Surface hydrophobicity: effect of alkyl chain length and network homogeneity

Understanding the nature of hydrophobicity has fundamental importance in environmental applications. Using spherical silica nanoparticles (diameter = 369 ± 7 nm) as the model material, the current study investigates the relationship between the alkyl chain network and hydro-phobicity. Two alkyl silanes with different chain length (triethoxymethylsilane (C1) vs. trimethoxy(octyl)silane (C8)) were utilised separately for the functionalisation of the nanoparticles. Water contact angle and inverse gas chromatography results show that the alkyl chain length is essential for controlling hydrophobicity, as the octyl-functionalised nanoparticles were highly hydrophobic (water contact angle = 150.6° ± 6.6°), whereas the methyl-functionalised nanoparticles were hydrophilic (i.e., water contact angle = 0°, similar to the pristine nanoparticles). The homogeneity of the octyl-chain network also has a significant effect on hydrophobicity, as the water contact angle was reduced significantly from 148.4° ± 3.5° to 30.5° ± 1.0° with a methyl-/octyl-silane mixture (ratio = 160:40 µL·g−1 nanoparticles).