738
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 FRED 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
ELECTROLYTES I N HYDROPHOBIC SYSTEMS
739
has justified the recognition given to it in some of the more recent theories of coagulation (4, 8). The effects of electrolytes on the mobility and the stability of a manganese dioxide sol and of an arsenic trisulfide sol of different concentrations were studied in the present investigation. The same stock samples of sols were used in this work as mere employed in the critical mobility studies previously reported (5), and, in both cases, the experimental techniques were the same. RESULTS
Precipitation data for a manganese dioxide sol are shown graphically in figure 1, in which the percentage concentration of the sol is plotted
CONCENTRATION OF SOL, PER CENT FIG 1 Precipitation data for a manganese dioxide sol
against the ratio of the flocculation valde of each electrolyte, a t a given concentration of sol, to the flocculation value of the 100 per cent sol. Similar data for an arsenic trisulfide sol are shown in figure 2. Illustrative data showing the effects of electrolytes on the mobility of manganese dioxide sols of different concentrations are reported in figures 3, 4, and 5. In these figures, curve I represents the behavior of the 100 per cent sol (0.6 g. of manganese dioxide per liter), and curve I1 that of the 25 per cent sol, while curve I11 shows the mobility behavior of the 6.25 per cent sol with electrolytes. Curves showing the effect of barium chloride upon the mobility of the arsenic trisulfide system a t different concentrations are plotted in figure 6, where curves I, 11, and I11 refer to the 100 per cent sol (4.0 g. of arsenic
740
FRED HAZEL
trisulfide per liter), 50 per cent sol, and 25 per cent sol, respectively. Similar data with potassium chloride are shown in figure 7.
CONCENTRATON OF
SOL, PER CENT
FIG.2. Precipitation data for an arsenic trisulfide sol
MILLIMOLES KC PER LITER
FIG.3. Effect of potassium chloride on the mobility of manganese dioxide sols. Curve I, 100 per cent sol; curve 11, 25 per cent sol; curve 111,6.25 per cent sol.
Critical mobility and flocculation data for manganese dioxide sols and arsenic. trisulfide sols of different sol strengths are summarized tables
MILLIMOLES AqNO, PER
LITER
FIG.4. Effect of silver nitrate on the mobility of manganese dioxide sols. Curve I, 100 per cent sol; curve 11, 25 per cent sol; curve 111,6.25 per cent sol.
MILLIMOLES CrCI3 PER LITER
FIG.5. Effect of chromium chloride on the mobility of manganese dioxide sols. Curve I, 100 per cent sol; curve I I , 2 5 per cent sol; curve III,6.25 per cent sol. 741
742
FRED HAZEL
MILLIMOLES
h C l 2 PER LITER
FIG.6. Effect of barium chloride on the mobility of arsenic trisulfide sols. Curve I, 100 per cent sol; curve I I , 5 0 per cent sol; curve 111,25 per cent sol.
FIG.7. Effect of potassium chloride on the mobility of arsenic trisulfide sols. Curve I, 100 per cent sol; curve 11, 50 per ccnt sol; curve I I I , 2 5 per cent sol.
1 and 2. In the tables, mobilities are expressed in p per second per volt per centimeter, while flocculation values are reported as millimoles of electrolyte per liter.
743
ELECTROLYTES IN HYDROPHOBIC SYSTEMS DISCUSSION
Weiser and Milligan (10) have recently studied the effect of dilution on the adsorption of precipitating ions during the coagulation process. These investigators, while demonstrating that the amount of polyvalent ion adsorbed per gram of adsorbent increases with dilution of the sol, have called attention to the common observation that proportionately more electrolyte is required to precipitate dilute sols than concentrated ones, regardless of the valence of the precipitating ion. TABLE 1 Manganese dioxide sol 100 PER CENT SOL
I
6.25 PER
25 PER CENT SOL
CENT SOL
ELECTROLYTE
--i -millimoles I rrisec. / a per liter I m.
t/sec./u./ m.
KaOH. . . . . . . . . . . . . . . . . . . . . . . . . KC1. . . . . . . . . . . . . . . . . . . . . . . . . . . HCl.. . . . . . . . . . . . . . . . . . . . . . . . . . AgSOs. . . . . . . . . . . . . . . . . . . . . . . . BaCl,.. . . . . . . . . . . . . . . . . . . . . . . CrClS.. . . . . . . . . . . . . . . . . . . . . . . .
2.5 2.5 2.3 1.6 1.2 1.2
I
16.0 6.0 3.2 0.6 1 0.14 , 1 0.0521
1 ~
~
2.3 2.3 1.8 1.3 0.9 1.1
iI
___ iillimoles per liter
I'r/sec./r./ cm. , millimoles per liter
I
15.0 7.2 3.0 I 0.6 0.072i 0.022' ~
2.1 2.1 1.3 1.2 0.9 0.8
12.0 8.0 , 2.2 1 0.54 1 0.048 0.010 ~
~
~
TABLE 2 Arsenic trisulfide sol
1 1w PER CENT SOL
50 PER CENT SOL
25 PER
CENT SOD
ELECTROLYTE
r / s e c . / v . / millimoles cm. I perliter
KCI . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.0
BaCI,.. . . . . . . . . . . . . . . . . . . . . . . CrCls . . . . . . . . . . . . . . . . . . . . . . . .
1.4 1.3
64 '
1
0.32 0.036
The stabilizing effect of dilution can be interpreted in the light of the data in tables 1 and 2, where it is shown that the critical mobilitier of all ions (except that of potassium chloride with arsenic trisulfide) decrease with dilution. This behavior strongly suggests that the repulsive forces operating between the particles must be reduced t o a lower value, during the coagulation of dilute sols, in order to compensate for the decreased chance of collision of the particles (6). From the above considerations there appear to be two opposing factors which determine the effect of dilution on the coagulating power of ions: namely,-(I) the increased adsorption of ions with dilution, and ( 2 ) the lower critical mobilities M hich accompany dilution. Both of thcsc factors
744
FRED HAZEL
operate when sols of different strengths are coagulated with polyvalent ions. Thus, the increased adsorption of polyvalent ions with dilution of the sol tends to decrease the repulsive forces through the mechanism of reduction of the electric charge. This factor increases the precipitating power of polyvalent ions. On the other hand, the coagulating effect of these ions is opposed by dilution, since under this condition more adsorption is necessary in order to meet the requirement of a lower critical potential.' An additional factor and one of fundamental importance in determining the effect of dilution on the coagulation concentrations of polyvalent ions is the effect of dilution on the adsorbability of the ions. An inspection of the data in the paper by Weiser and Milligan (10, Table 111) shows that the adsorbability of the sulfate ion (as well as ferricyanide ion) by a ferric oxide sol is independent of the sol concentration. Thus, the same percentage of the flocculating concentration of this ion is adsorbed by the most dilute sol as by the most concentrated one. This behavior might lead one to expect that proportionately the same amount of the ion would be required to precipitate the sol, irrespective of its concentration. Such was not found to be the case, however, owing to the fact that a higher concentration of sulfate had to be adsorbed (presumably because of a lower critical mobility) to coagulate the particles of the dilute sol. The results show that, while the sol concentration was decreased nineteenfold, the flocculation value decreased by only approximately twelvefold. Data from the same paper show that, although the precipitation valuesol concentration curves for barium ions with both copper ferrocyanide and arsenic trisulfide sols decrease on dilution, the decrease in flocculation value on dilution is not as marked as in the case just cited. The data show, in this case, that the adsorbability of the barium ion decreased strongly with dilution of the sols: 84.5 per cent of the barium ions added in the coagulation of the sol containing 20 g. of copper ferrocyanide per liter was adsorbed, while only 32 per cent of the flocculating Concentration was adsorbed in the coagulation of the sol containing 1 g. per liter. Similarly, 61 per cent of the flocculating concentration of barium ions was adsorbed in the coagulation of a sol containing 30 g. of arsenic trisulfide per liter, while only 13 per cent of the flocculating concentration was adsorbed in the coagulation of the sol containing 1 g. per liter. Quite obviously, the greater the decrease in the adsorbability of a polyvalent coagulating ion on dilution of the sol, the greater will be the proportionate increase in stability of the sol, on dilution, toward that ion. 1 The greater adsorption of polyvalent ions by particles in dilute sols may be regarded as a necessary consequence of the lower critical mobilities which obtain under these conditions, since it is by a process of adsorption that the mobility is depressed to the critical value.
ELECTROLYTES IN HYDROPHOBIC SY6TEMS
745
The marked decrease in adsorbability of barium ions on dilution of the arsenic trisulfide sol, studied by Weiser and Milligan, appears to explain their observation that a t the highest dilution the precipitation value-sol concentration curve turned upward (9). As opposed to the characteristics which are associated with the coagulation of a sol of a given concentration with a strongly adsorbable polyvalent ion, the phenomena associated with the flocculation of the same sol by an electrolyte of the potassium chloride type are as follows: (1) a high concentration of electrolyte is required; and ( 2 ) coagulation occurs a t a high critical mobility. These phenomena have been discussed in a previous communication ( 5 ) , where it w&s proposed that compression of the electric double layer a t high electrolyte concentrations played an important r6le in the precipitation of sols with univalent ions of low adsorbability. From a consideration of those cases where dilution of the sol raises the flocculation value of a uni-univalent electrolyte, it is possible to differentiate, on the basis of electrophoretic behavior, between (a) systems in which there is no preferential adsorption of the ion of like sign and ( b ) systems in which a stabilizing ion is preferentially adsorbed on addition of the coagulating electrolyte. Examples of these two cases will be discussed in turn. Figure 3 shows that the addition of potassium chloride to manganese dioxide sols produced only a decrease in the mobility of the particles, indicating that potassium ions, and not chloride ions, were preferentially adsorbed. The adsorption of potassium ions was so moderate, however, judged from the effect on the mobility of the particles, that the systems would have remained stable had it not been for the electrolyte concentration effect in decreasing the thickness of the double layer. Although it is difficult to evaluate the bearing of this latter effect on the relationship of sol concentration to flocculation value, since both the probability of collision of the particles and the critical mobility are decreased by dilution, it is possible that the net result is manifest by an increase in flocculation value for the more dilutc. sols. Other factors which are forced upon this colloidal system by dilution, and which may contribute to the increase in flocculation value of potassium chloride, are the decrease in adsorbability of the (poorly adsorbable) potassium ion, and the lower critical mobilities of the more dilute systems which, accordingly, require greater adsorption of potassium ions for coagulation. While no direct experiments were conducted to determine whether chloride ions were preferentially adsorbed during the coagulation of the arsenic trisulfide sols with potassium chloride, the mobility curves in figure 7 give strong indications that this was the case. Since the adsorption of potassium ions can be regarded as negligible in comparison to the adsorption of chloride ions, the precipitation with potassium chloride may be
746
FRED
HAZEL
considered as being due to compression of the double layer a t high electrolyte concentrations. When this sol is diluted, both the increased adsorption of chloride ions and the decreased probability of collision stabilize the system toward the precipitating action of potassium chloride. Furthermore, since dilution of the sol does not cause a lowering of the critical mobility, the thickness of the double layer around the particles must be decreased to a greater degree in order for coagulation to occur. This is accomplished by an increase in the electrolyte concentration, SUMMARY
1. The effects of electrolytes on the mobility and the stability of a manganese dioxide sol and of an arsenic trisulfide sol of different concentrations have been studied. 2. A proportionate increase in stability upon dilution was observed with all electrolytes for both sols. 3. It was shown that dilution of the sols lowered their critical mobilities toward the electrolytes (with the exception of that of potassium chloride for arsenic trisulfide). 4. On the dilution of a sol, the proportionate increase in stability with strongly adsorbable polyvalent ions was ascribed to the decrease in adsorbability of the coagulating ions (although the amount adsorbed per gram may be greater) and/or to the lower critical mobility a t which coagulation occurs. Moreover, these same effects contributed to the increase in stability with moderately adsorbable univalent coagulating ions. 5 . The decreased probability of collision which accompanies dilution was regarded as of particular importance in the coagulation of sols with uni-univalent electrolytes whose stabilizing ions are preferentially ad-, sorbed. Since the dilution of the sol does not cause a lowerkg of the critical mobility] the double layer around the particles must be decreased to a greater degree for coagulation to occur; thus a greater addition of electrolyte is required. REFEREKCES
(1) BURTON AND BISHOP: J . Phys. Chem. 24,701 (1920). (2) BURTON AND MACINNES: J. Phys. Chem. 26, 517 (1921). (3) FISHER AND SORUM: J. Phys. Chem. 44,62 (1940). AND HIRSHON: J. Phys. Chem. 43, 1015 (1939). (4) HAUSER (5) HAZEL: J. Phys. Chem. 46, 731 (1941). (6) KRUYTAND VAN DER SPEK:Kolloid-2. 26, 3 (1919). J. Am. Chem. SOC.37, 2024 (1915). (7) MUKOPADHYAYA: (8) OSTWALD:Kolloid-2. 76, 39 (1936). (9) Cf., however, OSTWALD:Kolloid-2. 80, 304 (1937). (10) WEISERAND MILLIGAN: J. Am. Chem. SOC.62, 1924 (1940).
