How do rod-like molecules freeze and arrange in mesopores?

We present an x-ray diffraction study on rod-like molecules, i.e. the normal alkanes C₁₉H₄₀ and C₉H₂₀ condensed in nanoporous Vycor glass. Temperature-dependent diffraction patterns elucidate how the structure and phase behaviour of these molecules are affected by the random substrate disorder and the geometric confinement. For the medium-length alkane C₁₉H₄₀ a quenching of the lamellar ordering is observed, whereas this geometric ordering principle survives in the case of the short-length C₉H₂₀, although in a modified fashion.