One-Step Hydrophobic Silica Nanoparticle Synthesis at the Air/Water Interface

A template-free, single-precursor method is presented for constructing hydrophobic silica nanoparticles (NPs) without organic solvent in a Langmuir trough divided into two isolated interfaces connected by a mildly acidic subphase. A water-insoluble trimethoxysilane with an n-propyl side group (nPM) spread at one interface is solubilized upon hydrolysis and diffuses through the subphase to the clean air/water interface where a wire probe tensiometer measures the interfacial sol-gel reaction. Adsorption and assembly of hydrolyzed nPM at the air/water interface is reflected by a steady increase in surface pressure followed by a plateau. Transferred films characterized with FTIR, XRD, SEM, AFM, and water contact angle confirm self-assembly of hydrolyzed nPM into an similar to 1 nm thin film at the air/water interface that undergoes polycondensation into sub-10 nm "seed" particles, which further assemble into amorphous, similar to 150 nm hydrophobic NPs. Films transferred through the Langmuir-Schaefer method yield modestly hydrophobic surfaces (water contact angles of 115 degrees), whereas aspirated films deposited as a multilayer coating yield superhydrophobic surfaces with contact angles greater than 165 degrees. BET surface area analysis indicates nonporous particles. Longer alkyl-chain organo-silanes (up to C8) also form NPs through this method but with no greater hydrophobicity than the C3 nPM silane. This method provides a facile approach to prepare hydrophobic organo-silane particles using a single short-chain alkyl-silane precursor under purely aqueous conditions.