Solid State Radical Recombination and Charge Transfer across the Boundary between Indomethacin and Silica under Mechanical Stress

A composite of indomethacin (IM) and SiO2 was prepared in a solid state by cogrinding. Dehydration was observed as a consequence of a chemical reaction between silanol groups of SiO2 and carboxyl groups of IM. Several specific radical species found on separately ground SiO2 disappeared preferentially after cogrinding as a result of their recombination. These chemical interactions under mechanical stress bring about a charge transfer, as confirmed by X-ray photoelectron spectroscopy, leading to Si–O–C bridging bond formation at the IM/SiO2 interface.