Effect of Negative Surface-charge Densities of Smectite Clays on the Adsorption Isotherms of Racemic and Enantiomeric Tris(2,2'-bipyridyl)ruthenium(ii) Chloride

The level of adsorption of enantiomeric tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)3 2§ by seven different smectite samples was found to be controlled by the total surface area of the clay. The level corresponded to one monolayer. The adsorption of the racemic mixture was lower for all samples, being limited to saturation of the clay's cation-exchange capacity. The addition of NaCl increased the adsorption of the racemic RuCopy)32§ tO the same level as the enantiomers. The dependence on the clay's surface charge density, expected on the basis of the limited face-to-face aggregation model, was not found. The sharp decrease in adsorption of the enantiomers by low charge-density clays and the corresponding sharp increase in adsorption of the racemate by high charge density clays were not observed. These results do not necessarily invalidate the model. The range of negative charges examined may not have been wide enough. Also, charge distribution in smectite is generally not uniform. Key Words--Adsorption, Enantiomorphs, Racemic pairs, Smectite, Surface charge, Tris(2,2'-bipyridyl)rnthenium(II). I N T R O D U C T I O N Clay minerals display unusual and unexpected molecular recognition properties. Among these properties is the ability of smectite to distinguish between the racemic mixture and the enantiomers of cations of the type M(bpy)32§ and M(phen)32~ (M = Fe, Ru, Ni; bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline) (Yamagishi, 1987). The selection of optical isomers by clays could be very important, especially in connection with the postulated role of clay minerals in prebiotic evolution (Ponnamperuma et al., 1982). Smectite is not optically active (Cairns-Smith, 1982). The source of this chiral discriminating power is not well understood, but may be based on a packing phenomenon. Yamagishi (1987) proposed that the adsorption of this type of cation occurred in racemic pairs, which he assumed packed more efficiently than enantiomeric pairs. Thus, more of the racemic mixture could be intercalated in the limited space available between the clay layers. Aside from the fact that a more compact adsorption of the racemic cations has never been satisfactorily demonstrated, this hypothesis leaves several unanswered questions. First, how can the differences observed at low loading levels be accounted for? At small cation/clay ratios, differences exist in the absorption and emission spectra of Ru(bpy)3 z+ and Ru(phen)32+. If the cations are highly diluted in the clay, packing efficiency should not matter; yet, these differences ini Present address: Department of Chemistry, University of New Brunswick, Bag Service #45222, Fredericton, New Brunswick E3B 6E2, Canada. Copyright 9 1990, The Clay Minerals Society crease with a decrease of the cation/clay ratio (Joshi and Ghosh, 1987). Second, the amounts of the different forms of the cation adsorbed by smectite samples can differ by as much as 2.5 times. How can small differences in packing efficiency have such a major effect on the adsorption isotherms? Furthermore, unlike what has been reported for Ru(phen)32+ (Yamagishi, 1985), for Ru(bpy)a 2§ it is the enantiomers that are adsorbed in larger amounts. Various proposals have been made to reconcile these observations with the racemic-pairs adsorption hypothesis. Spectral differences have been interpreted in terms of a higher local concentration of the racemic cations. An attraction force between the optical antipoles has been postulated to cause aggregation of the racemic cations, even if they were highly diluted in the clay colloids (Joshi and Ghosh, 1989). The large difference in the levels of adsorption of the enantiomers and the racemic mixture was attributed to specific adsorption sites based, for example, on the partial penetration of the cation ligands in the clay surface hexagonal holes (Fitch et al., 1988). Neglected in these models is the "nonrigid" nature of the clay particles. The size and shape of the particles in clay colloids are dependent on many factors and can easily be changed (Nadeau, 1987). In a recent paper (Villemure and Bard, 1990), an extension of Yamagishi's racemic-pairs adsorption hypothesis was proposed to take into account the effect of the adsorption of Ru(bpy)32. on the clay morphology. The adsorption of racemic Ru(bpy)32§ was assumed to cause more extensive face-to-face aggregation of the clay particles than adsorption of enantiomeric Ru(bpy)32~. Because an intercalated cation is in contact with two clay layers,