Determination of the crystal structure of anhydrous sodium dodecyl sulphate using a combination of synchrotron radiation powder diffraction and molecular modelling techniques

The structure of anhydrous sodium dodecyl sulphate (SDS) was determined usi ng a combination of high resolution, synchrotron, powder X-ray diffraction and molecular modelling techniques, including the use of a systematic searc h method to select suitable inter-molecular packing configurations for subs equent Rietveld refinement. Anhydrous SDS is monoclinic, space group P2(1)/ c, the unit cell dimensions are a =38.915 Angstrom, b = 4.709 Angstrom, c = 8.198 Angstrom and beta = 93.29 degrees and the asymmetric unit is compris ed of a single SDS molecule. The packing motif consists of double layers of molecules. Molecules in adjacent layers are related by 2(1) axes, whilst w ithin layers the adjacent molecules are related by c glide planes. The sulp hur atom in the head group is displaced from the (100) plane by 2.1 Angstro m and the methyl carbon is displaced by a perpendicular distance of 1.2 Ang strom from the (200) plane. Electrostatic interactions between the headgrou ps dominate the packing and largely determine the alignment of the hydrocar bon chains with respect to the long axis of the unit cell. The average area per polar head group is 19.4 Angstrom (2). The present structure is compar ed with previously determined structures for three hydrated phases of SDS i n terms of the observed intermolecular packing motifs. (C) 2000 Elsevier Sc ience B.V. All rights reserved.