EQUILIBRIUM PHENOMENA 1Y COAGULATION O F COLLOIDS’ B Y E. F. BURTOh- AXD MAY ANNETTS
I. Introduction The study of phenomena observed in the light scattered from liquids has resulted in bringing the technique of these observations into a high state of perfection.* As a consequence this method has already been applied by various workers to follow changes taking place during coagulation of colloidal solutions. 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. The following account consequently consists of four different parts, L’ZZ.,
I. Light scattered by aqueous sols of gum mastic and arsenious sulphide during coagulation. Energy transmitted by the above sols during coagulation. 2. 3. Stages of partial coagulation of various sols by small traces of electrolytes. 4. Observations on the structure of arsenious sulphide.
11. Light scattered by Sols during Coagulation Light scattered by a sample of colloid was studied by the use of a pyrex glass cross similar t o that used by Martin in the study of pure liquids. The intensity of the scattered light was compared with that of the incident light by the method fully described by Martin and by S w e i t ~ e r . ~ (i) Mastic Sol. This sol was pre--c s-+ pared by dissolving gum mastic in absolute alcohol and adding a small quantity of the alcoholic solution to a large amount of water. Small quantities of electrolyte were added to various I O O cc samples and the variation in the intensity of the FIG.I scattered light was observed. In every
LgAt
1 This work was carried out in part with the aid of a Scholarship from the National Research Council of Canada. 2 Martin: Alexander’s “Colloid Chemistry,” I , 340 (1926). J. Phys. Chem., 31, 1150 (1927).
EQUILIBRIUM PHENOMENA IK COAGULATION O F COLLOIDS
49
case this intensity decreased to a value which remained constant until the particles began t o settle visibly. The time required for the intensity to reach this steady value depended on the amount of electrolyte added, but in a curious manner. Table I and Fig. 2 record this action for the salts N ’ j aluminium nitrate, S/5 aluminum sulfate and N magnesium sulfate.
Cc. of Elect. added to 100cc Mastic
Time in minutes necessary to reach Steady Value N MgS04.7HgO
N / j A ~ ( N OgH20 ~ ) ~ N / j AI2(SO4),.8H20
0.25
53.5
0.5 0.75
20.3
14.0
38
1.5
8.7 8 .o
.o
9.3
6
I
2
.o
8
2 . j
3 .o 3.5 4.0
22
.o
9
18.0 23 . o
I2
I2
4.5
16 21
j .O
23.7
5.5 6 .o
20.0
I8
9.0
I1
.o
8
6.5 7 .o 7.5 8 .o
34 24
23
18
22
I1
9.5
15.0
8.5
13 16
16 I5 I8 21
30 34
19 . o 8 .o
7
9.5 IO
.o
4.7
d
I2
I1
.o
15
.o
4.0
4
4
9.0
23 7
Fig. 2 shews the existence in this case of a curious zonal effect during the process of coagulation, which would be missed entirely by any less sensitive method. At first sight one might think this effect spurious; with the aluminum nitrate curve, the observation was repeated after an interval of some weeks, the two separate observations being indicated by the points distinguished by circles and squares respectively. It is to be noted in addition, that whereas the trivalent ion (Al) gives two maxima and minima, the di-
50
E. F. BURTON AND MAY ANNETTS
valent coagulating ion (Mg) shews only one maximum and minimum. Apparently here we have some antagonizing effect of the positive and negative ions. (ii) Arsenzous Sulphide. This sol was prepared by the ordinary method of bubbling hydrogen sulphide gas through a solution of arsenious acid and clearing of excess hydrogen sulfide by extended bubbling of hydrogen gas
Q VA N T I T Y OF ELECTROL YTE tee) FIG.2
through the sol. The addition of electrolyte not sufficient to bring about immediate flocculation (about I to 5 cc N / ~ o o oaluminum nitrate per I O O cc As&& sol.) a t once caused the particles to scatter much more light-a change of the order of five times the total change recorded in the mastic experiments. After the sudden increase, a further very gradual increase in the intensity of the scattered light continued until the sol was obviously coagulated, a t which time the amount of scattered light rapidly decreased. Fig. 3 illustrates this phenomenon. There was no indication of the zonal effect such as given by mastic.
0
z
3
Ll tu) 4
8
ow
111. Energy transmitted by Sols during Coagulation Having made some observations on the scattered light it was natural to follow up with measurements of transmitted energy of radiation on similar sols under similar conditions of coagulation. The simple method of measuring the transmitted energy by means of a sensitive thermocouple was used; the E.M.F. of the thermocouple was measured by a Wolff potentiometer capable of measuring to O.OOOOI volt. I n essence these measurements are not nearly as sensitive as the measurement of scattered light. The arrangement of
I L
-
l
w
V
FIG.4
52
E. F. BURTON AND MAY ANNETTS
EQUILIBRIUM PHENOMENA IN COAGULATION O F COLLOIDS
53
apparatus is shewn in Fig. 4, where P indicates a 1000-watt lamp, T the thermocouple, and V the parallel-sided cell of glass containing the sample of sol. B is blackened pipe for protection. W is a water filter to absorb the long heat rays. A typical set of readings is shewn graphically in Fig. 5 , where the thermocouple E.M.F. is plotted against the time elapsed since the addition of the electrolyie. It is apparent that there is a rather sudden drop in the E.M.F. to a minimum in about four minutes, corresponding to a decrease in the transmitted radiation. In the case illustrated, which is the effect of adding 0.5 cc N/5 aluminum nitrate to 7 0 cc mastic sol, the transmitted radiation then increases rather irregularly until t equals 65 min., when a very sudden increase in the transmission takes place showing a clearing of the sol and a return to practically the value of the transmission of pure water. Further experiments were carried out to learn how the phenomena occurring in the initial period varied as varying amounts of electrolyte were added. The details are given in Table I1 and illustrated in Fig. 6, where again the E.M.F. of the thermocouple is plotted against the time in minutes.
TABLE I1 Elect. in 70 cc 801.
0 .I
cc K/IO
Time after
Pot. reading Elect. added (volts)
(min.) 0 .o
8.5 0 .o
0.00127
21
4 4.5
0.0011g
31 .o
0.001 I 2
6.5 8.5 13.5
0 .oo108
36.0 43 .o 48 .o
o.oo1og 0.00106 0.00100 0.00098 0.00097 0.00096
.o
0.00094
19.5
0.00095
71 .o
0.00092
24.5 30.5 33.5
0
2 .o
4.0
cc N/25
(min.)
0.00129 o.00090 0.00088 0.00088 0.00088
I .o
0 .I
Elect. in 70 cc. Timeafter Pot.reading sol. Elect. added (volts)
2
o ,00106 0.00099
0.1
cc K / j o
0.0
I
.o
0.00128 0.00124
5 .o
0.00121
.o
0.00114
1 7 .o
I2
j I
.o
.ooog1
o .00089
0.00088
They shew that with smaller amounts of electrolyte not only is a longer time required to reach the steady reading (at which the colloid remains until settling out commences) but that this steady state is different. With very small quantities of electrolyte, the sol reaches a steady state for which transmission is higher than for larger quantities of electrolyte; from this condition one gets only partial coagulation.
54
E. F. BURTON AND MAY ANNETTS
EQUILIBRIUM PHENOMENA IN COAGULATION OF COLLOIDS
55
IV. Coagulation by Stages During the course of the preceding experiments it was noticed, f i s t with arsenious sulphide, that, when very small traces of electrolyte were added to a given sample of sol, some of the colloidal material was precipitated, but by no means all. Such a state of affairs is suggested, for example, by the curves in Fig. 6. It was thought at first that this partial coagulation might be due to the presence of various sized particles, Le., that the first amount of electrolyte carried down only the particles above a certain size. But it was found that centrifuging (at 2000 r.p.m.) when continued several hours, so that an appreciable number of particles were thrown out of suspension, did not alter the phenomenon. A series of experiments was carried out with samples of mastic, gold, and arsenious sulfide. To a comparatively large amount of the original sol a small trace of the precipitating electrolyte was added and partial coagulation completed in the course of a few hours; the uncoagulated supernatant portion was drawn off and additional traces of electrolyte added, causing a second partial coagulation. These stages of partial coagulation actually represented states of equilibrium, as such samples shewed no indication of progressive coagulation when kept even for months. I n the case of the arsenious sulphide, partial coagulation was repeated until four or five stages were completed. At each stage the concentration of the supernatant sol was measured by weighing the dry residue left after heating samples in an oven a t 110' for three hours. Measurements were also made of the cataphoretic mobility of the particles in the sol a t each stage. A typical set of results with arsenious sulfide is recorded in Table I11 and illustrated by the curves in Figs. 7 and 8. TABLE I11 No. of treatments with Elect. (0.1 cc N/IW AI(N0j)s
Amt. of Material present in sol 100 cc (mgms)
Cataphoretic mobility cm/aec/volt
0
607 580 566 556
0.00056
I 2
3
4 5
0.00044 o.00029
0.00014
260 0
The phenomena recorded above afford conclusive evidence that as regards concentration, electric charge, etc., the colloid is in a state of equilibrium with the surrounding medium, particularly in relation to the amount of electrolyte present. The question a t once arose as to whether or not the partial coagulation was due to the complete absorption of the precipitating ion by the colloidal material in the precipitated portion. I n order to test this, the precipitation was carried out with small quantities of ferric chloride, as iron lends itself to qualitative estimation in very small proportions. The ferric chloride was added and after the partial precipitation was complete,
56
E. F. BURTON AND MAY ANNETIS
the supernatant colloid was tested for iron, which was found to be present in distinctly definite quantities. The decrease in the cataphoretic mobility of the particles as stage after stage of precipitation occurs shews that the charge on the colloidal particle is reduced step by step as the coagulation proceeds.
V. The Constitution of Arsenious SuEde Sol It is generally accepted that the charge on the particles of the sulphide sol is primarily due to the hydrogen sulphide which has been adsorbed by the particles during preparation. The particle may be schematically represented‘ as follows:
**
tx
c(t
55 FIG.9
As pointed out already by W e i ~ e rthere , ~ is very little to be gained by trying to give a specific formula to any particular sol as the composition varies with preparation, age, etc., We may indicate the composition by the adjustable formula : (nAs2Ss.mHS)- mH+
+
This suggests that hydrogen sulphide after being intimately adsorbed by the colloidal particle a t least partially dissociates leaving the ions HS- as the charge on the surface if the particle and the Hf ions as the diffuse outer layer of the Helmholtz double layer. This theory affords a possible explanation of a strange phenomenon observed during this work. A sample of arsenic trisulphide sol (zoo cc) was left standing in a loosely covered graduate. After a few days the upper portion of the sol began to lose its colour dividing the liquid into distinct levels near the top. As time went on the levels gradually merged into one, dividing the liquid very sharply into two sections: the upper one a very pale yellow, almost colourless, the lower one a very dark orange. This level continued to fall with time but no sediment appeared in the bottom of the vessel, and, Kruyt: ‘“‘Colloids,”74. Weiser: The Colloidal Salts,” 24-25 (1928).
EQUILIBRIUM PHENOMENA IN COAQULATION OF COLLOIDS
57
when tiewed both in the microscope and the ultramicroscope a large number of particles could be seen in samples taken from both layers. This phenomenon has been repeated with samples of four different arsenic t,risulphide solutions including one stabilised by driving off the excess H2S by bubbling oxygen through. Further it has been shewn that this phenomenon is dependent on exposure to the air, for no settling has been observed in
FIQ.IO
any sample of sol set up in a tube of the same diameter as used above, but having the top sealed off. I n one case a sample of sol which had stood two weeks thus without shewing any trace of settling was set u p in air in an open tube and shewed a definite settling level in three days. If we suppose that we have complex polysulphide particles in equilibrium with adsorbed ions, ions in the solution, and HIS, then some of the H2S will gradually escape to the air, displacing the equilibrium in the direction causing more ions to go into solution and the particles to break down into simple sulphides. Thus arbitrarily designating the particles by (n As2S3)mHS- the equations representing the reactions are ( n A s , S a ) m H S - e (nAs2S3) (m- I)HSHS- HS- H+=HzS I n the case of coagulation (previously referred to) with ferric chloride, it was found that shortly after coagulation the colloid gave the test for iron
+
+
58
E. F. BURTON AND MAY ANNETTS
in the ferric form, but after a few hours it responded only to the test for the ferrous form. This is in agreement with the above theory of ionisation at the surface of the particle, for in the presence of HS- ions we would expect the reaction zFe+++ HS- ---t 2Fe++ SI H+
+
+ +
The change of the particles from complex polysulphides to simple sulphides may be regarded as releasing hydrogen sulphide which diffuses up
through the solution. The slow fall of the sharp level separating the two layers is determined by the rate of diffusion of gas through the liquid column above. Suppose the concentration of hydrogen sulphide just at the surface of separation of the two layers is C, and that that at the air-liquid surface is zero. Then if y is the depth of the “pale” layer and D is the diffusion constant, Q the quantity of hydrogen sulphide passing in unit time through unit area perpendicular to the direction of flow, is equal to so that Q d t = Q D C/y d t
D(C - 0) ~
Y
(approx.)
EQUILIBRIUM PHENOMENA IN COAGULATION O F COLLOIDS
59
This hydrogen sulphide must come from the “dark” layer and is the difference between M, the total quantity of material given off in time dt, and the amount used to raise the concentration of this newly formed layer (depth dy) to C. (M-C)dy = D C/y d t DC y d y = -d t M - C t = kiy2 kz
+
One set of such observations is shewn in Table IV while in Figs. I O and I I, t has been plotted against y and against y2. (for nine random values of y and t , )
TABLE IV Time in days
Distance fallen
(mms)
8
Time in days
Distance fallen (mms)
17.0 18.0
55
Timein Distance fallen days (mms)
3 .o 3.5 4.25
IO
I3
19.0
5.25
20
20.0
22
21.25 22.0
29.0
78
32.0
84
14.0
36 39 44 47.5
61 62 67 68
85
15.0
51
33.0 35.0
36.0 38.25 39.0 40.0 42.0 45.0 46.0 47.0 49.0
89
50.0
6.0 11.0 12.0
13.0
58
90 94 95 96 99 I02
104 IOj
I08 I IO
summary 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. 3. Suggestions are made as to the constitution of the arsenious sulphide particle. I.
