Phase Engineering of Hydrophobic Meso-Environments in Silica Particles for Technical Performance Enrichment

Abstract

Hexadecyltrimethylammonium bromide (CTAB) was utilized to template the growth of mesoporous silica particles via ammonia-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) in the reaction solutions with varied volume fractions of ethanol ( f). The use of 9,10-bis(phenylethynyl) anthracene (BPEA) as a fluorescence probe unraveled a clear difference in interior structure between the CTAB micelles confined at different f. At f of 0.3, the confined CTAB micelles consisting of regularly and densely packed alkane chains, which created crystalline interiors, in which the doped BPEA molecules were effectively isolated in the monomeric form and well protected against aggressive attack from the surrounding environment. At f of 0.4 or 0.5, the confined CTAB micelles consisting of less regularly but densely packed alkane chains created glassy interiors, which enabled reversible aggregation of the doped BPEA in response to the surrounding environmental change, for instance, the ethanol content in the particle dispersion. At f of 0.6 or 0.7, the confined CTAB micelles consisting of loosely packed alkane chains created amorphous interiors, which offered sufficiently large free spaces to facilitate the material exchange with the surrounding environment, as evidenced by noticeable intake of the Pyronin Y molecules present in the particle dispersion. The revealed phase modulation of the interiors of surfactant micelles, confined in the pores of mesoporous particles, from crystalline to glassy and amorphous structures, which were scarcely reported in literature, will inspire rational design of mesoporous silica particles with desired technical performance according to the purposes of the practical application.

Water Interactions with Hydrophobic versus Hydrophilic Nanosilica

“…Various hydrophobic silicas are of importance from a practical point of view − and produced by many companies for a variety of applications. − Many problems appearing upon the use of hydrophobic materials are caused by features of their interactions with water, the degree of hydrophobicity, durability of a hydrophobic functional layer, etc. − Interaction of water with a solid surface depends not only on the type of functionalization (hydrophobization), but also on the surface topology and confined space effects, as well on the presence of coadsorbates and a dispersion medium type. ,− On the other hand, water can strongly affect the organization of fumed metal or metalloid oxides (FMO) with respect to the secondary particles such as aggregates of nonporous nanoparticles (NPNP) and agglomerates of aggregates. ,, Additionally, in the systems with hydrophobic particles and water, air bubbles can play an important role in flotation and decrease in the wettability of a particle surface and whole secondary structures with hydrophobized NPNP. , Features of interactions of water with hydrophobic and hydrophilic particles allow one to create “dry” water or the reverse system with hydrophobic cores and hydrophilic shells containing bound water. − Note that water bound to hydrophilic components of complex materials (such as polymers, rubbers, and paints filled by pigment particles, etc.) can lead to some negative effects.…”

Synthesis of Mesoporous Silica Using the Sol–Gel Approach: Adjusting Architecture and Composition for Novel Applications

“…Furthermore, some studies have also demonstrated that confining functional elements in the pores of mesoporous silica not only leads to better functionalization, but also contributes to novel features. For instance, Yang and co-workers reported that mesoporous silica prepared by co-dissolving 9,10-bis(phenylethynyl)anthracene with CTAB molecules in a mixed solvent with different volume ratios of ethanol to water exhibited unique properties; the fluorescent probe not only clearly detected the evolution of CTAB micelle phases in the mesopores, but also established a correlation between the crystalline state of CTAB (crystalline, glassy, and amorphous) in mesoporous particles and the color rendering of the fluorescent probe [ 42 ].…”