Exploring hydrophobic surface modifications in silica and alumina nanomaterials

Superhydrophobic materials have attracted significant attention in various applications including leather manufacturing, water-resistant textiles, self-cleaning surfaces, anti-icing coatings, etc... In the present work, we elucidated the functionalization of silica nanoparticles using isostearic acid, a cheap, non-toxic and highly branched molecule, substituting the conventional toxic fluorinated molecules, in comparison with the alumina counterpart and commercial hydrophobic materials. In this respect, the performance of isostearic acid is compared to the commercially used hexamethyldisilazane, a rigid and bulky moiety of high toxicity and environmental persistence. The functionalized surfaces were characterized by infrared spectroscopy, nuclear magnetic resonance spectroscopy, N2 physisorption, field emission scanning electron microscopy, and thermogravimetric analysis. Microgravimetric and microcalorimetric measurements were also used to describe the surface interaction with water, while the water-repellency at the macroscopic level was monitored through contact angle measurements. Both silica surfaces functionalized with isostearic acid and hexamethyldisilazane show a collective superhydrophobic behavior, but the flexibility of the isostearic acid chains, bonded in low surface density (0.7 units per nm2) through alkyl chains, allow for diffusion of water vapor at the molecular level, while this is not observed in the hexamethyldisilazane case due to the smaller molecular size of the rigid moiety, coupled to a higher surface density (3.5 units per nm2). The results revealed that a material does not necessarily expose micro-nano hierarchical structures to reach an ultimate waterproof capability with a contact angle higher than 150 degrees.