Organic nanocrystals: an NMR study of cyclohexane in porous silica

The formation of cyclohexane nanocrystals in porous silica (60 Angstrom, 200 Angstrom and 500 Angstrom nominal pore sizes) of well characterized geometry has been studied using T-2 and T-1 rho nuclear magnetic resonance (NMR) measurements over a temperature range from 295 K to 77 K. The confined solid was found to behave in a substantially different way from the bulk material, and exhibited pore size dependency. Below the melting point, multi-component relaxation times indicated a mixed phase system, with evidence for the coexistence of an ordered nanocrystal and a highly disordered region. The disordered region is of considerable interest since it is highly mobile and preliminary measurements have indicated anomalously fast diffusion. Measurements also demonstrated enhanced anisotropic reorientational motion, the suppression of the formation of the rigid lattice and the loss of the solid-solid phase transition for smaller pores.