The crystal structure of sodium diheptylsulphosuccinate dihydrate and comparison with phospholipids

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Abstract
Sodium diheptylsulphosuccinate can be crystallised in two polymorphic modifications, a monohydrate and a dihydrate. The dihydrate gave crystals of sufficient quality for a single-crystal structure analysis and its full structure has been elucidated. The crystal is monoclinic, spacegroup P21/a, with cell dimensions a= 7.79 (1)Å, b= 9.80 (1)Å, c= 32.63 (3)Å,β= 92.5 (1)°. There are 4 surfactant and 8 water molecules per unit cell. The crystal is racemic, i.e. there are two molecules of each optical enantiomer of the sulphosuccinate per cell. The water molecules form hydrogen-bond links between sulphonate oxygens of adjacent molecules. The discrimination in the crystal between the two optical enantiomers is through these hydrogen bonds. One of the water molecules causes enantiomer pairs to form, straddling two monolayers. The other water molecule is involved in the formation of infinitely long ribbons of alternating enantiomers in one monolayer in the direction of the a axis of the crystal. In the dihydrate, the aliphatic chains have a virtually zero angle of tilt with respect to the normal on the monolayer plane. From X-ray long spacings it is deduced that the monohydrate crystal has a tilt angle of ca. 38°. The molecular conformation, the packing in the crystal, the array of hydrogen bonds and the tendency to crystal polymorphism show a great similarity with recently elucidated crystal structures of phospholipids. It is suggested that the polymorphic transition between crystal types finds its parallel in monolayer-covered surfaces of sulphosuccinate solutions. Here, slow surface-tension changes could be ascribed to cooperative changes in tilt angle for rows of molecules, as found for phospholipid monolayers. Subsequent rapid formation of intermolecular hydrogen bonds would then consolidate this new conformation.

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