Microdynamics and phase equilibria in organic nanocrystals.

The effects of confinement on various organic molecules have been studied by nuclear magnetic resonance (NMR) relaxation techniques (T2 and T1 rho) between room temperature and 77 K. Cyclohexane, pentadecane, squalane, and squalene have been constrained within 60 A porous silica, and the behaviour of the resulting nanocrystals has been compared to that of the bulk material. In all cases the molecular dynamics of the confined material were found to be significantly different from the bulk, and there is evidence to suggest that the behaviour is largely dependent on the size and shape of the molecules and the resultant structural disorder present when constrained within the pores.

Diffusion processes in confined materials.

The diffusion coefficient for cyclohexane confined within pores of diameter 40 to 500 A has been measured as a function of temperature between 296 and 180 K, and is compared to values obtained for the bulk material. A substantial liquid-like signal is observed in the region of the depressed freezing points and a diffusion coefficient is measurable in all samples to well below these temperatures. The diffusion data appear to be continuous over the freezing region. These observations suggest persisting molten layers at interfaces which exchange with crystals forming within the silica pores. The diffusion coefficient of the molecules in the surface layer is three orders of magnitude larger than in the plastic phase of bulk cyclohexane.