COMPARISON OF ENANTIOMERIC AND RACEMIC MONOLAYERS OF 2-HYDROXYHEXADECANOIC ACID BY EXTERNAL INFRARED REFLECTION-ABSORPTION SPECTROSCOPY

Surface pressure/area (II/A) isotherms and infrared reflection-absorption spectra (IRRAS) of racemic and enantiomeric 2-hydroxyhexadecanoic acid (HHDA) Langmuir films showed that the presence of bivalent cations in the aqueous subphase gives rise to considerable compression (Ca2+ > Pb2+ similar to Zn2+) and to increased chiral discrimination (Ca2+ < Pb2+ similar to Zn2+) in HHDA monolayers. Although both in the presence of Pb2+ and Zn2+ cations the II/A isotherms of the L-enantiomer exhibit the more condensed characteristics, the IRRAS measurements show that Pb2+ cations induce, against expectation, heterochiral discrimination in compressed HHDA films, while the presence of Zn2+ cations leads to preferred homochiral interactions. In the latter two instances, the conformational order of the alkyl chain is nearly independent of the compressional status of the monolayer; i.e., the optimum order induced by the presence of Pb2+ and Zn2+ cations is already attained at very low surface pressures around 1 mN m(-1). The different influence of bivalent cations on chiral recognition is assumed to be caused by different complexes formed by the respective cation and the functional group of the film-forming substances. Hence, in addition to van der Waals and electrostatic interactions as well as hydrogen-bond formation, potential complex formation between bivalent cations and the head group appears to play an important role for heterochiral and homochiral discrimination in chiral Langmuir films.