Abstract. Na+.C12H25OaS-.H2 O, M r = 306.4, trielinic, Pi, a = 10.423 (4), b = 5.662 (3), c = 28.913 (12) A, ct=86.70(4), #=93.44(4), y=89.55 (4) ° , V= 1700(1) A 3, Z=4, D x=l.20gcm -3, 2(CuKa)= 1.5418A, #=20cm -~, F(000)=664, T=290-294 K, R = 0.11 for 4457 observed reflections. Each asymmetric unit is made up of two water and two sodium dodecyl sulfate (SDS) molecules with different conformations. The SDS molecules form a lamellar structure with regions alternately polar and apolar. Adjacent polar heads of the same monolayer are displaced in a zigzag fashion along the a axis forming a rippled structure. Facing monolayers are held together by Coulombic interactions not screened by the water molecules. Weak van der Waals interactions are present in the apolar region. From comparison of the present phase with other SDS crystal phases differing in water content it is possible to deduce that the higher the water content, the larger the average area per polar head and the smaller the lamellar thickness. In all the crystal phases interdigitation of hydrocarbon chains does not take place, whereas the polar heads can give rise to planar or rippled monolayers.
Crystals of 5e-androstan-3,17-dione are monoclinic, space group C2, Z=4, a--12.70, b = 6.19, c= 21.34 /~., ,8= 91 ° 16'. The structure was solved by potential-energy calculations coupled with a minimum residual phasing technique. Three-dimensional X-ray data were used in the refinement, which was carried out by least-squares methods. The crystal packing is characterized by molecular layers, normal to the b axis, tied together by van der Waals, dipole-dipole and n-re interactions.
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