Surfactant Control of Phases in the Synthesis of Mesoporous Silica-Based Materials

TLDR

Low‑temperature liquid‑crystal‑like arrays of inorganic–organic complexes, governed by surfactant packing parameters, provide a convenient route to mesostructured silica materials. The study investigates how the molecular shapes of organosilanes, quaternary ammonium surfactants, and mixed surfactants under varied reaction conditions can be harnessed to synthesize specific silica‑based mesophase configurations. By tuning solvent polarity, hydrophobic/hydrophilic balance, and surfactant functional groups, the authors direct the liquid‑crystal‑like solid phase to produce the desired mesophases. The authors demonstrate that selecting appropriate surfactants and synthesis conditions yields predicted mesophases, including room‑temperature MCM‑48 and a low‑temperature, low‑pH route to high‑quality MCM‑41 with pores up to 60 Å and volumes of 1.6 cm³ g⁻¹.

Abstract

The low-temperature formation of liquid-crystal-like arrays made up of molecular complexes formed between molecular inorganic species and amphiphilic organic molecules is a convenient approach for the synthesis of mesostructure materials. This paper examines how the molecular shapes of covalent organosilanes, quaternary ammonium surfactants, and mixed surfactants in various reaction conditions can be used to synthesize silica-based mesophase configurations, MCM-41 (2d hexagonal, p6m), MCM-48 (cubic Ia3d), MCM-50 (lamellar), SBA-1 (cubic Pm3n), SBA-2 (3d hexagonal P63/mmc), and SBA-3 (hexagonal p6m from acidic synthesis media). The structural function of surfactants in mesophase formation can to a first approximation be related to that of classical surfactants in water or other solvents with parallel roles for organic additives. The effective surfactant ion pair packing parameter, g = V/a0l, remains a useful molecular structure-directing index to characterize the geometry of the mesophase products, and phase transitions may be viewed as a variation of g in the liquid-crystal-like solid phase. Solvent and cosolvent structure direction can be effectively used by varying polarity, hydrophobic/hydrophilic properties and functionalizing the surfactant molecule, for example with hydroxy group or variable charge. Surfactants and synthesis conditions can be chosen and controlled to obtain predicted silica-based mesophase products. A room-temperature synthesis of the bicontinuous cubic phase, MCM-48, is presented. A low-temperature (100 °C) and low-pH (7−10) treatment approach that can be used to give MCM-41 with high-quality, large pores (up to 60 Å), and pore volumes as large as 1.6 cm3/g is described.