Factors Affecting the Hydrophobic Property of Stearic Acid Self-Assembled on the TiO2 Substrate.

Abstract

The self-assembled monolayer (SAM) on inorganic metal oxides is highly applicable in making different kinds of surface phenomena such as superhydrophobicity, functional group-modified surfaces, corrosion resistance, and so on. The formation of stearic acid SAMs on the TiO2 substrate depends on a few factors, and the cleanability of the substrate surface can be considered as the critical criterion for the formation of the SAM layer. The solvent, concentration of the adsorbate, immersion time, and temperature can be identified as other factors that are crucial for growing a uniform and highly dense monolayer. SAM layers always build up spontaneously on a suitable substrate, but the growth rate and arrangement can be changed by varying the external factors. These factors highly affect the chemisorption of stearic acid molecules onto the TiO2 substrate and building a well-ordered pattern on the surface without defects. This study mainly focuses on identifying the critical conditions of the external factors in obtaining a high-performance superhydrophobic surface. The crystal structure and surface morphologies of the substrate materials are characterized by powder X-ray diffraction and scanning electron microscopy, and the surface wettability is characterized by contact angle measurements. High superhydrophobicity is observed at the optimum conditions of the factors. Ethanol is used as the solvent; the temperature is about 40 °C; and 600 ppm of stearic acid is the critical concentration in obtaining a superhydrophobic surface with 100 min of immersion time, while the contact angle is 151.38°. Simultaneously, if the concentration is 1000 ppm and the immersion time is 120 min, the surface shows high superhydrophobicity with a contact angle of 162.06°. These critical conditions are found to be adequate for building well-ordered stearic acid SAMs on the TiO2 substrate.