Threshold hydrophobicity for inhibition of salt scale formation on SAM-modified titania nanotube arrays

Fouling of solid surfaces is a ubiquitous problem in industrial processes. As unwanted material accumulates on components, their function is impaired and costly repairs are required. Most fouling occurs from impurities present in water that are deposited when the water contacts a solid surface. It follows that if the adhesive forces at the water-solid interface are minimized, less fouling will result. In this study, we present a novel method to minimize fouling by fabricating a hydrophobic surface based upon self-assembled monolayer (SAM)-modified titanium dioxide nanotube arrays (TNTAs). We demonstrate a direct correlation between hydrophobicity and the formation of scale from dissolved salts by comparison of the surface's static contact angle and degree of precipitation deposition. Furthermore, by tailoring the surface hydrophobicity through employment of a wide variety of SAMs with different alkyl chain lengths, we determine the threshold level of hydrophobicity that inhibits fouling in the SAM-TNTA system; surfaces with a static contact angle greater than 144 degrees display vastly increased fouling resistance. The surface morphology, surface composition, and stability of the alkyl phosphonic acid- and perfluoroalkyl phosphonic acid-SAM-TNTAs were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).