286
SISTER AGNES ANR GREEN AKD JAMES W. MCBAIN
SOLUBILIZATION OF WATER-INSOLUBLE DYE BY PURE SOAPS AXD DETERGEKTS OF DIFFEREST TYPES' SISTER AGNES ANN GREES Department of Chemistry, Immaculate Heart College, Los Angeles, California ASD
JAMES W . McB.4IS Department of Chemistry, Stanford University, California Received August 8 , 1946 INTRODUCTION
Among the interesting and characteristic properties of detergents, the phenomena of solubilization are a t present the most challenging from the viewpoint of theory. Emphasis is placed upon the need for interpretable data by the rapidly increasing use of solubilization in such industrial processes as emulsion polymerizations, the dyeing of rayon, and the preparation of lubricants. Solubilization (1) consists in the spontaneous passage of a substance insoluble in a pure solvent into a dilute solution of a detergent in that solvent to form a thermodynamically stable solution. The saturation value is constant a t any given concentration of detergent under conditions of constant temperature and pressure, whether the end point is approached from undersaturation or oversaturation. The immediate purposes of the following experiments were to study the effect of changes in the detergent molecule itself on solubilizing power, and to observe the effects of the presence of certain organic liquids. To this end, a study of several aqueous systems of pure soaps, comnercial detergents, and a non-ionic detergent was made in the presence of the same saturant, Orange OT (l-o-tolylazo-2-naphthol). This dye was chosen as a non-polar crystalline substance, insoluble in water but whose solubility in the detergent solutions is great enough to be easily measured, while not of sufficient magnitude to alter the properties and the,micellar constitution of the solutions. The experimental method was that described in a former paper (3). EXPERIbfEXTAL RESULTS
Potassium and sodium soaps as solubiliters
Measurements were made on potassium and sodium laurates and on potassium and sodium oleates oyer as complete a concentration range as the solubility of the soaps themselves would permit. The results are given in tables 1 t o 4. h comparison of the solubilizing capacity of the two laurate soaps below 0.25 N is represented in figure 1. In this range it is plainly evident that the sodium laurate is more effective per gram-molecular \\-eight than the potassium laurate. 1 Presented a t t h e TIventieth S a t i o n a l Colloid Symposium, which was held a t LIadison, Wisconsin, May 28-29, 1946.
287
SOLUBILIZATION O F DYE BY SOAPS
TABLE 1 Solubilization of Orange OT in aqueous solutions of potassium lawate at 86%'. voLum
C o ~ ~ l R * T 1 n NO F
I
DYE PEP
100 CC. OF
7
mg
0 001
0
I
o oin
0 025 0 049 0 098 n 125 0 250 0 500 0 820 1 000
1
SOLUTION
DYE PEP MOLE OF
I
n nz 0 2 8 12 25
I
~
I
~
s5 40 107 60
Y 0.001 0,005 0.010 0.025 0.050 0.075 0.100 0.150 0,200
w. 0.03 0.095 0.260 0.950 3.65 i.45 14.45 23.06 35.10
io,
DYE/SOAP
o 076
0 020 o 176 0 462 0 833 0 970 1 038 1 048
44
28 15 12 95
I
KCu
grams
n
67 1 76 3 18 3 70 3 96 4 00 4 10
grams
1I
0 30 0 19 026 0 380 0730 0 998 1 445 1 535 lim
'
1.14: 0.12 0.99 1.45 2.78 3.80 5.50 5.85
6.61t
* Believed t o include suspending action ( 3 ) ,
t From supersaturation.
,
T.iBLE 3 Solnbzlzzatzon o f Orange OT z n aqueous solirlions of potasszum oleate at 25°C 'nLCuT. CosCEsTRAT1oN
-
OF SOiP
\-
"'g.
0.0001 ~~.0010 0.00'5 0.0125 0.125 0.250 0.500
* Believed
DYE P E R 100 CC. OF S O L U I I O I
0.04 0.21 0.68 3.3s 33.75 6i.00
141.00
t o include suspending action
~
I
DYE PER MOLE O F
KCis ~
YOLAP PATIO DYEISOAP 108
x
granis
4.00 2.10 2.72 2.70 2.70 2.67 2.82
l5,25* 8.00 10.31 10.29 10.29 10.21 10.75
TABLE 4 Solubilization of Orange OT i n aqueous solutions of sodium oleate atdb'C. VOLUYE CONCENTRATION OF SO*P
DYE PEP MOLE OF
N
W.
gram:
o.ooo1
0.01 0.14 0.56 1.25 2.90 7.50 15.00 30.0 59.0 100.5
1.00 1.40 2.24
0.0010 0.0025 0.005 0.010 0.025 0.050 0.10 0.20 0.30
YOLAP M P I O DYE/SOAP
NaCn
3.81 5.34
8.54 9.53
2.50 2.93
11.06 11.44 11.44 11.44 11.26 12.77
3.00
3.00 3.00
2.95 3.35
I
1
I
1
K
1
I
0 05 0.05
0 IO 0.10
LAUR
1
CONCENTRATION
0.15
OF
0.20
5
SOAP
FIQ.1. Solubilization of Orange OT b y sodium and potassium laurates 288
289
SOLUBILIZATION OF DYE BY SOAPS
A steep rise in the curve, marking the greatest increase in the amount of dye solubilized per mole of potassium laurate, occurs in the interval from 0.01 N to 0.10 N, the same region where the conductance (8) and osmotic coefficients (2) indicate a transition from the behavior of a strong electrolyte to that of e colloidal electrolyte. In the case of the sodium laurate, it is difficult to decide whether its own limited solubility prevented it from reaching an even higher maximum representing full colloidal form. Its greatest rate of change in solubilizing ability per mole also occurs in the same concentration range as that of potassium laurate. A comparison of the relative solubilizingpower of potassiumand sodium oleates below 0.125 N is given in figure 2, again illustrating the superior solubilizing power of the sodium soap. The greater regularity of the sodium oleate curve I
I
I
I
1
1
-
x
-
L
9
I2
..
*A
P
-
IO Y
I(
obtm
OLFATL
-
2 ‘ L
-
a 4 J
0
I
-
,
.+ 0
I 001
I OD4
I O.o#
I
I
0.0.
0 10
1 0.I t
I
ni 4
FIG.2. Solubilization of Orange OT by sodium and potassium oleates
is attributed to the greater purity of this product, which was prepared from specially purified oleic acid (13). The other soaps were prepared from Kahlbaum’s best fatty acids without further purification. In figure 3, both the oleates and the laurates, within the range 0.01 N to 1.0 N, are represented on the same graph. This brings out the superiority of the oleates, representing greater chain length and unsaturation in the soap molecule. This figure also shows the greater effectiveness of both the sodium soaps as compared with the potassium soaps. Apparently, then, solubilization is not entirely dependent upon the length of the hydrocarbon chain of the detergent molecule. Since this difference can only be due to the polar end of the molecule, some dipole attraction must enter into the process of solubilization, if only in its influence on the colloidal structure of the soap solutions.
290
SISTER AQNES ANN GREEK AKD JAMES W. MCBAIN I
I
I
I
14 0
s
x
I2
a 6 0 VI
IC
r
OLEATE
E W %
8
0 NA
-
0
LAURATE
6
I-
6
a
4
a 6
-J
0
I
2
w n
04 0:
a 4
0 J
I 0
I
1
01
oIa
I
0.)
I
0 4
I
0 5
I
0.6
I
01
-
08
These measurements showed that the cation-active detergent, dodecylamine hydrochloride, possesses greater solubilizing ability than that of the neutral anion-active detergents, potassium and sodium laurates, containing the same number of carbon atoms. It was only slightly more effective than lauryl sulfonic acid, which was also better than the laurates.
293
SOLUBILIZATION O F DYE BY SOAPS
A non-ionic detergent, Triton X-100 Triton X-100 is a commercial product (Rohm and Haas Company), a highmolecular-weight derivative of polyethylene oxide, which does not ionize in TABLE 7 Solubilization of Orange O T in aqueous solutions of Triton X-100 I
PERCENTAGE COYPOSITlON BY WEIGHT
Dm
*"
lW cc'
I
W.
0.0176 0.10 0.50 1 .00 2.00 5.00 10.00 30.00
0.305 , 1.95 ! 4.05 8.50 21.65 44.87 142.5t
DYE PER GRAM OF DETEXGENT
APPROXIUATE NORMALITY'
DYE PER YOLE O F DETERGENT
W.
Y
groms
0.0003
0.04 1.83 2.34 2.43 2.55 2.60 2.69 2.85 11.69
0.68 3.05 3.90 4.05 4.25 4.33 4.49 4.75
0.0017 0.0083 0.0167 0.033 0.083 0.167 0.5
MOLAR RATIO DYE~DETERGENT
x
108
0.156 6.98 8.92 9.27 9.72 9.91 10.25 10.8 44.57
* Calculation of normality based upon molecular weight of 600, determined by freezingpoint measurements in benzene. Unpublished work of Dr. E. Gonick. t A measurement b y Arthur G . Wilder, Stanford Cniversity, which fits the extrapolated curve for these data, shown in figure 7. f Units of this measurement are milligrams of dye per 100 g. of detergent. "0 X L
z W
U
a IW
n
I a a L
W
a
PER
CENT
TRITON
X-100
FIG. 6.Solubilization of Orange OT by aqueous solutions of Triton X-100
solution. However, the measurements given in table 7 proved its solubilizing capacity to be high, and the shape of the curve below 10 per cent, shown in
294
SISTER AGNES ANN GREEN AND JAMES W. MCBAIN
figure 6 , is very similar to those of the colloidal electrolytes. A measurement was made of the solubility of the dye in the pure Triton X-100, a liquid, which showed its solvent power to be four or five times its solubilizing power, per weight
I-
z W c)
a W
IW
0
-I
0
1 K
w
a
W
>. 0
rl
I 4
K
u
PERCENTAGE
COMPOSITION
BY
WEIGHT
FIG.7. Solubilization versus solvent action in solutions of Triton X-100 and acetone. Solubility of Orange O T in acetone from measurements by R . C. Nerrill, J r .
of detergent. This is shown in figure 7 , with an extrapolated line for the intermediate compositions, many of which set into rigid gels. In this figure, the solubility of Orange OT in pure acetone and in aqueous solutions of acetone is also represented, in order to bring out the difference bekeen the behavior of a solvent and of a solubilizer.
295
SOLUBILIZATION OF DYE BY SOhPS
The solubilizing pover of aqueous solutions of Triton X-100 indicates some kind of colloid formation. This is supported by unpublished measurements by Dr. Gonick in these laboratories. For example, one of these measurements showed that a 0.100 m solution, assuming the molecular weight to be 600, produced a freezing-point lowering in water solution of 0.032', indicating a molecular weight of 3484, definite evidence of a high degree of association. Although solutions of Triton X-100 were found to be excellent solubilizers, a 2 per cent solution of carbowax, also a polyethylene oxide polymer, showed no solubilizing action on Orange OT, but exhibited instead a high foaming and flotation action. TABLE 8 Eqect of organzc lzquzds on solubilization of Orange OT by 0.2 T . potassaum laurate ADDED HYDROCARBON.. .
......
.....
NOM
BENZENE
Amount of hydrocarbon in 25 cc. of solution, cc. . . . . . . . . . . . . . . . ...... 0.094 D y e per 100 cc. of solution, milligrams. , . 27.50 D y e per mole of soap, grams.. . . . . . . . . . . 1.375 Molar ratio dye/soap X 103.. . . . . . . . . . . 5.24 0.21 Molar ratio hydrocarbon/soap , . . , , , ,
0 052 29 25 1 462 5 57 0 098
I
0 098 33 50 1 675 6 38 0.15
~
1 25 17 50 0 921 3 51 4.28
20.46 1.023 3.90
* .4bsolute ethyl alcohol. TBBLE 9 Solubilitii of Oranpe OT i n t h e pure oiqanic solvents SOL\XNI
_______
-i Benzene ................ Toluene . . . . . . . . . . . . . . . ..I Alcohol . . . . . . . . . . . . . . . I n-Hexane. . . . . . . . . . . . . . . . ~
I
78.11 92.13 46.07 86.14
0 879 0 866 0 789 0 662
88 106 58 130
9 4 4 1
gromr
gronlr
62 65 2 5
5.55 6.92 0.16 0.71
5 0 i25 438
21.2 26.4 0.61 2.70
Effect of solvents on the solubilizing ability o j soap solutions It is a well-established fact ( 1 , 7 ) that many organic liquids have a measurable solubility in soap solutions. In order to determine whether detergent solutions containing solubilized organic liquids are better or poorer solvents for a materinsoluble crystalline dye, the folloning measurements were made. The organic liquid was incorporated in 0.2 S potassium laurate solutions, in amounts in the case of hydrocarbons equal t o 75 per cent of the saturation value determined by Richards ( 7 ) . The dye was added to these clear solutions, and the saturation value determined in the usual manner (3). The results are given in table 8. Similar measurements were made with 0.1 potassium oleate, and the same trends were observed. The solubility of the dye in the pure solvents themselves was determined and is recorded in table 9; the results brought out the fact that the variations apparent in table 8 were not due to actual solution of the dye in
296
SISTER AGNES ANN GREEN AND JAMES W. MCBAIN
the added solvent. Alcohol, a good solvent for the dye, caused a decrease in the solubilizing power of the soap solution. The hydrocarbons, which themselves are solubilized, caused a notable increase when present only in small amounts. Solubilization by three sodium naphthenates The following experiments were carried out in order to observe how naphthenate soaps of different molecular weights compared among themselves and with other detergents in the solubilization of Orange OT. The soaps were prepared by neutralizing with sodium hydroxide three fractions of naphthenic acids obtained from the Standard Oil Company of California. The acid numbers and the average equivalent weights of these fractions were determined by electroTABLE 10 Solubilization o.f Oranoe " OT bu three sodium naohthenates o f d CONCENTION OF NAPB TEENATE
N 0.005 0.01 0.05 0.10 0.15 0.u) 0.25 0.50 0.75 0.99 1 .oo 1.27
went equivalent weights
LIOEI NAPTEENATE
YEDIUY NAPETEENAIE
AEAVY NAPETEENATE
(236.4 equivalent weight)
(303 equivalent weight)
(354.9 equivalent weight)
1
1
Dye per Ratio X IC Dye per equivalent lwcc' weight
Dye per
mg.
mg.
O
I
0.275' 0.055 1.25 0.125 2.05 0.137 3.25 0.163 5.60 0.224 11.75 0.235 25.00 0.333 35.68 0.361
1
0.21 0.48 0.52 0.62 0.85 0.90 1.30 1.38
0.55: 1.53 3.25 6.69 24.5 47.0
0.98 0.94
3.74 3.58
98.0 132.5
0.98 1.047
3.74 4.91
* Believed to include suspending action
latio X
Dye per loo cc.
100 cc.
___
10'
grams
0.77! 1.60 8.00 15.50
1.55 1.60 1.60 1.55
5.91 6.10 6.10 5.91
41.25 102.50
1.65 2.05
6.29 7.82
-
(3).
titration and were found to have the following equivalent weights: 214.4, 281.0, and 332.9. The procedure for the colorimetric determination of the dye had to be modified because of the color of the naphthenate solutions. The dye was extracted with benzene in the case of the naphthenate of lowest molecular weight. This procedure was unsatisfactory for the other two, owing to the formation of permanent emulsions which could not be broken conveniently by centrifuging. Hence, a colorimetric comparison of a dilution of the solution saturated with dye was made with the same dilution of the original solution. The results are given in table 10, and also shown graphically in figure 8. The solubilizing power of the naphthenates was found to increase very greatly with their equivalent weights, in a manner similar to the behavior of the soaps of the fatty acids (3, 4), but not in the same series. The naphthenates are not as
o
297
SOLUBILIZATION OF DYE BY SOAPS
effective as the fatty acid soaps, since the naphthenate with the average equivalent weight of 303 has a saturation ratio of 3.74 X lo-$ at 0.25 N , which compares with 3.96 X 10-3 for 0.25 N potassium laurate, a soap of lower molecular weight (238.4). Hence the condensation of the saturated paraffin chain into rings decreases the solubilizing ability of the soap. There is an advantage, however, in the greater solubility of these sodium soaps, in that higher concentrations can be employed. a
E
l
U
e
*
d
.
4
2 0
2
CONCENTRATION
OF
SODIUM
NAPliTHENATL
FIG.8. Solubiliaation of Orange OT by three sodium naphthenates of diff went molecular weights.
Solubilization in benzene Many detergents which are solubilizers in water have also been shown to be effective in other solvents (5). The following simple experiment was performed to show that the amount of methylene blue which can be solubilized by soap in benzene is much greater than that which could possibly be dissolved by the benzene in a colorless form (11). Excess methylene blue was agitated in thiophenefree benzene, and then allowed to stand for 20 hr. The dye settled, and half of the colorless upper portion, 20 cc., was then removed by pipet and placed in a second bottle. The same amount of sodium oleate, 0.05 g., was added to both bottles, which were then agitated for 24 hr. at 25°C. A blue-violet color developed in the first bottle, which contained the excess dye in contact with the benzene-soap solution; no color was visible in the bottle containing the benzene, which v a s removed from contact with the dye. A small amount of water extracted the color in its normal hue from the former, but remained colorless in the latter.
298
SISTER AQNES ANN QREEN AND JAMES W. MCBAIN SUMMARY
A study has been made of the comparative effectiveness of potassium and sodium soaps as solqbilizers. Measurements were made on potassium and sodium laurates and oleates. The sodium soaps were found to be better solub i l i z e ~for the water-insoluble dye 1-o-tolylazo-2-naphthol (Orange OT) per gram-molecular weight of soap. Measurement of the solubilizing ability of the cation-active detergent, dodecylamine hydrochloride, showed it to be greater than that of the neutral anion-active detergents, the laurates, containing the same number of carbon atoms. Its effect is comparable to that of lauryl sulfonic acid, which was reinvestigated in order to obtain a complete curve over its entire range of solubility at 25°C. The solubilizing power of the non-ionic detergent, Triton X-100, was studied, and the similarity of its solubilization curve to that of the colloidal electrolytes indicated the highly colloidalnature of its water solutions. Its solubilizingpower per unit weight of detergent in a 10 per cent solution is only about one-fourth its solvent power for the dye in anhydrous form. The effect of simultaneously solubilized organic liquids on the solubilization of a dye was found to be dependent on the nature of the added liquid. Benzene, toluene, and n-hexane in small amounts were found to increase the solubilization of the dye from 30 to 50 per cent (synergistic effect); 5 per cent ethyl alcohol was found to decrease it (antergistic effect), although anhydrous alcohol itself is a good solvent for the dye. The relative solubilizing abilities of three sodium naphthenates of different molecular weights were measured. They varied with their equivalent weight, the higher having very much the greater solubilizing power for the dye. Compared with the fatty acid soaps of similar molecular weights, the naphthenates are not as effective as solubilizers, but they have a higher range of solubility. REFEREKCES (1) MCBAIN,JAMESW . : “Solubilization and Other Factors in Detergent Action,” in Advances i n Colloid Science, E. 0. Kraemer (Editor), Vol. I, pp. 99-142. Interscience Publishers, Inc., New York (1942). (2) MCBAIN,JAMESW., AND BOLDUAN, 0. E . A . : J . Phys. Chem. 47. 94 (1943). (3) MCBAIN,JAMESW., AND GREEN,SISTERAGNESANN:J. Am. Chem. Soc.68,1731(1946). K . E . : J. Am. Chem. Sot. 66,9 (1914). (4) MCBAIN, JAMESW., AND JOHNSOX, (5) MCBAIN,JAMESW . , MERRILL, R . C., JR., ASD VIKOGRAD, J . R . : J . Am. Chem. SOC. 62, 2880 (1940). (6) MCBAIN,JAMESW., MERRILL, R . C., J R . , AND VINOGRAD, J. R . : J. Am. Chem. Sot. 83, 670 (1941). (7) MCBAIN,JAMESW., A N D RICHARDS, PAULH . : Ind. Eng. Chem. 38, 642 (1946). (B) MCBAIN,M. E. LAING:J . Phys. Chem. 47, 196 (1943). S. A , :J. Am. Chem. SOC.61,3210 (1939). (9) McBAIN,M.E . L . , D Y E ,W. B . , AND JOHNSON, (10) NOLLER, CARLR . , A N D GORDON, J . J . : J . Am. Chem. SOC.66, 1090 (1933). (11) PALIT,S. R . : Nature 153. 317 (1944). E . J.: J. 4 m . Chem. SOC.64,97 (1912). (12) RALSTON, A. W . , HOERR,C . W . , AND HOFFMAS, (13) VOLD,ROBERTD . , A N D MONTGOMERY, R E B A :Communication from the Cniversity of Southern California, 1945.
