Surface chemical composition and morphology play an important role in determining the superhydrophobic behavior of solids. To provide insights on the influence of surface chemical composition and roughness on superhydrophobic behaviour of tetramethoxysilane based silica aerogel materials over a wide range, we have carried out a quantitative study of the effect of hydrophobic agent on contact angle and surface free energy of silica aerogels. The apparent surface free energy values were calculated using Zisman method and validated by Fowkes-Girifalco-Good theory.