The application of layered solids for molecular recognition is summarized. By using layered solids (silicates, aluminosilicates, titanates, hydroxides, and so on), ions and molecules can be concentrated from aqueous and vapor phases. The large surface area and tunable surface properties derived from the layered structures contribute to molecular recognition. The choice of materials and modification of the nanostructure were carefully investigated to optimize the performance based on molecular recognition (selective adsorption, substrate selective reaction, detection, etc.). The progress made in materials syntheses (variation of layered materials, sophisticated modification, controlled morphology, and processing) has made the design of materials more attractive and realistic.

This study investigates the adsorption of caffeine in water on organically modified clays (a natural montmorillonite and synthetic saponite, which are smectite group of layered clay minerals). The organoclays were prepared by cation-exchange reactions of benzylammonium and neostigmine with interlayer exchangeable cations in the clay minerals. Although less caffeine was uptaken on neostigmine-modified clays than on raw clay minerals, uptake was increased by adding benzylammonium to the clays. The adsorption equilibrium constant was considerably higher on benzylammonium-modified saponite (containing small quantities of intercalated benzylammonium) than on its montmorillonite counterpart. These observations suggest that decreasing the size and number of intercalated cations enlarges the siloxane surface area available for caffeine adsorption. When the benzylammonium-smectite powders were immersed in water, the intercalated water molecules expanded the interlayer space. Addition of caffeine to the aqueous dispersion further expanded the benzylammonium-montmorillonite system but showed no effect on benzylammonium-saponite. We assume that intercalated water molecules were exchanged with caffeine molecules. By intercalating benzylammonium into smectites, we have potentially created an adaptable two-dimensional nanospace that sequesters caffeine from aqueous media.

The utilization of smectite clay, swelling layered silicate, as scaffolds for designing functional nanostructures was overviewed. Surface modification of smectites with organoammonium ions has given hydrophobic and microporous nature to uptake nonionic organic contaminants from environments. The states of the adsorbed nonionic organic compounds have been altered and varied by the modification of smectites as shown by the controlled release and specific catalytic reactions. Cationic species have been easily concentrated on smectites from aqueous phase and the states (orientation and distribution) have been controlled by the co-adsorption of both cationic and nonionic species. The functions of smectite-organic intercalation compounds derived from the precisely controlled nanostructures were introduced in this review.

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The adsorption of 2,4-dichlorophenol onto the 1,1'-dimethyl-4,4'-bipyridinium-saponite, a smectite group of clay minerals, from dilute aqueous solution occurred effectively through charge-transfer interactions and such interactions resulted in the change in the color of the 1,1'-dimethyl-4,4'-bipyridinium-saponite.