Silika aerojellerin hexametildisilazan-karbon dioksit karişimlariyla yüzey modifikasyonu ve hexametildisilazan-karbon dioksit karişimlarinin faz davranişlari

There are several advantages to using supercritical CO2 (scCO2) as solvent in chemical modification of the metal oxide surfaces. In particular, the low viscosity and low surface tension of the super critical fluids leads to enhanced deposition kinetics over conventional solution methods. The application areas include polymer modification, construction of low-energy hydrophobic surfaces, preparation of stationary phases for reversed-phase chromatography, making biocompatible surfaces, and forming monolayers for lithography, micropatterning, and sensors where alkylsilanes are generally used as the modifying agent.In this study, we rendered the surface of monolithic silica aerogels hydrophobic by hexamethyldisilazane (HMDS)-carbon dioxide mixtures. The silica aerogels were prepared by two step (acid-base) sol-gel method using 50 wt. % tetraethylorthosilicate (TEOS) and ethanol as solvent. The effects of HMDS concentration in the fluid phase and the reaction time were investigated at a fixed temperature of 333.15 K and a fixed pressure of 20.68 MPa. The treatment led to hydrophobic silica aerogels which are as transparent as untreated aerogels. The extent of hydrophobicity was determined by contact angle test and contact angle was found to be 130° at different conditions. FTIR spectra confirmed the hydrophobicity by indicating a reduction in hydrophilic surface silanol groups and the emergence of hydrophobic methyl groups.The second part of the work deals with the vapor-liquid equilibria study of the binary mixture of HMDS-CO2. The bubble point pressures of the HMDS-CO2 system were obtained at temperatures 298.15 K, 313.15 K, 327.65 K and 341.95 K and at various concentrations. At a fixed temperature, the bubble point pressure decreased as the concentration of HMDS increased. At a fixed composition, bubble point pressure increased as the temperature increased. The bubble point pressures were modeled using the Peng-Robinson Stryjek-Vera equation of state (PRSVEOS) and compared well with the experimental data.