Probing the Diffusion Mechanism of n-Alkanes in Mesoporous Confinement Using Pulsed Field Gradient NMR

Abstract

Pulsed field gradient NMR experiments are used to probe the diffusion mechanism of liquid n-alkanes, with carbon numbers in the range of 8−40, confined in five mesoporous silicas of average pore diameters 6.9, 13.9, 19.9, 25.0, and 38.7 nm. A novel method, based on the Cohen−Turnbull−Bueche model of diffusion, employs mixtures instead of pure liquids to achieve separation of free volume and diffusion mechanism effects on diffusion coefficients. Changes in the diffusion mechanism as a function of confinement, defined as the ratio of average pore diameter to average molecular size (C f ), are observed. The Zimm mechanism is identified in bulk liquids, while for all C f values less than 9, diffusion occurs by the reptation mechanism. Between C f values of 9 and 34, the data are consistent with the Rouse mechanism for chain lengths in the range of 8−16 and the coexistence of both Rouse and reptation mechanisms for longer chain lengths of 21−40. The critical entanglement chain length, where the dominant mechanism changes from Rouse to reptation in nalkanes, is shown to significantly reduce in confined liquids relative to bulk. The critical entanglement chain length reduces by a factor of at least 12 in pore diameters of 6.9 nm.

Advances in Biocarbon and Soft Material Assembly for Enthalpy Storage: Fundamentals, Mechanisms, and Multimodal Applications

“…Typically, soft materials under strong intermolecular interaction with supporting materials could change their molecular packing and/or segment dynamics compared with the performance observed in bulk material. [87][88][89] Such confinement effect can control the intermolecular motion and surface properties of soft material that significantly influence the intrinsic enthalpy storage mechanism and stability. For example, Atinafu et al [90] proved that standard biochar (obtained from oilseed rape feedstock) assembled with 1-dodecanol (a phase change material) showed high intermolecular interaction and thus exhibited lower enthalpy storage (73.6 kJ kg −1 ) than that of biochar with n-dodecane (90.5 kJ kg −1 ).…”