T H E INFLUEXCE OF COXCENTRATION OF A SOL O X I T S STABILITY B Y S U B O D H G O B I S D A CHAUDHURY
The effect of dilution of the colloid on the coagulating concentration of an electrolyte has been recognized long ago. Thus Woudstralfound that the greater the concentration of the colloid, the smaller the concentration of an electrolyte necessary to precipitate it. This is the reverse of what Freundlich2 has observed with arsenious sulphide sols. Mukherjee and Sen3observe ( I ) that the nature of the cation of the electrolyte and the extent of dilution determine whether the diluted sol is more or less stable than the original sol; and that ( 2 ) a diluted sol of arsenious sulphide is mostly more stable than the undiluted sol, when the precipitating cation is univalent, and always less stable when the cation is aluminium (trivalent) or thorium (tetravalent). When the cation is divalent either a diminution or an increase in stability may be observed. At moderate dilutions, the sol becomes unstable depending on the degree of dilution. Cupric and mercuric sulphide sols always showed an increase in stability on dilution. They have considered firstly the effect of decrease in the colloid-liquid interface which takes place on dilution and would under certain conditions diminish the stability and secondly an increase in the distance between the particles, i.e. the decrease in the number of particles in a given volume consequent on dilution, which adds to its stability. The observed effects are referred to the combined influence of these factors. Kruyt and van der Spek4 simultaneously came to similar conclusions. A few years later Burton and his co-workers5 reported that for monovalent precipitating ions the concentration necessary for coagulation increases with dilution of the sol. I n the case of divalent precipitating ions, values of coagulating power remain approximately the same in spite of the change in the concentration of the sol, whereas in the case of trivalent ions there is a direct proportionality between the concentration of the sol and the concentration of the precipitating ion. They postulated a relationship between the effect of dilution and the valency of the coagulating ions. From the observation of Mukherjee and Sen that a diminution or increase in stability is observed against coagulation by barium chloride depending on the extent of dilution, it is obvious that no such relationship can be postulated. The obvious explanation is that given by Mukherjee and Sen and by Kruyt and Spek. In a series of papers Dhar, Ghosh and Sen6 have studied the influence of the Z. physik. Chem., 61, 607 (1908). Z. physik. Chem., 44, 139 (1903). J. Chem. SOC., 115, 461 (1919);see also J. Am. Chem. SOC., 37, 2024 (1915). 'Kolloid-Z., 25, I (1919). J. Phys. Chem., 24, 701 (1920); 25, 5 1 7 (1921). J. Phys. Chem., 26, 701 (1922); 28, 313 (1924); 29, 435, 659 (1925); Kolloid-Z., 34, 262 (1924); 36, 219 (1925).
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SUBODH GOBISDA CHAUDHURY
change of concentration of a sol on its coagulation and have formulated what they call a general dilution rule (Ghosh and Dhar) viz. that, the greater thedilution of a sol, the less is the amount of electrolyte necessary for coagulation, provided that the sol does not appreciably adsorb ions carrying the same charge as the sol from the coagulating electrolytes. On this basis they have classified these colloids into normal and abnormal ones‘. According to them normal colloids are those which follow this dilution rule. Those colloids which require a higher end concentration of the electrolyte for the coagulation of a more dilute sol are called abnormal colloids and they attribute the abnormality to the adsorption of similarly charged ions. That the similarly charged ions play a part in the coagulation of colloids has long been recognised by Freundlich2 and Bancroft3. On considering the classification of these colloids into normal and abnormal ones, we find that there are cases where the socalled abnormal colloids show normal behaviour, e.g. As& sol and AgN03 and also that normal colloids show abnormal behaviour, e.g. positively charged MnOz sol against CuC12 and AgN03. Examples might be multiplied. Thus we can a t best speak of a normal and abnormal behaviour of colloids in the above sense. The above classification into normal and abnormal colloids is thus unjustifiable. The adsorption of the similarly charged ions was first given detailed attention by Weisef and he pointed out the fact that the influence of the ions having the same charge as the colloid cannot be disregarded entirely in any case and may be quite marked if the electrolyte precipitates the colloid a t high concentration. From an investigation of the concentration of sols on their precipitation by electrolytes the following conclusions were formulated by Weiser5 in 192 I . ( I ) The manner in which the precipitating value of an electrolyte varies with the concentration of the colloid is determined to a large extent by the relative adsorbability of the precipitating ion and the stabilising ion.
If the adsorption of the stabilising ion of an electrolyte is negligible (2) and the adsorption of the precipitating ion is very large the precipitation value varies almost directly with the concentration. Weisere in a recent communication observes: “Indeed Ghosh and Dhar found that the rule was followed with positive ferric oxide sol using potassium chloride but was abnormal if aluminium nitrate was the precipitating electrolyte. . . . Such a rule as Dhar has set down seems to me to be a positive menace rather than a help, if its limitations and the principles on which it is based are not recognised clearly”. Regarding the experimental evidence for the statement that the adsorption of ions carrying the same charge as the particles increases on dilution, one finds only a single instance. The J. Phys. Chem., 31, 187, 666 (1927). “Kapillarchemie”, 352 (1909); Z. physik. Chem., 44, 104 (1903). 3 J. Phys. Chem., 19, 363 (1915). J. Phys. Chem., 24, 30 (1920). 5Weiser and Sicholas: J. Phys. Chem., 25, i p (1921). 6 J. Phys. Chem., 30, 20 (1926). 1
2
ISFLUESCE O F COXCENTRATIOS O F SOL O S STABILITY
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increase of the ratio of the adsorption of the similarly charged ion to that of the precipitating ion on dilution has been recorded in the case of positive MnOs sol and CuC12 (Ghosh and Dhar). The chances of error in analytical measurements are very great and these experiments do not appear to be sufficiently convincing. Besides it has been pointed out by Mukherjee and Ghosh' that analytical measurements do not in reality give an idea of the type of the adsorption as would affect the charge but they give an idea of the exchange of ions between the double layer as a whole and the solution. I n order to explain facts relating to coagulation with mixtures of electrolytes, Mukherjee and Ghosh have also been led to assume the adsorptionof similarly charged ions. Electrical measurementsof the charge of colloids under these conditions are necessary to throw further light on the subject. The author in a joint paper2 has measured the variation of the rate of migration, i.e. the charge of colloidal particles, with dilution in the case of arsenious sulphide, copper ferrocyanide and gold sols. I n each case it has been found that the charge of the colloidal solutions decreases on dilution. The data in Table I have been taken from the above paper. The rates have been corrected for viscosity taking that for water a t the temperature as unity. TABLE I AS& sol Temp. 35'C. Dilution in ratio of volume of original sol to that of water added to it.
Rate of migration in cms. per sec. per volt/cm.
Pure sol
6 0 . 1 X IO-; 5 8 . 9 X IO-^
I : I I
I I
5 5 , 7 X IO-^ 1 7 . j X IO+ 1 2 . 2 X IO+
:3 : IO : 20
All the previous workers in this field have overlooked the possibility of the decrease of the charge with dilution in the case of a t least some colloids, which would tend to sensitise the sol on dilution against electrolytes. The question naturally arises whether the relative increase in adsorption of the similarly charged ion suffices to counteract the decrease in charge on dilution. Measurements of the charge of colloids diluted and undiluted, keeping the electrolyte concentration the same, were done three years back3. The data were taken with arsenious sulphide sol and hydrochloric acid. TABLE I1 of stock colloid
C.C.
C.C. of
water
Dilution
C.C. of Mobility a t 30" in cm. S / z j HCl per sec. per volt per cm.
25
nil
I
25
15
IO
1.66
25
IO
15
2.5
25
3 8 . 9 X IO-; 3 8 . 9 X IO-^ 3 9 . 1 X IO+
J. Indian Chem. SOC.,1 , 213 (1924).
* Mukherjee, Chaudhury and Roy Chaudhury: Quart. J. Chem. SOC., 125, 79 (1924).
J. Indian Chem. SOC.
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I t seems likely from these results that the relative adsorption of anions increases, since the colloid without any electrolyte shows a decrease in its charge on dilution. But the increased adsorption does not increase the charge sufficiently to make it stable towards univalent electrolytes in the case of As2S3sol, for the values are very nearly equal. We are therefore forced to the conclusion that greater distance between the colloidal particles in a diluted sol is at least partly if not mostly responsible for stabilisation against monovalent electrolytes in the case of A%S3sol. In a recent paper Ghosh and Dhar’ write “that the view advanced by Kruyt and Spek and by Mukherjee and Sen that the decreased chance of collision amongst the colloidal particles is an important factor that prevents a weaker sol of As& from coagulating is not corroborated by experiment.” The experimental data given here show that the relative increase in the adsorption of anions alone is not sufficient to make the sol stable on dilution but that the increase in the distance of the colloidal particles on dilution is equally, if not more, potent in bringing about stabilisation. This, it seems t o the author, has been experimentally proved from the data given in this paper. Ghosh and Dhar forget that the particles must meet together before they can agglomerate into bigger particles, and the rate of such collision must be taken into consideration. The mean distance between the particles determines among other factors the number of such collisions. The point need not be pressed further in view of the classical works of Smoluchowski* and of Zsigmondys. The observations up to date on the variation of the coagulating concentration of different electrolytes on dilution of the colloid might be fully accounted for in the following manner:-On diluting a colloid, both the charge and the total surface of the colloidal particles decrease, whereas the distance between the particles increases. Diminution of charge and also of the total surface tend to make the sol unstable against coagulation by electrolytes provided it i s assumed that the potential at which a sol coagulates does not change on diluting the sol and that the relative adsorption of all ions on the surface remains the same. The greater distance between the particles of a diluted sol tends to make it more stable. That in a number of cases, the sol becomes unstable on dilution, is presumably due to preponderance on the first two effects over the third. Now when the ratio of the amount of adsorption of similarly charged ions to that of the precipitating ions increases, we meet with another factor that tends t o make it more stable on dilution. The increased adsorption may be attributed t o one or more of the following possibilities: (a) A relative increase in the adsorbability of the similarly charged ions. 1
J. Phys. Chem., 31, 649 (1927).
physik. Chem., 92, 129 (1917). Z. physik. Chem., 92, 500 (1918).see also Mukherjee and Pspaconstantinou: Phil. Mag., 44,302(1922);Hatschek, Mukhe;jee and Masumdar: 3. Chem.SOC.,125, 785 (1924); Anderson; Gamer and Lema. 2Z.
a
ISFLUESCE OF COXCENTRATION OF SOL O S STABILITY
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(b) Increase in the equilibrium concentration of the electrolyte consequent on the decrease in the colloid-liquid interface In this case it will be necessary to assume further that with the increase in concentration the relative adsorption of the similarly charged ions increases. The second possibility may be neglected, for in general we have found that the manner of variation of the charge with the concentration of electrolyte does not warrant this assumption. The adsorption theory of Freundlich predicts that dilution of the sol will always diminish its stability and in the limiting case of uniunivalent salts, this theoretical diminution may not be perceptible. Diminution of charge on dilution shows that a consequent change takes place on the surface of the colloidal particles, on dilution, and probably this is the reason why relative adsorbability also changes with dilution. I t is also possible on the views of adsorption of similarly charged ions put forward by Mukherjee' to attribute the increased relative adsorption of the similarly charged ions to a diminution in the charge itself, which increases considerably the number of collisions of the similarly charged ions on the surface, assuming that the concentration of these ions remains constant. The main reason for the difference in the behaviour of a colloid on dilution against coagulation by electrolytes--which as we have seen above depends on the nature of the coagulating electrolyte--is perhaps to be sought in the difference in the coagulating concentrations. When the concentration is high, the effect of diminution of interface on dilution is negligible, and the increased distahce between the particles has a dominating influence. The reverse is the case when the coagulating concentration is very low. I n the present paper, we have in the absence of relevant charge measurements assumed that coagulation always takes place at a definite potential of the double layer for the same sol. This is not always true, as would appear from the measurements of Mukherjee and ChaudhuryZand Mukherjee, Chaudhury and Roy Chaudhury3. My thanks are due to Prof. J. iX.Mukherjee, D.Sc., for his kind interest in the subject. Physatzl Chemistry Laboratory, Untversity College of Sctence, Calcutta.
*Phil. Mag., 44,321 (1922). J. Indian Chem. Soc., 2, 296 (1926). Communicated for publication.
