[CONTRIBUTION FROM
THE
RESEARCH LABORATORY O F ARMOURAND COMPANY]
T H E SOLUBILITIES OF T H E NORMAL SATURATED FATTY ACIDS A. W. RALSTON
AND
C. W. HOERR
Received August 8, i9@
The solubilities of the lower members of the normal saturated fatty acid series in a wide variety of solvents have long been known with considerable accuracy, and complete phase diagrams with most of the ordinary solvents have been constructed. While extensive data have been reported for various derivatives of the higher acids, such as the soaps and esters, the solubilities of the acids themselves, from caproic acid upward in the series, have been determined in relatively few solvents and considerable disagreement exists among the recorded values. Seidell (l),Brown (2), and Lewkowitsch (3) have compiled some of the solubility data for the saturated acids, and, in addition, the literature contains many references to the solubility of certain acids in specific solvents. No solubility data have as yet been available for several of the acids. Since a wide variety of experimental procedures, such as surface studies, extraction, and fractional crystallization, require accurate solubility data, determination of the solubilities of the higher acids in common solvents is of appreciable value. This paper presents a comprehensive study of the solubilities of the normal saturated fatty acids, from caproic to stearic acid, inclusive, in water, ethanol, acetone, 2-butanone, benzene, and glacial acetic acid from 0” (slightly below in several cases) to the boiling points of the solvents, except in the case of water and of glacial acetic acid. In connection with the investigation of the solubility, a study of the state of the acids in solution can readily be made by calculation of the apparent molecular weight from the depression of the freezing point of the solvent by addition of known amounts of acid. Numerous recent papers have reported association of the fatty acids in benzene (4), cyclohexane ( 5 ) , quinoline (6), and in pyridine (7). This paper presents a further consideration of this problem with a discussion of the apparent association of the fatty acids in benzene and in glacial acetic acid. EXPERIMENTSL
The acids employed in this investigation were of the same lots as were used in recent studies in this laboratory, and their preparations have been reported (8, 9). The freezing points of these acids are listed in Table I. The water used was freshly distilled conductivity water. The ethanol was commercial “absolute” (99.4% by weight). The various dilutions which were used were determined by measurement of the density at each dilution and interpolation of these values with known densities at given concentrations (10). The acetone was redistilled from potassium permanganate as was the 2-butanone. The benzene mas Baker C.P. thiophene-free grade and was dried over sodium wire. The glacial acetic acid was U.S. P., 99.5%. The solubility of the fatty acids in water was determined by shaking flasks containing acid-water samples in a constant temperature bath for 2-4 days. The desired temperatures were arrved a t from higher and from lower temperatures in order to assure that equilibrium had been attained. Solution was removed from the flasks by means of a small, fine sintered Pyrex glass filter sealed in a glass tube. The solutions were titrated with standardized 546
547
SOLUBILITIES O F FATTY ACIDS
Ba(@H)zusing phenolphthalein as the indicator, The less soluble samples, from lauric acid upwards in the series, were titrated conductimetrically with the conductivity apparatus described elsewhere (11). -4microburette was used to measure the amounts of Ba(OH)z. The solubilities of the fatty acids in the organic solvents were determined in sealed tubes by the method and with the apparatus previously described (8, 12, 13). TABLE I FREEZING POINTS OF PURIFIED FATTYACIDS
~ ~ _ _ _ _ ~ ~~
~~
Caproic. . . . . . . , . . . . . . . . . . . . Heptylic . . . . . . , . . . . . . . . . . . . , Caprylic. . . . . . . . . . . . . , . . . . . . Konylic . . . . . . . . . . . . . . . . . . . . Capric. . . , , . , . . . . . . . , . . , . . . , Undecylic . . . . . . . , . . . . . . . . . . Lauric. . , . . . . . . . . . . . . , . . . . . .
6 7 8 9 10 11 12
1
16.30 Pentadecylic., , . . . . . . . . , . . 12.24 Palmitic, , , , . . , . . . , . . , . . . . 30.92 ' Heptadecylic.. . . , . . . , . . . . . 28.13 Stearic.. . . . , . . . . . . , . . . . . , , 43.86
j
13 14 15 16 17 18
~
41.76 53.78 52.49 62.41 60.94 69.20
TABLE I1 SOLUBILITIES OF THE FATTY ACIDSI N WATER NO. OF
c ATOMS 0.00
6a 7b SC 9 10d
11 12 13 14 15 16 17 18"
0.864 ,190 .044 .014 ,0095 .0063 .0037 .0021 ,0013 .00076 .00046 .00028 .00018
30.0'
20.0'
0.968 ,244 .068 .026 .015 .0093 .0055 ,0033 .0020 .0012 .00072 .00042 .00029
1.019 0.271 ,079 .032 .018 .011 .0063 .0038 .0024 ,0014 .00083 .00055 .00034
45.0'
1.095 0.311 ,095 ,041 .023 ,013 .0075 * 0044 .0029 .0017 .0010 .00069 .00042
60.0'
1.171 0.353 ,113 .051 ,027 .015 .0087 .0054 .0034 ,0020 .0012 .00081 .0w50
0.882 g. per 100 g. H20a t 15' (3), 1.08 a t 20" (14); b0.22 a t 15" (14), 0.241 a t 15" (15); 0.079 a t 15" (3), 0.25 a t 100" (3); dO.l a t 100" (3); e 0.031 a t 25" (16), 0.0033 a t 25" (17), 0.1 a t 37" (18), 0.0165 a t 50" (17).
a
The depression of the freezing points of benzene and of glacial acetic acid by the addition of known amounts of fatty acids was determined using 25-g. portions of solvent. A Beckmann thermometer which had been calibrated by the National Bureau of Standards was employed. RESULTS
Solubilities in water. The solubilities of the normal saturated fatty acids in water are listed in Table 11.
548
A. W. RALSTON AND C. W. HOERR
The reproducibility of the results presented are shown by the following examples: for caprylic acid a t 20.0" the experimental results gave a value of 0.068 =t0.002 g. acid per 100 g. water, an average of 7 separate determinations; for lauric acid a t 45.0",0.0075 j=0.0005 g. for 6 determinations; and for heptadecylic acid a t 30.0°, 0.00055 f 0.00003 g. for 8 determinations. The values for the other acids a t a given temperature are of similar reproducibility for averages of 3 to 8 separate determinations of each acid a t each temperature. With these values for the solubilities of the fatty acids in water and those previously reported (8) for the solubility of water in the fatty acids, the phase diagrams of the acids can now be fairly completely constructed.
WEIGHT PER CENT. ACID
FIG.1. SOLUBILITIES OF THE FATTY ACIDSIN 95.0% ETHANOL (BY WEIGHT) The numbers on the curves refer to the number of carbon atoms in the acid molecule
Solubilities in ethanol. The solubilities of the fatty acids in ethanol were determined rapidly, and with the minimum of heating to high temperatures in order to reduce the possibility of esterification. The results are listed in Table 111, and are shown graphically' for 95.0% ethanol in Fig. 1. Comparison of the values in Table 111 with those previously recorded in the literature shows relatively good agreement among a number of the values. I n general, the agreement is much better at lower temperatures than a t higher temperatures. Some of these reportedly higher solubilities were.,investigated by 1 In the figures in this report, the solubilities are shown in terms of weight percentage rather than in terms of g. per 100 g. of solvent because of the more convenient scale for diagrammatic presentation.
549
SOLUBILITIES OF FATTY ACIDS
preparing a number of samples a t the recorded values. Upon cooling, most of the samples crystallized before the reported temperature was reached. CrystalTABLE I11 SOLUBILITIES OF
__% ETHANOL
NO. 01 c
(BY WEIGHT:
ATOPS
99.4
125 14* 17d 188
95.0
91.1
80.8
8 9 10 11 12' 13 14 15 160 17 18b
THE
FATTY ACIDS IN ETHANOL G . ACID PEE
0'
20.4 7.07 1.89 2.04 0.42
100
41.6 9.77 3.20 2.98 1.09
20
100 G. SOLVENT
30'
400
~.
600
105 23.9 7.21 6.62 2.25
292 84.7 23.9 22.2 5.42
m
m
m
m
m
m
m
m
m
m
8980
m
m
m
m
m
m
OD
m
m
m
00
262 1035 3230 393 60.6 93.5 85.2 190 15.2 34.0 15.5 34.5 3.86 7.64 7.18 3.82 2.10 0.85 1.68 1.03 0.24 0.65
440 706 91.2 104 18.9 19.5 4.93 4.17 1.13
16' 18;
0.76 0.13
1.94 0.35
4.60 0.66
18
ca. 0.06
0.10
0.20 ,
1540 263 94.2 110 22.7
soo
1410 260 336 6560 238 68.7 78.5 295 16.7 73.4 84.2 15.3 3.42 17.1 15.3 2.30 0.81
m
1560 320 388 105
1485 2460 287 344 83.9
m m
2600 8230 400
00
m
2280 6560 365
...
...
...
13.5
68.7
...
50.8
238
3.20
0 20.5 at 0" (19), 25.8 at 0" (20), 45.6 a t 8" (20), 61.2 a t 12" (20), 92.1 a t 16.5" (20), 57.3 a t 21" (19). b7.14 a t 0" (19), 31 at 21" (19). c 2 a t 0" (19), 1.93 a t 0' (20), 1.45 a t 0" (21), 2.0 a t 5" (21), 4.0 a t 10" (21), 3.5 a t 10" (22), 6.5 at, 15" (21), 6.86 a t 17" (20), 9.9 a t 20" (21), 11.6 a t 20" (22), 10.1 a t 21" (19), 16.8 a t 25" (21), 11.0 a t 25" (23), 29.0 a t 28" (21), 81.0 a t 36" (21), 41.3 at 40" (22). d 1.53 a t 0" (21), 2.42 a t 5.4" (21), 4.12 a t 10" (21), 6.72 a t 15" (21), 13.4 a t 21" (21), 32.14 a t 28" (21). e 0.37 a t 0" (21), 0.51 a t 5" (21), 1.10 a t 10" (21), 0.90 a t 10" (22), 1.6 a t 15" (21), 2.5 a t 20" (21), 4.9 a t 25" (21), 8.30 a t 25" (16), 6.0 a t 28" (21), 20.0 at 36" (21), 17.9 at 40" (22). f 35.0 a t 8" (20), 45.2 at 12" (20), 69.7 at 16.5" (20). 2.21 a t 8" (ZO), 2.72 a t 12" (20), 5.90 at 20" (20), (94.0% ethanol) 0.70 a t 0" (24). h 5.55 a t 25" (16), (94.6% ethanol) 0.1515 a t 0" (25), (94.4%) 0.188 a t 0" (26), (94.0%) 0.15 a t 0" (24). (87.6% ethanol) 0.451 a t 0" (27). i (91.6% ethanol) 0.112 a t 0" (3), (91.53%) 0.136 a t 0" (28), 0.481 a t 10" (as), 2.23 at 25' (29), (90%) 3.30 at 25" (16), (87.6%) 0.89 a t 0" (27), (86.16%) 0.073 a t 0" (28), 0.278 at 10" (38).
lization of the other samples was induced by seeding. This behavior of undercooling and supersaturation, indicating the necessity of maintaining equilibrium
550
A. W. RALSTON AND C. W. HOERR
conditions, has been discussed by Emerson (25) with regard to the solubility of stearic acid in ethanol a t 0". Some of the divergent solubilities which are recorded may indicate contamination of a given acid by its homologs. Investigation of the solubilities of binary mixtures of the acids shows that their values fall between those of the pure comTABLE I V SOLUBILITIES OF THE FATTY ACIDSI N ACETONE NO, OF
0.ACID P E E 100 0.ACETONE
c
ATOMS 00
100
221 356 45.3 50.2 8.95 7.52 2.75 2.20 0.60 0.40 0.21
8 9 10 11 12 13 14 15 16 17 18a
975 3740 112 149 21.9 20.2 6.50 5.27 1.94 1.50 0.80
1
56.5'
40'
200
30'
m
m
m
m
m
m
m
X
X
oc
4660
407 706 60.5 78.6 15.9 13.8 5.38 4.28 1.54
m
m
m
218 316 42.5 49.3 15.6 14.6 4.93
1 ~
1590 8230 149 183 58.0 67.5 17.0
m m m m
880 1330 220
4.96 at 25" (16).
TABLE V FATTY ACIDS I N
SOLUBILITIES OF THE NO. OF ATOMS
0.
c 00
10 11 12 13 14 15 16 17 18
42.4 47.9 11.5 11.9 4.28 4.28 0.90 0.71 0.25
2-BUTANONE
ACID PEB 100 0.%BUTANONE
100
200
100 139 24.7 29.5 8.46 8.70 3.09 2.88 1.01
318 521 64.7 95.0 18.5 20.2 8.57 7.41 2.99
30'
40'
50'
60'
7040
m
m
m
m
m
m
1825 8230 189 257 66.1 77.7 24.8
m
m
m
m
1230 2530 228 288 84.7
m
m
202 315 54.3 70.4 20.6 20.3 8.34
m
2390 6560 344
ponents. A mixture containing 5% palmitic and 95% stearic acids dissolves in twice its weight of 95% ethanol at about 1.5" lower than does an equal concentration of pure stearic acid in 95% ethanol. Solubilities in acetone and 2-butanone. The solubilities of the fatty acids in acetone and in 2-butanone are listed in Tables IV and V, respectively, and are shown graphically in Figs. 2 and 3. As in the case of the acids in ethanol, the curves for acetone and 2-butanone
SOLUBILITIES O F FATTY ACIDS
551
are also paired. The acids of higher molecular weight are the more soluble of each pair, with certain obvious exceptions. All of the acids investigated are more
WEIGHT PER CENT. ACID
FIG.2. SOLUBILITIES OF TEE FATTY ACIDSIN ACETONE
WEIGHT PER GENT ACID
FIG.3. SOLUBILITIES OF THE FATTY ACIDSIN 2-BUTANONE
soluble in 2-butanone at a given temperature than in acetone at the same temperature, except capric and undecylic acids, which are more soluble in acetone.
552
A. W. RALSTON AND C. W. HOERR
TABLE VI FATTY ACIDSIN BENZENE^
SOLUBILITIES OF NO. OF
G. ACID
c
ATOMS
100
8 9 10 11 12b 13 14 15 16 17 18C
THE
20'
770 2680 145 208 32.3 42.4 6.95 8.84 1.04 1.52 0.24
1
PEP 100 0 . BENZENE
1
30"
I
40'
m m
663 93.6
m
260
50"
1
60"
l
m
m
m
l
m
m
m
I
1390 7600 239 295 105 121 51.0
m
m
1
0 Powney and Addison (31) investigated the solubilities of several acids (caprylic, lauric, myristic, palmitic, and stearic) in benzene, but, unfortunately, no values were tabulated, and the solubilities cannot be read accurately from their small diagrams. b 186 at 25" (30), m at 40" (30); e22 at 23" (3).
P
is
E
-201
I
I
1
I
20
FIQ4. LIQUIDUSCURVESFOR
I
I
40 60 WEIGHT PER CENT. AGIO
TEE
I
I
BO
1
100
FATTY ACID SYSTEMS WITH ANHYDROUB BENZENE
553
SOLUBILITIES OF FATTY ACIDS
Solubilities in benzene. The solubilities of the fatty acids in benzene are listed in Table VI and the liquidus curves of the systems are shown in Fig. 4. TABLE VI1 SOLUBILITIES OF THE FATTYACIDS I N GLACIAL ACETICACID NO. OF
G. ACID P E E 100 G. GLACIAL ACETIC ACID
c ATOMS 200
10 11 12 13 14 15 16 17 18
567 800 81.8 96.8 10.2 8.76 2.14 1.27 0.12
30"
40'
500
60'
m
00
W
m
m
297 395 51.1 62.0 8.11 6.52 1.68
1480 8230 289 350 51.7 61.0 7.58
m m
m m m m m
8230
~
~
1410 2600 313 384 74.8
2280 6560 485
; . I
-2 Iw
@
20
40
WEIGHT
FIG.5.
LIQUIDUS C U R V E S FOR T H E
60
80
100
PER CENT. ACID
FATTYACIDSYSTEMS
WITH GL.kCIAL ACETIC k ! I D
Again pairing of adjacent homologs occurs. It is of interest that in this case no pairs of curves intersect, as with the preceding solvents discussed. Each of the acids forms a eutectic with benzene, these eutectics occurring a t the following concentrations and melting at the following temperatures : Cs a t 50.4% and
554
A. W. RALSTON AND C. W. HOERR
-10.5"; CS,54.0% and -13.1'; CL0,34.5% and -2.0'; Cll, 38.9% and -4.0'; CIE,11.2% and 4.5'; C13, 14.6% and 3.7'; c14,2.88% and 5.20'; C15, 3.65% and 5.15'; C16, 0.19% and 5.40'; CI7, 0.42% and 5.35'; and C18,0.015% and 5.50'; These values agree reasonably with those of Powney and Addison (31). Calculations from the freezing point data show that a t the freezing point of benzene, the ratio of apparent to true molecular weight (M/Mo) for caprylic acid approaches a value of 2 at about 0.5 molal, indicating association in solution. In solutions of lauric acid in benzene, the values of M/Mo approach 2 at 0.3 molal. Investigations a t higher temperatures by other methods (4) indicate that the values of M / M o for the fatty acids in benzene approach limits somewhat less than 2, and their limiting value is approached in relatively higher concentrations. Solubilities in glacial acetic acid. The solubilities of the fatty acids in glacial acetic acid are listed in Table VII, and the liquidus curves of the systems are shown in Fig. 5. Again pairing occurs, with the acid of higher molecular weight being, in general, the more soluble of a given pair. Each of the acids forms a eutectic with glacial acetic acid, these eutectics having a composition and melting point as follows: Cg, 97.7% and -5.4'; C7, 98.8% and -6.5'; Cs, 80.0% and 3.1'; C9, 83.6% and 1.6'; Clo, 55.1% and 8.6'; Cll, 57.5% and 8.0'; (212, 17.3% and 12.8'; C13, 15.9% and 13.1'; c14, 5.3% and 15.18'; CIS, 4.3% and 15.40'; (216, 1.23% and 16.17'; C17,0.58% and 16.34'; and CIS,0.03% and 16.48'. Calculations from the freezing point data indicate that the values of M / M o of lauric acid in acetic acid approach 1.65 at about 0.2 molal. Thus, as in the case of benzene solutions of the acids, there appears to be some formation of double molecules of the fatty acids in glacial acetic acid. SUMMARY
The solubilities of the normal saturated fatty acids from caproic to stearic, inclusive, have been determined in water, ethanol, acetone, 2-butanone, benzene, and glacial acetic acid. CHICAGO, ILL. REFEREXCES (1) SEIDELL,"Solubilities of Organic Compounds," 3rd Ed., Vol. 11, Van Nostrand Co., Inc., New York, 1941. (2) BROWN,Chem. Rev.,29, 333 (1941). (3) LEWKOWITSCH, "Chemical Technology and Analysis of Oils, Fats and Waxes," 6th Ed., Vol. I, McMillan and Co., Ltd., London, 1921. (4) MEISENHEIMER AND DORNER, Ann., 623, 299 (1936); HYRNAKOWSKI AND ZOCHOWSKI, Ber., 70,1739 (1937); DUNKEN, Z.physik. Chem., B46,201 (1938). (5) BROUGHTON, Trans. Faraday Soc., 30, 367 (1934). (6) MATAVULJ, Bull. SOC. chim.roy. Yougoslav, 10, 33 (1939). (7) MATAVULJ AND KHOJMAN, Bull. soc. chim.roy. Yougoslav, 10, 49 (1939). (8) HOERR,POOL, AND RALSTON, Oil and Soap, 19, 126 (1942). (9) POOL AND RALSTON, Ind. Eng. Chem., 34, 1104 (1942).
SOLUBILITIES OF F,4TTY ACIDS
555
(10) “International Critical Tables,” Vol. 111, p. 117, McGraw-Hill Book Co., Inc., New York, 1928. (11) RALSTON, HOERRAND HOFFMAN, J . Am. Chem. SOC., 64, 97 (1942). AND SELBY, J. A m . Chem. SOC.,63, 1916 (1941). (12) HARWOOD, RALSTON, J . A m . Chem. SOC.,64, 1516 (1942). (13) RALSTON, HOERR,AND HOFFMAN, Tsnetnye Metal., 594 (1931). cf. Seidell (1). (14) LIPETZA N D RIMSKAYA, (15) LTJMSDEN, J. Chem. SOC.,89, 90 (1905). (16) SEIDELL,Bull. No. 6Y,Hygienic Laboratory, U . S . Public Health Service, 1910. (17) MTJKHERJEE AND DATTA, J . Indian Chem. SOC.,16,573 (1939). AND WILSON,Biochem. J., 12, 347 (1909). (18) MOORE,HUTCHINSON, (19) TIMOFEIEW, Dissertation (Kharkow) 1894, c f . Seidell (1). (20) EKWALL AND MYLITJS, J. prakt. Chem., 136, 133 (1933). (21) RTJTTAN, 8th International Congress of Applied Chemistry, Vol. 25, p. 341, New York, 1912. (22) FALCIOLA, Gam. chim. ital., 40, 11, 217 (1910). (23) KROBER,2. physik. Chem., 46, 305 (1919). (24) KREISA N D HAFNER, 2 . Untersuch. Nahr. u Genussm., 6,22 (1903); Ber., 36, 2766 (1903). (25) EMERSON, J . A m . Chem. Soc., 29, 1750 (1907). (26) HEHKERAND MITCHELL, Analyst, 21, 323 (1896). (27) Kn, Ind. Eng. Chem., Anal. Ed., 9, 103 (1937). (28) THOMAS AND MATTIKOW, J . A m . Chem. SOC.,48, 968 (1926). (29) THOMAS AND PTJ,J. A m . Chem. SOC., 46, 117 (1923). (30) hlcB.41~AND EATON,J . Chem. SOC.,2166 (1928). (31) POWKEY AND ADDISON, Trans. Faraday SOC.,34, 625 (1938).
