Janus silica film with hydrophobic and hydrophilic surfaces grown at an oil–water interface†

We report a new methyltrimethoxysilane (MTMS) based route to growing a Janus silica film at the oil–water interface, which upon drying shows anisotropic wetting by water on its two surfaces. The contact angle of water on the surface grown in contact with the oil-side is found to be 150 , but it is much smaller, 65 , on the side which grew in contact with the aqueous phase. This large difference in the contact angle is found to be primarily because of two reasons: (i) orientation of hydrophobic methyl groups towards the oil-side of the film as confirmed by micro-Raman spectroscopy, and (ii) microstructural differences in the oil and water-side surfaces of the film. The inherently hydrophobic silica cluster network on the oil-side surface also exhibits larger pores that provide an air cushion for the water droplet and engenders a large contact angle. Effects of oil–water interfacial tension on the film growth and on its wetting and microstructural properties are also investigated by addition of cationic and anionic surfactants in the aqueous subphase. Static and dynamic wetting properties of the oil-side surface indicate that these do not change significantly due to variations in either the microstructure or chemical nature of the surface alone, but is a combined effect of both. Interestingly, the Janus films showing asymmetric surface properties can also be grown directly and thus integrated with a variety of porous surfaces like cotton, paper, hydrogel and ceramic substrates by having these surfaces straddle an oil–water interface.