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

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 (Cf ), are observed. The Zimm mechanism is identified in bulk liquids, while for all Cf values less than 9, diffusion occurs by the reptation mechanism. Between Cf 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 n-alkanes, 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.