Effect of concentration of trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDZ) silylating agents on surface free energy of silica aerogels

The surface free energy of a solid determines its surface and interfacial behavior in processes like wetting and adhesion which is crucial for silica aerogels in case of organic liquid absorption and transportation of chemicals at nano-scale for biotechnological applications. Here, we have demonstrated that the surface free energy of aerogels can be tuned in wide range from 5.5892 to 0.3073 mJ/m(2) by modifying their surface using TMCS and HMDZ silylating reagents. The alcogels were prepared by two step acid-base catalyzed process where the molar ratio of precursors Tetraethoxysilane (TEOS):Methanol (MeOH):Oxalic acid:NH4OH:NH4F was kept at optimal value of 1:2.7:0.18 x 10(-4):0.02:0.22 x 10(-3), respectively. To modify gel surfaces, TMCS and HMDZ concentration have been varied from 3% to 12% and such alcogels were dried at ambient pressure. It is observed from FTIR for aerogels that increase in concentration of silylating reagent resulted increase in hydrophobicity. This leads to increase in contact angle for water from 123 degrees to 155 degrees but leads to decrease in surface free energy from 5.5892 to 0.3073 mJ/m(2). As there is not direct method, we have used Neumann's equation of state to estimate surface energy of aerogels. (C) 2011 Elsevier Inc. All rights reserved.