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

Abstract 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 (Tmax) 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 (Tmax ≈ 92–93%) over a wide range of conditions, whereas the glass/P-SiO2/HMDS coating maintained high optical stability (Tmax ≥ 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.