Crystallization of medium-length 1-alcohols in mesoporous silicon: an x-ray diffraction study

The linear 1-alcohols n-C(16)H(33)OH, n-C(17)H(35)OH, n-C(19)H(39)OH have been imbibed and solidified in lined up, tubular mesopores of silicon with 10 and 15 nm mean diameters, respectively. X-ray diffraction measurements reveal a set of six discrete orientation states ("domains") characterized by a perpendicular alignment of the molecules with respect to the long axis of the pores and by a fourfold symmetry about this direction, which coincides with the crystalline symmetry of the Si host. A Bragg peak series characteristic of the formation of bilayers indicates a lamellar structure of the spatially confined alcohol crystals in 15 nm pores. By contrast, no layering reflections could be detected for 10 nm pores. The growth mechanism responsible for the peculiar orientation states is attributed to a nanoscale version of the Bridgman technique of single-crystal growth, where the dominant growth direction is aligned parallel to the long pore axes. Our observations are analogous to the growth phenomenology encountered for medium length n -alkanes confined in mesoporous silicon [A. Henschel, T. Hofmann, P. Huber, and K. Knorr, Phys. Rev. E 75, 021607 (2007)] and may further elucidate why porous silicon matrices act as an effective nucleation-inducing material for protein solution crystallization.