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

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

Automated summary

Pulsed field gradient NMR experiments were conducted to investigate the diffusion mechanism of liquid n-alkanes confined in mesoporous silicas of different pore diameters. A novel method using mixtures instead of pure liquids was employed to separate the effects of free volume and diffusion mechanism on diffusion coefficients. The diffusion mechanism was found to change with confinement, with Zimm mechanism in bulk liquids, reptation mechanism for C-f < 9, and coexistence of Rouse and reptation mechanisms for C-f between 9 and 34. The critical entanglement chain length significantly reduces in confined liquids compared to bulk.