Interfacial phenomena at a surface of partially silylated nanosilica

• Effects of silica silylation on the interfacial behavior of co-adsorbed water and light gases.
• Confined space and hydrophobization effects on freezing-melting temperatures of bound water.
• Enhancement of light gases co-adsorption due to frozen water bound to TMS-nanosilica.

Unmodified pyrogenic silica PS300 and partially silylated nanosilica samples at a degree of substitution of surface silanols by trimethylsilyl (TMS) groups ΘTMS = 27.2% and 37.2% were studied to elucidate features of the interfacial behavior of water adsorbed alone, or co-adsorbed with methane, hydrogen, or trifluoroacetic acid (TFAA). In the aqueous suspension modified PS300 at ΘTMS = 37.2% forms aggregates of 50–200 nm in size and can bind significant amounts of water (up to ∼5 g/g). Only 0.5 g/g of this water is strongly bound, while the major fraction of water is weakly bound. The presence of surface TMS groups causes the appearance of weakly associated water (WAW) at the interfaces. The adsorption of methane and hydrogen onto TMS-nanosilica with pre-adsorbed water (hydration degree h = 0.05 or 0.005 g/g) increases with increasing temperature. In weakly polar CDCl3 medium, interfacial water exists in strongly (SAW, chemical shift δH = 4–5 ppm) and weakly (δH = 1–2 ppm) associated states, as well as strongly (changes in the Gibbs free energy −ΔG > 0.5–0.8 kJ/mol) and weakly (−ΔG < 0.5–0.8 kJ/mol) bound states. WAW does not dissolve TFAA but some fraction of SAW bound to TMS-nanosilica surface can dissolve TFAA.

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