SOAPS : ELECTRIC CHARGE EFFECTS AND ... - ACS Publications

J. Phys. Chem. 1935.40:821-832. Downloaded from pubs.acs.org by FLORIDA STATE UNIV on 09/09/15. For personal use only.

SOAPS : ELECTRIC CHARGE EFFECTS AND DISPERSING ACTION W. M. URBAIN

AND

L. B. JENSEN

Swift & Company, Chicago, Illinois Received April 84, 1058

INTRODUCTION

When a soap acts as a detergent, it functions in a number of ways. Two of its most important functions are its action as an emulsifying agent and as a deflocculating agent. By emulsification, soap suspends oily materials ; by deflocculation, it suspends “inert” materials. Essentially both processes are the same in effect, for each results in the dispersion of the foreign material in the soap solution in such fashion that it can be removed with the soap solution or rinse water. While oily material is suspended almost entirely by emulsification, some solvent action by the soap solution on oils undoubtedly occurs (11, 13). However, in the relatively dilute solutions used in most washing operations, the solvent powers of soap solutions are small. The purpose of this investigation was to consider the mechanism whereby soaps render emulsions of oily materials and suspensions of “inert” materials relatively stable. I n this way it was thought possible to arrive at a more complete picture of the detergent action of soap. THEORETICAL

I n the case of oily materials emulsified by soap solutions in a washing operation, it is doubtful that water-in-oil emulsions are ever formed; only oil-in-water emulsions result. Ellis (3) and Powis (12) have shown that the stability of oil-in-water emulsions is dependent upon the value of the electric charge carried by the oil droplets. The higher the value of the charge, the more stable is the emulsion. The oil droplets of an oil-in-water emulsion usually carry a negative charge. Since the hydrocarbon tail of the fatty-acid ion of a soap resembles the molecules of an oily material, the two should be more or less mutually soluble. If an oil droplet acquires the negative ion of a soap by dissolving this hydrocarbon tail, it also acquires the charge carried by the ion. I n this way the negative charge of the oil droplet may be built up, and hence the stability of the emulsion increased. 821


822

W. hl. CRBAIN AND L. B. JEhTSEN

In the case of the “inert” materials, it seems probable that a similar phenomenon would occur. Xost particles, when suspended in water, assume a potential negative to that of water. The adsorption of a negative ion by such a particle serves to increase the negative potential. The magnitude of this effect on the potential increases with the valence of the ion adsorbed. For instance, the ferrocyanide ion increases the negative potential of graphite more than does the chloride ion (1). McBain and his coworkers (8) have explained the anomalous conductivity of soap solutions by assuming the formation of colloidal micelles bearing a high electric charge-density. The formation of thew micelles involves the fatty-acid ions of the soaps, and the micelles bear a negative charge. This high charge-density is analogous to a high negative valence. In vie\y of the valence effect of ions on the electric potential, referred to above, it seems probable that, owing to the adsorption of these highly charged micelles, a soap should have a large effect on the potential of particles of inert materials suspended in a solution of the soap. TABLE 1 Fatty aczds used zn the preparation of soaps IODINl NDMBER

ACID

Caprylic ............................... Lauric. ................................ Myristic ............................... Palmitic ............................... Stearic ................................. Oleic.. .................................

665

272.7 1116 1213 402

53.8 61.9 68.5

7.1

196.3 197.5

0.54 92.9

The effect of soap solutions on the charge of both the oil droplets of an emulsion and the particles of a suspension of an inert material can be determined by measurement of the zeta potential in an electrophoresis cell. This has been done in this investigation, and the results are reported below. An attempt has also been made to correlate the stability of the suspension of an inert material in soap solutions with the value of the zeta potential. MATERIALS

Table 1 lists certain fatty acids from which the sodium and potassium salts were prepared according to the method of Ferguson and Richardson (4). It also includes the available constants and information as to sources of the acids. Two oils were investigated, one a paraffin oil and the other a cottonsepd oil. Neither oil contained more than 0.03 per cent free fatty acid calculated as oleic acid.

SOAPS: ELECTRIC CHARGE EFFECTS

823


The inert material used in the majority of the electrophoresis experiments was a bleaching carbon Nuchar GL, made by the Industrial Chemical Sales Company of Chicago. It contained 1.85 per cent water-soluble ash and was grease-free. This carbon was screened to remove particles larger than 1 mm. in diameter. APPARATUS

The electrophoresis apparatus was of the Northrop-Kunitz (9) type, and was obtained from the Arthur H. Thomas Company of Philadelphia, Pa. In order to work a t elevated temperatures, a jacket was built around the apparatus and the desired temperature maintained by a thermoregulator and electric heater. Extensions were attached to the stopcock handles so that they could be manipulated from outside the jacket. A potentiometer, used in conjunction with an assembly similar to that suggested by Gibbard (5), was employed to measure the potential drop across the electrophoresis cell. EXPERIMENTAL PROCEDURE

The solutions were prepared directly from weighed amounts of the dry salts, including soaps, and freshly boiled distilled water which had not been allowed to cool. The solutions were brought directly to the desired temperature. In order to prepare the emulsions, 1 cc. of the oil was shaken with 100 cc. of the water or solution. The excess oil was removed before the emulsion was used. To prepare the suspensions of inert materials, 0.1 g. of the solid was shaken with 100 g. of the solution. When the electrophoretic velocity was to be determined, the suspension or emulsion was placed immediately in the electrophoresis cell, the velocity of the particles or globules determined with a stopwatch in the usual manner, and the potential drop across the cell measured. The microscope was focused at the level given by the equation 2 =

(

d 1/2--

7

!2v3

where d is the depth of the cell and 2 is the lower stationary level (a level at which the velocity of the liquid is zero).l Measurements of velocities were made only on particles from 1 to 3 micra in diameter and only on oil droplets of about 3 micra in diameter. Particles and globules of these sizes could be easily chosen in the microscopic field. The formation of a gel in the electrophoresis cell was found to cause mechanical disturbances 1 The cell employed in this investigation was 0.940 mm. deep, and, following the above formula, the microscope was focused 0.198 mm. up from the bottom of the cell.


824

W. M. URBAIN AND L. B. JENSEN

which interfered with the motion of the particles. Neasurement of the velocity a t 60" or 75°C. avoided this difficulty. At these temperatures, none of the solutions investigated formed gels. When the actual stability of the suspensions formed was to be determined, the solutions containing the dispersed carbon were placed in 4-08., oil-sample bottles, stoppered, and allowed to stand sixteen hours in an air thermostat a t approximately 60°C. At the end of this period, the relative stability of the suspensions could be determined by visual inspection. The maintenance of a temperature of 60°C. prevented the formation of a gel by the soaps. All pH measurements were made with a glass electrode. CALCULATIONS

The electrokinetic potential can be calculated from the electrophoretic velocity by the equation

in which { is the electrokinetic potential, v the viscosity of the medium, V the velocity of the particle, X the potential gradient, and D the dielectric constant of the medium. All units are C.G.S.E. units. Burton (2) has shown that, in the case of a silver sol, variation of the viscosity by changing the temperature produces a corresponding change in

( V ) is a constant. Gilthe velocities of the particles so that the product 9 -

(X)

ford (6) has shown that, for a number of substances, temperature variation of the electrophoretic velocity depends only on the change of viscosity of the liquid phase.

(n

If the product q -is constant, then the product ( D

(X)

is also constant. Since D varies with the temperature, zeta must also vary. Hence, the value of zeta may be calculated from equation 2 for any temperature a t which D is known. The potentials (in millivolts) reported below are calculated to 25°C. The value of D a t 25°C. was taken t o be 78.5 (14). Although a comparison is made between these potentials a t 25°C. and the stability of the suspensions obtained a t 60"C., no discrepancy is introduced. Calculation of the potentials to 60°C. instead of 25°C. would shift all values by the same percentage, so that the relative order would remain the same; in this comparison only the relative order is considered. Besides the potentials calculated from equation 2, the velocities actually observed, and from which the potentials were calculated, are listed. Since the velocities were obtained a t various temperatures, these data cannot be compared with one another. The velocities are reported in micra per second per unit potential gradient.

825



RESULTS

The effect of sodium oleate on the zeta potentials of the oil droplets of two emulsions, one of a paraffin oil and the other of a refined cottonseed oil, was determined. Table 2 lists the results. The oil droplets bore an initial negative charge; this was increased by the presence of sodium oleate in the aqueous phase, In the light of the work of Ellis (3), Powis (12), and others, these results indicate that the stabilization of oil-in-water emulsions by soap is due to the ability of soap to increase the negative charge of the oil droplets. Powis has shown that an emulsion is relatively stable if the oil/water TABLE 2 Effect of sodium oleate on zeta potential of oil droplets of two oil-in-water emulsions OIL PHASE

VELOCITY AT

AauEoua PHARE

28%.

I

lETA POTENTIAL

slsec.lvoltlcm.

millivolfa

7.1 12.5 6.1 11.6

-151 -74

Paraffin oil., . . . . . . . . . . . . . . . . . Water only Paraffin oil. . . . . . . . . . . . . . . . . . . 0.0036M sodium oleate Cottonseed oil. . . . . . . . . . . . . . . . Water only Cottonseed oil.. . . . . . . . . . . . . . . 0.0036M sodium oleate

-86

-140

Effect of a soap solution on zeta potential of different materials WATER MATERIAL

A carbon black. ............................ h water-insoluble dye.. .................... Ferric oxide ................................ A strain of staphylococci bacteria.. . . . . . . . .

8 0 A P BOLUTION

Velooit at WC?

Zeta potential

Velocity at 28%

Zeb, potential

volt/cm.

slaec.1

milliuolfs

ooltlcm.

P/8@./

millioolta

4.5 4.6 2.1 2.8

-60 -62 -28 -34

5.3 5.9 5.8 4.1

-71 -79 -78 -49

potential difference is greater than a definite critical value of approximately 30 millivolts (plus or minus). If the potential falls below this critical value, the emulsion breaks. The extraordinarily high values of the zeta potential obtained with the emulsions made with sodium oleate listed in table 2 indicate that these emulsions are very stable. This result is in agreement with the exceptionally good emulsifying powers of soaps. It was next desired to determine the general effect of soaps on the charge of various inert particles. Table 3 lists the velocities and potentials observed for a number of different materials, first in distilled water and then in a dilute solution of a commercial soap. All these materials showed an Ancrease in the negative zeta potential of the particles when-placed in the

826



soap solutions over that observed in water. The data of table 3 indicate, therefore, that these particles adsorb the negative constituents of the soap. The remainder of the study was continued with a single material, the bleaching carbon described under “Materials”. While it is known that the value of the zeta potential depends upon the history of the carbon (lo), nevertheless a single carbon can be used to determine the relative effect of TABLE 4 E f e c t of concentration o j soaps o n zeta potential of carbon particles (a) Sodium oleate a t 28OC. (b) Sodium palmitate at 6OoC. CONCENTRATION

VELOCITY

ZETA POTENTIAL

CONCENTRATION

VEUXITY

ZETA POTENTIAL

-

moles per liter

r/sec.luoltlcm.

nillitolts

moles pm liter

p/aec.lroltlcm.

millsolls

0.0007 0.0013 0.0023

-78 -80

0,0007 0.0014 0.0021 0.0036

10.7 11 .o 10.7 10.2

-73 -74

0.0026

6.5 6.6 6.7 6.9

-73 -69

0.0033 0.0039 0.0049 0,0056

7.3 7.4 7.4 7.2

-88

0,0054 0 .0072 0.107

12.0 11 .o 11.2

-81 -74 -76

0.0066 0.0082 0,0099

6.9 6.6 6.8

-81 -83

-89 .- 89 -87 -83

-80 -82

TABLE 5 Effect of various salts o n zeta potential of carbon particles COMPOUND,

0.0036 M

Water. ............................................ Sodium acetate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sodium sulfate., .................................. Trisodium phosphate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VELOCITY AT

28-c.

ZETA POTENTIAL

plsec.lvoltlcm.

millhofts

4.3

-52

4.7 5.2

-41* -57 -63

-60

-83

* From velocity measurement

at 75%.

different salts on the potential. This was done in order to obtain the data reported below. Table 4 lists the data showing the effect of concentration of two different soaps on the electrophoretic velocity and zeta potential of the carbon particles. The effect of concentration over the short range investigated is small, as the data of table 4 show. Previously published data for salts


827


!

(1) show that the relationship between the potential of the suspended particle and the concentration of the dissolved salt is complex. I n the range of concentration studied in this investigation, concentration is not an important factor in the value of the zeta potential, and as the data of table 4 show, no significant differences were observed. The concentrations investigated center about those used in laundry practice. The data listed in both tables 3 and 4 are in agreement with a hypothesis that the negative ions or negatively charged ionic micelles of soaps are adsorbed by an inert surface. This, in itself, is not unexpected, for the adsorption of ions of salts by inert surfaces is a general phenomenon. However, in order for this adsorption to result in it stable suspension of the

FIQ. 1. Effect of increasing length of the fatty-acid chain on the zeta potential.

particles, the electric charge developed on the particles must be fairly high. Therefore, the magnitude of the effect of soaps on the seta potential was compared with that of other salts. In table 5 is shown the effect of salts containing negative ions of different valences on the zeta potential of the carbon particles suspended in their solutions. It is evident that the increase in potential produced by sodium oleate was appreciably greater than that produced by the other salts. I n table 6 are shown the values of the zeta potential obtained when the carbon was suspended in solutions of the sodium and potassium salts of some of the saturated fatty acids. These data are plotted in figure 1, showing that the effect on the zeta potential is greater with the salts of the higher homologs (the soaps) than with the lower members of the

828

W. M . URBAIN AND L. B. JENSEN I


series, and that the effect increases in a fairly regular order as the length of the carbon chain of the fatty acid increases. Reference to table 7 shows that the alkalinity of the solutions of the sodium salts of the saturated fatty acid series increases with the length of the carbon chain of the fatty acid. It was necessary to determine whether the potentials observed for the soap solutions could be ascribed to this TABLE 6 Zeta potentials obtained with sodium and potassium salts of certain .f a t t.y acids COMPOUND,

0.0036 M

ZBTA POTENTIAL

e

rlsec.lvoltlcm.

nkllitolls

acetate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . butyrate.. ................................ caprylate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . laurate.. .................................. myristate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . palmitate.. ............................... stearate., .................................

11.2 12.3 15.4

-61 -67 -84

Potassium acetate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Potassium caprylate.. ............................. Potassium laurate.. ............................... Potassium myristate. ............................. Potassium palmitate. ............................. Potassium stearate. ...............................

7.4 7.6 11.2 10.3 12.3 12.7

-41 -42 -61 -56 -67 -70

Sodium Sodium Sodium Sodium Sodium Sodium Sodium

COMPOUND.

7.4 7.6 9.5 9.1

0.0036 fif

-50

PH

Sodium acetate.. Sodium butyrate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sodium caprylate., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.......................

-41 -42 -52

....

Sodium myristate. . . . . ...................................... Sodium palmitate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sodium stearate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7.8 7.4 7.6 8.3 10.0 10.8 10.7

alkalinity. To do this, the effects of the addition of hydrochloric acid and of sodium hydroxide to solutions of sodium acetate and of sodium palmitate were determined. The data are listed in table 8. The alkalinity of the soap solutions is not sufficient to account for their large effect on the zeta potential of the carbon particles suspended in them. The sodium acetate solutions did not have as great an effect as did similar solutions of sodium palmitate with the same concentration of hydroxyl

829

SOAPS: E L E C n I C CHARGE EFFECTS


ions. One should note, however, that in order to obtain the high potential with sodium palmitate, it is necessary for the solution to be alkaline. The effect of mixtures of fatty acids comprising the soap is shown in table 9. In this table are presented data obtained for soaps made from mixtures of oleic and stearic acids. It is apparent that there is no essential TABLE 8 Effect of varying the pH of solutions of sodium acetate and of sodium palmitate on the zeta potential (b) (a) Sodium acetate, 0.0036 M . . Sodium palmitate, 0.0036 M p~

REAQENT ADDED

-4.1 5.4 8.0 9.1 10.0 10.2 11.4

HC1 HCl NaOH NaOH NaOH NaOH

VELOCITY AT 60%.

EETA POTENTIAL

rlsec.lvoltl cm.

millmolts

4.5 5.6 7.2 7.5 8.1 7.5 7.8

11

p~

1

REAQENT ADDED

I

VELGCITY ATM*C

ZET.4 IPOTENTIAL

Irlsee.holtl

milliDolfs

cm.

-30

2.8 3.8 4.7 7.6 8.5 9.3 10.1 10.8

-38 -49 -51

-55 -51 -53

HC1 HC1 HCl HCl HCl HC1 HCl

3.9 7.0 7.4 11.8 10.1 11.3 11.3 10.3

-26 -47 -50 -80

-68 -72 -72 -70

TABLE 9 Efect of soaps made from mixtures of oleic and stearic acids on zeta potential STEARIC ACID I N FATTY ACID MIXTURE MAKINQ UP SOAP

per cent

VELOCITY AT 7 5 ° C .

POTENTIAL

p/aec./nolt/cm.

milliuolls

0 10 20 30 40

13.2 13.4 14.9 13.6 14.4

-72 -73 -82

50 60 70 80 90

13.4 14.2 12.6 13.2 14.0

-73

100

15.4

-84

-75 -79 -78 -69

-72 -77

difference in the effects observed for the various mixtures. The potentials are of the same order of magnitude as those observed for pure sodium palmitate or stearate. By the method described above (experimental procedure), a rough correlation was obtained between the stability of the suspension of the carbon


830


particles in various soap and salt solutions and the zeta potential. In figure 2 is shown a comparison between the potentials observed with the sodium salts of the fatty acid series (table 6) and the relative stability of the suspension of the carbon in solutions of the sodium salts of this series. Although the method of determining the stability was very rough, the relative order of the stabilities of the suspensions formed could be ascertained easily.

FIQ.2. A comparison of the values of the zeta potential with the stability of the suspension formed. The curve represents the values of the zeta potential for carbon particles immersed in the indicated solution, The heights of the heavy bars represent the stability of the suspension formed. The relative order of stability only is indicated.

Compared with the soaps, the inorganic salts listed in table 5 were not good suspending agents. This is in agreement with the relatively low zeta potentials observed. DISCUSSION O F RESULTS

Emulsification of oily material and deflocculation of inert material constitute two of the primary functions of a detergent after the dirt has been removed from the surface to which it was attached. The data presented above indicate that soap solutions perform both of these important functions by essentially the same mechanism. The results indicate that



83 1

both the globules of an oil-in-water emulsion and the particles of a suspension of an inert material show an increase in the values of the zeta potential when a soap is present in the aqueous phase. This increase in potential is definitely larger than that observed with other salts. While the numerical value of the zeta potential is not always very much greater in the presence of a soap, nevertheless it is possible that only a certain critical potential must be reached to form a relatively stable suspension. Powis (12) has shown this to be true of emulsions. It also appears to be true for the carbon black investigated here. The value of this critical potential probably varies with the kind of material and size of the particles, according to Stokes’ law. This increase in the zeta potential must be due to the acquisition of the negatively charged constituents of the soap, either the single ions or, more likely, the ionic micelles postulated by McBain (7). The actual mechanism of the acquisition of these negative constituents may be different in the two cases. In the case of the emulsion of an oil, mutual solubility of the hydrocarbon tail of the fatty-acid ion of the soaps and of the oil may be the means of the acquisition. When inert materials are involved, an adsorption at the surface may take place, or a process involving the capillary action of the soap solution may occur. I n either case, the result is apparently the same. The high charge resulting from the acquisition of the negative constituent of the soap stabilizes the emulsion or suspension, and in this way the dirt can be removed from the vicinity of the surface to which it was attached. The deflocculatiiig action of soap solutions has been suggested by McBain (8) and others as a means of evaluating the detergent powers of soaps. The methods proposed consist in the determination of the actual amount of a given material that can be held in suspension by a soap solution under certain standard conditions. If, as is here suggested, the ability of the soap solution to hold the material in suspension depends upon the magnitude of the effect on the zeta potential of the particles of the suspension, then measurement of the zeta potential affords a rapid method of evaluating a detergent. The ability to emulsify can also be determined in this way. However, the method will require considerable refinement to be of practical value. SUMMARY

The effect of soaps on the zeta potential of the oil droplets of typical oil-in-water emulsions and on the zeta potential of certain “inert” materials has been investigated. In both cases the presence of soap increased the value of the (negative) zeta potential. Very high values of the zeta potential were obtained for oil globules of emulsions, and these are considered sufficiently high to account for the stability of the emulsions. The TEE JOURNAL OF PHYSICAL CHEMI0TRY. VOL.

40, NO. 6


832

W. M. URBAIN AND L. B. JEh-SEN

zeta potentials of the particles of an inert material suspended in soap solutions were appreciably higher than those obtained in solutions of other salts. In the case of the salts of the fatty-acid series, over the range investigated, the potential increases with the length of the carbon chain of the fatty acid present in the salt. It has been shown that the free alkali present in soaps is not sufficient to account for the large effect of soaps on the zeta potential. At 75"C., soaps prepared from mixtures of oleic and stearic acids have the same effect as soaps prepared from the individual fatty acids. It has been shown that soaps producing a high negative potential on certain carbon particles also form a stable suspension of the carbon. Conversely, salts which do not alter the potential of the carbon particles to any great extent do not form stable suspensions. This effect of increasing the zeta potential of the oil droplets of an emulsion and of the particles of a suspension is suggested as the mechanism whereby soaps act as emulsifying and deflocculating agents. Measurement of the effect of a solution of a soap on the zeta potential of the oil droplets of an emulsion or of the particles of a suspension is suggested as a means of evaluating the detergent powers of a soap. Our thanks are due to Professor T. F. Young of the University of Chicago for his kind interest and advice in the preparation of this material for publication. REFERENCES (1) ABRAMSON, H. A.: Colloid Symposium Monograph 1934, p. 277. The Williams & Wilkins Co., Baltimore (1934). (2) BURTON,E. F.: Phil. Mag. 67, 587 (1909). (3) ELLIS, R.: Z. physik. Chem. 78,321 (1912); 80,597 (1912); 89, 145 (1914-15). A. S.:Ind. Eng. Chem. 24, 1329 (1932). (4) FERUUSON, R. H., AND RICHARDSON, (5) GIBBARD, J.: Science 72, 398 (1930). (6) GILFORD,C. L. S.: Phil. Mag. 19, 853 (1935). (7) MCBAIN,J. W., AND SALMON, C. S.: J. Am. Chem. Soc. 42, 426 (1920). (8) MCBAIN,J. W., HARBORNE, R. S.,AKD KING,A. M.:J. SOC.Chem. Ind. 42, 373T (1923). (9) NORTHROP, J. H., AND KUNITZ, M. J : J. Gen. Physiol. 7, 729 (1924-25). (10) OLIN,H. L., LYKINS,J. D., AND MUKRO, W. P.: Ind. Eng. Chem. 27,690 (1935). (11) PICKERING, S. U.:J. Chem. Soc. 111, 86 (1917). (12) POWIS,F.: Z. physik. Chem. 89, 91, 179, 186 (1914-15). (13) SMITH,E. L.: J. Phys. Chem. 96, 1401 (1932). (14) WYMAN,J., JR.: Phys. Rev. 36, 623 (1930).

SOAPS : ELECTRIC CHARGE EFFECTS AND ... - ACS Publications

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