Types of Solubilization in Solutions of Long-Chain ... - ACS Publications

1350

WILLIAM D. HARKINS, ROSE MITTELMANN, AND M. L. CORRIN

Waterman, Mrs. Bernice Platt, Mrs. Martha Atkins, Mrs. Elizabeth Green, and Mr. Harold Lewis, who made most of the thermal measurements, Dr. Kenneth Palmer and Miss Merle Ballantyne, who took the x-ray powder photographs, Mr. Lawrence White and Miss Elizabeth McComb, who made analyses establishing the compositions of the hydrate crystals, and Mr. Jay T. Allison and Mr. Eugene J. Gatze, who did much of the photographic work. REFERENCES BABINSKI,J.: Gae. Cukrownicea 38, Nos. 34-8; Centr. Zuckerind. 32, 782 (1924). COLE,W. C.: J. Agr. Research 66, 137 (1938). FREUNDLICH, H., AND SCHNELL, A , : 2. physik. Chem. 133, 151 (1928). GRUT,E. W.: 2. Zuckerind. Eechoslovak. Rep. 61,356 (1937). GUTHRIE,F.: Phil. Mag. [5] 2,216 (1876). H R U BR., ~ , AND KASJANOV, V.: 2.Zuckerind. Eechoslovak. Rep. 63,187 (1939); Intern. Sugar J. 42, 21 (1940). (7) JONES, H. E., AND GETMAN, F. H.: Z. physik. Chem. 49,385 (1904). (8) KOLTHOFF,I. M.: Verslag. Akad. Wetenschappen Amsterdam 36, 281 (1926). R., AND EITEL,H.: Rec. trav. chim. 42, 539 (1923). (9) KREMANN, (10) LEIGHTON,A.: J. Dairy Sci. 10, 219 (1927). (11) MCBAIN,J. W., AND KISTLER,S. S.: J. Phys. Chem. 33, 1806 (1929); Trans. Faraday SOC.26, 157 (1930). (12) MONDAIN-MONVAL, P.: Compt. rend. 181, 37 (1925). W., AND DIEHL,H. C.: Western Canner and Packer 36 f4), 55 (April, 1944). (13) RABAK, G.: J. Am. Chem. SOC.43, 2406 (1921). (14) SCATCHARD, (15) SHORT,B. E.: The Specific Heat of Foodstuffs.I . Publication No. 4432, Bureau of Engineering Research, University of Texas, Austin, Texas (August 22, 1944). (16) SUGDEN, J. N.: J. Chem. SOC.129,174 (1926). (17) VAND,V.: J. Phys. Colloid Chem. 62, 314 (1948). (18) VERHAAR, G.: Areh. Suikerind. Nederland en Ned.-Indie 1, 324 (1940); Intern. Sugar (1) (2) (3) (4) (5) (6)

J. 43, 50 (1941).

TYPES OF SOLUBILIZATION I N SOLUTIONS OF LONG-CHAIN COLLOIDAL ELECTROLYTES WILLIAM D. HARKINS, ROSE MITTELMANN,

AND

M. L. CORRIN

George Herbert Jones Chemical Laboratory, University of Chicago, Chicago, Illinois Received January 13, 194.9 I . INTRODUCTION : PRINCIPAL TYPES OF SOLUBILIZATION

Solubilization is a term used to indicate the process by means of which substances which are not very soluble in a given solvent dissolve in solutions of long-chain electrolytes to an extent appreciably greater than in the solvent itself, The solubility is the amount dissolved at equilibrium with the solute. 1 This investigation was carried out under the sponsorship of the Reconstruction Finance Corporation, Office of Rubber Reserve, in connection with the synthetic rubber program of the United States Government.

TYPES OF SOLUBILIZATION

1351

In an earlier communication (10) the different types of solubility in soap solutions which have been most distinctly recognized are classified as follows, except for terminology: 1. Solubility in non-micellar solutions. 2. Kon-polar solubilization: The solute molecules are not oriented with respect to the water, but are possibly oriented with respect to the inner portion of the micelle. This behavior is exhibited by hydrocarbons. The x-ray M-band indicates a thickening of the micelle, and the energy relations indicate that the solubilization is in the middle of the micelle.

4.0 POTASSIUM MYRISTATE ON GRAPHITE 35oC.

I-

z

y

a

3.5

8 3.0

!? 0

E

25

v1

9

"I- 2.0 a

a

2

1.5

d

I 1.0

0.5

0

Qo2 Qo3 404 EOUlLlBRlUM CONCENTRATION, MOLAL

FIG.1. Change in the amount of soap adsorbed on graphite as a function of the concentration of the soap solutions. The kink in the curves occurs a t the critical micelle concentration. At higher concentrations at which micelles are present the amount adsorbed is shown not to be constant, as it is often assumed t o be.

3. Polar-non-polar solubilization : The solute molecules are oriented with respect to the water, and solubilization involves penetration of the hydrocarbon chains between those of the soap. The type of solubilization considered in Section I. 4. Adsorption: The type of solubilization exhibited by some dyes. I n types 2 , 3 , and 4 the solubility includes that in the water in addition to that in the micelles, since no experimental method is known by means of which the effect can be partitioned between the two regions. The solubility in the water when micelles are present is different from that in water alone or in non-micellar

1352


solutions, and also varies with soap concentration when micelles are present, since, as shown by experiments on the adsorption of soap from its aqueous phase on graphite, the amount adsorbed may vary greatly with concentration above as well as below the critical micelle concentration (figure 1). 11. SOLUBILIZATION BY ADSORPTION

The evidence for the existence of solubilization by adsorption is less extensive than that for the other types. If it is a true adsorption, the adsorbed material lies between the aggregate and the region of the diffuse ionic layer. I n 1918 McBain and Bolam (14) suggested that a water-soluble substance could have its solubility increased by sorption on the exterior of the micelle. The exact reason for using the indefinite term “sorption” instead of “adsorption” is not apparent. In the solubilization of long-chain polar-non-polar substances the polar groups lie between the ionic heads of the long soap ions and the hydrocarbon chains of the polar-non-polar molecules lie between those of the soap. If the chain is very short, as in methyl alcohol, this type of solubilization (type 3) differs only slightly from adsorption (type 4). However there appears to be a real difference, since methyl alcohol lowers the critical micelle concentration considerably a t low soap concentrations. Thus these two types overlap. Of the four types of solubilization considered in this paper, that by adsorption is the most difficult to distinguish. That it exists seems to be shown by the two related types of evidence presented below. Solutions of pinacyanol chloride in water on the order of 1 X 10-4 molar are reddish blue in color; such solutions are known to contain a polymeric form of the dye. When this solution is agitated with heptane, in which the dye is insoluble, the only visually perceptible color becomes concentrated a t the interface; this interfacial color is similar to that in alcoholic solutions of the dye in which only the monomeric form is present. Similar behavior is noted when pinacyanol chloride solutions are agitated with silica gel and titanium dioxide. The adsorption of pinacyanol chloride a t the micelle-water interface has been postulated by Sheppard and Geddes (17) and of other dyes by Stauff (18). Arkin and Singleterry in an unpublished communication report that rhodamine B is slightly soluble and non-fluorescent in benzene, but fluoresces strongly when as little as moles per liter of calcium or sodium aryl stearate is added, thus producing micelles. That solutions of rhodamine 6G and other fluorescent dyes fluoresce when micelles are present and almost not at all just below the C.M.C. (critical micelle concentration) was discovered by Corrin and Harkins (1). The change is sharp and thus gives an excellent method for the determination of the critical micelle concentration. Just below this the single ions of the soap act as a salt, which quenches the fluorescence. It was assumed in this work that the dye is adsorbed on the micelle. Arkin and Singleterry give evidence that this is true. They find that a solution of 2 X 10-6 moles of dye and 5 X moles of the soap when excited by polarized light of 546 mp wave length emits light which is 28 per cent polarized

TYPES O F SOLUBILIZATION

1353

whereas that from a solution of the same concentration of the dye in methyl alcohol is only 2.3 per cent polarized. This, they show, is evidence that the dye in the micellar soap solution is in the adsorbed state.

FIG.2a. Solubility of oils in solutions of potassium tetradecanoate (continuous lines) and potassium dodecsnoate (dashes). The increase by two carbon atoms in the chains from benzene to ethylbenzene has the same effect as decreasing the number of carbon atoms in the soap by two. 111. SOLUBILIZATIOhT O F NON-POLAR SUBSTANCES

The solubilization of non-polar substances in micelles has been the subject of many investigations. In an earlier paper from this laboratory (19) it was shown that in this type of solubilization of normal hydrocarbon oils (figures 2%and 2b) : ( I ) The solubility increases rapidly with the length of the hydrocarbon chain of the soap. (2) The rate of increase of solubility (ds) with increase in concentration (de) of the soap increases mjth e, or (as/ac). increases with c . In figure 2a the dashed lines represent the solubility in solutions of potassium

1354

WILLIAM D. HARKINS, ROSE MITTELMAN”, AND M. L. CORRIN

dodecanoate, the continuous lines that in solutions of potassium tetradecanoate. The substitution of the ethyl group in benzene lowers the solubilization in the one case shown, to the same extent as the subtraction of two carbon atoms from the length of the hydrocarbon chain. As is well known, carbon atoms in the benzene ring are not so effective in lowering the solubility as those in a normal hydrocarbon chain. 1.0

0.9 0.6

0.7

-00.6 J

THYL BENZENE

u.

0

0.5

i

U A a4 0

I

0.3 0.2

0.I

MOLALITY

OF SOAP

FIG.2b. Solubility of oils in solutions of potassium tetradecanoate. The solubility of decanol is higher than that of heptane, but the solubility of decane is very much lower than that of heptane. This exhibits the difference between polar-non-polar and nan-polar solubilization.

X - r a y evidence The only direct evidence for the thickening of the micelle by non-polar solubilization is that given by one of the spacings revealed by the x-ray bands. On the basis of extensive work in this laboratory two of these bands have been designated as the interinicellar or I-band and the micelle diameter 01’ 114-band. This work shows that the I-spacing ( d r ) is always the larger, and decreases, becoming closer to that of the M -band as the concentration of the colloidal electrolyte increases. The I-band is related in an unknown way to the mean distance between the micelles. The existence of the micelle diameter band in the x-ray photographs was dis-


1355

covered only about two years ago (15,20),although its existence was suspected much earlier. Its detection was due to a microphotometer, constructed by R. S. Steams, which revealed the band undetected by the commercial instrument employed earlier. The solubilization of a hydrocarbon increases both the I - and the M-spacings, the former more than the latter. The most extensive and best x-ray work on soap solutions prior to that of this laboratory was carried out by a group of German investigators (references to this work are given in an earlier paper (9)). They considered that the value of Ad1 g.ives the thickness of a solubilized oil layer, and obtained in one case in this way a value of 36 A. as the thickness of this layer. This and their other values are entirely too large, partly because their lamellar model of the micelle is incorrect, and partly because no method is known by means of which their I-bands could be translated into correct I-spacings; i.e., the theory has not been developed. The values of the M-spacings indicate that the diameter of the micelle is independent of soap concentration, but that it increases with the length of the soap molecule, and also increases by an amount Ad, upon the solubilFation of a hydrocarbon oil. The value of A d , was found to be 11.2 8. and 11.4 A. for ethylbenzene and n-hexane, respectively, when a 0.33 molal solution of potassium dodecyl sulfate a t 25°C. was made 0.290 molal with respect to the former and 0.160 molal with respect to the latter. These may be considered as the increases in the diameter of the micelle due to the solubilization of these hydrocarbons. Since the interface hydrocarbon-water is the locus of a high free energy, the solubilized hydrocarbon molecules bury themselves in the hydrocarbon portion of the micelle in order to keep the area of the hydrocarbon-water interface of the micelle as small as possible. A further consideration of these relations is presented later. IV. EVIDENCE FOR THE ORIEKTED SOLUBILIZATION O F POLAR-NON-POLAR MOLECULES

(FIGURE3)

The evidence which shows that in the micelle normal long-paraffin-chain alcohols exhibit essentially the same orientation and position as the soap molecules may be considered under four categories. 1. Energy relations f o u n d in connection with the development of the theory of

molecular orientation in interfaces These relations are given in many papers by Harkins and collaborators (3, 6, 8, l l ) , of which only a few can be referred to here. The papers of Langmuir (12, 13) are also of value in this connection, but do not deal so directly with the energy relations involved. An extensive discussion of the orientation theory cannot be given here, but it is the fundamental basis of all theories of micellar structure. Consider the increase in molecular potential energy involved in certain interfacial changes (table 1).

1356


0.2

0

go.t

c-I

?

r;

01 .

0.2

0.4 0.6 0.8 MOLALITY POTASSIUM MYRISTATE

FIG.3. Exhibits the great difference between polar-non-polar and non-polar solubilization. , TABLE 1 Increase of potential energy when a liquid is pulled apart or from another liquid Ergs for a bar of liquid 1 cm.2 in cross-section, for the separation of t h e upper from the lower liquid a t 20°C.

n-Octyl alcohol 101 n-Octyl alcohol n-Octyl alcohol 165 Water n-Octyl alcohol

55.1 n-Octyl alcohol n-Octyl alcohol 91.8 Water

The high values of the adhesional energy relate to the separation of the alcohol from the water, in which case a hydrogen bond is involved.

1357


The origin of both polar and non-polar solubilization in micelles lies in the fact that the energy to separate water from itself is much larger (ec = 237, W c = 144 ergs than for the separation of hydrocarbon chains from water (eA = 107, W A = 43). Thus if a hydrocarbon molecule or the hydrocarbon chain of an alcohol removes itself from water to go into a hydrocarbon environment it does not do so because it is attracted more by the hydrocarbon than by the water, but on account of the large decrease of free energy due to the coming together of the water to fill the hole left by the removal of the hydrocarbon chain. TABLE 2 Effect of benzene on the critical micelle concentration

I

BENZENE

MOLAR C.M.C.

Sodium decyl sulfonate g./IOOO

g.

solution

0.0

4.00 X

0.340 0.550 0.89 1.22 1.47

3.8, 3.86 3.89 3.82 3.85

Mean. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.85 f 0.02 X 10-2

Potassium dodecanoate 0.0

2.40 X lo-'

0.44 0.61 1.00 1.39 1.50

2.30 2.34 2.30 2.29 2.30

Mean. ...................................

2.31 f 0.01

x lo-*

Molecules of an alcohol, amine, etc. have a highly polar end which forms a hydrogen bond with water. This end does not leave the water entirely if the polar group is oriented toward the water. Thus this group is, in the micelle, oriented toward the water. If it were to go into the hydrocarbon part of the micelle, the free energy would be much higher. It is true that in the above discussion macroscopic relations are applied to exceedingly minute systems. However, much experience has shown that deductions made in this way are in general qualitatively correct. 9. X-ray evidence

The spacing of the micelle diameter or M-band is found to be (a) increased by as much as 12 d. by non-polar solubilization, and ( 6 ) not increased by oriented

1358


or polar-non-polar solubilization. At some higher soap concentrations decreases of as much as 5 or 6 A. were found. These results show that there are two entirely di$erent types of solubilization inside the micelle.

'08.

4I,49IOXANE

.

ETHYLENE GLYCOL/

!- .06 3

ETHANOL

h

\9

I

0

1.0

PROPANOL

1

I

I

I

I

3.0 4.0 5.0 6. C 0N C E NT RAT I ON OF ADD IT I V E (M0LES/I> 2.0

'

FIG.4 . Effect of the addition of alcohols on the critical micelle concentration of deoyltrimethylammonium bromide.

The only position which a polar-non-polar long-chain molecule can take without increasing the thickness of the micelle is essentially the same as that of a soap molecule, with its polar group oriented toward the water and its hydrocarbon chain lying between those of the soap. There is an important difference, however, in that the area occupied by an alcohol molecule is much smaller than that occupied by a soap molecule. With aJong paraffin chain the area per molecule of alcohol on water is about 20 sq. A. at 20°C. a t a moderately high film pressure. Although the alcohol molecules occupy the same type of position as the soap molecules, they do not appear to reduce their number.


1359

3. Evidence in the effect of alcohols and hydrocarbons upon the critical micelle concentration (C.M.C.)

Hydrocarbons when solubilized have very little efect upon the critical micelle concentration. Table 2 exhibits the lowering of the C.M.C., produced by benzene in solutions of sodium decyl sulfonate (lowering 0.15 X molar) and potassium domolar). The table indicates that the lowering decanoate (lowering 0.09 X produced is essentially independent of the amount of benzene solubilized. Alcohols have a very great effect. Thus they lower the C.M.C. and the decrease in C.M.C. per mole of alcohol increases with great rapidity with the length of the hydrocarbon chain of the alcohol. Figure 4 exhibits these relations for the effect of normal primary alcohols of from one t o four carbon atoms upon the C.M.C. of decyltrimethylammonium bromide. With another soap heptyl alcohol was found to lower the C.M.C. very much more rapidly than butyl alcohol. These are exactly the relations to be expected if hydrocarbon or non-polar molecules are solubilized in the middle of the micelle, and the polar-non-polar molecules align themselves between the soap molecules, with their polar groups oriented toward the water.

4. Monolayer penetration Less conclusive, but giving support to the relations already considered, is the phenomenon of monolayer penetration. However, it was this which led to the initiation of researches on this problem. This phenomenon was discovered by Schulman and Hughes (16), who found that if a solution of hexadecyl sulfate is injected under a monolayer of the alcohol, the molecules of the soap penetrate the monolayer and increase the film pressure, From sharp kinks in the curve Schulman considered that definite compounds are formed, but in the work equilibrium was not attained. Harkins, Copeland, and Gordon (7) constructed a recording film balance, and found, using 12 hr. t o attain equilibrium, that no kinks were present. At 25°C. 3X molar detergent increased the film pressure fro? 1 dyne per centimeter to 45 dynes per centimeter at a molecular area of 26 A.2 for the alcohol. This extremely great increase in pressure, by a factor of 45, indicates a large decrease in free energy in the process. Thus the large change in free energy on the mixing of soap and alcohol in the monolayer seemed to make it probable, but does not, prove, that a long-chain alcohol would form a similar mixed film by penetration of the alcohol into the soap film. V. DISCUSSION

In Section IV the penetration of alcohols was consiaered from the standpoint of work and energy of adhesion. A consideration of the free interfacial relations gives further evidence. At 20°C. the free surface energy of the interface between normal hydrocarbons and water is almost constant from n-hexane (y = 50.92, B = 74.0) to n-hexadecane (y = 54.85, B = 72.0) (5). Although these specific values cannot be carried over to the energy between a

1360


single molecule of hydrocarbon and water, it was estimated that the decrease of free energy when the hydrocarbon chain of an alcohol moves from water into a hydrocarbon environment should be of the general order of 1000 cal. per mole for each carbon atom in the chain. I n the work of Corrin and Herzfeld (4) (figure 5 ) it was found that with pure ordinary soaps with seven to fourteen carbon atoms in the chain, a reduction by unity in the number of carbon atoms multiplies the critical micelle concentration by 2. From this Debye (2) calculated the corresponding decrease in free energy in the formation of a soap micelle as 1280 cal. per mole per carbon atom,

0.002

t

I 7.

I 8

1 1 I I 9 IO II 12 NUMBER OF CARBON ATOMS

.

1

I

I I

13

14

FIG.5. Effect of an increase in the number of carbon atoms in the hydrocarbon chain of a soap upon the critical micelle concentration (Herafeld). which should not be extremely different for an alcohol. Thus the hydrocarbon chain gives a large decrease in free energy when the alcohol molecule enters the micelle. If the hydroxyl group moves from water into the hydrocarbon portion of the micelle, a large increase of free energy is involved. However a large part of this is avoided if the hydroxyl group in the micelle remains adjacent to the water, especially since here a hydrogen bond may be formed. The alcohol molecule in the micelle takes a position which is on the whole the same as that of a soap ion, but its non-ionic nature involves a difference in free energy. The relations of the amines are essentially the same. VI. SUMMARY

Evidence is presented which indicates the existence of four types of solubilization in solutions of colloidal electrolytes: (1) solubility in the water, which,


1361

when micelles are present, cannot be determined; ( 2 ) solubilization of non-polar substances in the non-polar interior of the micelle; (5) solubilization of polarnon-polar molecules in a position similar to that of the soap molecules; and (4) adsorption at the surface of the micelle. The most conclusive evidence for types 2 and 3 is obtained from the x-ray scattering, by means of which i t is shown th?t type 2 solubilization increases the diameter of the micelle by as much as 12 A., whereas in type 3, when alcohols ar: used, there is either no increase or a decrease in diameter by as much as 5 or 6 A. In addition, in type 2 solubilization hydrocarbons have very little effect on the critical concentration for the presence or absence of micelles, whereas in type 3 solubilization, normal alcohols lower this concentration. The decrease per mole of alcohol increases with great rapidity with the length of the hydrocarbon chain. Thus the alcohol, or another linear polar-non-polar substance, increases the lowering of free energy which accompanies micelle formation. This indicates a more intimate relation to the soap than in type 2 solubilization. The variation of the adhesional energy between water and hydrocarbons (type 2) or polar-non-polar substances (type 3) gives support to the point of view which is presented, and the solubilization relations in types 2 and 3 are shown t o be very different. The evidence for solubilization by adsorption is not extensive, and rests upon very few facts. REFERENCES (1) CORRIN, M. L., A N D HARKINS, W. D . : J. Chem. Phys. 14, 641 (1946). (2) DEBYE,P. : Private communication. (3) HARKING, W. D.: I n Surface Chemistry,pp. 40-87 including the bibliography; Pub. Am. Assoc. Advance. Sci. No. 23 (1943). (4) HARKINS, W. D . : J. Am. Chem. SOC.69, 1434 (1947). W. D . , AND ADINOFF,B.: Unpublished paper. (5) HARKINS, W. D . , BROWN, F. E., AND DAVIES,E. C. H. : J. Am. Chem. SOC.39,354 (1917). (6) HARRINS, (7) HARKINS, W. D . , COPELAND, L. E., AND GORDON, S. : Symposium on Surface Chemistry, held in September, 1941, by the American Association for the Advancement of Science and the University of Chicago in commemoration of the first contribution by William D. Harkins in the field of surface chemistry; Pub. Am. Assoc. Advance. Sci. No. 21, p. 79. (8) HARKINS, W. D., DAVIES, E. C. H., AND CLARK, G. L. : J. Am. Chem. SOC.39,541 (1917). (9) HARKINS, W. D . , MATTOON, R . W., AND CORRIN,M. L . : J. Am. Chem. SOC.68, 221 (1946), references 5 to 13. (10) HARKINS, D., MATTOON, R. W., AND MITTELMANN, R. : J. Chem. Phys. 16,763 (1947). L. E.: J. Am. Chem. SOC.44, 653 (1922). (11) HARKINS, D., AND ROBERTS, (12) LANGMUIR, I.: J. Am. Chem. SOC.39, 1848 (1917). (13) LANGMUIR, I.: In Colloid Chemistry,edited by J. Alexander. The Chemical Catalog Company, Inc., New York (1926). (14) MCBAIN,J. W., AND BOLAM, T. R.: J. Chem. SOC.113, 825 (1918). (15) MATTOON, R . W., STEARNS, R. S., A N D HARKINS, W. D. : J. Chem. Phys. 16,209 (1947) ; 16, 644 (1948). (16) SCHULMAN, J. H., AND HUGHES, A. H . : Biochem. J. 29, 1243 (1935). (17) SHEPPARD, S.E . , AND GEDDES,A. L.: J. Chem. Phys. 13, 63 (1945). (18) STAUFF, J.: Z. physik. Chem. A191, 69 (1942). (19) STEARNS, R. S., OPPENHEIMER, H., SIMON, E., AND HARKINS, W. D.: J. Chem. Phys. 16, 496 (1947). (20) STEARNS, R. S.: Private communication, July, 1946.

w. w.

Types of Solubilization in Solutions of Long-Chain ... - ACS Publications

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