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Superhyrophobicity and structures of adsorbed silane coupling agents on silica and diatomaceous earth

Perera, M. A. Helanka Jayani
Superhydrophobic coatings were prepared using fluorosilane treated diatomaceous earth (DE) with either polyurethane or epoxy binders and the results were compared. The water contact angles were studied as a function of the amount of fluorosilane on DE and the amount of treated DE particle loadings in the coatings. The contact angles exceeded 150° for coatings with at least 0.02 mass fraction of fluorosilane on the DE and with 0.2 mass fraction particle loadings of fluorosilane-treated DE particles. The water contact angles of the surfaces were dependent on the nature of the binder below 0.2 mass fractions loading of the treated DE particles, while they were independent of the type of the binder after attaining superhydrophobicity.
The wettability of treated DE was studied as a function of the chain lengths and adsorbed amounts of alkyltrimethoxysilanes (C3, C8, C12, C16, and C18) with the help of water contact angle measurements. Temperature modulated differential scanning calorimetry (TMDSC) results showed that silanes on the treated DE became crystalline with increasing chain length (C ≥ 12) and adsorbed hydrocarbon amounts (adsorbed amount ≥ 2.2 mg/m2). The formation of a crystalline/ordered structure from low-surface energy material led to the formation of superhydrophobic coatings on treated DE. At similar adsorbed hydrocarbon amounts, as the carbon chain length of coupling agents increased, we observed a concomitant increase in the water contact angle.
The structural assemblies of hexadecyltrimethoxysilane (HDTMS) on silica/DE particles were studied by TMDSC, thermogravimetric analysis, and Fourier transform infrared spectroscopy (FTIR). HDTMS molecules adsorbed at very small adsorbed amounts molecules were directly bound to the silica/DE surface as isolated molecules, and their aggregates were more likely to be amorphous. These molecules were found to have very small enthalpies for both melting and crystallization of HDTMS hydrocarbon chains. These enthalpies were found to increase linearly with adsorbed amounts for the silica surface. With further increased adsorbed amounts of HDTMS, melting and crystallization enthalpies increased exponentially and approached the bulk HDTMS enthalpy for samples with a distance constant of around 1.6 and 2.9 mg/m2 for silica and DE, respectively.