A facile preparation of transparent methyltriethoxysilane based silica xerogel monoliths at ambient pressure drying

The highly transparent and hydrophobic methyltriethoxysilane (MTES) based silica xerogel monoliths with excellent integrity and low thermal conductivity were facilely synthesized by a novel method without complex steps of surface modification and solvent exchange via ambient pressure drying. The effects of water to MTES molar ratios, namely, n (Water)/n (MTES) = 8:1, 9:1, 10:1, 11:1, 12:1 and 13:1, on the pore structure, surface morphology, water contact angle, optical transmittance and thermal conductivity were systematically investigated. It was found that the average transparency and hydrophobicity of the silica xerogels decreased and increased linearly with the increase of n (Water)/n (MTES) value, respectively. Furthermore, the thermal conductivity was also strongly dependent upon the n (Water)/n (MTES) value which directly influenced the completion degree of MTES hydrolysis. A simplified pore model is suggested based on the experimental results to correlate the formation of pore structures with the property of silica xerogels. The average transparency of 65-69% in the visible light range of 300-800 nm with the thermal conductivities of 0.048-0.052 W/(m.K) and water contact angles of 105 degrees similar to 110 degrees could be simultaneously achieved with the silica xerogel monoliths by controlling the n (Water)/n (MTES) values of 10:1 and 11:1.