Glass Surfaces Rendered Hydrophobic by Silylation and Sol-Gel Coatings: Optical, Wetting, and Chemical Stability

This study presents a comparative evaluation of three strategies for hydrophobizing glass surfaces: direct surface silanization with dimethyldichlorosilane (DMDClSi), the application of a hybrid silica coating (H-SiO₂), and the use of a hexamethyldisilazane (HMDS)-modified silica coating (P-SiO₂/HMDS). The glass/H-SiO₂ and glass/P-SiO₂/HMDS coatings were prepared by a sol-gel process and deposited onto glass substrates via dip-coating using alcohol-based precursor sols. All surface treatments resulted in hydrophobic surfaces, with advancing water contact angles exceeding 90° and very low contact angle hysteresis (∼1°), indicating uniform and stable water-repellent surfaces. Surface free energy analysis confirmed the predominantly nonpolar character of the modified surfaces, with the lowest values observed for the DMDClSi-treated samples (18 ± 1 mN m^-1). Both silica-based coatings also increased the light transmittance of the glass substrates compared to the bare glass, achieving maximum transmittance (T_max) of ≈99.6-99.7% and average transmittance increases of 6.0-6.5% (400-800 nm), together with controlled porosity (34-35%) and uniform thickness values (127 nm). Ellipsometric porosimetry revealed marked differences in pore structures between the silica-based coatings: the glass/H-SiO₂ coating exhibited a high-surface-area porous network (1124 m² cm^-3, 38% porosity), whereas the glass/P-SiO₂/HMDS coating showed higher porosity (48%) with larger mesopores and a lower specific surface area (245 m² cm^-3), indicating a more open porous structure. The stability of hydrophobic and optical properties was investigated under combined chemical and thermal stress using a three-factor Box-Behnken design. The results identified pH as the main degradation parameter, while temperature and exposure time acted as accelerating factors. Under alkaline-thermal conditions, degradation of Si-CH₃ groups was identified as the primary mechanism responsible for hydrophobic failure in the glass/H-SiO₂ system, as confirmed by ATR-FTIR analysis. These results suggest that the high-surface-area pore network of the glass/H-SiO₂ coating is more susceptible to the loss of Si-CH₃ functionalities. Wettability measurements demonstrated that the glass/DMDClSi surface exhibited the highest resistance to degradation throughout the investigated design space. Among the silica-based coatings, the glass/P-SiO₂/HMDS sample showed markedly improved hydrophobic stability compared to glass/H-SiO₂, retaining water-repellent properties under most conditions and undergoing substantial hydrophilization only under the most severe alkaline-thermal exposure. Light transmittance measurements showed significant optical degradation for glass/H-SiO₂ (T_max ≈ 92-93%) over a wide range of conditions, whereas the glass/P-SiO₂/HMDS coating maintained high optical stability (T_max ≥ 99.1%) across most of the investigated pH-temperature-time combinations, with deterioration observed only under the most severe conditions. Although the DMDClSi treatment provided the highest wettability stability, the glass/P-SiO₂/HMDS coating combined high optical stability with durable hydrophobicity, and improved resistance to environmental stress.