Nanostructured Amorphous Silicas Hydrophobized by Various Pathways.

Various nanostructured amorphous silicas [fumed silicas such as crude (A-300), hydro-compacted (cA-300, TS 100), and precipitated silica Syloid 244] were modified by different polydimethylsiloxanes such as PDMS5, PDMS100, PDMS200, PDMS1000, and PDMS12500 (the label numbers show the viscosity (η) values) using dimethyl carbonate (DMC) as a siloxane-bond-breaking reagent. In addition, hexamethyldisilazane was used to modify fumed silica cA-300. The nanocomposites were characterized using microscopy, infrared spectroscopy, thermodesorption, nitrogen adsorption-desorption, solid-state NMR spectroscopy, small-angle X-ray scattering, and zeta-potential methods. It was found that the morphological, textural, and structural characteristics of silicas grafted with PDMS depend strongly not only on the type and content of the polymers used but also on the organization of nonporous nanoparticles (NPNP) in secondary structures (aggregates of NPNP and agglomerated aggregates, ANPNP), as well on the reaction temperature (T r). Specifically, we determined that ANPNP with a macro/mesoporous character are favorable for the effective modification of the silicas studied with short polymers and no DMC addition but at higher temperatures or for a longer silicone polymer with the presence of DMC and at lower temperatures. In particular, the PDMS/DMC-modified silicas are of great interest from a practical point of view because they remain in a dispersed state with no strong compaction of the secondary structures after modification, and this corresponds to a better distribution of the modified nanoparticles in polymeric or other matrices.

Water Interactions with Hydrophobic versus Hydrophilic Nanosilica.

It is well-known that interaction of hydrophobic powders with water is weak, and upon mixing, they typically form separated phases. Preparation of hydrophobic nanosilica AM1 with a relatively large content of bound water with no formation of separated phases was the aim of this study. Unmodified nanosilica A-300 and initial AM1 (A-300 completely hydrophobized by dimethyldichlorosilane), compacted A-300 (cA-300), and compacted AM1 (cAM1) containing 50-58 wt % of bound water were studied using low-temperature 1H NMR spectroscopy, thermogravimetry, infrared spectroscopy, microscopy, small-angle X-ray scattering, nitrogen adsorption, and theoretical modeling. After mechanical activation (∼20 atm) upon stirring of AM1/water mixture at the degree of hydration h = 1.0 or 1.4 g of distilled water per gram of dry silica, all water is bound and the blend has the bulk density of 0.7 g/cm3. The temperature and interfacial behaviors of bound water depend strongly on a dispersion media type (air, chloroform, and chloroform with trifluoroacetic acid (4:1)) because the boundary area between immiscible water and chloroform should be minimal. Water and chloroform molecules are of different sizes affecting their distribution in pores (voids between silica nanoparticles in their aggregates) of different sizes. Structural, morphological, and textural characteristics of silicas, and environmental features affect not only the distribution of bound water, but also the amounts of strongly (frozen at T < 260 K) and weakly (frozen at 260 K < T < 273 K) bound and strongly (chemical shift δH = 4-6 ppm) and weakly (δH = 1-2 ppm) associated waters. Despite the changes in the characteristics of cAM1, it demonstrates a flotation effect. The developed system with cAM1/bound water could be of interest from a practical point of view due to controlled interactions with aqueous surroundings.