X-Ray Diffraction in Liquids: Primary Normal Alcohols

Abstract

Evidence for a molecular space array in liquid primary normal alcohols, methyl to lauryl.—The x-ray diffraction in liquids has long been known. The suggestion has been repeatedly made that the effect is caused by fragmentary crystals. The viewpoint here taken is that there is a molecular space array, not crystalline, which is named cybotaxis. Evidence of the cybotactic state in liquid primary normal alcohols, methyl to lauryl, is adduced. By Mo $K\alpha$ x-ray diffraction ionization curves, two significant distances are measured, the first linearly dependent upon the content of carbon atoms and the second practically independent of this change in molecular length. The latter is thus the distance of separation perpendicular to the chain, and is 4.6A with lauryl, decreasing slowly to 4.4A with butyl and then rapidly to 3.8A with methyl. The value 4.6A is in striking agreement with the work of Adam on surface films of saturated fatty acids. The increase of the first distance, linearly with carbon content, is in harmony with the work of Müller and Saville and others, in that the increase is about 1.3A per carbon atom, and leads to the conclusion that the diffraction is produced by planes containing the polar groups, which are not perpendicular to the direction of the parallel chain molecules. Comparison of peaks in the liquid and solid states show that the spacings are not the same and the phenomena is not caused by crystal fragments. The cybotactic state permits mobility, but not random motion and is peculiar to the substance. The distances computed are the most probable spacings. The molecules may be regarded as having the same orientation in a small group, too small to give optical anisotropy. The discussion does not, therefore, extend to liquid crystals but adheres to the more general condition. The conception of the cybotactic state is helpful in an understanding of solutions and other liquid phenomena. The theories of Raman and Ramanathan, Debye and Zernike and Prins are suggestive in that they indicate that a definite molecular space array is not necessary for the production of a single diffraction halo. But they in no sense negate the evidence here adduced for a cybotactic state.