The application of antireflective coatings to the glass covers of solar thermal collectors, allows increasing the efficiency of the whole system. The work presented here describes the room temperature synthesis of highly transparent and hydrophobic silica coatings using tetraethylorthosilicate (TEOS) as a precursor and polydimethylsiloxane (PDMS) as a hydrophobic modifying agent via a simple dip coating technique. Then, the films were characterized by measuring contact angle and optical transparency, Fourier transform infrared (FTIR) spectroscopy, Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) measurements. A new two-step method by double layer of acidic and basic sols was employed. Results showed the maximum transmittance of the antireflective solar glass with double layer coating is about 95.02% at 565 nm wavelength, which is about 4% higher than the substrate glass. The durability of the materials used in solar systems is a key point since they should keep their initial properties during the operational lifetime. In this work, the stability of the optical properties of the films after one year at room temperature in an environment has been achieved, thanks to the application of a hydrophobic treatment and two step-catalyzed sols to the coating. The addition of PDMS to the silica sols improved the hydrophobicity of the coating, and prevented to some extent the coating from cracking which occurred in a pure inorganic thick antireflective coating. It was observed that the obtained silica films become hydrophobic with the introduction of the hydrophobic organic group and static water contact angle (97°) was obtained for the silica film prepared with double layer of acidic/basic coating.
