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

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