TEOS based water repellent silica films obtained by a co-precursor sol–gel method

The nature of interfaces between water and hydrophobic materials has been a subject of great interest. Experimental results from tetraethoxysilane (TEOS) based hydrophobic silica films synthesized by the two step sol?gel process using hexadecyltrimethoxysilane (HDTMS) as a co-precursor are described. The molar ratio of TEOS, methanol (MeOH), acidic water (0.001?M, oxalic acid), and basic water (8?M, NH4OH) was kept constant at 1:66.66:6.76:6.66, and the molar ratio of HDTMS/TEOS (M) was varied from 0 to 22.9 ? 10?2. The maximum contact angle of 125? was obtained for M = 22.9 ? 10?2. It is observed that the water contact angle value on the HDTMS modified films remained stable at 125?, after heat treatment up to 235??C. However, the water contact angle value decreased to 52? in the case of unmodified films, indicating the improvement in the thermal stability using HDTMS as a co-precursor. The hydrophobic silica films retained their hydrophobicity up to a temperature of 235??C and above this temperature the films became hydrophilic. The hydrophobic silica films were characterized by surface roughness studies, scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, contact angle measurements, and % optical transmission.