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ABSTRACT
In this study, caffeine was adsorbed onto montmorillonite and the surface of the montmorillonite was observed before and after the adsorption. Observations were performed by X-ray diffractometry (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and scanning probe microscopy (SPM). Measurement of the interlayer distance by XRD revealed that it narrowed to 1.09 nm after caffeine adsorption. Adsorption of a small amount of caffeine causes a broadening of d001 peaks. This suggested that caffeine molecules were adsorbed into the interlayer space in part, resulting in an irregular layer stacking. The results of SPM observations of caffeine-adsorbed montmorillonite showed the existence of protrusions that are different from minerals on the surface of montmorillonite and suggested a possibility that the protrusions were caffeine molecules adsorbed onto the surface of montmorillonite. DRIFTS demonstrated that the intensity of peaks assigned to Si–OH and siloxane decreased with the increase in the amount of caffeine adsorbed. These results suggested that caffeine molecules were adsorbed both into the interlayer space and onto the surface and interacted with Si–OH and siloxane, in particular. These findings will help us to select ions to be held in the interlayer space and organic compounds to improve the amount of adsorption when montmorillonite is used in the manufacture of caffeine-free drinks and foods.
Acknowledgments
The authors thank Kenji Shiota and Ryoto Yoshida of Kyoto University and Aruto Yoshida, Takahiro Eguchi, and Yuko Yotsumoto of Research Laboratories for Beverage Technologies, Research and Development Division, Kirin Company, Limited for their support for experiments and useful discussions. The authors also thank Shimadzu Corporation for their help in acquisition of part of experimental data.