We report the immobilization of a fulleropyrrolidine, bearing a dec-9-ynyl functionality, on silicon surfaces through a thermal hydrosilylation protocol. Contact angle measurements on porous silicon ( PS) surfaces reveal an unusual dependence of the angle with the PS roughness that apparently contradicts Wenzel's formula. This result has been explained by an extension of Wenzel's model in which the critical angle, which discriminates between the hydrophilic/hydrophobic character of a solid material, is substantially reduced below 90 by surface roughness.