably without any formation of a salt, and the adsorption is reversible. Since the colloids are positively charged and since no salt formation can be traced, it is better to assume that the H+ ion is preferentially adsorbed which stabilises the colloid or suspension. It has been found that this adsorption of the hydrogel~ion increases with the increase in concentration of the acid and reaches a maximum, and hence this adsorption view can explain the greater stability of the colloids with the increase in the amount of the stabilising agent without assuming the formation of any stable oxy-salts. The amount of adsorption will depend both on the amount of the colloidal subetance as also on the amount of the acid present, and hence no simple relation exists between the two. Hence the composition of the colloid will vary between wide limits. It is an well known fact that when the concentration of the adsorbable solute is low, the adsorption is practically complete. This is known in the case of charcoal and iodine and has been found by me to be the case also with acids and hydrated oxides. The great difficulty of removing the last traces of HCI from ferric hydroxide sol by dialysis, a fact which has been supposed to agree very well with the assumption that a definite oxychloride exists, probably hinges on the same phenomenon as that in which we find that in small concentrations of an adsorbable solute practically the whole of it is adsorbed by a substance say hydrated ferric oxide. From my experimental results, it will be observed that with aluminium hydroxide suspension, the greater the concentration of the suspension, the greater is its stability towards electrolytes. These results are in agreement with the law enunciated in a foregoing paper that the greater the concentrotion of the sol, the greater the stability towards electrolytes, the purity of the sol remaining the same. Summary I n investigating the stability of sols towards electrolytes, both the (I) concentration as well as the amount of peptising agent are important factors. 1
Pauli and Matula: Kolloid-Z. 21, 49
(1917).
STABILITY O F COLLOIDAL SOLUTIONS
103 5
It is shown in the case of aluminium hydroxide suspensions, that (2) with benzoic, acetic and propionic acids, the stability of the suspension depends both on the concentration of the hydroxide as well as on the amount of the acid present. (3) With increase in concentration of the acid, the stability gradually increases to a maximum. With the same purity of the suspension, the etability decreases with dilution. This is in agreement with the law that the greater the concentration of a sol, the greater is its stability towards electrolytes.
My best thanks are due to Professor N. R. Dhar for his interest in this work. Chemical Laboratory University oj Allahabad, Allahabad. April 10, 1914.
