ASPECTS OF SPONTANEOUS SEPARATION OF ENANTIOMERS IN 2-DIMENSIONAL AND3-DIMENSIONAL CRYSTALS

Spontaneous separation of enantiomers in two- and three-dimensional cr ystals is driven by the same thermodynamic and kinetic factors. Howeve r, amphiphilic crystalline monolayers at an interface cannot possess a center of inversion, the most common symmetry element in bulk crystal s. This fact should, in principle, lead to better chances for spontane ous separation in the Langmuir or Langmuir-Blodgett monomolecular film s. On the other hand, the monolayers of most amphiphiles studied to da te incorporate long aliphatic chains that have an intrinsic tendency t o pack in a herringbone motif requiring glide plane symmetry, thus cre ating a bias towards racemate formation. Moreover, 2-D crystals suppos edly have a much higher degree of molecular and therefore enantiomeric disorder compared to bulk crystals. All these factors necessitate a c areful choice of molecules to guarantee enantiomeric separation in two dimensions. Unambiguous detection of spontaneous resolution in 2-D ap pears to require atomic resolution of molecular packing arrangement, w hich can in principle be obtained by grazing incidence X-ray diffracti on or atomic force microscopy, whereas in bulk solids spontaneous reso lution can be easily detected by various macroscopic methods. This sho rt review provides analogies between spontaneous separation in 3-D and recent examples in 2-D, showing that spontaneous separation generally depends upon subtle differences in molecular structure. (C) 1998 Wile y-Liss, Inc.