Organoclays in Water Cause Expansion That Facilitates Caffeine Adsorption Tomohiko Okada,* ,† Junpei Oguchi, † Ken-ichiro Yamamoto, § Takashi Shiono, § Masahiko Fujita, ∥ and Taku Iiyama ‡,∥ † Department of Chemistry and Material Engineering, Faculty of Engineering, and ‡ Center for Energy and Environmental Science, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan § Kirin Company, Ltd., Namamugi 1-17-1, Tsurumi-ku, Yokohama 230-8628, Japan ∥ Department of Chemistry, Faculty of Science, Shinshu University, Asahi 3-1-1, Matsumoto 390-0802, Japan * S Supporting Information ABSTRACT: 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 counter- part. 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. ■ INTRODUCTION Adsorption of specific molecular species onto solid surfaces is currently exploited in a wide range of scientific and practical applications such as removing toxic compounds and recovering desired substances. The periodic structures of appropriately designed nanostructures are ideally suited for selective adsorption, and well-established microporous and mesoporous solids (e.g., zeolites and nanoporous silicas) have been extensively researched for this purpose. 1−5 Another class of nanostructured materials with regularly arranged organic moieties is porous inorganic−organic hybrid solids (e.g., metal−organic frameworks: MOFs and porous coordination polymers: PCPs). In these compounds, the desired organic molecules are concentrated by interaction with moieties. 6−8 On the other hand, inorganic ultrathin layers act as useful nanospace scaffolds because they incorporate organic moieties as so-called “pillars” into their two-dimensional expandable interlayer spaces. 9−12 In such inorganic−organic hybrid systems, the nanospace can be tuned by spatially controlling the number and size (molecular structure) of the organic moieties, which affects their spatial distribution. 12,13 Nano- structural versatility encourages us to seek further applications for these systems in selective adsorption, separation, and catalysis. 12 Among the expandable layered inorganic solids, the smectite group of layered clay minerals has been most extensively studied. 14,15 Smectites are composed of ultrathin (ca. 1.0 nm) crystalline silicate layers separated by hydrated interlayers. 15,16 The cations in the interlayer spaces that compensate the negatively charged silicate layers are readily exchanged with various organic cations. Cation-exchange reactions with a relatively small size of organoammonium cations, such as tetramethylammonium and trimethylphenylammonium ions, create nanospaces surrounded by cation−silicate layer structures. This technique has been used to produce inorganic−organic microporous clays. 12,17−19 In nanospace engineering, the structure of the pillaring agents and charge density of the smectites can be varied to control the adsorption of nonionic aromatic compounds. 20−26 However, although the relationship between structure and absorptive properties affects the adsorption behavior of organic molecules in organoclay Received: September 17, 2014 Revised: December 1, 2014 Published: December 6, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 180 dx.doi.org/10.1021/la503708t | Langmuir 2015, 31, 180−187