Nanostructured surfaces

Design of MCM‑41‑type silicas with molecular template roughness surface for selective adsorption of azo dyes
Roik N.V., Belyakova L.A., Dziazko M.O.
Chuiko Institute of Surface Chemistry of NAS of Ukraine, 17 General Naumov Str., Kyiv, 03164, Ukraine
E-mail: roik_nadya@ukr.net

Molecular imprinting is widely used to prepare silicas with size and shape selective cavities due to the simplicity of concept as well as high efficiency of resulting materials. Applying of bulk molecular imprinting strategy to creation of selective sites within matrix of periodic mesoporous silicas of MCM‑41 type has some advantages, the main of which is formation of highly accessible open imprints localized on the surface of cylindrical pore channels or deeper in the pore walls.
Our research was focused on the non‑covalent bulk imprinting of MCM‑41‑type silica with methyl red (MR) and study of resulting material recognition ability towards azo dyes with different structure. Imprinting of MR moieties within silica matrix was realized in the process of sol-gel condensation of tetraethoxysilane with (3‑aminopropyl)triethoxysilane in the presence of long chain alkylammonium salt  cetyltrimethylammonium bromide. The influence of MR on mesoporous structure and recognition ability of resulting imprinted material towards azo dyes was elucidated and compared with corresponding parameters for control non‑imprinted silica. It was found that introduction of MR in sol-gel synthesis results in noticeable improvement of mesoporous structure hexagonal arrangement and increase of surface area. Comparative sorption of different azo dye pollutants by synthesized silica materials from phosphate buffer solutions with pH 4.8 proved that selective adsorption process may be optimized by the control of the molecular template roughnesses range on the silica surface and protolytic state as well as geometrical size of acid-base indicator dye. The caverns formed on silica matrix by MR molecules introduced in sol-gel synthesis as co-template do not influence sorption removal of this azo dye in undissociated and partially dissociated forms. At the same time, the noticeable suppression of adsorption was observed for negatively charged dissociated indicator dyes whose geometrical parameters correspond to the surface molecular imprints as well as undissociated ones with substantially different geometry. So, obtained results could be useful in design of selective adsorbents for azo dye pollutants removal from their diluted solutions and optimization the separation process.