We use the adsorption of systematically substituted silanes with either simple alkyl or alkyl phenyl ether chains onto oxidized Si to study the electronic effects of such molecular monolayers on Si. While there is no significant effect of distance of the substituents from the surface, a strong effect of what we interpret as depolarization is found for layers made up of molecules with high (>5 D) free molecule dipole moment. This is also apparent from differences in UV-visible and Fourier transform infrared (FTIR) spectral features, suggesting changes in molecular conformation, and, especially, from the measured contact potential differences. These reflect the modified surface's electron affinity and, thus, the effective dipole moment of the monolayer. The effect is ascribed to the system's response to the energetic price of dipole-dipole repulsion.