The Diffraction of X-Rays in Liquid Normal Monobasic Fatty Acids

Abstract

Eleven of the normal monobasic fatty acids in the liquid state have been investigated with $\mathrm{Mo}K\alpha$ radiation. The results are similar to those previously reported for the liquid primary normal alcohols by Stewart and Morrow. There are two spacings found by the application of Braggs' law, one of which is the lateral separation of the molecules which lie in parallel collinear chains and the other the separation of the planes passing transversely through the chains and containing the diffraction centers of the molecules. The lateral spacing is 4.55A for the samples having more than five carbon atoms to the molecule. This is in agreement with the values found by Stewart and Morrow for the primary n-alcohols and with the work of Adam on monomolecular films. The longitudinal spacing varies linearly with the carbon content, the increase being 2.00A for two molecules. In the molecular arrangement, the chains are not normal to the transverse planes and there are two molecules in series, with associated COOH groups, for each spacing. A comparison of data for the acids in the liquid and crystalline states shows that the arrangement of molecules in the cybotactic state is not that found in the crystalline state.