The first stage functionalization of hydrogenated (111) Si surface with methyl-terminated monolayers to form hydrophobic coatings has been studied by accurate ab initio density functional total energy calculations. The first stage adsorption events involving one or two deposited n-alkane and n-alkyl-silane molecules have been characterized from the geometrical and the energetic points of view; the ground-state adsorption configurations together with the geometrical and the energetic parameters relevant to self-assembling processes have been obtained, such as the polymers tilt angles, the rotation energy barriers, the binding energies, and the electrostatic interactions. The above-mentioned quantities have been related to the stability properties of self-assembled monolayers and have been recognized to affect critically the stability and the uniformity of the hydrophobic film elucidating the reasons why some commonly used polymers behave differently.