1334
LEONARD N. RAY, JR., AND A. WITT HUTCHISON
fairly well with a relationship derived by averaging molecular areas. Similar agreement was reported previously for the system carbon tetrachloride-methanol. All the partial-pressure data for a given component were found to fit common curves within experimental error, regardless of surface composition, when the fraction of the surface covered (0 = na/+)was plotted against the relative partial pressures (pressures relative to those a t 0 = 1). This is shown to imply that deviations from the two-dimensional gas law for a given component (i.e., the value of FiS/n,RT) are a function of 0 only and are independent of the composition of the surface. Total spreading pressures have been evaluated from the semi-log partialpressure isotherms. The data are discussed from kinetic and molecular standpoints. REFERENCES (1) FELLER, M., AND MCDONALD, H . J.: Anal. Chem. 22, 338 (1950). W. D., A N D JURA, H. J.: J. Am. Chem. SOC.66, 1362 (1944). (2) HARKINS, (3) INNES, W. B., AND ROWLEY, H. H . : J. Phys. & Colloid Chem. 61, 1154 (1947). (4) LEE, S. C.: J. Phya. Chem. 36, 3558 (1931). (5) NIINI, A.: Ann. Acad. Sci. Fennicae A49, 16 (1938). (6) ROWLEY,H. H., OLNEY,R. B., AND INNES, W. B.: J. Am. Chem. SOC.72, 5180 (1950). (7) SCATCHARD, G . , WOOD,S. C., A N D MOCHEL,J. M.: J. Am. Chem. SOC.68, 1957 (1946). (8) SCHMIDT, G. C.: 2. physik. Chem. 121, 221 (1926).
ELECTROPHORETIC MOBILITIES OF CARBOX I N DILUTE SOAP SOLUTIONS LEONARD N . RAY, JR.:
AND
A. W I T T HUTCHISON
School of Chemistry and Physics, The Pennsylvania Slate College, State College, Pennsylvania Received August 17, 1960
The formation of relatively stable suspensions of solids in solutions of soaps and other detergents is of both theoretical and practical interest. It has been suggested that one of the principal factors involved in suspending power is the zeta potential of the particles in such suspensions. Urbain and Jensen (16), for example, measured the electrophoretic mobilities of carbon particles suspended in solutions of the sodium salts of a number of fatty acids and found a correlation between the suspending power of these solutions as visually observed and the mobilities of the particles therein. More recently, Greiner and F'old (7) examined the suspending power with respect to particles of manganese dioxide of solutions of sodium oleate and a number of synthetic detergents. F'old and Presented in part before the Division of Colloid Chemistry a t the 116th Meeting of the American Chemical Society a t Atlantic City, iSew Jersey, September, 1949. Fellow, Ellen H. Richards Institute, 1948-50.
ELECTROPHORETIC MOBILITY OF CARBON IN SOAP SOLUTIONS
1335
Konecny (17) measured the suspensibility of carbon in simiiar solutions and concluded from the nature of the relationships between suspending power and concentration obtained in these investigations that the zeta potential was one of the dominant variables. X o direct measurements of potentials, however, were made in these studies. In these laboratories Stubblebine (14), in connection with a general study of certain factors involved in detergency, measured the electrophoretic mobilities of carbon and cellulose particles in aqueous solutions of a commercial soap, with and without the addition of various alkaline builders. His results were correlated with those reported by Oesterling (12) in laboratory tests of soil removal from fabrics by these same detergent solutions. In the present work, the electrophoretic mobilities of carbon particles suspended in dilute solutions of a commercial soap were determined with a vertical microelectrophoresis cell over a concentration range extending to 1.0 per cent soap a t 25OC. In addition, the suspending power for carbon of the same solutions was measured. The results obtained show the zeta potential to be a principal factor in these suspensions. That a correlation of zeta potential with suspensibility is not necessarily valid in all suspensions has been pointed out recently by Doscher (4), whose results in studies with carbon suspensions in cationic and nonionic detergent solutions showed that other factors were operative in these systems. MATERIALS AND APPARATUS
The soap used in this study was a low-titer soap commercially available from the Proctor and Gamble Company and marketed under the name of P & G Olate Flakes. It was made from a mixture of oils consisting of approximately 75 per cent oleic acid, 15 per cent linoleic acid, and 10 per cent saturated acids. The following has been given as a typical analysis: real soap, 94.2 per cent; water, 4.6 per cent; salt 1.2 per cent; free alkali as NalO, less than 0.01 per cent; titer, 12°C. The carbon utilized was a sample of Korit C obtained from the American Norit Company. It was reported (8) as consisting largely of particles between 2 and 10 microns in diameter (of which only 3 t o 4 per cent exceeded 44 microns in diameter) and to be grease-free. Preliminary experiments on the mobility measurements were made in a horizontal-type cell as described by Briggs (2). In investigating suspensions of carbon which were relatively unstable, however, it was felt that the settling of carbon on the bottom of this cell during the course of the measurements was responsible for difficulties experienced in correlating data obtained a t different depths with the theoretical values for such an apparatus. As a consequence, an apparatus was constructed in which the measurements were made in a cell arranged vertically. The apparatus resembled substantially that described by Abramson, Moyer, and Voet ( 1 ) . The inside dimensions of the cell itself were a s follows: length, 55 mm.; width, 22.5 mm.; depth, 0.923 mm. The electrodes by
1336
LEONARD N . RAY, JR., AND A. WITT HUTCHISOX
which the potential was supplied consisted of a pool of mercury covered with a saturated solution of mercuric nitrate. Connection to external leads was made by means of tungsten wires sealed through the glass. Potential was supplied by six 45-v. B batteries, and a DPDT switch in the circuit permitted a convenient reversal of polarity. A multirange microammeter in series with the cell provided a measure of the current flowing through the cell. The motion of the particles in the cell was observed with a microscope equipped with an 8-mm. objective and a l o x ocular fitted with a scale which had been calibrated with a stage scale. The cell was illuminated with a high-power microscope fitted with a water cell to eliminate heating effects. The conductivities of the suspensions of carbon in the dilute soap solutions a t 25°C. were measured in a Freas-type cell with a Leeds 6: Northrup precision conductance bridge. In connection with the studies of suspending power use was made of a Fisher electrophotometer for the determination of the amount of carbon in suspension. EXPERIMENTAL METHODS
Soap solutions of the desired concentrations were made by the dilution of a stock solution. The carbon suspensions were prepared by adding 0.2 g. of the carbon, weighed to 1 mg., to 100 ml. of the soap solution in a small bottle. The bottle was shaken and allowed to stand for 30 min. before removal of samples for the conductivity and mobility measurements. This procedure eliminated the grosser particles or agglomerates from the samples to be studied. The electrophoresis cell was prepared for use by cleaning with chromic acid solution and rinsing with tap and distilled water. The cell was loaded by the slow displacement of the water in the cell with the carbon suspension to be studied. The cell then was shut off from the rest of the apparatus by means of the three-way stopcocks, and 0.01 N potassium nitrate solution was drawn up the arms of the apparatus. The electrode chambers then were inserted, and electrical contact was established by proper adjustment of the stopcocks. In using the vertical cell, the effect of gravity mas eliminated by timing the same particle when its direction of motion under the electrical field first was aided and then was opposed by gravity, recording the total time for the completion of the round trip. In general, observations were made on about twenty particles a t the 21 per cent level with a like number a t the 79 per cent level. These were found to be the levels a t which effects due to electroendosmosis are eliminated in a cell of the type used in this assembly (10, 13). In these experiments, measurements of the velocities were made a t the temperature prevailing in the laboratory. In the calculation of the mobilities, however, the specific conductance of the solution a t 25°C. was utilized. If the reasonable assumption is made that the only effects of importance associated with a temperature change are those dependent on the viscosity of the medium, and if, further, the effect of changing viscosity on the conductance and on the velocity
ELECTROPHORETIC MOBILITY O F CARBON I N S0.4P SOLUTIOKS
1337
of motion of the particle is the same, a value of the mobility calculated in this manner should represent substantially the mobility a t 2 5 T . The influence of temperature on viscosity and on velocities of suspended particles has been discussed by Burton (3) and by Gilford (6). That such a simplification is possible in the treatment of the data of these experiments was shown by making actual measurements on the velocity of motion of the particles a t about 25°C. in a particular experiment and then comparing mobility values based on these data with others obtained by velocity measurements a t 16°C. and 30°C. The results were in good agreement. The preparation of the carbon dispersions for the determination of the suspending power of soap solutions of various concentrations was precisely the same as that outlined for the mobility measurements. 500
=
00
01
02 03 PCRCCNT 30AP
04
05
00
01
02
03
5
PEKCLNT S O A P
FIG.1 FIG. 2 FIG.1. Electrophoretic mobilities of carbon in dilute soap solutions FIG.2. Plot of carbon-suspending power against concentration of soap solutions
Two different experimental methods were used in determining the amount of carbon suspended. I n the first method, 50 ml. of the suspension mas placed in 50-ml. Kessler tubes; the tubes were stoppered and allowed to stand undisturbed for 48 hr. At the end of this time 10 ml. of the suspension was withdrawn from the upper part of the tube. The turbidities were read with a Fisher electrophotometer. In the gecond method the remainder of the suspension was filtered through qualitative filter paper. The filtrates then were diluted and their turbidities determined with the electrophotometer. RESULTS
The values obtained for the mobilities of carbon are listed in table I ; and in figure 1, the values up to 0.5 per cent are represented graphically. The data from 0.5 per cent to 1 per cent were omitted from the figure because it was felt that, on the extended scale required for the graph, the data for the lower concentrations could not be represented satisfactorily. The inclusion of these data yields a smooth curve showing a small decrease in mobility a t higher concentrations. In all cases investigated, the carbon had a negative charge and moved toward the anode under applied potential. The striking feature of the mobility-concen-
1338
LEONARD N . RAY, JR., AND A. WITP HUTCHISON
tration curve is the very rapid rise of mobility in dilute solutions. From a value of 1.65 in distilled water, which is in agreement with that reported by Tsai and Chiang (15) and by Stubblebine (14), the mobility has risen rapidly and a t 0.1 per cent has attained a value of 5.0, which is near the maximum of 5.5 found a t 0.5 per cent. The mobility begins to decrease a t concentrations greater than 0.5 per cent, resulting in a value of 5.36 a t 0.75 per cent and continuing to decrease to 5.10 a t 1.0 per cent. It can be seen in figure 2 that, when values of the photometer reading of the carbon suspensions are plotted against the concentration, the resulting curves show the same rapid rise to about 0.1 per cent soap. Then, as in the case of the TABLE 1 Mobilitu- of. carbon varlicles i n soav solutions SOAP
wcighl pn c e d
0.00 0.005 0.010
0.020 0.025
0.035 0.050
MOBILITY
r/scc./r./cm.
1.65 2.27 2.80 4.23 4.37 4.51 4.74
SOAP
wcighl per c c d
0.075 0.100 0.150 0.250 0.500
0.75 1.00
MOBILITY
p/sec./n./cm.
4.86 5.01 5.16 5.42 5.49 5.36 5.10
mobility-concentration curve, the values maintain a flat maximum over approximately the same range of concentrations. A slight decrease in suspending power a t higher concentrations of soap follows. Essentially the same results were obtained by both methods of measuring the suspending power. The curves of mobility-concentration and carbon suspended-concentration bear a close resemblance to a typical adsorption isotherm. I t was noted on shaking soap solutions to which carbon had been added, that no stable foam persisted in the solutions with initial concentrations less than 0.1 per cent, and that, even a t this concentration, the foam lasted only a few minutes. Solutions as dilute as 0.005 per cent produce a stable foam when no carbdn is present. These observations indicate that the carbon had adsorbed most of the soap from the dilute solutions. If a specific surface area of GOO sq. m. for the carbon and a cross-sectional area of 46 A,z (11) for the soap are assumed, calculation shows that 0.2 g. of the carbon will require all of the soap in 100 ml. of 0.13 per cent solution in order to become covered with a monolayer. As was stated previously, the mobility has reached nearly its maximum value at this concentration. These findings are in agreement with the suggestion of Kathju (9) that a complete monolayer is necessary to give the most stable dispersions. The experimental results of this study are in general agreement with the findings of Urbain and Jensen (16), who worked with carbon in solutions of sodium oleate made according to Ferguson and Richardson (5). Their studies,
ELECTROPHORETIC MOBILITY OF CARBON IN SOAP SOLUTIONS
1339
however, were not extended to the concentrations in which the mobilities were found to decrease in solutions of P & G Olate Flakes. DISCUSSION
Urbain and Jensen (16) appear to have been the first to suggest that the suspending power of soap solutions is primarily dependent upon the zeta POtential. The results of the present investigation clearly confirm this conclusion in the case of the soap solutions examined. The recent work of Greiner and Vold (7) on suspensions of manganese dioxide powder and of Vold and Konecny (17) on carbon in various detergent solutions yielded results which these authors interpreted in terms of a mechanism of deflocculation of aggregates by the action of the detergent. I n this deflocculation, the zeta potential on the particle is presumed to play a dominant role. This conclusion was reached from a consideration of the general character of the curves obtained for suspending power versus detergent concentration; and it is of interest that the present work appears to provide additional experimental confirmation of this concept in the single example studied. N o detailed comparison of the two studies has been made, since the concentration ranges studied as well as the detergents used are not identical. In the light of the recent work of Doscher (4)it is probable that in solutions of higher concentration than those of the present study the nature of the adsorbed species may become an important factor. Since the above papers include an extended discussion of other factors which may be of interest in the problem of suspending power, together with a full bibliography of references to much of the pertinent earlier literature, these will not be repeated here. SUMMARY
This study has shown the applicability of the vertical microelectrophoresis cell for measuring the mobility of carbon particles in suspensions. Values were obtained for the electrophoretic mobility of carbon in dilute soap solutions. The ability of these same solutions to suspend carbon was studied, and the results were correlated with the mobility measurements. It was concluded that the zeta potential is a primary factor in producing stable suspensions of carbon in soap solutions. REFERENCES (1) A B R A M S O N &TOYER, ,
VOET:J. Am. Chem. SOC.68, 2362 (1936). (2) B R I G G SInd. : Eng. Chem. 2, 703 (1940). (3) B U R T O S :Phil. M a g . 67, 557 (1909). (4) DOSCHER: J. Colloid Sci. 6, 100 (1950). AID R I C H A R D S OInd. N : Eng. Chem. 24,1929 (1932). (5) FERGUSON : Mag. 19, 853 (1935). (6) G I L F O R DPhil. (7) G R E I X E R AND VOLD:J . Phys. & Colloid Chem. 69, 67 (1949). (8) K A A T Z(The American Norit Company) : Private communication. AND
1340
A. L. G . REES
I
