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

Abstract

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 Å 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 intra-crystalline 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.