Hydrophobization of Silica Nanoparticles in Water: Nanostructure and Response to Drying Stress

We report on the impact of surface hydrophobization on the structure of aqueous silica dispersions and how this structure resists drying stress. Hydrophilic silica particles were hydrophobized directly in water using a range of organosilane precursors, with a precise control of the grafting density. The resulting nanostructure was precisely analyzed by a combination of small-angle X-ray scattering (SAXS) and cryo-microscopy (cryo-TEM). Then, the dispersion was progressively concentrated by drying, and the evolution of the nanostructures as a function of the grafting density was followed by SAXS. At the fundamental level, because the hydrophobic character of the silica surfaces could be varied continuously through a precise control of the grafting density, we were able to observe how the hydrophobic interactions change particles interactions and aggregates structures. Practically, this opened a new route to tailor the final structure, the residual porosity, and the damp-proof properties of the fully dried silica. For example, regardless of the nature of the hydrophobic precursor, a grafting density of 1 grafter per nm² optimized the interparticle interactions in solution in view to maximize the residual porosity in the dried material (0.9 cm³/g) and reduced the water uptake to less than 4% in weight compared to the typical value of 13% for hydrophilic particles (at T = 25 °C and relative humidity = 80%).