Relaxation and diffusion studies of cyclohexane confined in MCM-41 by NMR

In this work the rotational and translational dynamics of cyclohexane confined in MCM-41 have been studied as a function of temperature by measuring intra-crystalline diffusivities and T-1 and T-2 relaxation times. The results are compared with values obtained for bulk cyclohexane and cyclohexane confined in 40 Angstrom silica. T-1 was measured for three different samples, with three different filling grades of cyclohexane. The T-1 curves are continuous over the studied temperature region, and the slope is practically unchanged. However, a marked increase in T-1 with the degree of pore filling is seen owing to an increasing contribution from the bulk-like molecules at the interior of the pores. Only one surface component, attributed to the molecules at the pore walls, was observed when measuring the T-2 relaxation. The relatively narrow spin-echo signal and the long T-2 indicate that the liquid adsorbed on the pore surface does not freeze at all, even at temperatures far below the plastic-brittle solid transition point of the bulk material. The true intracrystalline diffusivity was obtained by using the short diffusion time model and extrapolating to zero observation time. A high diffusion rate was observable over a wide temperature region, and the measured diffusivity is about three orders of magnitude larger than in the plastic phase of bulk cyclohexane. (C) 2001 Elsevier Science B.V. All rights reserved.