Chemical Sciences Division, National Institute of Standards and Technology , 100 Bureau Drive M.S. 8320, Gaithersburg, Maryland 20899, United States.
NIST Center for Neutron Research, National Institute of Standards and Technology , 100 Bureau Drive M.S. 6102, Gaithersburg, Maryland 20899, United States.
Langmuir. 2017 Dec 19;33(50):14252-14262. doi: 10.1021/acs.langmuir.7b03406. Epub 2017 Dec 11.
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.
利用蒙特卡罗和分子动力学模拟，我们研究了甲烷在圆柱形二氧化硅介孔中的吸附，以试图理解在 MCM-41 和 SBA-15 二氧化硅中吸附甲烷的可能相变，这种相变先前通过小角中子散射（SANS）[Chiang，W-S.，等，Langmuir 2016，32，8849]测量到在毛细凝聚后吸附流体密度的意外增加而被识别。我们的初步模拟结果表明，对于压力增加的等温线或温度降低的等压线，液态吸附相的密度大致增加了 10%，并且这种致密化与吸附等焓的局部最大值有关。通过计算吸附流体特定环形层的键取向有序参数对模拟流体进行了后续分析，结果表明，这些层经历了从无序、无定形状态到具有二维六方结构的有序转变。此外，这种流体的二维重构发生在与吸附等焓的上述致密化和局部最大值相同的热力学状态点。因此，我们得出结论，流体的致密化是结构重组的结果，这由等焓的最大值来指示。由于模拟和实验甲烷流体中的结构转变具有定性相似性，我们提出这种六方重组作为 SANS 测量中观察到的致密化的合理解释。最后，我们推测这种结构转变如何影响吸附流体的输运性质。
