Crystal Dewetting at the Water–Guest Interface in Macropores Sidesteps the Hydrate Mass-Transfer Bottleneck

Abstract

Unprecedented massive growth of fibrous cyclopentane hydrate on an activated carbon was shown recently to circumvent the mass-transfer bottleneck caused by hydrate crust at guest−host interfaces. Here, we generalize to other carbons and to porous silicas, confirming hydrate formation in surface macropores. Experiments with hydrophilic or -phobic porous substrates and with single glass capillary model pores elucidate the growth process. Surprisingly, wettability is of indirect importance. Detailed observation of unlimited single fibers with the better optical access and well-defined geometry of the glass model pores reveals the growth mechanism: crystal dewetting at pinned guest−host interfaces, in surface macropores in the case of porous substrates, where hydrophobicity serves only indirectly, to conduct the guest to the active interface. Fiber growth is closely analogous to the dewetted Bridgman process observed decades ago in microgravity. "Micro" or negligible gravity is provided here by the small size of the pores compared to the capillary length. Transposing the theory to the present system, we correlate fiber diameter and dynamic contact angles on the fiber and the pore wall, which in turn depend on the growth rate, hence on the supercooling. The model prescribes for generalization to further combinations of guest and porous substrates.