Silica Nanoparticles Synthesized from 3,3,3-Propyl(trifluoro)trimethoxysilane or n-Propyltrimethoxysilane for Creating Superhydrophobic Surfaces

Superhydrophobic silica (SHS) nanoparticles (NPs) with fluoro- or alkyl-surface functionalization are explored for a wide range of potential applications. Here, the aqueous synthesis of SHS NPs from single precursors, 3,3,3-propyl(trifluoro)trimethoxysilane (3F) and n-propyltrimethoxysilane (nPM), are presented. The 3F and nPM precursor silanes were solubilized by acid hydrolysis followed by base condensation to synthesize NPs ranging from 50 to 300 nm, dependent on silane concentration. Dynamic light scattering and atomic force microscopy revealed that NP size decreased with precursor concentrations. The 3F NPs displayed lower size, polydispersity, and surface roughness compared to nPM NPs synthesized at the same precursor concentrations, indicating distinct sol–gel kinetics for the two hydrophobic silanes. Attenuated total reflection-Fourier transform infrared spectroscopy revealed free silanol groups on fluoro-NPs and powder X-ray diffraction showed distinct peak intensities for fluoro- and alkyl-NPs supporting a greater propensity toward fully condensed oligomeric silsesquioxane (POSS) cage structures for nPM than for 3F. Space-filling models attributed these structural differences to steric constraints of the terminal CF3 moiety on 3F contrasted with CH3 on nPM. A difunctional analog of 3F was unable to assemble into particles during the condensation process, supporting the role of POSS cages as the fundamental assembly unit. Both NPs formed superhydrophobic thin films demonstrating water contact angles >165° and sliding angles <5°. The aqueous synthesis of SHS NPs from either fluoro- or organo-silane single-source precursors provides alternative pathways to constructing SHS NPs for applications requiring superhydrophobicity.