COMMUNICATIONS TO THE EDITOR X-RAY STUDIES OF FATTY ACIDS AND OF MIXTURES OF FATTY ACIDS
In view of the time-consuming nature of this work a preliminary report is given a t this time. X-ray studies have been made of samples of the saturated normal fatty acids with ten to eighteen carbon atoms, and the large crystal spacings have been measured with great accuracy. (These samples, which represent the purest acids now available, were prepared by Dr. J. Ruhoff and Dr. J. D. Meyer with Professor E. E. Reid in this laboratory in the course of an investigation which will be published shortly.) In comparison with similar studies (Francis, F., Piper, S. H., and Malkin, T.: Proc. Roy. SOC.A128, 214 (1930)) made with the purest acids previously available, we find definite deviations which do, however, not exceed one per cent. Francis, Piper, and Malkin have made a very interesting study of equi1 carbon atoms. Their molar mixtures of fatty acids containing n and n results, expressed as simply as possible, indicate that a characteristic long spacing is obtained for such mixtures and that the spacing is the mean value of the spacings the pure components would have if they crystallized in the same modification (same tilt of the fatty acid molecules, which are parallel to the C-axes of the crystals). Since the building elements of such crystals are double molecules they propose the following explanation. I n the case of the mixtures the double molecules are obtained essentially by combination of a long and a short single molecule. This theory did not appeal to us for various reasons. The experimental test is given by a study of mixtures in non-equimolar proportions. I n the case of the mixture of Clo and Cll for instance, we have studied the whole range in ten equal steps. We find the equimolar mixture to be the actual mean of the Clo and spacings, but any of the other mixtures gives also single, sharp spacings, which follow roughly a linear relation with the composition. Thus, it is obvious that the theory proposed does not hold, but that the phenomenon is analogous to ordinary mixed crystal behavior, the relation being approximately expressed by Vagard’s rule. That our point of view is correct is also indicated by the fact that previous work on various non-equimolar mixtures of CISand Cls gives definite spacings varying according to composition between the values of the two components. (Piper, S. H., Malkin, T., and Austin, H. E.: J. Chem. SOC.1926,2310.) In this and other cases for mixtures of n and n 2 we have found similar results.
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COMMUNICATIONS TO T H E EDITOR
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For mixtures of the n and n 3 acids we find, similar to Francis, Piper, and Malkin, mixed-crystal spacings. Acoording to these authors such combination does not occur in mixtures of n and n 4 acids. This is contrary to our results; the equimolar mixture of CI4and CIS,for instance, gives the spacing of the CI6acid. For two-component mixtures of acids of still larger differences in chain length we have not been able as yet to obtain appreciable mixed-crystal formation. It seemed of interest to study mixtures of a large number of components, for it was anticipated that in such cases mixed-crystal formation would be possible over a much wider range. Actually it was found that approximately equimolar mixtures of the acids with n = 11, 13, 15, and 17, n = 10, 12, 14, 16, and 18, and finally a mixture of n = 10, 11, 12, 13, 14, 15, 16, 17, and 18 in each case gave rise only to one definite combination spacing, this value lying between the two corresponding extremes and closer to the higher one. It seems that these experiments are important for the interpretation of x-ray diagrams of highly polymerized substances, Such bodies are in many cases assumed now to consist of chain-like molecules of varying lengths. Such systems were up to now not expected to yield any x-ray interferences connected with the chain length, Our results seem to indicate that this point of view must be modified. In an earlier paper (Ott, Emil: Z. physik. Chem. BO, 378 (1930)) it was shown that definite chain lengths could be assigned to certain polymer formaldehydes. Although the author was a t that time inclined to his present outlook, it seemed safer to conclude, in agreement with the prevailing opinion, that the result indicated that such polymers were built up essentially of molecules of a given length only. At present it appears quite reasonable to consider the chain length mentioned as corresponding to an “average” length. This is of course in better agreement with the chemical experience. It is probably for the latter reason that Sauter (Z. physik. Chem. B18, 417 (1932)) attacks the view of the author; his evidence, however, is essentially negative. This point shall be tested further in due time. At present it seems to us most essential to increase the knowledge concerning x-ray diffraction of such well-defined systems, which may serve as a model for high polymers. The results thus far obtained in this direction seem to favor our point of view. These investigations are being continued and with other coworkers extended to other groups of long-chain compounds. EMILOTT. F. B. SLAGLE. Department of Chemistry Johns Hopkins University Baltimore, Md.
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