Tan Shiliang Johnathan, Loi Quang K, Do D D, Nicholson D
School of Chemical Engineering, University of Queensland, St. Lucia, Qld 4072, Australia.
School of Chemical Engineering, University of Queensland, St. Lucia, Qld 4072, Australia.
J Colloid Interface Sci. 2019 Jul 15;548:25-36. doi: 10.1016/j.jcis.2019.04.018. Epub 2019 Apr 5.
Kinetic Monte Carlo simulated isotherms calculated in the canonical ensemble, at temperatures below the critical temperature, for bulk fluid, surface adsorption and adsorption in a confined space, show a van der Waals (vdW) loop with a vertical phase transition between the rarefied and dense spinodal points at the co-existence chemical potential, µ. Microscopic examination of the state points on this loop reveals features that are common to these systems. At state points with chemical potentials greater than μ the microscopic configurations show clusters, which coalesce to form two co-existing phases along the vertical section of the loop (the coexistence line). As more molecules are added, the dense region expands at the expense of the rarefied region, to the point where the rarefied region becomes spherical (cylindrical for 2D-systems) with a curvature greater than that of the coexisting phases. This results in a decrease of chemical potential from µ to the liquid spinodal point where the rarefied region disappears. With a further increase in loading, the chemical potential and the density increase. The existence of a vdW loop is the microscopic reason for the hysteresis observed in the grand canonical isotherm, where the adsorption and desorption boundaries of the hysteresis loop are first-order transitions, enclosing the vertical section of the vdW loop of the canonical isotherm. However, a first-order transition is rarely observed in experiments where transitions are usually steep, but not vertical. From our extensive simulations, we provide two possible reasons: (1) the finite extent of the system and (2) the existence of high energy sites that localize the clusters. In the first case, the desorption branch, and in the second case the adsorption branch, either comes close to, or collapses onto the coexistence line. When both occur, the hysteresis loop disappears and the isotherm is reversible, as often observed experimentally.
在正则系综中，于低于临界温度的温度下，针对体相流体、表面吸附以及受限空间内的吸附情况，通过动力学蒙特卡罗模拟计算得到的等温线显示出范德瓦尔斯（vdW）环，在共存化学势μ处，稀薄和致密旋节线点之间存在垂直相变。对该环上状态点的微观检查揭示了这些系统共有的特征。在化学势大于μ的状态点，微观构型呈现出团簇，这些团簇沿着环的垂直部分（共存线）合并形成两个共存相。随着更多分子的加入，致密区域以稀薄区域为代价进行扩展，直至稀薄区域变为球形（二维系统中为圆柱形），其曲率大于共存相的曲率。这导致化学势从μ降低至稀薄区域消失的液体旋节线点。随着负载进一步增加，化学势和密度增大。vdW环的存在是在巨正则等温线中观察到滞后现象的微观原因，其中滞后环的吸附和解吸边界是一级相变，包围了正则等温线的vdW环的垂直部分。然而，在实验中很少观察到一级相变，实验中的相变通常很陡峭，但并非垂直。基于我们广泛的模拟，我们给出两个可能的原因：（1）系统的有限范围；（2）存在使团簇局部化的高能位点。在第一种情况下，解吸分支，在第二种情况下，吸附分支，要么接近共存线，要么与共存线重合。当两者都发生时，滞后环消失，等温线是可逆的，这在实验中经常观察到。
