Evolution of solution structures during the formation of the cubic mesoporous material, KIT-6, determined by double electron-electron resonance

Double electron - electron resonance (DEER)is employed to explore the evolution of solution structures during the formation of the bicontinuous cubic (Ia3̄d) mesoporous material KIT-6. We focused on the early stages of the reaction, where micellar structures are not well-resolved in the micrographs of cryogenic transmission electron microscopy (cryo-TEM). KIT-6 is synthesized with Pluronic P123 block copolymers, PEO₂₀PPO_70- PEO ₂₀, as a template. Initially, the aqueous solution, held at 40 °C, consists of Pluronic P123 micelles, which are characterized by a hydrophilic corona, comprising the poly(ethylene oxide) (PEO) blocks, and a hydrophobic core, consisting of the poly(propylene oxide) (PPO) block. The variations in the volume of the hydrophobic core of the micelles as a consequence of the addition of the silica source (TEOS, tetraethoxyorthosilane) were evaluated using a hydrophobic spin-probe, 4-hydroxy-tempobenzoate (4HTB), which is localized in the hydrophobic core of the micelles. The measurements were carried out on solutions that were freeze - quenched at different times after the addition of TEOS (tetraethoxyorthosilane). New details on the formation of KIT-6 were obtained, revealing swelling of the hydrophobic core during the first ∼10 min of the reaction followed by a contraction, close to the original size. The swelling was attributed to penetration of the TEOS and its hydrolysis products into the micelles.