Diffusion-based evidence for hyperskin in CMS membranes

Carbon molecular sieve (CMS) membranes are emerging as high-performance materials for molecular separations. For CMS membranes the existence of a dense surface layer—termed the “hyperskin”—has been postulated in the literature. In this study, we provide direct, transport-based evidence for hyperskin existence and quantify hyperskin permeance in CMS membranes. This was achieved by comparing microscopic self-diffusion coefficients from pulsed field gradient (PFG) NMR with macroscopic corrected diffusivities from permeation and vapor sorption. The study was performed for methanol, p-xylene, and o-xylene in flat-sheet CMS membranes formed via pyrolysis of crosslinked poly(vinylidene fluoride)(PVDF) films. While PFG NMR was used to measure self-diffusion coefficients in the membrane bulk, isolated from surface effects, the diffusion data from permeation included all contributions including the surface resistance. In all cases, self-diffusivities from PFG NMR exceeded the corresponding corrected diffusivities from permeation by over an order of magnitude, clearly indicating the presence of a surface transport barrier. Quantitative analysis revealed that hyperskin permeance decreases systematically with molecular size—from 8.6 × 10 -8 m/s for methanol (3.6 Å) to 1.3 × 10 -8 m/s for o-xylene (6.5 Å). Our findings demonstrate that thin surface layers can dominate overall membrane resistance, and must be explicitly considered in modeling and performance optimization. This work introduces a new experimental framework to quantify surface transport resistances in CMS membranes and informs strategies for membrane design and processing. • Sorbate diffusion in CMS was quantified using permeation and PFG NMR. • Comparison of permeation and PFG NMR data yielded hyperskin permeance. • Hyperskin permeance decreased with increasing sorbate molecular size • Hyperskin determined permeation rates for the studied sorbate-CMS systems.