ABSTRACT
Silicas (bulk SiO2 with surface silanols) are the most popular oxide materials including porous (silica gels and ordered mesoporous silicas), nonporous highly disperse nanomaterials, microparticles, microspheres, nanotubes, etc. SiO2 is also a component of many natural minerals. Silica materials have a relatively simple structure of bulk and surface, including siloxane bonds ≡Si-O-Si≡, terminal single ≡SiOH, and twin =Si(OH)2 silanols (Iler 1979, Legrand 1998). Silanols and siloxane bonds, which are characterized by very different activity in adsorption and chemical reactions and have clear spectral difference in infrared (IR), 29Si and 1H nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS), are very convenient objects in investigations of silica surfaces and interfacial phenomena there. Despite composition simplicity, highly disperse silicas (nanosilicas), as well as silica gels and other silicas, possess many useful properties for practical applications, and some of these properties can be quite unusual when nanoscaled effects appear and become essential for the interfacial behavior of adsorbates. Notice that such a simpler compound as water, both bulk-free and adsorbed (i.e., interfacial), possesses much more unusual properties than silica (Gun’ko et al. 2005d, 2009d, Chaplin 2011). Therefore, silica with bound water represents an interesting combination, properties of which play an important role in the nature and human life. These properties can be very unusual, especially after addition of another solvent (e.g., nonpolar, weakly polar, or polar organics) to water and/or surface modication of silica by polar or nonpolar compounds creating a mosaic hydrophilic/hydrophobic surface (Gun’ko et al. 2001e, 2003g, 2005d, 2007d,f,i, 2009d). Analysis of the interfacial phenomena at a silica surface could be started from nanosilicas composed of nonporous primary nanoparticles.
