A Phase Studv of Commercial J
Soap and Water JAMES W. McBAIN, MARJORIE J. VOLD, AND JOHN L. PORTER Stanford University, California
Typical commercial soaps may be studied under the assumption that they behave like two-component systems consisting of a soap and water, in fair accordance with the principles of the phase rule. Most of the techniques thus far developed for the study of the phase behavior of soap have been applied to (a)a typical sodium soap, (b) a soap made from the same fat stock but in which 25 per cent of the sodium has been replaced by potassium, and (c) synthetic mixtures of single pure soaps having the same average molecular weight and iodine number. For isotropic liquid, liquid crystalline middle soap, soap-boiler's neat soap, superneat soap, and
T
HE purpose of this investigation is to study the extent to which a typical commercial soap resembles the salts of single fatty acids in phase behavior. I n certain regions of composition and temperature such a complex mixture of fatty acid salts can be treated as a single component, and comparison may be made of the diagram in these regions with similar diagrams for the single soaps. The typical phase-rule diagrams with water and with both water and salt are important in the interpretation and scientific control of the changes occurring during the manufacture of soap by the boiling process. The first major advance in the transformation of soapmaking from a craft to chemical industry in the modern sense may be considered to be the successful interpretation of the changes that occur in the various stages of soapboiling in terms of phase diagrams (4, IO). While the general validity of this description was confirmed in an investigation of a portion of the phase diagrams for tallow and coconut oil soaps a t a single temperature (W), twenty years of research on soap have not yielded published extensions of this interpretation. Recently (6, 9, 11, l a ) , however, much new information has been obtained. We are indebted t o Lever Brothers Company and the National Oil Products Company for samples of special soaps. Sodium soap A has the composition of a typical fitted and settled household soap, made from fatty acids of average molecular weight 264 and iodine value 36.6; the original soap contained 63.4 per cent fatty acids with 0.093 excess sodium oxide and 0.41 per cent sodium chloride. Sodium laurate (€4, palmitate (9),stearate (8), and oleate (12)are the same preparations used in previous work. Phases of Anhydrous Sodium Soap The single anhydrous sodium salts of fatty acids pass through a sequence of stable forms as the temperature is raised
probably also the neat and subneat forms of anhydrous and nearly dry soap, the phase-rule behavior is qualitatively the same as that of single pure soaps. For other regions of the diagram, however, some fractionation seems to occur. Solubilities of various pure and commercial soap are each characterized by a value of Ts,the temperature of ready solubility which can be precisely defined by reference to the lower temperature boundary of moderately concentrated isotropic solution. As an example, for a palm oil soap Tsis 52" C. Above this temperature as much as 30 grams of soap will dissolve in 70 grams of water. Only 2' C. below T , this solubility has fallen to 5 grams in 95 grams of water.
(11, IS, 14). The phases which occur between monoclinic crystal and liquid are subwaxy, waxy, superwaxy, subneat, and neat; the last two are semifluid, translucent, doubly refracting liquid crystals. The complete sequence, however, is not realized for all soaps (11). Dry sodium soap A melts t o form liquid sharply a t 285" C., a value well within the range (241" for sodium oleate to 336" for sodium laurate) found for single soaps. Between 285" and 240" C. it exists as neat soap, similar in appearance to the corresponding phase in single soaps, and between about 200" and 248" C. it exists as subneat soap. Photomicrographs of the characteristic appearance of these two phases between crossed Nicols are shown in Figure 1. The transition temperatures were determined both by visual observation of the microscopic appearance as the temperature of the whole sample was raised, and with the hot wire technique (11) in which the sample was subjected t o a temperature gradient and the positions were determined of the boundaries which form between different phases on account of their differences in structure, opacity, and specific double refraction. Below 200" C. no sharp boundaries are visible with the hot wire technique although the appearance of the sample varies from that of' the opaque crystalline curd to that of the nearly transparent, only slightly anisotropic (in thin layers) superwaxy form. Viewed in the polarizing microscope between crossed Nicols a t any temperature between about 60" and 200" C., the soap appears heterogeneous and consists of dark and light patches all having the stippled appearance shown in Figure 2. The dependence of specific volume on temperature, determined as described previously (14),shows that phase changes occur a t about 65", 106-126", and 183-185" C. A typical curve is given in Figure 3. The heterogeneous appearance of the soap and the wide temperature range over which transitions occur makes it seem probable that the various constitu1049
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INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE1. PHOTOMICROGRAPHS ( X 150) OF ANHYDROUS SODIUNSOAPA
BETWEEN
Vol. 33, No. 8
CROSSED NICOLS
Lejt, a t 273' C (neat soap); right. at 210' C . (subneat soap).
waxy) are still t o be found, but the various constituents of the mixture have probably partially separated in the phaserule sense, though not on a macroscopic scale.
Fluid Phases of Aqueous Soap Systems
l
158' C. (WAXY F 2. ANHYDROUS ~ ~ SODIUM ~ SOAP ~ A AT ~ SOAP)BETWEEN CROSSED NICOLS( X 150)
ents of the mixture are partially fractionated. The change near 65" C. is probably associated with a fraction rich in sodium oleate. Between 65' and 126' C. the soap is probably a mixture of crystalline curd fiber phases and subwaxy forms, with waxy soap beginning t o form somewhere above 106' and being completely formed by 126' C. The transition near 185" C. is associated with the formation of superwaxy soap from waxy. Thus in the dry state only a t relatively high temperatures does this typical commercial mixture behave as a single pure soap. At lower temperatures all of the characteristic soap phases (crystalline curd, subwaxy, waxy, and super-
The incorporation of only a small amount of water (5 to 15 per cent) into the single soaps (8, 12) results in a complete and fundamental change in behavior; for where a sequence of as many as five intermediate forms exists for the dry soaps and for soaps containing small amounts of water, only two, or a t most three, liquid crystalline forms exist in more dilute systems. These forms seem t o be equivalent for all sodium soaps and may have no relation t o the forms of anhydrous soap. This behavior for soap A is shown in the phase-rule diagram, Figure 4. The region of existence of the different phases was determined by the standard techniques developed in this laboratory (7). The Ti curve gives the temperatures at which liquid crystal first forms from isotropic liquid on cooling; the T, curve gives the temperature a t which all traces of solid opaque curd disappear on heating; To is the temperature, observed microscopically, a t which isotropic liquid just begins to form on heating. The data are given in Table I. The two maxima in the T,curve (points S and M ) occur a t somewhat higher concentrations than the values for the corresponding points obtained by averaging the values for the single soaps and weighting each in accord with its abundance in the mixture; this is illustrative of the generalization (6, 9 ) that in a mixture the more soluble soaps exert a greater influence on the phase-rule behavior than do the less soluble soaps. Thus average molecular weight and iodine value alone are not sufficient to characterize the solubility properties of a soap. A synthetic mixture having the same molecular weight and iodine number was prepared, and its T , and Tocurves were determined. The mixture contained 43.8 per cent sodium oleate, 26.2 sodium palmitate, and 15.0 each of sodiumlaurate and sodium stearate. The data are presented in Table I1 and plotted in Figure 5. The diagram for this mixture is qualita-
~~
Wt.% Soap
Ti
9.9
.... 51
68
91 132 155 166 170 166 158 155 195 210 215 225
TO
Ti
Wt.% Soap
4g.b
*.
*.
52 52 53
.. .. ..
89.1 90.8 91.7 94.0
273 270
94.6 96.8 99.0
273 281 286
Wt.% Soap
To
1.5 13 22 35.5
279 273 278 258
.. ..
.... ..
AND
To DATA FOR
TC
W t . % Soap 20 29.9 34:9 40.0 45.0 47.4 48.4 49.6
To
48
48 48.5 48.5 49 50 51
~
TABLE11. Ti
SOAPA TABLEI. Td,To,AND To DATAFOR SODIUM
25.9 27.2 28.1 34.6 39.7 44.6 46.5 47.6 49.6 50.0 51.0 61.9 52.0 54.8
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INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1941
To
..
SYNTHETIC MIXTURE Wt.%s o a p TI TC
THE
51.9 59.3 69.3 74.3 82.1 89.4 94.0 99.0
47 48 49 50 52 53 53 57
80 116 145
148 150 148 152
195 242 271 276 270 267 264 282
59 63 66 70
..
.. .. ..
TABLE111. COMPARISON OF PHASE-RULE DIAGRAM FOR Two WEIUKT AND IODINE SODIUM SOAPSOF THE SAMEMOLECULAR NUMBER,WITH THE RANGEOR SPREADOBSERVEDBETWEEN VARIOUS PURESOAPS Composition, W t . % Range for Synthetio single SoapA mixture soaps
Point M.P.
... 93
NS
:;54
S
75.5
IMN M
ICM
26.0
Temperature,
... 94.4
... S
Soap A. 28s 269
77.5 49.0 44.5 27.0
4 5.5 4.5 10
279 150 171 48
Synthetic mixture 284 262 274 147 151 48
C. Range for single soaps 95 86 34 89 3s 44
201
3 0 0 175
taining 40 per cent sodium oleate and 20 per cent each of the other three soaps was found to give closer agreement a t M . Figure 4 shows that the field of middle soap for a mixture, like middle soap for the single constituents, is not continuous on the phase diagram with any form of anhydrous soap. It is not yet certain for the mixture, or for any of its single constituents, whether soap-boiler's neat soap and superneat soap are separate phases and whether they have any anhydrous counterpart.
cd
0
O0 0
0 15c
O0 0
.rat U
Y
5 IO( k-
9
E
7!
5(
8
2
-0
n
0.b5 hV
O!l 0 CC/CRAM
O:i5
FIQURE3. TYPICAL DILATOMETER CURVEFOR ANHYDROUS SODIVM SOAPA Arrows indicate transitions.
tively the same as that for soap A. Table I11 contains a quantitative comparison of the diagrams a t corresponding points: the melting point of the dry soap, m. p.; the points where isotropic liquid is in equilibrium with neat soap and superneat, INS; the maximum melting point of superneat, 8; the point where isotropic liquid is in equilibrium with soapboiler's neat soap and middle soap, IMN;the maximum melting point of middle soap, M ; and finally the point I C Mwhere isotropic liquid is in equilibrium with middle soap and curd fiber phase. Column 4 gives the range of compositions found for these points in phase-rule work on four of the principal constituents, sodium laurate, sodium palmitate, sodium oleate, and sodium stearate. The agreement obtained between the two soaps in regard to composition of phase change, while quite close, is not very significant in view of the similarity of behavior of the single soaps. The agreement in regard to temperature is much more striking. Only at point M is there any considerable difference. A second synthetic mixture con-
io
.u W LL
I :z
300
a: 4
m*p.mIuR'$
w
BE&i
soap
A
t 601
93
4
A 100
8b COMPOSITION,
' 6 0 ' WEIGHT PER
4'0 CENT
Zb SOAP
FIGURE4. PHASE-RULE DIAGRAM OF A TYPICALLSODIUM SOAPWITH WATER 0 Ti; 0 Tc; =results obtained by several methods on'dry soap. + To. points from viewing experiments. Q point from layer sep)arstionl A dilatometrio data, To;A dilatbmetric TI: dilatometric Ts.
IN D U S T R I A L A N D E N G I :J E E R I N G
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CHEMI
STRY
Vol. 33, No. 8
,J50U 3
+ 4
o!
100w LL
I W
c 50I
I
100
60
80
40
WEIGHT PER
CENT
20
0
SOAP
FIGURE5 . PHASE-RULEDIAGRAM FOR SYNTHETIC MIXTURE
62.0 %SOAP
r,.55O
THE
Phase-Rule Behavior at Ordinary Teniperatures and Concentrations The region of the phase-rule diagram which is of greatest technical importance lies between 20 O and 20.2 SOAP 100' C. and between about 70 and 85 per cent soap. Tc v In its upper part it is traversed by the T, curve. VOLUME I N C R E A S E (ARBITRARY UNITS) Above the T,curve the soap is relatively fluid and translucent. Below the T , curve it rapidly assumes FIGURE 6. DILATOMETER CURVXi FOR SAMPLE0 O F SOAP -4 AND WATER the typical appearance of ordinary bar soap. Arrows indicate phase changes, Vertical aria is temperature from 20° t o . 100: 6. Horizontal axis is dilatometer reading (arbitrary units). Compositions are in welght Two sets of experiments were designed further per cent soap. to investigate this region, which is incompletely understood even for sinele soaus. First. a series of samples was allowed-to s t a h for 24 to 48 hours a t several temperatures, and as much as possible about the phases present in each was determined by inspection. The results of these experiments are given in Table IV. The observations confirm the position of the T , curve and extend i t somewhat. The single tube, 51.9 per cent soap A, separated into nearly equal layers of neat soap and isotropic liquid a t 161' C.; this shows that the two-phase region
TABLE Compn., Wt.% 39.7 46.G 47.6 50.0 61.9
I)-.
-
INBPECTION EXPERIUEXTS'
84 4
31 M
M RI L
59.4 64.4 69.4 79.2 84.7 89.1
L
91 2 bI
96 2
RI
31
31
31 RI L
RI
L
L
hl
L
L H N Cp*'
H H N
C
C
H H N CN C
91 7
C
c
94 0
c
c
C
C C
968
C
O S AQUEOUS SODIUM
SOAPA
CK
c
Tempeiature, C.: 109 0 115 5 126 2 31 AI 31 31 AI 11 >I hl 31 31 &I >I H L H H H
w W
161
..
31
AI
41
2
H H H
NI H H
H N N
H H H
S N N
WN
ws
WE
N
W
WN W
W
N
"
WN
144 31 31 AI
wri
H
wx
FIGURE7. TYPICAL DILATOWETER C U R V ~FOR SonIuxi SOAPA AND WATER(70.6 PERCENTSOAP)
ws
between soap-boiler's neat soap and middle soap is narrow, as i t is for sodium stearate (8),rather than wide as for sodium oleate (12). The appearance and fluidity of soap-boiler's neat
August, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
1053
has begun t o occur. The exact temperature a t such points is not sharply defined by the experimental curves. A few reasonably satisfactory values are recorded as TIin Table V. When the curves are plotted on a larger scale, many additional inflections are obtained below T , (Figure 7), indicative of further phase changes not yet completely interpreted. Since the various constituents of a commercial soap are probably partially separated in the region of the diagram below and to the left of the T,curve, this part of the diagram cannot be accorded its full phase-rule significance. It is only superficiallyrelated to a true binary phase-rule diagram, since the laws of tie lines are not strictly applicable in this restricted region.
2a
15c
d; Y
3
2 10 P
z
;i
TABLEVI. Wt.% Soap 100
96.9 94.0 91.0 85.1 83.5 79.0 73.7 69.5 66.0 59.5 56.8 54.3
5
FIGURE 8. DILATOMETER CURVEOF A MIXEDSODIUM AND POTASSIUM SOAP
Ti 318 308 302 301 298 299 295 288 279 265 240 227 192
Ti AND T , DATAFOR SOAPR TO Wt. yo Soap T, 1351 1291 1291 114 92 85 71 56 51 50 45 41 41
53.9 49.9 44.7 39.4 34.4 29.9 23.8 19.5 10.0 5.2 1.9 0.9
173 177 180 154 114
.. .. ..
... ... ... .,. ...
TC 41 41 41 41 43 43 43 43 43 41 39 34
Arrows indicate transitions.
soap changes considerably with varying composition, but it has:not been established that this variation has any phaserule significance. A series of curves of specific volume as a function of temperature were also determined for several aqueous systems, of about one gram in weight, by essentially the same technique as described for dry soap (14). I n general these curves were not carried to high temperatures on 'account of the formation of steam in the dilatometer bulb. At temperatures below T , the expansion arises partly from temperature increase and partly from the gradual conversion of the more solid phases to the less dense soap-boiler's neat soap. When these phase changes ar8 complete, the rate of expansion drops sharply. Values of T , determined dilatometrically are given in Table V and shown graphically in Figure 6. A few pronounced changes of this type, which do not correspond to T,but possibly to temperatures a t which part of the soap has been converted to subwaxy or waxy forms, are recorded as T*in Table V. Conversely, a sudden increase in the rate of thermal expansion shows that a phase change accompanied by a density decrease
Phase-Rule Behavior of the Mixed Sodium and Potassium Soap B The phase-rule behavior of a soap made from the same fat stock as soap A, but containing 25 per cent potassium with the sodium, mas also determined by the same experimental procedures. The data for Ti and T,curves are given in Table VI; dilatometer curves for the dry soap and for two samples containing water are given in Figures 8 and 9. These data were used to construct the phase-rule diagram of Figure 10.
TABLE v. TEMPERATURES" OF PHASE CHANGE I N AQUEOUS SODIUM SOAPA, DETERMINED DILATOMETRICALLY Wt.% Soap 93.6 90.4 84.1 81.3 80.2 78.9 76.6 70.9 70.6 62.0 58.5 49.8 20.2
TI 2'2
Tc
TI
Tz
70
63 87
..
R
R
.. ..
R
About 4 i About 40 42 R
R
R
.. .. .. R .... ..
..* . ..
TO
..
R
92 '88 78 (72)
R
63 60 (55) 55 67 52 47
= temperature of positiye ohsnge in slope of dilatometer curve;
P
FIGURE9. DILATOMETRIC BEHAVIOR OF A MIXED SODIUM AND POTASSIUM SOAPCONTAINING WATER R
means thie change i s too rounded to fix its temperature negative ohenge in slope of dilatometer curve, that does not oorrespond t o T S negative change In slope of dilatometer curve corresponding to visual T .
The partial replacement of sodium by potassium raised the temperature of complete melting by 33' C. The temperatures of formation of other intermediate phases seem to have been altered so much that direct correlation of transitions be-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 33, No. 8
tween the two soaps is not possible on this eo . basis alone. However, both seem to be largely 0S T E 4 R 4 T C P 0 PALMITATEconverted to waxy soap in the same region of temperature (by 126' C. for soap A, by 10 . 113" for soap B). Soap B does not show a minimum value . u 60 . of T,which would separate the region tentatively labeled "superneat" from the region of neat soap. I n this respect it differs qualitatively from soap A. However, the sharp inflection in the Ti curve at 301' C. makes it seem unlikely that there is continuity between c neat soap and superneat soap. A comparison of corresponding composi30P OLIX tions and temperatures for soaps A and B is -0---- - - _ _ _ _ _ _ D OLEATE given in Table VII. It is apparent that the 20 introduction of the potassium has stretched out the phase-rule diagram, as far as temperatures are concerned, by raising TS and lowering T,. I n general, also the effect of 30 20 10 COMPOSITION (WEIGHT PER CENT SOAP) the potassium is t o render the soap more soluble. FIGURE 11. T,CURVESFOR VARIOUBSODIUM SOAPS The shape of the T,curve for soap B shows that the solid soap is too heterogeneous to be therefore pseudo binary in that slight fractionation must have considered as a single component in the phase-rule sense, for it is not possible to construct the horizontal isothermal tie lines occurred. It might be possible to consider the sodium and potassium soaps separately and treat the system as one of that would be required if the phases present just under the three components. curve between 70 and 100 per cent water were actually only curd fibers and isotropic liquid. This portion of the diagram is
-
_x__mr)
TABLE 1'11. COMPARISON OF SOAP B Point
M. P.
Compoaition, Wt.% A B
..
IMN 1 W
93 75.6 50.5 46.4
ICM NMC
26.0 63
Tc (70%) T i (86%) Compn. for Tc 3 1000 c.
86
INS
S
.. ..
AND
SODIUM SOAPA C
Temperature,
A
....
288
B 318
5i:O
160
ili
46.5
30.0 57
269 279 171 48 52
..
181 43
60 96 88
41 53 91
..
Comparison of the Solubility of Various Soaps
Soaps, in general, show a marked change in solubility over a very narrow range of temperature. The rounded elbow in the T , curve for sodium soaps has commonly been called the "Krafft point" (I), and it is taken to be practically the same w
h 3
as the melt'ing point of the corresponding fat'ty acid. HOWever, it is not sufficiently precise to serve as a good measure of
solubility from soap to soap. Likewise it is very different for sodium and potassium soaps. Furthermore, later authors have overlooked the fact that, even for the various pure , sodium soaps, Krafft and Wiglow (3) observed temperaw I ,, c tures from 9" to 23' C. below the melt'ing points of the pure acids. 80The most satisfactory procedure is to refer to the lower left-hand corner of the field of isotropic solution on the phase diagram. Here T , has become practically horizontal and may be referred t o as the temperature of ready solubility. Above this temperature, T,,the soap is fairly soluble in amounts up to a t least 15 per cent. Below T,the solubility falls rapidly to very small values. This corner is also referred to as Icni, 100 8'0 60 4b 26 the concentration of isotropic liquid a t which it is in equilibriuln COMPOSITION, WEIGHT PER CENT SOAP with curd fibers and middle soap in a binary diagram. T, thus supplies a definite means of' comparison of the various FIGURE10. PHASE-RULE DIAQRAM OF A MIXED SODIUM soaps. AND POTASSIUM SOAP(SOAP E) I-
2120w
August, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 11 shows such solubility or T,curves from which T, values are obtained for a number of single soaps and commercial mixtures. While there is a wide spread in the values of T,for the single soaps, those for the commercial mixtures, except for sodium olive oil soap, cluster in the 10" range between 45" and 55" C. Indeed, in the past one of the principal arts of the soap boiler was so to blend varying materials available at different times as to produce a uniform product.
Literature Cited (1) Chwala and
Martina in Hefter-Schonfeld's "Chemie und Teohnologie der Fette und Fettprodukte", Vol. IV, p. 47, Vienna, Julius Springer, 1939. (2) Ferguson, R. H., Oil & Soap, 9, No. 1,5-8,25 (1932) ; Ferguson, R . H., and Richardson, A . S., IND. ENC. CHEM., 24, 1329 (1932). (3) Krafft. F., and Wiglow, H., Bw.,28, 2566 (1895).
1055
McBain, J. W., in Alexander's "Colloid Chemistry", Vol. I. p. 137 (1926). McBain, J. W., Brock, G. C., Vold, R. D., and Vold, 21. J., J.A m . Chem. SOC.,60, 1870 (1938). McBain, J. W., Elford, W. J., and Vold, R. D., J. SOC.Chem. Ind., 59, 243 (1940). McBain, J. W., Lararus, L. H., and Pitter, A. V., Z . physik. Chem., A147, 87 (1930). McBain, J. W., Vold, R. D., and Frick, M., J. Phys. Chem., 44, 1013 (1940). McBain, J. W., Vold, R. D., and Jameson, W. T., J . A m . Chem. Soc., 61, 30 (1939). McBain, J. W., and Walls, E., 4th ReDt.. Brit. Assoc. Advancement Sci., 1922, 244. Vold, M. J., Macomber, M . , and Vold, R. D., J. 4 m . Chem. Soc., 63, 168 (1941) Vold, R. D., J . Phys. Chem., 43,1213 (1939). Vold, R . D., Rosevear, F. B., and Ferguson, R. H., Oil & Soup, 16, 48 (1939). Vold, R. D., and Vold, M . .J., J. .4m. Chem. Soc., 6 1 , 808 (1939).
Electrostatic Charges on Coal Particles in Oil Relation of the Charge to Particle Stability J. E. HEDRICK, A. C. ANDREWS, AND J. B. SUTHERLAND Kansas State College, Manhattan, Kans.
SERIOUS disadvantage to the practical use of colloidal fuel is the difficulty in preparing stable suspensions of the coal particles in oil. Aside from this difficulty, the fuel has several qualities that might make i t preferable to heavy fuel oil. I n Great Britain, where petroleum is inconvenient to secure, attention has already been given to the possible use of colloidal fuel. I n the United States consideration will undoubtedly be turned to this fuel as the need for conservation of our petroleum reserves becomes more apparent. Another probable use of oil-coal suspensions is in the chemical utilization of coal. It seems logical to believe that such processes as halogenation and hydrogenation of coal could best be carried out when the coal particles were suspended in oil. I n most aqueous suspensions of colloids the stability is closely related to the charges on the particles. It is difficult to predict whether this relation would apply also to colloidal particles in nonaqueous media. The literature on this subject is almost nonexistent. Soyenkoff @) has studied some of the less complex hydrocarbons as dispersion media for colloidal graphite. His conclusion was that stability is not related to the particle charge. However, since coal particles are not pure carbon and since commercial petroleum oils are not only mixtures of hydrocarbons but contain small amounts of impurities, i t was thought that the problem here might be quite different. The cost of reducing coal to a size such that the particles are truly colloidal would be prohibitive. For commercial use the particles in so-called colloidal fuel are considerably
A
Coal particles in oil assume a lines-of-force arrangement under the impress of a current. Both positive and negative charges exist in the same suspension, not only on different particles but on different areas of the same particle. Surface-active agents affect the charges on coal particles, tending to make them more uniform and thus preventing agglomeration. The electrostatic charge does not affect the stability of suspensions except as it influences flocculation of the particles.
larger. It is to be expected, then, that their electrophoretic responses would be considerably more sluggish than for true colloids. On the other hand, opposite charges on these particles in the same suspension would cause gradual flocculations, where stability would otherwise be complete. This work was undertaken t o determine whether such charges as exist on coal particles in oil will influence dispersion and flocculation. Since preliminary work showed that both positive and negative charges existed in the same mixture, a further object was to study the influence of small percentages of addition agents. I n the latter case it was thought that such agents as could be selectively adsorbed on the coal particles would have an effect on the electrophoretic response.
Samples and Procedure The apparatus was arranged so that the oil-coal suspension could be placed in an electrical field and the particles observed through a microscope. The coal used was a washed midwest
