Ambient Pressure Hybrid Silica Monoliths with Hexamethyldisilazane: From Vitreous Hydrophilic Xerogels to Superhydrophobic Aerogels

Hybrid silica-based monoliths were synthesized at ambient pressure, using minimum amounts of the silylating agent hexamethyldisilazane (HMDZ). Depending on the synthesis approach, the materials ranged from dense and vitreous xerogels to transparent and superhydrophobic aerogels. Emphasis was given to understanding the role of the silylating agent, its content and incorporation process on the final morphology, and properties of the xerogels/aerogels. It is proven that as a coprecursor, increasing HMDZ content contributes to increase the lipophilic/hydrophilic balance, induce high surface areas, and decrease densities, but there is a maximum usable content for producing monoliths. Conversely, as a postsynthesis modifier, there is an optimum HMDZ content that maximizes hydrophobicity (water contact angle of ∼144°) and induces high surface area (∼700 m2·g-1), keeping the density low (∼300 kg·m-3). It is proven that the aging period in the hydrophobizing solution is a crucial parameter. The most superhydrophobic xerogels were obtained using HMDZ as a postsynthesis modifier, achieving values of water contact angles as high as ∼173°, at the cost of density increase to ∼600 kg·m-3 and decrease of the surface area to ∼300 m2·g-1. The best compromise between low density, high surface area, and superhydrophobicity is obtained using HMDZ both as a coprecursor and as a postsynthesis modifier, in a low HMDZ/tetraethoxysilane total molar ratio (<0.2), with an aging period of 16-24 h. The use of subcritical drying, along with the minimization of the expensive organic modifier quantities, allows envisaging a safe and low-cost large-scale production of a variety of materials, including superhydrophobic aerogels with potential distinctive applications.