Effects of confinement on critical adsorption: absence of critical depletion for fluids in slit pores.
Effects of confinement on critical adsorption: absence of critical depletion for fluids in slit pores.
复制标题
限制对临界吸附的影响：狭缝孔隙中的流体不存在临界消耗。
DOI：
10.1103/physreve.60.7105
复制
发表时间：
1999
期刊：
影响因子：
--
通讯作者：
N. Wilding
中科院分区：
文献类型：
--
作者：
A. Maciołek;R. Evans;N. Wilding
利用密度泛函理论和Lennard-Jones流体的蒙特卡罗模拟方法，研究了狭缝孔隙中近临界流体的吸附。我们的工作受到最近对SF6在介孔玻璃中吸附的实验的刺激，实验表明了显著的临界耗竭现象，即吸附过剩伽马首先增加，但随后迅速减小到负值，随着体相临界温度T(C)从上方沿着近临界等温线逐渐接近。相反，我们的密度泛函和模拟结果表明，对于一系列强吸引的壁流体势，伽玛随着温度沿临界(Rho=Rho(C))和亚临界等温线(Rho<；Rho(C))向T(C)降低而单调增加并最终饱和。这种行为是由于密度分布从墙壁到狭缝中间的衰减越来越慢所致，如T--&T(+)(C)。对于Rho<；Rho(C)，我们发现在流体中，有利于脱附的负体积场不足以支配有利于吸附的表面场的影响。我们将这种情况与具有恒定(负)体场的格子气体模型的早期结果进行了比较，发现了临界耗尽。在对中等和弱吸引壁面流体势的模拟中，发现了密度分布和吸附的性质不同的行为，但我们在实验中没有观察到临界耗尽。我们的结论是，后者不能用单孔模型来解释。
The adsorption of a near-critical fluid confined in a slit pore is investigated by means of density functional theory and by Monte Carlo simulation for a Lennard-Jones fluid. Our work was stimulated by recent experiments for SF6 adsorbed in a mesoporous glass, which showed the striking phenomenon of critical depletion, i.e., the adsorption excess Gamma first increases but then decreases very rapidly to negative values as the bulk critical temperature T(c) is approached from above along near-critical isochores. By contrast, our density functional and simulation results, for a range of strongly attractive wall-fluid potentials, show Gamma monotonically increasing and eventually saturating as the temperature is lowered toward T(c) along both the critical (rho=rho(c)) and subcritical isochores (rho<rho(c)). Such behavior results from the increasingly slow decay of the density profile away from the walls, into the middle of the slit, as T-->T(+)(c). For rho<rho(c) we find that in the fluid the effective bulk field, which is negative and which favors desorption, is insufficient to dominate the effects of the surface fields which favor adsorption. We compare this situation with earlier results for the lattice gas model with a constant (negative) bulk field where critical depletion was found. A qualitatively different behavior of the density profiles and adsorption is found in simulations for intermediate and weakly attractive wall-fluid potentials, but in no case do we observe the critical depletion found in experiments. We conclude that the latter cannot be accounted for by a single pore model.