1H NMR relaxation and diffusion studies of cyclohexane and cyclopentane confined in MCM-41

The rotational and translational dynamics of cyclohexane and cyclopentane confined in a MCM-41 solid have been studied as a function of temperature by measuring the T-1 and T-2 relaxation times and diffusivities, and the results are compared with values obtained for the bulk substances. Filling degrees of 60%, 80% and 104% were used when investigating the confined samples. The T, behaviour is remarkably similar for confined cyclohexane and cyclopentane although we compare compounds exhibiting different bulk phases at corresponding temperatures. T-1 and T-2 increase clearly with increasing pore filling, presumable as a result of a reduced contribution from surface relaxation. The self-diffusion rate of confined cyclopentane, however, decreases markedly with increasing pore filling in contrast to the situation for cyclohexane. In the former case, however, the diffusion experiment detects the diffusivity of all the liquid in the pores, while in the latter case only the diffusivity of the surface layer is monitored below the depressed melting point. The thickness of the surface layer inside the pores is approximately one molecular layer for both adsorbates, i.e.. 0.65 and 0.52 nm for cyclohexane and cyclopentane, respectively. (C) 2004 Elsevier Inc. All rights reserved.