ELECTROLYTES I N HYDROPHOBIC SYSTEMS
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EFFECTS OF ELECTROLYTES I N HYDROPHOBIC SYSTEMS. I
ELECTRIC MOBILITYAND STABILITY FRED HAZEL Department of Chemistry and Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania Received October $0,1940 INTRODUCTION
Following the concept introduced by Hardy (9), Powis (20) demonstrated in the coagulation of hydrophobic colloids by electrolytes that ( I ) it is not necessary to decrease the electric mobility of the particles to zero, and ( 2 ) the critical mobilities of monovalent ions are higher than those of polyvalent ions. Briggs (2, 12) conducted experiments which supported Powis’ observations but which made it clear that the higher critical potentials were associated with electrolytes, e.g., potassium chloride, which cause coagulation only in relatively high concentrations.’ The irregular behavior of electrolytes of the potassium chloride type is not confined to their high critical mobilities but is also shown in coagulation experiments with sols of different concentrations, where a marked decrease in precipitating power with dilution of the sol is observed (3, 4). Likewise, in the precipitation of sols by mixtures of electrolytes, potassium chloride frequently (14, 16), although less generally (5, 21), increases the stability of the sols toward polyvalent ions. The anomalies encountered 13 ith monovalent ions have stimulated new theories (8, 15, 19) for the process of coagulation by electrolytes and in these stress has been placed on the properties of the dkpersion medium. Hauser and Hirshon (10) have proposed a general theory t o explain the effects of monovalent ions in colloidal systems. After a review of the literature, these investigators, while approving the view which stresses the importance of the countw ions in the dispersion medium, have pointed out White and Monaghan (ZZ),in a paper dealing with the ratio of electroosmotic t o electrophoretic velocities as a function of salt concentration, have reviewed the literature on the subject of critical potentials. Based upon a concept of polarization of the electric double layer in dilute salt solutions, the conclusion was reached t h a t polyvalent ions which coagulate sols a t low concentrations will appear to have a lower critical potential than monovalent ions which require much higher concentrations t o produce coagulation. The lower critical potentials of polyvalent ions were considered t o be a n “artifact” inherent in the electrophoretic method; i.e., the critical potentials of monovalent ions were not considered as being abnormally high, b u t rather the critical potentials of polyvalent ions were considered t o be superficially low.
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FRED HAZEL
the difficulties encountered in attempting a quantitative solution from the standpoint of the Debye-Huckel theory of electrolytes. The contribution of Hauser and Hirshon can be indicated by the following quotations from their paper: “Since the dispersion medium may be considered as possessing an excess of ions of the same charge, due to the adsorption effects of the colloidal particles, these may interact and form groups of ions which have a greater charge associated with them as pointed out by Gurney (7). Thus these groups may act as nuclei about which the colloidal particles may condense (and coagulate), owing to forces of an electric nature.’’ The property of the dispersion medium to act as an oppositely charged electrical field was considered to be the controlling factor in sol behavior only with coagulating ions that were poorly adsorbed, Le., monovalent ions. The classical theory was considered to give an adequate explanation for coagulation by polyvalent ions. In this case coagulation was assumed to be “due mainly to neutralization of the charges on the particles,” which allowed them to be impelled together by their kinetic energy. The higher critical mobilities of monovalent ions were cited as evidence for a different mechanism of coagulation by these ions. The S-shaped mobility curve obtained by Kruyt and Briggs (12) with potassium chloride for an arsenic trisulfide sol was explained satisfactorily by this theory. The present investigation is concerned with the relationship of electric mobility to stability, with particular emphasis on the behavior with monovalent ions. The colloidal systems that were employed in this study,manganese dioxide and arsenic trisulfide,-are the same systems that will be described later in the investigations of the relationship of sol concentration to stability, and of the effects of mixtures of electrolytes on the stability of the particles. EXPERIMENTAL
The manganese dioxide sol was prepared by adding 0.75 M ammonium hydroxide dropwise a t the rate of one drop every 10 sec. to 0.01 M potassium permanganate a t 90°C. The sol was purified by dialyzing a t room temperature for 10 days with Visking casing. The purified sol had a manganese dioxide concentration of 0.6 g. per liter. The arsenic trisulfide sol was prepared by dropwise addition of a halfsaturated solution of arsenic trioxide to a solution of hydrogen sulfide through which hydrogen sulfide gas was passing. The sol was beated to boiling and then dialyzed for 5 days a t room temperature. The arsenic trisulfide concentration of the purified sol was 4.0 g. per liter. Mobility measurements were made by an ultramicroscopic method described previously (23).
733
ELECTROLYTES IN HYDROPHOBIC SYSTEMS RESULTS
The mobilities of the manganese dioxide particles a t the flocculating concentrations of several different electrolytes are recorded in table 1. These data are for sols of three different strengths. The last column records the concentrations of the electrolytes, in millimoles per liter, required to produce coagulation of the 100 per cent sol in 24 hr. Similar data for colloidal arsenic trisulfide are reported in table 2. TABLE 1 Manganese dioxide
1
801
CRlTICAL YOBILITI, )r PER SECOND PER VOLT PER CENTIYDTER
ELECTROGYTB ~
100p;lcleent
25 par cent 801
8.25 per wnt
FLaYIULATION VALUE; 100 PER CENT SOL
sol
millimdsr pr
*I
N a O H . . ....................... K C l . .......................... H C l . .......................... AgNOa.. . . . . . . . . . . . . . . . . . . . . . . . BaClo.. . . . . . . . . . . . . . . . . . . . . . . . .
2.5 2.5 2.3 1.6 1.2
2.3 2.3 1.8 1.3 0.9 1.1
2.1 2.1 1.3 1.2 0.9 0.8
16.0 6.0 3.2 0.6 0.14 0.052
TABLE 2 Arsenic trisulfide sol ELBZROLYTE
c m n c n r , MOBILITY, p PER SECOND PER VOLT PER CENTIMETER
100 per cent sol 1 W per wnt a01
1 25 per cent sol
FLOCCULATION VALUE; 100 PER CENT SOL
millimdsr psr liler
KCI. .......................... BaClz.. ........................ CrCla. . . . . . . . . . . . . . . . . . . . . . . . . .
5.9 1.6 1.8
6.1 1.5 1.5
6.0 1.4 1.3
48 0.36 0.064
The effects of the monovalent ions on the mobility of the 100 per cent sols are shown, up to the flocculating concentrations, in figures 1 and 2. Figure 2 also shows the effect of barium chloride on the mobility of the arsenic trisulfide sol. Coagulation is indicated on the graphs with arrows. Considerable difficulty was experienced in making mobility measurements with the arsenic trisulfide sols, particularly when the systems contained no added electrolytes. Thus, it was found that when a potential gradient was applied at once to a sample of sol, freshly introduced into the electrophoresis cell, a counter flow was developed which greatly modi-
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FRED HAZEL
fied the mobility of the particles. I t was discovered subsequently that, if the samples were allowed to stand in the cell for about 1 hr. before
MILLIMOLES ELECTROLYTE PER LITER
FIG.1. Effect
of monovalent ions on the mobility of manganese dioxide sols
MILLIMOLES BACI, PER LITER
MILLIMOLES KCI PER LITER
--+ FIG.2. Effect of monovalent ions and of barium chloride on the mobility of arsenic c
trisulfide sols.
mobility measurements were attempted, the anomalous behavior was absent. The cause for the anomaly was not determined, nor was it present in systems containing added electrolytes. However, in some of the systems
ELECTROLYTES II1’ HYDROPHOBIC SYSTEMS
735
the mobility of the particles was found t o be heterogeneous, particles with different velocities being observed. Accordingly, the averages of a number of determinations were taken to obtain the values that are reported. The results with arsenic trisulfide, as shown by the curves in figure 2, were checked against two other preparations of this sol, made by the same method, and were found to be in close agreemenL2 DISCUSSION
The results of the present investigation can be explained on the basis
of the following considerations: 1. The mobility of the particles in an electric field is a reflection of the effects of both the charge and the thickness of the diffuse double layer. 2. Both the electric charge and the thickness of the diffuse double layer are subject to change on the addition of electrolytes. 3. Alteration in the magnitude of the electric charge on the addition of electrolytes is controlled by the adsorption of ions. 4. Either of the ions of an electrolyte may be adsorbed preferentially. 5. Adsorpbion of ions of opposite sign decreases the electric charge of the particles, while adsorption of ions of the same sign increases the charge of the particles. 6. The thickness of the double layer is determined by the concentration of the added electrolyte (1,8). 7 . In dilute solutions, e.g., in coagulation with polyvalent ions, the ionic atmosphere is diffuse. 8. In more concentrat>edsolutions, e.g. , as encountered during coagulation with potassium chloride, the ionic atmosphere is compressed. 9. In dilute solutions, electrophoretic effects of the counter ions are small and the niobility of the particles is determined essentially by the electric charge of the particles.
* While i t is recognized t h a t colloidal arsenic trisulfide is not a simple system, since i t is subject t o chemical change, e.g., hydrolysis, it has been studied extensively, with the result t h a t its behavior has contributed significantly t o colloidal theory. I t is difficult t o evaluate whether or not this has been a wise procedure. D a t a from different laboratories reveal that there is good agreement in the mobility behavior of the sol with barium chloride. This is notwithstanding the fact t h a t the sols were prepared by different investigators working under different conditions and were studied by different techniques. On the other hand, the disagreement in the results t h a t have been reported with potassium chloride are quite as striking. Thus Kruyt, Roodvoets, and van der Willigen (13) obtained a curve similar to t h a t shown in figure 2, in which the mobility of the sol particles increased continuously on the addition of potassium chloride. Freundlich and Zeh (6), however, found only a decrease in mobility on adding this electrolyte, while Kruyt and Briggs (12) obtained an S-shaped curve. Mukhcrjee, Roychoudhury, and Rajkumar (18) found t h a t the mobility decreased a t low concentrations and then increased without falling off a t the highest concentration. I t was on the basis of these behaviors t h a t Mulrherjee (17) rejected the whole concept of criticid mobility.
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FRED HAZEL
10. In concentrated solutions electrophoretic effects are more prominent and modify the effect of the electric charge on the mobility of the particles. 11. The stability of the system is greatest when both the electric charge of the particles and the thickness of the diffuse layer have high values. 12. When the double layer is thick, the electric charge must be decreased to a lower value to produce coagulation than in the case of a thin or compressed layer (8). 13. During coagulation with polyvalent ions which are strongly adsorbed, the electric charge is markedly decreased. This results in a correspondingly low electric mobility and, since this phenomenon occurs at a low concentration of added electrolyte, the ionic atmosphere is diffuse. Accordingly, the electric mobility must be reduced to a low value before coagulation occurs. 14. During coagulation with electrolytes of the potassium chloride type, where adsorption of the ions of opposite charge is less pronounced, neither the electric charge nor the mobility is reduced to an appreciable extent. Accordingly, coagulation is brought about in this case by a compression of the double layer a t high electrolyte concentrations. The above considerations explain why monovalent ions coagulate colloidal suspensions a t higher critical mobilities than polyvalent ions. The curves in figure 1 may be explained from the same standpoint. The polarizable silver ion was strongly adsorbed, thereby decreasing the mobility to a marked extent, which fact enabled coagulation to occur at a low concentration of electrolyte. The potassium ion was less strongly adsorbed and the mobility of the particles was decreased to a smaller extent. Coagulation occurs a t the critical mobility in this case by virtue of a compression of the diffuse layer a t the higher electrolyte concentration. The results in figure 1 are in general agreement with those of Briggs (2) in respect to the relationship between the critical mobilities of monovalent ions of different. adsorbabilities and their coagulating concentrations. I n order to account for the mobility curve of arsenic trisulfide with potassium chloride observed in the present work, it is possible to assume strong preferential adsorption of the chloride ion, which caused an increase in the mobility of the particles far above the water value. It may be observed from figure 2 that the major part of the mobility increase was effected when only one-fourth of the flocculating concentration of potassium chloride was present. On further addition of the electrolyte, the major effect was, presumably, one of compressing the double layer until a t the flocculating concentration the layer of counter ions was crowded close to the particle. Coagulation occurred under these conditions, in spite of the high charge (with the resulting repulsive forces which opposed coagulation), because the mean electrical center of gravity of the counter ions was of the same order of distance away from the particles as the distance over which the attractive forces operated.
ELECTROLYTES IN HYDROPHOBIC SYSTEMS
737
In order to reconcile the above mobility curve with the proposals of Hauser and Hirshon, it is necessary to assume that the property of the dispersion medium to act as an oppositely charged electrical field was a t a maximum when no added electrolyte was present. The continuous increase in mobility on the addition of potassium chloride then can be explained by assuming that neither of the ions of the electrolyte was preferentially adsorbed and, therefore, both ions were mixed in the dispersion medium. The decrease in the “electroviscous effect” which accompanied the reduction of the electrical potential of the dispersion medium was then manifest by an increase in the mobility of the particles. While these investigators selected S-shaped curves of monovalent ions from the literature for testing their theory, they recognized that other types of curves have been reported. Accordingly, they outlined the conditions under which these departures may be expected. As evidenced by the data in figures 1 and 2, all of the mobility curves obtained in the present investigation represent departures in the above sense, and it should be pointed out that the systems were of such nature that none would be anticipated. It should be mentioned also, that the experiments cited by these investigators with bentonite (11) were with a highly purified and dilute system (0.024 g. per liter), where one might expect, on the basis of the above, an opposite heharior to that found. SCMMARY
1. The critical mobilities of several monovalent ions of different co-
agulating power have been determined for colloidal manganese dioxide. 2. These values were found to be higher the lower the coagulating power of the ions; and these results were found to be in agreement with those of Briggs for an arsenic trisulfide sol. 3. The fact that polyvalent ions have lower critical potentials than monovalent ions was wrified for manganese dioxide and arsenic trisulfide sols. 4. An explanation was offered to account for the higher critical mobilities of monovalent ions. 5. Compression of the double layer by monovalent ions as a function of their concentrations was considered to be of first importance in the coagulation of sols by ions of this valence type. 6. Reduction in the charge of the particles by adsorption was considered to be the controlling factor in coagulation by ions of higher valences. 7 . Some recent theories treating the connection between electric mobility and stability were discussed. REFEREKCES (1) ABRAMSOX: Elektrokznelzc Phenomena, p. 101. The Chemical Catalog Company, Inc.,S e a York (1931). (2) BRIGGS:J. Phys. Chem. 34, 1326 (1930).
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FRED HAZEL
BURTON AKD BISHOP:J. Phys. Chem. 24,701 (1920). FISHER AND SORUM:J. Phys. Chem. 44, 62 (1940). FREUNDLICH AND SCHOLZ: Kolloid-Beihefte 16, 267 (1922). FREUNDLICH AND ZEH: Z. physik. Chem. 114, 65 (1925). GURNEY:J. Chem. Phys. 6,499 (1938). HAMAKER: Hydrophobic Colloids. The Nordemann Publishing Company, Inc., New York (1938). (9) HARDY:Z. physik. Chem. 35, 385 (1900). (10) HAWSER AND HIRSHON:J. Phys. Chem. 43, 1015 (1939). (11) HAZEL:J. Phys. Chem. 42,409 (1938).
(3) (4) (5) (6) (7) (8)
(12) KRUYTAND BRIGGS:Proc. Roy. Acad. Amsterdam 52, 384 (1929). (13) KRUYT,ROODVOETS, AND V A N DER WILLIGEN: Colloid Symposium Monograph 4, 304 (1926). (14) KRUYTAND V A N DER WILLIGEN:Proc. Roy. Acad. Amsterdam 29,484 (1926). J. Chem. Phys. 6, 873 (1938). (15) LANGMUIR: J. Chem. SOC.67, 67 (1895). (16) LINDERAND PICTON: (17) MUKHERJEE AND RAICHOUDHURI: h’ature 122, 960 (1928). AKD RAJKUMAR: J. Indian Chem. SOC. 10, 27 (18) MUKHERJEE,ROYCHOUDHURY, (1933). (19) OSTWALD: Kolloid-Z. 73, 301 (1935). (20) Powrs: J. Chem. SOC.109, 734 (1916). (21) WEISER: J. Phys. Chem. 28,232 (1924). J. Phys. Chem. 59, 925 (1935). (22) WHITEAND MONAGHAN: (23) WILLEYAND HAZEL:J. Phys. Chem. 41,699 (1937).
EFFECTS OF ELECTROLYTES I N HYDROPHOBIC SYSTEMS. I1
SOLCONCEKTRATION AND ELECTROLYTE STABILITY F R E D HAZEL
Department of Chemistry and Chemical Engineerzng, University of Pennsylvania, Phaladelphza, Pennsylvania Received October $0, 1940 INTRODUCTION
The influence of sol concentration on the flocculation values of electrolytes, while known for some time (7) and given prominence by Burton and coworkers (1, 2), has been the subject of recent investigations (3, 10). It is now definitely established that higher concentrations of electrolytes which have high precipitation values, e.g., certain uni-univalent salts, are required for the coagulation of dilute sols than are needed for the precipitation of more concentrated systems. This behavior appears to be general, however, only with regard to well-purified sols (3). The obvious bearing of this phenomenon on the problems of electrolyte coagulation
