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 using a combination of high resolution, synchrotron, powder X-ray diffraction and molecular modelling techniques, including the use of a systematic search method to select suitable inter-molecular packing configurations for subsequent Rietveld refinement. Anhydrous SDS is monoclinic, space group P21/c, the unit cell dimensions are a=38.915Å,b=4.709Å,c=8.198Åand β=93.29° and the asymmetric unit is comprised of a single SDS molecule. The packing motif consists of double layers of molecules. Molecules in adjacent layers are related by 21axes, whilst within layers the adjacent molecules are related by c glide planes. The sulphur atom in the head group is displaced from the (100) plane by 2.1Å and the methyl carbon is displaced by a perpendicular distance of 1.2Å from the (200) plane. Electrostatic interactions between the headgroups dominate the packing and largely determine the alignment of the hydrocarbon chains with respect to the long axis of the unit cell. The average area per polar head group is 19.4Å2. The present structure is compared with previously determined structures for three hydrated phases of SDS in terms of the observed intermolecular packing motifs.