By spatially resolved single-wavelength ellipsometry, we investigate the spreading on a solid substrate of low molecular weight trimethyl-terminated poly(dimethylsiloxane) droplets. Especially, we focus our study on the dependence of the diffusion coefficient of the first molecular layer on the substrate surface energy, characterized by the critical surface tension gamma(c) measured for the alkane series. Our experimental data show that the diffusion coefficient D calculated from the film length reaches a maximum value for surfaces of intermediate energies. This result is shown to be consistent with the predictions of the available theoretical approaches and agrees quite well with molecular dynamics simulations reported in the third part of the paper.