DETERMINATION OF THE CRYSTAL GROWTH UNITS BY GRAZING INCIDENCE X-RAY DIFFRACTION (GIXD) AND AFM. David Gidalevitz+, Robert Feidenhans'l[[daggerdbl]] and Leslie Leiserowitz+. +Dept. of Materials & Interfaces, The Weizmann Inst. of Science, Israel; [[daggerdbl]]Dept.of Solid State Physics, Risø National Laboratory, Denmark.
Molecular crystal growth in solution takes place at the crystal-fluid interface. The local structure at this boundary plays a primary role in determining the composition, growth and habit of the crystal. In this study we apply GIXD to probe the surfaces of organic molecular crystals and their growth and dissolution interfaces.
We shall show that GIXD and AFM may be used to advantage in a complementary manner to determine close to atomic resolution the structure of the crystal-fluid interface of [[beta]]-alanine and the [[alpha]]-form of glycine during growth and dissolution, and so provide information on the crystal growth units. We addressed principal questions of crystal growth as to the molecular arrangement of the growing crystal surface and the structure of the "building units" which dock from solution onto the growing crystal surface.
The crystals of [[beta]]-alanine (+H3NCH2CH2CO2-+H3NCH2CO2-) are composed of zwitterionic molecules interlinked by strong N-H...O hydrogen bonds into centrosymmetric bilayers. AFM revealed that both dissolution and growth proceed as step flow with an average minimal step height of 7+/-0.5Å for [[beta]]-alanine and 5.7+/-0.5Å for [[alpha]]-glycine, which corresponds to two molecular layers. The GIXD experiments on the (010) face of [[beta]]-alanine on dissolution and the (010) face of [[alpha]]-glycine on growth showed that during the X-ray data collection the surface termination of the crystals corresponded to bilayer surface truncation and did not change with time. The results described here are compatible with a model in which the molecules of [[beta]]-alanine and glycine effectively dock or leave the crystal surface as cyclic hydrogen-bonded dimers.