[CONTRIBUTION FROM TEE RESEARCH LABORATORY O F
ARMOUR AND
COMPANY ]
SOLUBILITIES OF BINARY MIXTURES OF THE SATURATED FATTY ACIDS A. W. RALSTON AND C. W. HOERR Received January 13, 1945
It is well known that the presence of even small amounts of one fatty acid influences the solubility of another. The authors have pointed out (1) that a mixture of stearic acid containing 5% palmitic acid dissolves in ethanol a t a temperature 1.5' lower than does an equal concentration of pure stearic acid. Waentig and Pescheck (2) had earlier stated that the solubility of palmitic acid in a solution of lauric acid in carbon tetrachloride is about 250% greater than in the pure solvent. This behavior was attributed by these investigators to the formation of an easily soluble bimolecular compound of the acids according to the equation (L)2 (P)2 @ (LPe). They claimed that the greatly increased solubility occurred in the solvents in which the acids associated, e.g., carbon tetrachloride, chloroform, benzene, heptane, and nitrobenzene, while they observed no increased solubility in solvents in which they assumed the acids to be monomolecular, e.g., ethanol, ethyl ether, ethyl acetate, and benzaldehyde. It has, however, been proved adequately that the determination of the degree of association of the higher aliphatic compounds by cryoscopic and ebullioscopic measurements, upon which Waentig and Pescheck based their conclusions, tends toward erroneous results (3), and the more recent concept of hydrogen bonding indicates that the fatty acids (at least the lower homologs) exist as double molecules in all states, even in solution. In order to explain more plausibly the increased solubilities of binary mixtures of fatty acids, this paper presents the sohbilities of a number of mixtures of stearic and palmitic acids in several solvents which possess widely differing physical properties.
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RESULTS
The fatty acid mixtures employed in this investigation were prepared from the highly purified stearic and palmitic acids which were used in the previous solubility studies (1, 4). Accurately weighed portions of the two components were combined, fused at a temperature above the melting point of stearic acid, and chilled rapidly t o avoid the separation of the components which would accompany slow cooling. The crystalline masses were then ground in a mortar to ensure complete mixture of the components. The solution temperatures of vario w concentrations of these mixtures were determined in several solvents in the manner described elsewhere (5). Figs. 1 and 2 represent graphically the solubilities of mixtures of stearic and palmitic acids in benzene and in acetone, respectively. Similar curves were obtained for these acid mixtures in chloroform and in ethyl acetate. These curves are so nearly the same a,s those shown that their presentation would be repetitious. These diagrams show strikingly that the mixture which is the most 170
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soluble in these solvents is not the one containing equimolar amounts of the acids, but is the one which contains 70% palmitic and 30% stearic acid. This most soluble mixture is, coincidentally, the minimum melting composition of the palmitic-stearic acid binary system shown in Fig. 3. Comparison of the temperature-concentration diagram of this system (6) with the solubility curves indicates that the chief factor which apparently governs the solubility of a given mixture is the melting point of that mixture, rather than the proportion of bimolecular complex which might be present. This phenomenon is understandable if solu-
PER CENT TOTAL ACID BY WEIGHT
FIG.1. SOLUBILITIES OF STEARIC-PALMITIC ACID MIXTURES IN BENZENE The numbers on the curves refer to the following: 1, pure stearic acid; 2, 90% stearic10% palmitic; 3, 80% stearic-20% palmitic; 4, 60% stearic-40’%palmitic; 5 , 50% stearic50% palmitic; 6, 40% stearic-60% palmitic; 7, 30% stearic-70% palmitic; 8, 20% stearic80% palmitic; 9,10% stearic-gO% palmitic; 10, pure palmitic acid.
bilization of a fatty acid is considered as a process of melting, such consideration being thermodynamically permissible since solubility is a function of the latent heat of fusion of a solute according to interpretation of Raoult’s law. It is evident in Figs. l and 2 that the solubility curves for mixtures containing a higher percentage of stearic acid tend to approach the curve for this pure component in the more dilute solutions, while those for mixtures rich in palmitic acid approximate the curve for the latter pure component, rather than the curves being parallel throughout the whole range of concentrations. It can be seen that the above relations of the solubility curves of the various mixtures are not altered appreciably by change of solvent. Benzene is a non-
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A. W. RALSTON AND C. W. HOERR
polar solvent presenting no hydrogen bonding capabilities; chloroform is a very slightly polar solvent containing highly active electron acceptor atoms ; ethyl
9
20
40
IO0
60
PER CENT TOTAL ACID BY WEIGHT
FIG.2. SOLUBILITIES OF STEARIC-PALXITIC ACIDMIXTURESIN ACETONE The numbers on the curves refer to the following: 1, pure stearic acid; 2,90% stearic10% palmitic; 3, SO% stearic-20% palmitic; 4, 607, stearic-40% palmitic; 5 , 50% stearic50% palmitic; 6, 40% stearic-60% palmitic; 7, 30% stearic-70% palmitic; 8, 20% stearicSO% palmitic; 9, pure palmitic acid.
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FIG. 3. THE BINARYSYSTEMSTEARIC-PALMITIC ACID. [SCHEUTTE AND VOGEL(6)]
acetate is moderately polar, and acetone is a relatively highly polar solvent, both of which contain electron donor atoms. Yet with these various hydrogen bonding potentialities the mixed fatty acid solubilities remain remarkably uniform in differing solvents.
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BINARY MIXTURES OF FATTY ACIDS I
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FIG,4. THEBINARY SYSTEM PALMITIC ACID-HEPTADECANE
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PER CENT TOTAL SOLUTE BY WEIGHT
FIG. 5. SOLUBILITIES OF PALMITIC ACID-HEPTADECANE MIXTURES I N %BUTANONE The numbers on the curves refer to the following: 1, pure palmitic acid; 2,80% palmitic acid-20% heptadecane; 3, 60% palmitic acid-40% heptadecane; 4, 40% palmitic acid-60% heptadecane;5,25% palmitic acid-75% heptadecane;6,10% palmitic acid-90%heptadecane; 7,5% palmitic acid-95% heptadecane; 8, pure heptadecane.
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I n order to investigate the solubility behavior of mixtures consisting of a fatty acid and an inert substance with which the acid molecules could not associate, the solubilities of several mixtures of palmitic acid and heptadecane (7) were determined in 2-butanone. The temperature-concentration diagram for this fatty acid-hydrocarbon system, shown in Fig. 4,consists of a smooth curve between the melting points of the pure acid and the pure hydrocarbon, with no eutectic formation. The solubilities of mixtures of these components are shown in Fig. 5. It can be seen from this diagram that in the absence of a eutectic (or minimum melting) composition, the solubilities of various mixtures lie between those of the pure components in direct ratio to the melting point of the mixture. SUMMARY
1. It has been found that the solubilities of binary mixtures of the saturated fatty acids are directly dependent upon the melting point of each given mixture, the most soluble mixture, therefore, being of that composition which possesses the minimum melting point of the system. 2. It has been shown that in binary aliphatic systems which have no eutectic or minimum melting mixture the solubilities of given mixtures lie between those of the pure components. 3. Bimolecular compound formation or intermolecular association between fatty acid molecules appear to have relatively little influence in altering the characteristic solubility behavior of their binary mixtures in solvents having diff erent physico-chemical properties. CHICAGO, ILL. REFERENCES (1) RALSTON AND HOERR, J . Org. Chem., 7,546 (1942). (2) WAENTIG AND PESCHECK, 2.physik. Chem., 99,529 (1919). (3) HILDEBRAND, “Solubility of Non-Electrolytes,” 2nd Ed., Reinhold Publishing Corp., New York, 1936. (4) HOERRAND RALSTON, J . Org. Chem., 9, 329 (1944). (5) HOERR, POOL,AND RALSTON, Oil and S o a p , 19, 126 (1942). (6) SCHEUTTE AND VOGEL, Oil and S o a p , 17,155 (1940). (7) RALSTON, HOERR,AND CREWS,J . Org.Chem.; 9,319 (1944).
