SYNCHROTRON HIGH-RESOLUTION POWDER STUDY OF MOLECULAR PACKING IN HYDRATE CALCIUM STEARATE

The structure of (C18H35O2)2Ca,H2O was completely solved using synchrotron ig resolution powder diffraction data. This work, which was long delayed because of severe overlapping in powder scans obtained on conventional diffraction instruments, was made possible by the use of synchrotron radiation and a suitable parallel beam geometry with a crystal analyzer. The unit cell is monoclinic, and consists in the stacking of four molecular layers, c being roughly 100 angstrom, with space group P2(1)/c. The organization of atoms is quite original: the local packing mode is monoclinic, which is very scarcely encountered; the chain orientation parallel to b reverses in the vicinity of polar groups, which are concentrated near z = 1/4 and z = 3/4 planes; the organization of hydrogen bonds is also explained.