Wettability evaluation of hydrophilic-hydrophobic nanohybrid silica thin films using picoliter water droplets.

Water wettabilities of hydrophilic-hydrophobic nanohybrid silica thin films were investigated by contact angles of extremely small size water droplets (8 pL-1 nL) and using an antifog analyzer. The nanohybrid silica was prepared via our unique sol-gel process based on tetramethoxysilane (TMOS) and metyltrimethoxysilane (MTMS) with a hydroxyacetone catalyst, in which hydrophilic portion was generated from hydrolysis of microporous silica prepared from TMOS and hydrophobic portion was directly prepared from MTMS. The sizes of these domains were controlled by the growth time of the microporous precursor polymers in solution. The hydrophilic and hydrophobic domains in the nanohybrid surface were visualized and confirmed by a lateral-mode friction force microscopy using the cantilever modified with self-assembled monolayers of mercaptohexadecanol. Using a small size of water droplets (< 100 pL), the contact angles for the nanohybrid silica films were lower than those for the wholly hydrophilic silica. The small size of the water droplet has a characteristic effect on the lower contact angle on the nanohybrid silica. The contact angles using the small size 70 pL of the water droplet on the nanohybrid silica films at [MTMS]/[TMOS] ratios from 0.25 to 0.75 were lower than those using the conventional size 2 microL. The dependence of the [MTMS]/[TMOS] ratio in the preparation on the antifog parameter was similar to that on the contact angle using the small size 70 pL of the water droplet. The use of the small water droplet for the contact angle measurement was suitable for the evaluation of the antifog films.

Microstructure investigation on micropore formation in microporous silica materials prepared via a catalytic sol-gel process by small angle X-ray scattering.

The so-called sol-gel technique has been shown to be a template-free, efficient way to create functional porous silica materials having uniform micropores. This appears to be closely linked with a postulation that the formation of weakly branched polymer-like aggregates in a precursor solution is a key to the uniform micropore generation. However, how such a polymer-like structure can precisely be controlled, and further, how the generated low-fractal dimension solution structure is imprinted on the solid silica materials still remain elusive. Here we present fabrication of microporous silica from tetramethyl orthosilicate (TMOS) using a recently developed catalytic sol-gel process based on a nonionic hydroxyacetone (HA) catalyst. Small angle X-ray scattering (SAXS), nitrogen adsorption porosimetry, and transmission electron microscope (TEM) allowed us to observe the whole structural evolution, ranging from polymer-like aggregates in the precursor solution to agglomeration with heat treatment and microporous morphology of silica powders after drying and hydrolysis. Using the HA catalyst with short chain monohydric alcohols (methanol or ethanol) in the precursor solution, polymer-like aggregates having microscopic correlation length (or mesh-size) < 2 nm and low fractal dimensions ∼2, which is identical to that of an ideal coil polymer, can selectively be synthesized, yielding the uniform micropores with diameters <2 nm in the solid materials. In contrast, the absence of HA or substitution of 1-propanol led to considerably different scattering behavior reflecting the particle-like aggregate formation in the precursor solution, which resulted in the formation of mesopores (diameter >2 nm) in the solid product due to apertures between the particle-like aggregates. The data demonstrate that the extremely fine porous silica architecture comes essentially from a gaussian polymer-like nature of the silica aggregates in the precursor having the microscopic mesh-size and their successful imprint on the solid product. The result offers a general but significantly efficient route to creating precisely designed fine porous silica materials under mild condition that serve as low refractive index and efficient thermal insulation materials in their practical applications.