Effect of annealing on improved hydrophobicity of vapor phase deposited self-assembled monolayers

The hydrolytic stability and the effect of annealing on the improved aging properties of self-assembled monolayer (SAM) hydrophobic coatings have been investigated. Surfaces coated with hydrophobic SAM coatings give higher flow rate and are of wide interest to researchers in the area of microfluidics. Hydrophobic SAM of fluorooctyltrichlorosilane (FOTS) was deposited on silicon by using a vapor phase technique. The aging of the hydrophobic property was examined by using contact angle measurements. It has been found that while such a monolayer suffers from a loss of hydrophobicity with time, annealing can partially recover the hydrophobicity. The mechanism of aging was investigated through Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), which revealed very little chemical change in annealed monolayers. Surface roughness and morphological changes due to hydrolytic attack was studied by atomic force microscopy (AFM). A spontaneous formation of silane mounds on the surface of the monolayers was found. These mounds have been irreversibly transformed from initially uniform hydrophobic surface layers. It is highly probable that the compliance of these mounds can reasonably allow hydrophilic microcapillaries to be located around the mounds. Interestingly, the density of these mounds decreases when the monolayer samples are annealed, with no significant chemical change. This novel finding explains the observed recovery of hydrophobicity of the monolayers as primarily a morphological effect that originates from the densification of the monolayers upon annealing.