Estimation of contact angle for hydrophobic silica nanoparticles in their hexagonally ordered layer

Wetting properties of bilayered nanostructured coatings were studied in this work. Coatings were prepared by stratifying a compact silica sol–gel (SG) film and a Langmuir–Blodgett (LB) layer of differently sized (84, 131 and 227 nm) Stöber silica particles onto glass substrates. Joint high temperature annealing of the layers increased the mechanical stability of the particulate film facilitating its further investigations. Regular structure of LB layers allowed us to carry out wetting model investigations. Close-packed arrangement of particles was reinforced by optical measurements. Transmittance spectra of LB films were taken and evaluated by a theoretical model. Resulting effective refractive index and film thickness values indicated a good agreement between the real and the supposed (hexagonally close packed, monodisperse spherical particles) layer structure. It was confirmed by field emission scanning electron microscopy (FESEM) images, too. Advancing and receding water contact angles (CA) on differently (with mono- and bifunctional chlorosilanes) hydrophobized SG and combined films were measured applying the sessile drop method. SG films showed approximately the same CA-s after different silylation processes without considerable contact angle hysteresis (CAH). In case of combined films ca. 30° higher advancing CA-s were measured at stronger silylation and only the milder silylation resulted in significant CAH. It was explained by the surface heterogeneity of constituent particles. Layers of differently sized particles showed the same wetting properties in all cases in agreement to the Cassie–Baxter equation which was used for the estimation of CA values for the individual silica particles (88–90° and 105–106° depending on the silylation conditions)