Hybrid Sol-Gel Superhydrophobic Coatings Based on Alkyl Silane-Modified Nanosilica

Hybrid sol-gel superhydrophobic coatings based on alkyl silane-modified nanosilica were synthesized and studied. The hybrid coatings were synthesized using the classic Stöber process for producing hydrophilic silica nanoparticles (NPs) modified by the in-situ addition of long-chain alkyl silanes co-precursors in addition to the common tetraethyl orthosilicate (TEOS). It was demonstrated that the long-chain alkyl substituent silane induced a steric hindrance effect, slowing the alkylsilane self-condensation and allowing for the condensation of the TEOS to produce the silica NPs. Hence, following the formation of the silica NPs the alkylsilane reacted with the silica's hydroxyls to yield hybrid alkyl-modified silica NPs having superhydrophobic (SH) attributes. The resulting SH coatings were characterized by contact angle goniometry, demonstrating a more than 150° water contact angle, a water sliding angle of less than 5°, and a transmittance of more than 90%. Confocal microscopy was used to analyze the micro random surface morphology of the SH surface and to indicate the parameters related to superhydrophobicity. It was found that a SH coating could be obtained when the alkyl length exceeded ten carbons, exhibiting a raspberry-like hierarchical morphology.