ORGANIC NANOCRYSTALS - AN NMR-STUDY OF CYCLOHEXANE IN POROUS SILICA

The formation of cyclohexane nanocrystals in porous silica (60 Angstro m, 200 Angstrom and 500 Angstrom nominal pore sizes) of well character ized 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 differen t way from the bulk material, and exhibited pore size dependency. Belo w the melting point, multi-component relaxation times indicated a mixe d phase system, with evidence for the coexistence of an ordered nanocr ystal and a highly disordered region. The disordered region is of cons iderable interest since it is highly mobile and preliminary measuremen ts have indicated anomalously fast diffusion. Measurements also demons trated enhanced anisotropic reorientational motion, the suppression of the formation of the rigid lattice and the loss of the solid-solid ph ase transition for smaller pores.