X-ray Diffraction Patterns of Montmorillonite Oriented Films Exchanged with Enantiomeric and Racemic Tris(2,2'-bipyridyl) Ruthenium(ii)

-X-ray diffraction patterns of oriented films of montmorillonite containing enantiomeric tris(2,2'bipyridyl) ruthenium(II) chloride (Ru(bpy)22§ show a peak corresponding to basal spacings of approximately 27 ,~. This peak is absent from the X-ray patterns of montmorillonite films containing racemic cations. A basal spacing of 27 ,~ is consistent with the adsorption of 2 layers of the enantiomeric cations in each interlayer space. Under the same condition only one layer of the raeemic cation was intercalated (basal spacings of 17.9 A). These results are in accord with previous reports that montmorillonite can adsorb more of the optical isomers than of the racemic mixture of Ru(bpy)32+. Addition of NaC1 to the mixtures resulted in an increase in the level of adsorption of the racemic cations and in the appearance of a peak at 27 A in the X-ray pattern. Key Words--Double layer, Enantiomorphs, Intercalation, Monolayer, Montmorillonite, Surface area, Tris(2,2'-bipyridyl)ruthenium(II), X-rays. I N T R O D U C T I O N There is as yet no satisfactory explanation for the ability of smectite clays to distinguish between the enantiomers and the racemic mixture of tris(2,2'-bipyridyl)ruthenium(II) cations (Ru(bpy)31+) (Yamagishi, 1987). Because smectites are not optically active (Cairns-Smith, 1982), the source of the difference in sorption has been attributed to a packing phenomenon. Yamagishi (1987) proposed that the adsorption of Ru(bpy)32§ by clay minerals occurred in racemic pairs, which he assumed packed more efficiently than enantiomeric pairs, making it possible to intercalate more of the racemic mixture than of the enantiomers in the space available between the clay layers. Aside from the fact that a more compact adsorption of the racemic cations has never been experimentally demonstrated, there are several problems with this hypothesis. First, it does not account for the differences seen at low loading levels. At small cation/clay ratios, differences are seen in the absorption and emission spectra of adsorbed Ru(bpy)32§ When the cation is highly diluted in the clay, packing efficiency should not matter. Yet, these differences increase with decrease of the cation/clay ratio (Joshi and Ghosh, 1989). Second, the max imum amounts of racemic and enantiomeric cations adsorbed by smectites can differ by as much as a factor of 2.6 (Villemure, 1990). How can small differences in packing efficiency have such major effects on the adsorption isotherms? Furthermore, unlike what was reported for Ru(phen)32§ (Yamagishi, 1985), it is the enantiomers of Ru(bpy)32+ that were adsorbed in larger amounts (Villemure, 1990). In an attempt to reconcile these observations with Yamagishi's racemic-pairs adsorption hypothesis, ViiCopyright 9 1991, The Clay Minerals Society lemure and Bard (1990) proposed that adsorption of racemic Ru(bpy)32+ caused more extensive face-to-face aggregation of the clay layers than adsorption of enantiomeric Ru(bpy)32+. A difference in the clay morphology is consistent with the interpretation of the spectral differences in terms of more extensive intercalation of racemic than enantiomeric Ru(bpy)32+ (Thomas, 1988). More extensive intercalation of the racemic cations would also explain why each adsorbed racemic Ru(bpy)32+ covers an average of 217 A2 of the clay surface, compared to only 122 ,~2 for each adsorbed enantiomeric Ru(bpy)32§ (Villemure, 1990). An intercalated cation is in contact with two clay layers, and is expected to cover approximately twice the area of an externally adsorbed cation. Differences in the clay aggregation should have an effect on its X-ray diffraction pattern. The present study was undertaken to test this hypothesis.