The azimuthal anchoring energy at the interface between the nematic liquid crystal pentylcyanobiphenyl (5CB) and an obliquely evaporated SiOxsubstrate is measured by using a reflectometric method. The time response of the director at the surface is investigated and two very different regimes are evidenced. The azimuthal anchoring energy coefficient exhibits an almost-critical behaviour near the clearing temperature and is almost independent on the thickness d of the SiOxlayer but it exhibits a sharp decrease for d ≤ dc≈ 25 Å. The anchoring function is found to depend on the azimuthal director angle ϕ as W(ϕ) = Wa/2 sin2(ϕ).