STABILITY OF LYOPHOBIC ORGAXOSOLS
BY HARRY B. WEISER AXD GCILFORD L. X 4 C K
Quantitative observations on the stability of organosols corresponding to the hydrosols are almost entirely lacking. Bikerman' measured the change in the (-potential of arsenic trisulfide particles dispersed in acetoacetic ester and in nitrobenzene on adding ferric chloride, copper acetoacetate, and tetraethyl ammonium iodide; and Errera* observed the time required to coagulate alcosols of mercuric sulfide, cupric sulfide, and platinum by the addition of liquids of varying dielectric constant. But, for the most part, investigations on the stability of sols in the presence of organic liquids have been confined to a study of the effect of the o r g h i c liquid on the stability of hydrosoIs. In the experimental part of this paper are reported some observations on the effect of adding water and organic liquids to organosols stabilized by preferential adsorption of ions, together with similar observations on the effects of adding organic liquids to hydrosols. This is followed by a discussion of the several factors which may influence the stability of such colloidal systems. As a measure of the stability of hydrosols toward electrolytes, it is usual to record the precipitation concentration of the electrolytes or to follow by cataphoresis experiments the change in the so-called {-potential of particles on adding the electrolytes in question. Both of these procedures have been used.
Preparations Observations previously reported3 show that some organosols are very sensitive to slight traces of moisture. Accordingly special precautions were taken t o prepare and preserve the organic liquids free from moisture. In all cases the purest material on the market was employed. The alcohols were all refluxed over lime for several hours and distilled, the first and last portions of the distillate being rejected. The middle portion was again refluxed over a small amount of lime for 8 to I O hours before distilling and only that part of the distillate coming over within 0.8' on either side of the correct boiling point was used. A good grade of anaesthetic ether was first dried over calcium chloride for several days, decanted and distilled off phosphorus pentoxide, practically the whole amount coming over within 0.5' of 35'. The benzene was anhydrous and thiophene free and distilled within one degree of 80.4. The water was double distilled in a block tin condenser. Solvents.
Z. physik. Chem., 115, 261 (1925).
* Kolloid-Z., 32, 240 (1923). 3
Weieer and Mack: J. Phys. Chem., 34, 86 (1930)
102
H.4RRY B. WEISER A S D GCILFORD L. MACK
Sols. Mercuric sulfide-methyl and propyl alcosols and ferric oxide-propyl alcosol were prepared as described in detail in the preceding paper.' Hydrosols of mercuric sulfide were prepared from mercuric cyanide (a) by a peptization method and (b) by a condensation method similar to that used in preparing the propyl alcosol.2
Salt Solulio~zs. For studying the precipitation of organosols by electrolytes, solutions of lithium chloride and of calcium chloride in both methyl and propyl alcohol were prepared. No suitable salt with a divalent anion could be found. The salts were purified by recrystallizing C.P. grade chemicals and dehydrating in a current of dry hydrogen chloride to prevent the formation of oxychloride. Any trace of HCl was removed by heating in a carefully dried current of air. The stock solutions were all made up by weighing both solute and solvent into cork stoppered Erlenmeyer flasks of about 50 cc capacity. The densities of each solvent for several temperatures were calculated from the equations given in International Critical Tables, Volume 111, pages 2 7 , 28. The values were plotted and smooth curves drawn from which the densities a t any particular temperature could be read off. From these data the concentrations of the solutions in milliequivalents per liter were calculated. The lower concentrations were made up by the method of successive dilutions either by direct weighing or by volume, using accurately calibrated pipettes.
Precipitation Experiments In all experiments six-inch Pyrex test-tubes previously steamed out and dried were employed. The stoppers were of cork purified by long boiling in distilled water and thoroughly dried. Five-cubic-centimeter portions of sol were first measured out, then sufficient alcohol to make the final volume I O cc after the addition of the salt solution, and finally, the salt solution. By using care in adding the alcohol and salt solutions it was possible to avoid premature mixing. As already noted, special precautions were taken to exclude water at all times. Mercuric Sulfide Methyl Akosol. After determining the approximate concentration of electrolyte necessary for complete coagulation of the sol, a series of concentrations in this region were thoroughly mixed with samples of the sol and were allowed to stand quietly for 2 hours. The tubes were centrifuged for I O minutes a t 2000 r.p.m. By this procedure the effect of differences in concentration as low as I X 10-6 equivalents per liter were easily distinguishable. The critical concentration of CaCL was found to be 0.095 milliequivalent per liter and of LiCl 4.7 milliequivalents per liter. These values are very much smaller than for similar salts on the mercuric sulfide hydros01.~ J. Phys. Chem., 34, 86 (1930). Schucht: Z. physik. Chem., 85,643 (1913). 8Cf. Freundlich and Schucht: Z. physik. Chern., 85,641 (1913)
* Freundlich and
STABILITY O F LYOPHOBIC ORGASOSOLS
103
Mercuyic Sulfide Propyl Alcosol. Since methyl alcohol is so hydroscopic it was very difficult to avoid contamination with water. Accordingly, subsequent experiments were carried out with dispersions in propyl alcohol. The sol was prepared by peptizing the washed precipitated sulfide with hydrogen sulfide and an aged preparation formed by precipitating the sulfide from mercuric cyanide solution at yoo, peptizing the precipitate with excess H2S and washing very thoroughly with hydrogen.' The precipitation concentrations were determined for the pure sol and for the sol to which water was added. Considerable difficulty was encountered in finding a sensitive and reliable end-point. The sol was so much more polydisperse than the methyl alcosol that a satisfactory procedure for the latter was not suitable for the former. With lithium chloride as precipitating agent it was found that good results could be obtained by measuring the time at which a clear supernatant liquid just appeared on adding varying amounts of electrolyte, plotting the time against the concentration, and reading off the concentration which would just cause coagulation in two hours. This time was selected as it was about the shortest which fell on the flat of the curve so that a small difference in concentration would produce a large variation in time. The results of a series of observations are given in Table I and shown graphically in Fig. I .
TABLE I Precipitation Concentrations of LiCl for HgS Propyl Alcosol, Pure and in the Presence of H 2 0 Precipitation concentration milliequivalents per liter
Percent water
0
3.3
22
6.4
5 '5
3.7
25
9.2
Percent water
Precipitation concen tration milliequivalents per liter
4,3
In order to get a satisfactory measure of the precipitation value ofCaC12 for the sol, pure and in the presence of water, it was found advisable to let the mixture of sol with electrolyte stand for 24 hours before centrifuging to determine whether coagulation was complete. The results are given in the second column of Table I1 and plotted in Fig. I . It will be noted that the addition of water to the alcosol stabilizes it to a marked degree. (Cf. Table YIII).
A second set of observations were made on a portion of the same sol that had been aged for one month. During this time there was a gradual transformation from the finely divided solvated form to a granular crystalline form as evidenced by the change in color and the decrease in stability shown by the results given in the third column of Table I1 and in Fig. I . The Cf. J. Phys. Chem., 34, 86 (1930).
HARRY B. WEISER AND GUILFORD L. MACK
104
somewhat higher precipitation value of the pure aged sol as compared x i t h the fresh sol may be due to dilution of the former as a result of appreciable coagulation during standing. The effect on the precipitation value of CaC12, of adding a liquid of lower dielectric constant than propyl alcohol to the mercuric sulfide propyl alcosol is given in Table I11 and shown graphically in Fig. 2. It will be seen that the addition to the propyl alcosol of isoamyl alcohol, dielectric constant 17, decreases the stability toward electrolytes while the addition of water, dielectric constant 80, increases the stability toward electrolytes.
O l O t
9 Q
."
i
0 2 0 ~
3$ B Y
aio $
0
Io
40
20
%WATER
FIG.I Precipitation Concentrations of Salts for HgS Propyl Alcosol-Kater Mixtures.
TABLE I1 Precipitation Concentrations of CaC12 for a freshly Prepared and Aged Propyl Alcosol Pure and in the Presence of R a t e r Percent water
Precipitation concentration milliequivalents per liter Freshly prepared Aged sol sol
Percent water
Precipitation concentration milliequivalents per liter Freshly prepared Aged sol sol
0
0.0044
0.012
20
0.0310
5 6
0.oo;o
25
o.oj20
30
0,0820
IO
0.01 19
___ 0.013 0.014
35
0.1300
15
0.0190
40
0.23 j
17
___
45
0.3350
0.OIjj
o
___
0.0185
___
0.0340 0.048; 0.0700
STABILITY O F LYOPHOBIC ORGANOSOLS
TABLE I11 Precipitation Concentration of CaClz for an Aged HgS Propyl Alcosol Pure and in the Presence of Isoamyl Alcohol Percent Precipitation concentration amyl alcohol milliequivalents per liter
Percent Precipitation concentration amyl alcohol milliequivalents per liter
0
0.0120
20
0.0098
5
0.011s
0.0080
IO
0.01IO
30 40
0.0065
FIQ.2 Effect of Dielectric Constant of the Medium on the Stability of Sols toward CaC12
TABLE IV Precipitation Concentration of CaClz on HgS Hydrosol Pure and in the Presence of Propyl Alcohol Percent by volume of propyl alcohol
Precipitation. concentration milliequivalents per liter
0
0.0212
5
0.014
IO
0.009
16
0.000
20 50
Precipitated, less than 4 hours Precipitated, less than j hours
\
I 06
HARRY B. WEISER A X D GUILFORD L. MACK
In the next experiments the effect on the precipitation concentration of CaCh of adding propyl alcohol to a mercuric sulfide hydrosol was determined. This is the reverse of the process of adding water to the alcoaol and the results show just the opposite effect on the stability, as indicated by the data given in Table IV and shown in Fig. 2. The effects are qualitatively the same as those obtained by adding amyl alcohol to the mercuric sulfide propyl alcosol, the lowering of the dielectric constant in both cases producing a sensitization toward electrolytes. This is in line with Errera’s observations.
Cataphoresis Experiments There are three general methods for the measurement of the migration velocity of colloidal particles under the influence of a potential gradient: The velocity can be calculated from the change in concentration of the sol in a Hittorf transference cell; the movement of the interface between the sol and the upper layers of the dispersion medium in a C-tube can be measured; and the motion of the individual particles in a capillary tube may be observed directly in the ultramicroscope. Svedberg and Anderson’ have reviewed the voluminous literature on these three methods and have discussed critically the sources and magnitudes of the errors in each. Not much can be expected from Duclaux’s transference method,* as the analysis seriously limits the accuracy and the electrolysis effects interfere with the change in concentration of the dispersed particles. The moving boundary method is very useful in certain cases, but it is subject to the same errors arising from conductance of the current by electrolytes, as in the former case. Since these errors are also present in the ultramicroscopic method, they may be enumerated and the conditions set down under which they will be a t a minimum. First, the ions present in the solution will move much faster than the colloidal particles, and as the ionic concentration in the surrounding medium changes the particle charge will also change. This is the most serious error involved, and it can only be lessened by taking shorter intervals of observation. Second, the polarization a t the electrodes introduces an error which may be eliminated by using a non-polarizing cell of the type introduced by Michaelis.3 The percentage error due to this source can be reduced by raising the potential gradient, and it becomes negligible with several hundred volts. Third, the heat generated by the electric current will affect the velocity. This may be reduced, but not obviated by either reducing the voltage or the length of time through which it acts. Thus there are two sets of conditions under which the combined effects due to the conduction of the current by electrolytes present in, or added to the sol will be at a minimum. The method developed by Kruyt makes use of macroscopic o b ~ e r v a t i o n . ~ A low potential is used to reduce the Joule effect Kolloid-Z., 24, 156 (1919).
* J. Chim. phys., 7, 405 (1909). Biochem. Z.,16, 81 (1908). Kolloid-Z., 37, 358 (1925).
STABILITY O F LYOPHOBIC ORGANOSOLS
107
and the polarization potential is absolutely eliminated. But the current must be allowed to flow for a considerable time, so that the change in ion concentration is a very serious error. The alternate procedure was chosen by Svedberg and Anderson.’ They used ultramicroscopic observation with a potential, thus obtaining small heat and polarization effects. The change in ion concentration is still appreciable, but is much less than in the former method because the observation time can be reduced to a small fraction of that necessary to produce a measurable migration in the U-tube. Kruyt objects to the very shallow cell and to the fact that gas bubbles may affect the results in a closed system. He devised a similar apparatus with an open cell, but was unable to obtain results with it which checked each other with an accuracy equal to that attained by Svedberg. If these were the only considerations, the U-tube method would probably be slightly preferable. But there is another very large error involved in the non-uniform gradient. Even when the upper liquid consists of theultrafiltrate from the sol, its conductivity is less. I n organic dispersion media this error appears to be greatly magnified, as evidenced from the results of Mukherjee2 and of K10sky.~ Other difficulties are the need of special apparatus for the observation of colorless sols and the inability to maintain a sharp interface a t the boundary, especially with large amounts of organic liquid present. On the basis of these considerations, the method of direct measurement of the velocity of individual particles was adopted. For this purpose the apparatus designed by Mattson was employed. This consists essentially of an ordinary ultramicroscope with a specially constructed cell.4 I n addition to the migration of the particles in a capillary tube, electro osmotic currents are set up by the external field of force. This motion of the liquid will affect the observed velocity of the particles. At the glassliquid interface, the Helmholtz double layer is formed. I n all the pure dispersion media employed, the glass is negatively charged but since small amounts of electrolytes easily change the sign of the charge, the adjacent liquid maymove toward either electrode. Since the system is closed it follows that an amount of water equal to that transported along the walls must return in the opposite direction through the center of the tube. The observed velocity V’ is therefore, given by the equation V’ = V v where V is the velocity of theliquid. Hence toobtainthe truecataphoreticvelocity, the motion of the liquid must be evaluated. This is accomplished indirectly by developing an equation expressing v in terms of the radius and the velocity a t a fixed point. From theoretical considerations Smoluchowski5developed an equation for the motion of a liquid in a rectangular capillary. For a round capillary this expression take st he form:^ = C(r2-a2/z)*wherer is the distance from the center of the capillary, a the radius of the capillary and C, a constant deter-
*
Kolloid-Z., 24, 156 (1919).
* J. Indmn Chem. Soc., 5, 697 (1928). J. Phys. Chem., 33, 621 (1929). For description, see J. Phys. Chem., 32, 1532 (1928). a
Graeta: “Handbuch der Elektrizitat und des Magnetismus,” 2, 1382 (1921).
I08
HARRY B . WEISER AND GUILFORD L. MACK
mined by the potential difference of the double layer. Now a t r = 0, v = vo and v = 2v0 (+/a* - 1 / 2 ) . Taking r a t various values of a in this equation, vis obtained as a function of vo. On putting these values of vo in the expression IT’= 1’ f v, a series of equations is obtained which may be solved simultaneously with the elimination of vo. V is thus obtained in any number of combinations of V’ a t two different depths of observations. Preliminary observations on a positively charged PBan de St. Gilles ferric oxide hydrosol showed that readings taken a t 1 / 2 and 3 / 4 of the radius gave the most consistent results. This compares favorably with the data of Svedberg and Anderson who found 1 / 2 a and 2 / 3 a to be the best for a rectangular tube. Similar observations on a negative mercuric sulfide sol in 50 percent propyl alcohol confirmed the observations that best results would be obtained by a combination of 1/2 a with 2/3 a or 3/4 a. For this reason Mattson’s method of taking a single reading at 0.707 a was not used. However, the method was tested using a negative silver sol protected by sodium “protalbinate”; and the results agreed quite well with Mattson’s for a quartz suspension. Knowing the migration velocity of the particles the electrokinetic potential, {, can be calculated from the Helmholtz-Perrin formula
where v = the velocity of the particles, H = the potential gradient in volts per centimeter, D the dielectric constant of the pure liquid, and 9 the viscosity of the liquid. I n the experiments to be described, all liquids were mixed by volume. I n every case an average of I O readings of the cataphoretic speed was taken a t each of the two depths. The dielectric constant of the mixtures was calculated on the assumption that i t is a linear function of the percentage composition by weight. The viscosities were taken from International Critical Tables. Moderate dilutions should not change the charge on the colloidal particle appreciably if the sol is highly pure. This is indicated by some observations with a ferric oxide sol containing 4 grams per liter prepared by the method of Sorum.’ The value of the {-potential in the original sol was 63.3 millivolts while in the sol diluted one-half it was 63.8 millivolts. This of course applies only to very pure sols since changing the concentration of electrolytes in contact with the particles will affect the charge on the particles. Thus Mukherjee found that the {-potential of the particles of a ferric oxide hydrosol containing free ferric chloride was increased by dilution. Propyl Alcohol. T h e first observations were Ferric Oxide Hydrosol made of the effect of adding pure liquids on the {-potential of the particles in sols containing only the stabilizing electrolyte in low concentration. The observations with a PBan de St. Gilles ferric oxide sol to which varying amounts of propyl alcohol and of acetone were added are given in Table T’. While the stability of the positive hydrosol is decreased by the addition of the
+
J. Am. Chem. Soc., 50, 1263 (1928).
I09
SThBILITY O F LYOPHOBIC ORGANOSOLS
TABLE V Effect on {-potential of adding non-Elect'rolytes to Ferric Oxide Hydrosol Non-electrolytes added percent by volume 0 2
jyc propyl alcohol
50Sc propyl alcohol 2 ~ acetone 7 ~ soyc acetone
Velocity of particles (r/sec) 7.60 2.46 I .48 3.69
11
Dielectric constant
f-potential millivolts
0.00846 0.0184
80 68
91 . o
0.0251
54
77.5
0.01161
66
70.8
2.98
0.01222
54
76.4
Viscosity
75.9
FIQ.3 r-potential of Particles on adding CaClr to HgS-Hydrosols-Propyl Alcohol Mixtures.
organic liquids, the decrease is not so marked as is usual with negative sols. Thus, in a previous experiment, 16ycalcohol was found t o precipitate completely a negative mercuric sulfide sol while the ferric oxide sol remained clear even when mixed with 50 percent of alcohol. Mercuric SulJide Hydrosol Propyl Alcohol and CaC12. Measurements were next made of the charge on the particles of mercuric sulfide and the precipitating action of electrolytes when propyl alcohol was added t o the hydrosol. For these experiments a sol was prepared according to the modified method of Lottermoser used in preparing the alcosol. I n order to prevent the ageing of the sol as much as possible and to ensure its stability, the
+
HARRY B. W E I S E R AND GUILFORD L. MACK
IIO
hydrogen sulfide was not removed completely from the preparation. The sol, alcohol, and electrolyte were mixed uniformly and allowed to stand two hours after which the mixture was thoroughly shaken and a sample transferred t o the cataphoresis cell for measurements. The effect of CaC12 on the {-potential of the particles in various mixtures is given in Table VI and shown graphically in Fig. 3.
TABLE TI Effect on the
