The Solubilization of Polycyclic Hydrocarbons. - The Journal of

SOLUBILIZATION O F POLYCYCLIC HYDROCARBONS

283

(8) HARKINS,W. D . , MATTOON,R . W., AND MITTELMANN, R.: J. Chem. Phys. 15, 763 (1947). (9) RALSTON,A. W.,AND EGGENBERGER, D. N.: J. Am. Chem. SOC.70,983 (1948). (10) RILSTON, A. w . , AND HOERB, c. W.: J. Am. Chem. soc. 68,851 (1946). (11) RALSTON, A. W., AND HOERR,C. W.: J. Am. Chem. SOC.68, 2460 (1946). (12) WARD,A. F. H . : Proc. Roy. SOC.(London) A176, 412 (1940).

SOLUBILIZATION OF POLYCYCLIC HYDROCARBONS’J H. B. KLEVENS Division of Agricullural Biochemistry, University of Minnesota, St. Paul, Minnesota Received August d d , 1948

I n the course of a study of a possible mechanism of transport of carcinogenic polycyclic hydrocarbons in the body, it was of some interest to determine their solubility in various media. Data are presented here concerning the solubility of various hydrocarbons in water and their solubilization in potassium laurate solutions. Systems in which salts of bile acids and mixtures of these salts with fatty acid soaps act as solubilizers will be described in a later report. These latter data indicate that the “choleic” acid principle also plays a part in solubilization. I t has been reported previously (15) that there are apparently a t least two types of solubilization : one involving hydrocarbons such as n-heptane and ethylbenzene in which there is a constant increase in the number of moles of compound solubilized with increase in soap concentration (MR) (7), and the other in which there is a constant M R or one which decreases slightly with increase in soap concentration. Examples of the latter are typical water-insoluble dyes, such as Orange OT (0-tolylazo-p-naphthol) and Yellow AB (phenylazo-p-naphthylamine) (18) and long-chain alcohols and amines (5). Data illustrative of these effects are seen in figure 1. Extensive x-ray studies on soap-hydrocarbon systems indicate that solubilization involves incorporation of the hydrocarbon in the region of the micelle center (18), as seen by an increase in x-ray spacing. Solubilization of long-chain alcohols involving palisade formation has recently been shown to cause no swelling of the micelle (6). Further, recent results indicate definitely two different loci of solubilization in the soap micelle, for the solubility of n-heptane and of 1-heptanol is shown not to be additive but, rather, the presence of long-chain alcohols greatly increases the solubilizing power of a soap solution for a hydrocarbon (16). The data presented below attempt to bridge, in part, the gap between the above phenomena. The importance of many of the compounds studied here and their derivatives as carcinogenic agents has been well established (4). 1 Presented a t the Twenty-third Pu’ational Colloid Symposium, which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society at Minneapolis, Minnesota, June 6-8, 1949. 2 Paper No. 2533, Scientific Journal Series, Minnesota Agricultural Experiment Station,

284

H . B. KLEVENS

EXPERIMEKTAL

Two experimental methods may be employed for the determination of the solubilization of polycyclic hydrocarbons. The first involves adding a known amount of an acetone or alcohol solution of the polycyclic hydrocarbon to a soap solution, agitating this mixture, and then evaporating off the organic solvent. A certain amount of water will be lost in this manner but this can easily be corrected for in subsequent calculations. Saturation of the soap solution by hydrocarbon can be determined by turbidity. In the second method, sealed ampules containing soap solution and solid polycyclics are shaken for periods up to three months, or until equilibrium results are obtained, a t 25OC. & 2". The ampules are then allowed to stand a t room temperatures for a few days, and centrifuged a t about 3000 g to insure settling out of residual polycyclic material. Samples are withdrawn through a plug in order to hold back any solid polycyclics, diluted in alcohol or some other suitable solvent, and the spectra determined in the region of the most intense band. In the cata-condensed polycyclics, this is the

WEIGHT NORMALITY

FIG.1. Solubility of ethylbenzene and Orange OT in potassium laurate solutions (25°C.)

transition (that associated with polarization along the long axis of the molecule), which is 100-500 times more intense than those of the weak, forbidden, long wave length 'A-lL, and 'A--'Lb transitions (13).This second method was found to be more satisfactory and was used for most of the data reported here.a Potassium laurate (KCI2)mas used as the solubilizing medium and mas prepared by saponification of the methyl ester which had previously been fractionally distilled. The soap was recrystallized a number of times from ethanol and acetone and then dried in Z'QCUO. The polycyclics were, for the most part, those obtained from Dr. R. S.,Jones and were purified by formation of the picrate complex followed by chromatogRecently Ekwall and Setala (Acta Chem. Scand 2, 733 (1948)) have determined the approximate solubility values of three polycyclic hydrocarbons, 1,2,5,6-dibenzanthracene, 20-methylcholanthrene, and 9,10-dimethyl-l,2-benzanthracene,by the measurement of their fluorescent intensities. This method of measurement is complicated by t h e quenching of fluorescence by the soap and other substances (e.g., oxygen) as well as b y self-quenching.

285

SOLUDILIZATION OF POLYCYCLIC HYDROCARBOXS

raphy. The spectra of these compounds have been reported by Jones (10) and others (13) and these data were used in the calculations of concentrations in those solubilization studies. SOLUBILITY I N WATER

The solubility of various polycyclics in water was determined by shaking crystals of each compound in 1 liter of water for as long as three months. Aliquots were removed and concentrations determined by spectra. For the sparingly soluble polycyclics, a special 1-meter cell with lenses as windows was used for TABLE 1 Solubility (8) of various hydrocarbons in water (BPC.) S

CObIpOUND

~

LOG

v

c pramr/lilrr

Benzene . . T o 1u en e . . . . . . . . . Ethylbenzene . . . . . n-Propylbenzene . . . . n-Butylbenzene . . . . . .

,860,000 500,000 175,000 120,000 50,000

Saphthalene Phenanthrene. . . . . . . . Anthracene. . . . . . Triphenylene . . . . . . . . . Pyrene. . . . . . . . . . . . Chrysene . . . . . . . 1,2-Bensanthracene., Saphthacene. . . . . . . .

1,000 370

12,500

97.5

1,600 75

9.0

43 175 6 10 (1.5)'

0.77 0.0276 0.0431 0.0066

,-3.00145 1-3.131156 -4.01!112

II6.73 -6.121159 ,-7.451178 ' 7.27182

118.14 ~

Fluoranthrene . . . . . . . .

(0.6)' 265

!

8.0

1-5,01151 6.34 142.6 110.5 9.5

!

1,2,5,6-Dibenzanthracene . . . . . . .

(9.0-9.5) 0.145 /(9.8-10.8)0.058

~

9.5 9.5 11.8 11.8 113.0 113.5

0,0109

-0.24 -0.69 1-0.85 1-1.24 '-1.96

0.00137 0.00006 1-2.86 -4.22

~

0.00014 0.0000031 O.ooO0089

0.002151 -8.67 1.32

1-5.88,163

9.4

0.0002

* Approximate values. the spectral analysis. The results obtained in this manner are collected in table 1. In the alkylbenzene series, the decrease in log S (solubility) or V (volume oil) is roughly linear with molar volume. Similar results as to log S are obtained if only the !inear polyacenes (benzene, naphthalene, anthracene, etc.) are considered. However, the nonlinear polycyclics do not follow this relationship. Complete density and molar volume data are not available for all the nonlinear polyacenes, so that a complete comparison of molar volume and solubility could not be obtained. However, by use of tables of bond distances and bond angles (21), it is possible to obtain the length of the various molecules along their

286

E. B. KLEVENS

major axis. The change in log S with length in A. is seen to follow a linear relationship in figure 2. A few of the points, those for the highly condensed pyrene and triphenylene, do not coincide with the curve, a result which indicates that this linear relationship may be valid only for the linear or near-linear polycyclics. In addition, 3,4-benzopyrene and perylene fall about as much below the curve as does pyrene; picene, cholanthrene, and methylcholanthrene, all somewhat less condensed than pyrene and perylene, fall on this curve (2). The naphthacene value is an approximate one because of the instability of this compound in solution. SOLUBILITY IN POTASSIUM LAUFLATE SOLUTIOKS

Data for the solubilization of benzene and various polycyclio hydrocarbons in various concentrations of potassium laurate solutions are collected in table 2

i21 \

t

’1 3 4

$8 0

c

0

I2

FIG.2. Variation of Solubility in water with length. 1, benzene; 2, naphthalene; 3, phenanthrene; 4, fluoranthrene; 5, pyrene; 6, anthracene; 7, triphenylene; 8, 1,2-bensantbracene; 9, chryeene; 10, cholanthrene; 11, naphthacene; 12, methylcholsnthrene; 13, 1,2,5,6-dibenzanthracene.

and some of these data are plotted in figures 3 t o 5. When all data are corrected for solubility of the hydrocarbons in water, the curves intercept the concentration axis a t about 0.25 M potassium laurate. This is the concentration a t which micelles form and can be compared with a value of 0.255 M found by refraction (12) and one of 0.23 M found by the dye-spectral method (1) for the same preparation of potassium laurate. The data in column 5 of table 2, molecules of oil per micelle, are based on a value of 150 molecules of potassium laurate per hydrocarbon-swollen soap micelle. This is an approximately average value of a number of different calculations for a soap of this chain length: ( 1 ) a value of 140 as calculated from x-ray evidence of swollen micelles (19),in which it is indicated that swollen micelles have about twice the number of soap molecules as the hydrocarbon-free micelles; (R) one of 130 calculated from equations describing the size of swollen micelles (14), using as a basis of the molecular weight values of nonswollen micelles as de-

287

SOLUBILIZATION OF POLYCYCLIC HYDROCiRBONS

TABLE 2 Solubility of various hydrocarbons i n potassium laurate (KClr) solutions ( W C . ) (Corrected for solubility in water) MOLES OF OIL PEP MOLE OF SOAP

KCii IN MOLES PER LITER

1

15 45 9 90 3 60

198 127 46 1

I

(n. ma

OIL

U T I P OF SOAP

PLP MxcmE

(X 101)

0.30 0 21 0 105

VOLmdE OF OIL

X O L E O L E S OF

118 111 99 90 66 50

660 605 438

SOLUIIOK)

123 119 108 103 97 83

34.3 26.9 17.3 11.1 4.02 1.73

Naphthalene

0.30 0.21 0.145 0.073 0.036

2.46 1.60 1.13 0.448 0.103

0.650 0.530 0.370 0.300 0.159 0.079

1.31 1.06 0.708 0.581 0.301 0.124

42.0 33.3 27.4 19.2 12.5 0.81

!

1

66.7 66.6 65.4 64.0 59.5 22.1

10.0 10.0 9.8 9.6 9.0 9.1 7.2 3.3

10.5 10.4 10.4 10.4 11.0 11.1 11.0 10.9

4.70 3.72 3.05 2.15 1.40 0.99 0.39 0.09

2.0 2.0 1.9 1.9 1.8 1.6

2.1 2.1 2.0 2.0 2.2 2.2

1.28 1.04 0.692 0.567 0.294 0.121

1.3 1.3 1.3 1.2 1.2 1.0

1.4 1.4 1.4 1.4 1.5 1.5

Aoenaphthene 8.50 6.91 4.60 3.75 1.95 0.81

13.1 13.0 12.4 12.5 12.3 10.3 Fluorene

0.650 0.530 0.370 0.300 0.159 0,079

0.956 0.790 0.533 0.424 0.219 0.0897

1.55 0.42 0.30 0.21 0.145 0.073 0.036

I

, ,

1

1.02 0.70 0.48 0.335 0.148 0.050

I

8.88

4.75 2.55 ~

0.54

8.53 6.65 5.60 3.83 2.64

6.83

13.5 13.3 13.3 12.8 12.6

1

1 ::: 1 1

2.0 1.9 1.9

2.1 2.1 2.1 2.1 2.1

1 1 i

1.22 1.03 0.865 0.593 0.408 0.285 0.126 0.0424

288

H. B. KLEVENY

TABLE 2-Continued

Anthracene 0.63 0.50 0.42 0.30 0.21 0.145 0.073

1.os 0.85 0.715 0.495 0.346 0.215 0.085

0.197 0.155 0.130 0.090 0.063 0.039 0.0155

1.71 1.70 1.71 1.65 1.65 1.48 1.16

0.26 0.26 0.26 0.25 0.25 0.22 0.17

0.27 0.27 0.27 0.27 0.25 0.27 0.27

0.157 0.123 0.103 0.0715 0.0502 0.0311 0.0123

0.91 0.90 0.84 0.84

0.355 0.272 0.124 0.0502

0.71 0.71 0.70 0.70 0.70 0.67

0.472 0.381 0.256 0.204 0.099 0.0378

0.20 0.20 0.21

0.135 0.102 0.0952 0.0734 0.0242

Fluoranthrene

0.37 0.30 0.159 0.079

~

0.65 0.53 0.37 0.30 0.159 0.079

0.428 0.335 0.153 0.062

1

0.601 0.484 0.325 0.260 0.126 0.048

5.73

j

0.86 ~

0.76 0.307 ,

4.77 3.88

2.98 2.40 1.61 1.29 0.623 0.238

4.58 4.53 4.36 4.30 3.93 ' 3.02 ,

~

~

I ~

i ~

0.59

0.68

:::: ~

~

0.65 0.58 0.45 , ~

Chrysene 0.597 0.50 0.418 0.299 0.119

0'172 0.143 0.121 0.0836 0.0308

0.597 0.50 0.418 0.299 0.119

0.170 0.145 0.119 0,0810 o.0288

1

0.755 0.627 0.530 0.369 0.136

i

0.746 0.635 0.525 0.356 I 0.127 1

~

~

I

0.19

1.27 1.25 1.27 1.24 1.14

~

~

0.19 0.19 0.17

, ;::; 1.25

~

1.19 1.06

I

0.19 0.19 0.19 0.18 0.16

Triphenylene 0.56 0 50 0 35 0 28 0 112

~

~

0.088 0 0765 0 0526 0 0421 0 0147

0 387 0 336 0 231 0 186 0 06451

0 10

0 660 0 665 0 577

1

0 10 0 10 0 09

1 ~

I 1 ~

I

' ~

0.20 0.20 0.20 0.20 0.20

0.11 0 11 0 11 0.11 0 11

1

1

''

I

1 I

0.137 0.117 0.0956 0.0650 0.0232

289

SOLUBILIZ.ITION OF POLYCYCLIC HYDROCARBOXS

..

TABLE 2-Concluded ~

AIeth\ lcholanthrene

__-

-

0,597 0.50 0,418 0.299 0.119

0 105 0 08i 0.0725 0 0520 0 0153

0 0 0 0 0

3ii 323 2iO

193 055

0 632 0 646 0 645 0 645 0 460

0 095 0 097 0 097 o Ogi 0.069

0 10 0 10 010 I o 11 009

1

1

______

1 2,5,6-Dibenaanthracene

0.63 0.50 0.42 0.21 0.145 O.Oi3 0.036

0.030 0.024 0.0196 0.00852 0.0061 0.0025 0.0012

0 108 0 0862 0 0702 0 0308 0 0221 0 0090 0 0043

0 0 0 0 0 0 0

171 172 168 148 152 124 120

0 026 0 026 0 025 0 022 0.023 0 019 0 018

-

---

-

0 02i 1 0 02i

0 0 0 0 0

027 025 027 034 058

1

termined by light scattering (3); and ( 3 ) one of 140-160 as obtained from preliminary light-scattering measurements on hydrocarbon-swollen micelles (17). The corrected value of molecules of oil per micelle is based on the assumption that 0.025 -11IiClz exists in all systems as nonmicellar (free) soap, and thus this amount does not play a part in solubilization. By use of this correction of "free" (nonmicellar) soap, it is possible to obtain a more accurate mole ratio of hydrocarbon t o micellar soap (the only soap which plays a role in solubilization). Plots of molecules per micelle, in the case of benzene, as a function of total soap concentration result in tn.0 fairly straight lines with the curve of the corrected values lying above and n.ith a somewhat smaller elope than the other curve. These will, of course, approach each other at higher soap concentrations, where the amount of "free" soap is negligible compared with the total soap present. Further. the use of the corrected data shows that the micelle has undergone its greatest change in solubilizing power per unit change in concentration up to a concentration of about 0.1 M .Above this concentration, the rate of change is constant, a small positive value in the case of the normal paraffins and the alkylbenzene series, and a value of essentially zero for the more complex polycyclics. Without consideration of this "free" soap, this concentration of maximum change in solubilization would appear to occur at about 0.2 J f , -1number of points regarding thedata in tahle2 and the corresponding figures 3 t o 5 ivill be considered helo\\.. Citrialitre of solithiIily cwws

The slope of solubility curves of simple aliphatic hydrocarbons, such as propene, n-hexane, n-heptane, etc.. and of benzene and the alkylbenzenes, such as toluene. ethylhensene, n-hutvlbenzene, etc., increases with increasing soap

290

H. B. KLEVENS

I

0.I

I

I

0.3

l / l

0.5

I

07

01

K tl2MOLES/ L I T E R

03

05

a7

KClp M O L E S l L l T E R

FIG.3 FIG.4 FIG.3. Solubility of various polycyclic hydrocarbons in potassium laurate solutions FIQ.4. Solubility of various polycyclics in potassium laurate solutions. 1, naphthalene; 2, accnaphthene, 3, fluorene; 4, phenanthrene, 5, anthracene.

0.1

0.1

0.3 0.5 KC,) M O L E S / L I T E R

0.7

I 0.1

I

I

I C 1 @ S i

0.3 0.5 KC,* M O L E S / L I T E R

0.7

FIG.5 FIG.6 FIG.5 . Solubility of various polycyclics in potassium laurate solutions. 1, fluoranthrene; 2, pyrene; 3, chrysene; 4, 1,Z-benzanthracene; 5, triphenylene; 6, methylcholanthrene; 7, 1,2,5,6-dibenzanthracene. FIQ.6. Chord plots of the data in figure 3 indicating the change, with soap concentration, of the increment in the number of molecules of hydrocarbon solubilized per increment in soap concentration.

29 1

SOLUBILIZ.&TION O F POLYCYCLIC HYDROCARBONS

concentration. This mould indicate that the expansion and probable increase in size of the hydrocarbon-swollen micelle are accompanied by an enhancement of the solubilizing power per soap molecule. This increase in slope with soap concentration observed in solubilization of the simpler hydrocarbons is accompanied by incorporation in the micelle at 0.63 N KC12 of about 20-125 molecules of hydrocarbon per micelle, as can be seen in table 3. Similar amounts of long-chain alcohols are incorporated per soap micelle, but in this case there ifi a decrease in slope (5).

___

TABLE 3 Effect of unsalioalioii and cyclzzation on solubzlzzation zn 0.85 N KCll GUMS PER LITER OF SOAP 5oLuno"I

COYPOUND

_ _ _ _ ~ _ _ _-

,

moL%x Pen LITER OF SOAP SOLUTION

MOLECULES OF EYDROCARBON PER MICELLE

15.3 33.8 41.5 36.0

0.178 0. A25 0.533 0,430

99 126 102

n-Heptane Toluene

12.5 37.5

0,125 0.403

96

n-Octane Ethylbenzene Styrene

12.0 29.5 34.5

0.105 0.28 0.332

24 66 78

8.5 18.7 5.38

0.058 0.147 0.042

35 10

n-Hexane Hexatriene Benzene Cyclohexane

n-Decane. n-Butylbenzene Naphthalene Phenanthrene Fluorene Anthracene o-Tolylazo-,+-naphtholt Dinitrodiphenylamine*

~

I

:

1.55 0.92 0.197 0.61 15.0

0,0085 0.0056 0.00108 0.0025 0.058

.42

30

14

A SLOPX 1

SOAP CONCLN"RAnON

+ + + + + + + + +

+0

2.0 1.3 0.26

0 0 0

0.60

0 0

14

* See reference 23. t See reference 18. As the size of the molecule is increased t o a system of two or more ringseither in the planar form, of which naphthalene is the simplest, or to a substituted biphenyl, as dinitrophenylamine (23)-the number of molecules per micelle decreases markedly and the change in slope becomes zero. This is found t o o x u r in the case of solubilization of the various polycyclics studied here. This is further exemplified by consideration of the chord plots in figure 6, which show the changes, withsoap concentration, of the increment in thenumber of molecules of hydrocarbon solubilized per increment in soap concentration. There is some indication of a lessening in the increase in slope with increase in

-

the length of the side chain on the benzene nucleus (H C2H, -+ C',H,) and an approach to a zero slope increment with the simplest polycyclic, naphthalene. All higher-order polycyclics show no change in slope with soap concentration, as can be seen in table 3. KOevidence is as yet available to indicate that the slope increment would approach or become equal to zero as the alkyl group on the benzene is increased in length to CS,G o , or CIS. On the basis of published x-ray evidence on hydrocarbon-swollen micelles ( S ) , it is to be expected that a certain number of molecules or portions of these long alkylated molecules would extend into the region between adjacent soap molecules in the micelle (the palisade of soap molecules). It seems well established that this occurs in the solubilization of long-chain alcohols (6). Charged dye molecules, after binding irith individual soap molecules, are probably solubilized also by inclusion in the palisade. As mentioned previously, these compounds upon solubilization show a zero or a negative change in slope. The lack of change in slope with concentration in the polycyclics does not necessarily indicate that the polynuclear hydrocarbon forms a part of the palisade of soap molecules in the micelle, but it may be esplained on the basis that little or no energy is contributed by the solubilized polycyclic as to van der Waals attraction to enhance the energy necessary for micelle formation. The increase in slope in solubilization of simpler hydrocarbons must be due, in part a t least, to this enhancement of van der Waals attractive forces, owing to the presence of the hydrocarbons in the region of the soap hydrocarbon tails. Effect of unsaturation and cyclization on solubilization

A comparison of the various groups of compounds in table 3 indicates that there is an enhanced solubilization upon unsaturation, as can be seen by a comparison of benzene and cyclohexane, hexatriene and n-hexane, and styrene and ethylbenzene. Further, one unsaturated ring in the compound increases its solubility when compared to an alkane of the same number of carbon atoms. Thus, benzene, toluene, etc. are almost three times as soluble in 0.63 A' KC12 as nhexane, n-heptane, etc. However, the presence of a second ring, as in the substituted biphenyl, or, more particularly, in the polycyclic, naphthalene, causes a change in the opposite direction so that these compounds are even less soluble than the normal paraffin of about the same molecular weight. Thus, naphthalene is much less soluble in KClz solutions than either n-decane or n-butylbenzene. Preliminary studies on tetralin and decalin indicate that both are more soluble than naphthalene, with decalin slightly more soluble than the two other compounds (17). Although cyclohexane is less soluble than benzene, owing probably to the more polar, partial double bonds in the latter compound (also possibly owing to the smaller size of the benzene molecule), it is much more soluble than the linear n-hexane. Effect of length and molar volume of moleciile oil solubility

No simple linear relationship as indicated betxeen volume of oil solubilized and the molar volume for the alkylbenzene and normal paraffin series (23) can be

SOLUBILIZATIOK OF POLYCYCLIC HYDROCARBOKS

293

obtained in the case of solubilization of the polycyclics. However, as can be seen in figure 7, a plot of the log solubility of various polycyclics as a function of length of the molecule along its major axis yields a fairly straight line. Deviations from linearity are most marked in the case of the more highly condensed polycyclics, for the lengths of these molecules are about equal to their more nearly linear “parent” polyacenes. Thus, the solubility per liter of 0.5 Ar and of naphthalene 33.3 X moles; KC12of acenaphthene is 6.48 X and that of triphenylene 0.336 X lo+, as compared that of pyrene 2.24 X moles. The two compounds with the solubility of phenanthrene of 6.65 X which are structurally very similar and also of the same length, chrysene and 1 ,%beneanthracene, are seen to have essentially the same solubility values.

-

110

90

MOLAR VOLUME 0 I30 IS0

170

\

0

3

FIG.7. Variation of solubility in potassium laurate solutions with molar volume and length. 1, benzene; 2, naphthalene; 3, acenaphthene; 4, phenanthrene; 5, fluorene, 6, fluoranthrene; 7, pyrene; 8, anthracene; 9, chrysene; 10, 1,2-benzanthracene; 11, triphenylene; 12, naphthacene; 13, 1,2,5,6-dibensanthracene.

There are two curves which may be drawn through the solubility-molar volume data, one for the linear polyacenes, benzene, naphthalene, anthracene, which will coincide essentially with the log S-length curve, and the other for the nonlinear cata- and the peri-polycyclics. Variations from these two curves are less than those of the highly condensed peri compounds from the log S-length curve. It should be noted that essentially the same division into two curves is required in the case of plots obtained from log S-molar volume data of the solubility of the polycyclics in water. A linear increase in expansion in A. per mole of hydrocarbon per mole of soap with molar volume of normal paraffins and another with the alkylbenzenes has been noted from the extensive x-ray studies on the increase in long spacing with added hydrocarbon of Hughes, Sawyer, and Vinograd (8). A similar attempt to treat solubilization data (volume of hydrocarbon per unit soap concentration as a

294

H. B . KLEVEKS

function of molar volume) resulted in considerable scatter of points about the linear plot (23). The volume of hydrocarbon solubilized in the polycyclic series falls rapidly with increase in number of rings in the compound, as seen in table 4, and no linear relationship can be observed. Attempts to find some linear dependence of log volume as a function of length or molar volume of the polycyclic resulted in much more scattering of points about the best line drawn through these data than that observed in a similar plot of solubility. No correlation of solubility could be found with area or volume occupied per polycyclic molecule, assuming that the area or volume had for its major axis the distance equivalent t o the length along the major axis. TABLE 4 Solubility of various hydrocarbons in 0.6 .M KC,: (86°C.) (Corrected for water solubility) l

OOy;,", M O" L 2 8 OF

cornmum

L,rEP

mT;~oy;oAp

s o ~ ~ ~I os o LNu r I n N 1 (X IC?

Benzene Ethylbeneene n-Butylbenzene Naphthalene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthrene Pyrene Chrysene 1,2-Benzanthracene Triphenylene Naphthacene

14.9 1.26 1 .oo 0.728 1.21 0.155 0.576 0.453 0.143 0.145 0.0765 (0 023)

MOLE-

CULES OF OILPEP

-0.41 116 -0.66 63 112 I -0 95 33 33.3 1 -1.46 10 6.46 -2.19 2.0 4.36 -2.36 1.3 6.65 -2.16 2.0 0.65 I -3.07 0.26 2.86 -2.54 0.66 2.24 -2.65 0.66 0.627 -3.20 0.19 0.635 -3.20 0.19 0.336 1 -3.47 0.10 (0 10): ( 4 0)* 0 0301 ~

~

MOLECOIFS OF

VOLUME OXOIL

OIL PEP MICELLE (CORX E C T ~

(MI. m a UTEPOI 1 SOAP SOLUTION)

123 34.3 66 25.6 36 17.3 10.4 3.72 0.975 2.1 1.4 1.03 2.1 0.27 0.123 0.90 0.469 0.71 0.345 0.20 0,102 0.20 0.117 0.11 0.03

The low concentrations of the polycyclic hydrocarbons with rt 2 4 (n = number of rings) solubilized even a t soap concentrations of 0.5-0.6 M indicate that a t the concentrations of materials (about 1 per cent) in the blood capable of acting as solubilizing agents (sterols, fatty acids, lipoproteins, etc.), there is only a minute amount of the polycyclic carried from point of primary injection by these agents to the region of the body where cancerous grox3th is noted. This concentration solubilized is probably somewhat larger than the amount soluble in the fatty acid soaps, for the solubility of 1,2-benaanthracene, for example, is about doubled when sodium taurocholate is used in place of potassium laurate. Further, it has been found that the "choleic" acid principle holds in the solubili-

295

SOLUBILIZATIOS O F POLYCYCLIC HYDROCARBONS

zation of the polycyclics, for certain mixtures of bile salts and fatty acid soaps are more active as solubilizers than either of the individual components (17). Extremely minute amounts of carcinogenic agents have been “dissolved” and then ut,ilized to bring about, tumor formation in the brain cavity even when the polycyclic hydrocarbons in the form of minute crystals have been injected through the skull and laid on the meninges (membrane covering the brain), a region of very high phospholipid concentration (20). This further indicates that very few carcinogenic hydrocarbons are necessary for collision and possible binding with that portion of the cell concerned \vith control of the kinetics of cell division and reproduction. The fact that the number of molecules of oil solubilized per micelle is constant in the case of the polycyclics and that there is a continuous decrease in this

TABLE 5 Probable size of micelle necessaru f o r solubilization of polycyclic hydrocarbons ( N = number of soap molecules per micelle) YOLECULES OF

D

MCELLE (150 MOLECULES)

~.-

Benzene . Saphthalene Acenaphthene Phenanthrene Bnthracene Fluoranthrene Pyrene Chrysene 1,2-Benzanthracene Triphen? lene Naphthacene Met hylcholanthrene lI2,5,6-Dibenaanthracene

-

-I

~-

, 1

’

A. 11s 10 2 0

20 0 26 0 86 0 68 0 19 0 19 0 10 0 030 0 10 0 026

i

’

150 1.50 150 150 600 175 320 790 790 1500 5000 1500 6000

,

60 60 60 60 120 65 73 138 138 190 360 190 390

constant value with increase in size of the hydrocarbon through phenanthrene indicates that micelles of about I50 soap molecules play a role in the solubilization of these polyacenes. The fact, as seen in tables 2 and 4, that for anthracene and for those polycyclics with n 2 4 only a fraction of a hydrocarbon molecule can be solubilized per 150-moleculemicelle, indicates that there must be present, in these systems, micelles of larger size. The number of these laiger micelles would be extremely small and probably would be found only when insoluble compounds ab the n 2 4 polycyclics are piesent The number of soap molecules per micelle, K ,necessary to account for the solubilization of these polycyclics $re seen in table 5 . On the basis that each soap molecule occupies about 30 A.? and that the soap yicelle is a cylinder composed of two layers of soap molecules, thediameters, D (A),of theseprobable aggregates hare been calculated and collected in table 5 The presence of any

296

H . B. KLEVENS

appreciable amount of particles of diameters of 300-400 -1.would result in a turbid system if the refractive indices of the large micelles were sufficiently different from those of the balance of the system (water, free soap, smaller micelles). These systems, a t equilibrium, were quite transparent, owing probably t o a negligible refractive index increment. It has been found, for example, that the refractive index of “free” potassium laurate is about 1.495, whereas that of the fully formed micelle (above about 0.15 M) is constant and equal to about 1.490 (11). If a molecule of polycyclic hydrocarbon were solubilized in this very large aggregate, the total refractive index of this latter particle would not be sufficiently different from the refractive index of the medium (water, “free” soap, normal micelles) t o result in any apparent opacity. An attempt has been made to extend these measurements to include the peripolycyclics, anthanthrene (n = 6) and coronene (n = 7 ) , but after shaking intermittently for 6 to 8 months, no consistent results have as yet been obtained. The solubility of these compounds in water was too small to determine by the present methods and this might be one of the factors complicating the determination of solubilization of these polycyclics. That there is no definite layering and orientation of polycyclic hydrocarbons such as naphthalene or benzene within the micelle can be assumed, for no observable differences in spectra could be found between naphthalene in a nonpolar solvent and in the micelle. Any observed spectral changes would be due to electronic resonance between molecules such as is observed in various cyanine dyes (9, 22) or in the starch-iodine complex (24). The complete absence of this type of spectral change would indicate that the density of the polycyclics in the micelle is equal t o or less than their bulk density. The possibility of micelles of about 5000 soap molecules nhich would he capable of solubilizing a molecule of dibenzanthracene is not difficult t o accept, for Professor Debye (private communication) has mentioned alkylamine hydrochloride systems containing an added electrolyte which show a blue Tyadall effect. This would indicate particles of the order of magnitude of 200-500 A . It is, of course, not known whether these systems are aggregates of micelles, not capable of acting like a single large micelle, which must be present just before salting-out occurs. S o quantitative data are available a t the present time regarding the solubility of the n 2 4 polycyclics in hydrocarbons such as n-hexane, which might present an atmosphere similar t o that which occurs in the micelle. I t is known, however, that the solubility of these polyacenes is extremely low in these solvents, but tlic order of magnitude is not known. I t does not seem necessary to look for another type of mechanism of solubilization to explain the apparently large micelles calculated here. Rather, it should be sufficient to recognize the fact that each molecule of dibenzanthracene, for example, requires a certain critical volume of hydrocarbon atmosphere for its solubility. This atmosphere can only be supplied by a large aggregate of soap molecules. The smaller aromatics, such as benzene and naphthalene, need a much

SOLUl3ILIZATIOS O F POLYCYCLIC HTDROCARBOXS

297

smaller volume of hydrocarbon per solubilized molecule and thus many more win fit into a single swollen micelle. SUI\fIUARY

The log of the solubility ( S )in water of the condensed polycyclic hydrocarbons from n = 1 to n = 5 (n = number of rings) decreases linearly with the length ( L ) of the molecule. In contrast t o the alkylbenzene series, where the volume of oil solubilized at one particular soap concentration decreases approximately linearly with L or the molar volume, a linear decrease for the condensed ring systems is obtained only with a log S L'S. L curve. Two curves are necessary t o obtain linearity of log S GS. molar volume, one for the linear polyacenes, benzene, naphthalene, anthracene, . . . ., and the other for the nonlinear polycyclics. The differential change in the slope of S as a function of concentration curves is jeen to be largest with benzene, decreasing with increase in substitution through toluene, ethglbenzene, and n-butylbenzene and becoming equal t o zero for the condenPed-ring polycyclics, similar to the results obtained in the case of dye solubilization. Solubilization data of the alkylbenzenes, naphthalene, phenanthrene, and dimethglaminoazohenzene are in acrord with the conwpt of the expanded hydrocarbon-containing micelle (as advanced by s-ray measurements), for one or more molecules of hydrocarbon can he solubilized per micelle. However, lies such as anthracene and those xith ti 2 4, a larger micelle than that of the ahove size \\.odd he required for soliibilization per polycyclic molecule. REFEREXCES CORRIN, AI. L., KLEVESS,H . B., .ASD HARKIXS,W .D . : J. Chem. Phys. 14, 480 (1946). DAVIS,W.IT., K R A H LM. , E . , A N D CLOTYES: G . H. A , : J. Am. Chem. Soc. 64,108 (1942). D E B Y EP , . : J. Colloid Sci. 3, 407 (1918); J . Phys. 8: Colloid Chem. 63, 1 (1919). For recent review. see H A D D O W A ,. : Brit. N e d . Bull. 4, 331 (1947). ( 5 ) HIRKISS, W.D . , . ~ N DOPPENHEIUER, H . : J . A m Chem. Soc. 71, 808 (1949). (6) HARKINS, \v. D . , A N D >IITTELIIASS, R . : J. Colloid Sci. 4, 367 (1919). (7) HELLER,W.,A N D XLEVESS,H . €3:J. Chem. Phys. 14, 567 (1946). (8) HUGHES,E. W.,S A W Y E R , T I ' > I , ,A N D YINOGRAD, J. R.: J. Chem. Phys. 13, 131 (1915). (9) JELLEY, E. E . : S a t u r e 139, 631 (1937). (10) JONES, R. S . :Cheni. Revs. 32, 1 (1043). (11) KLEVENS, H . B . : J. Cheni. Phys. 14, 5 6 i (1946). (12) KLEVESS,H . B . : J. Phys. R: Colloid Chem. 62, 130 (1948). H . B., A N D PLITT,J . R . : J. Cheni. Phys. 17, 470 (1949). (13) KLEYENS, (14) KLEVENS, H. B. : Paper presented at the Twenty-second Kational Colloid Symposium, which \\-as held under the auspices of the Division of Colloid Chemistry of the American Chemical Society a t Cambridge, Llassachusetts, June, 1918. (1.5) K L E V E K SH. , B . : Paper presented a t the 112th meeting of the American Chemical Society, which was held in S e w York C i t y , September 1>19,1947; J. Am. Oil Chemists' Soo. 26, 456 (1949). (16) K L E Y E NH~ ., B . : J. Cheni. Phys. 17, 1004 (1919). (17) KLEVEHS, H . €3.: Unpublished d a t a . (IS) MCBAIN,J. W . : Aduances in Colloid Science, Vol. I . Interscience Publishers, Inc., Xerv York (1942). (1) (2) (3) (4)

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(19) MATTOON, R. W.,STEARNS, R. S., AND HARKINS, W.D. : ,J. Chem. Phys. 16,644 (1948). (20) MOORE, G. E . : Science 106, 130 (1947); also private communication. (21) PAULING, L. S.: The Nature of the Chemical Bond, 2nd edition. Cornell University Press, Ithacs, New York (1940). (22) SCHEIBE, G.: Angew. Chem. 62, 631 (1939). (23) STEARNS, R . S.,OPPENHEIMER, H., SIMON,E . , AND HARKINS, U’.D . : J. Chem. Phys. 16, 496 (1947). (24) STEIN,R. S.,AND RUNDLE,R. E.: J . Chem. Phys. 16,195 (1948).