X-Ray Diffraction in Liquid Normal Paraffins

Abstract

X-ray diffraction as a function of angle in liquid pentane, hexane, heptane, octane, nonane, decane, dodecane, tetradecane and pentadecane.—In the ionization diffraction-angle curve, there was one diffraction peak only, instead of two as found by Stewart, Morrow and Skinner with the primary alcohols and normal fatty acids. The absence in the $n$-paraffins of the second peak, which, in the alcohols and fatty acids, indicated a longitudinal spacing, is explained qualitatively by consideration of the longitudinal molecular forces. These could not produce as uniform a longitudinal arrangement as occurs in the other compounds. The single peak was located at the same angle with each $n$-paraffin, excepting, at first, pentane and decane. Subsequent purification of the pentane produced no alteration in the peak position but when the sample was finally made synthetically, the peak came into coincidence with that of the other seven. The decane is believed to contain isomers. The lateral separation of molecules is approximately 4.6A and is essentially the same as in the liquids previously studied and referred to in this paper. In six of nine cases, the diffraction intensity does not decrease as one approaches 0°. The significance is not clear though one is reminded of total reflection and also of refraction. If one assumes the C atom to occupy a length in the chain of 1.3A, as found by other observers of x-ray diffraction in solids, and the length of H, 1.0A, and if one assumes the volume occupied by a molecule to be the square of the lateral spacing times the molecular length, then the computed values of densities of the seven normal paraffins are correct to within less than 4 percent or a mean of 2 percent. Two additional but faint peaks in the diffraction intensity curves are found with pentadecane and tetradecane. The corresponding spacings are 2.1A and 1.23A. The nearness of these to the repetitive values of 2.0A and 1.26A which are found in a diamond, is suggestive.