| Autor: |
Siderius DW; Chemical Sciences Division, National Institute of Standards and Technology , 100 Bureau Drive M.S. 8320, Gaithersburg, Maryland 20899, United States., Krekelberg WP; Chemical Sciences Division, National Institute of Standards and Technology , 100 Bureau Drive M.S. 8320, Gaithersburg, Maryland 20899, United States., Chiang WS; NIST Center for Neutron Research, National Institute of Standards and Technology , 100 Bureau Drive M.S. 6102, Gaithersburg, Maryland 20899, United States.; Department of Chemical and Biomolecular Engineering, University of Delaware , 150 Academy Street, Colburn Laboratory, Newark, Delaware 19716, United States., Shen VK; Chemical Sciences Division, National Institute of Standards and Technology , 100 Bureau Drive M.S. 8320, Gaithersburg, Maryland 20899, United States., Liu Y; NIST Center for Neutron Research, National Institute of Standards and Technology , 100 Bureau Drive M.S. 6102, Gaithersburg, Maryland 20899, United States. |
| Abstrakt: |
Using Monte Carlo and molecular dynamics simulations, we examine the adsorption of methane in cylindrical silica mesopores in an effort to understand a possible phase transition of adsorbed methane in MCM-41 and SBA-15 silica that was previously identified by an unexpected increase in the adsorbed fluid density following capillary condensation, as measured by small-angle neutron scattering (SANS) [Chiang, W-S., et al., Langmuir 2016, 32, 8849]. Our initial simulation results identify a roughly 10 % increase in the density of the liquidlike adsorbed phase for either an isotherm with increasing pressure or an isobar with decreasing temperature and that this densification is associated with a local maximum in the isosteric enthalpy of adsorption. Subsequent analysis of the simulated fluid, via computation of bond-orientational order parameters of specific annular layers of the adsorbed fluid, showed that the layers undergo an ordering transition from a disordered, amorphous state to one with two-dimensional hexagonal structure. Furthermore, this two-dimensional restructuring of the fluid occurs at the same thermodynamic state points as the aforementioned densification and local maximum in the isosteric enthalpy of adsorption. We thus conclude that the densification of the fluid is the result of structural reorganization, which is signaled by the maximum in the isosteric enthalpy. Owing to the qualitative similarity of the structural transitions in the simulated and experimental methane fluids, we propose this hexagonal reorganization as a plausible explanation of the densification observed in SANS measurements. Lastly, we speculate how this structural transition may impact the transport properties of the adsorbed fluid. |
