Do D D, Do H D, Nicholson D
Department of Chemical Engineering, University of Queensland, St. Lucia, Qld 4072, Australia.
J Phys Chem B. 2009 Jan 29;113(4):1030-40. doi: 10.1021/jp8046467.
We present a new approach to calculating excess isotherm and differential enthalpy of adsorption on surfaces or in confined spaces by the Monte Carlo molecular simulation method. The approach is very general and, most importantly, is unambiguous in its application to any configuration of solid structure (crystalline, graphite layer or disordered porous glass), to any type of fluid (simple or complex molecule), and to any operating conditions (subcritical or supercritical). The behavior of the adsorbed phase is studied using the partial molar energy of the simulation box. However, to characterize adsorption for comparison with experimental data, the isotherm is best described by the excess amount, and the enthalpy of adsorption is defined as the change in the total enthalpy of the simulation box with the change in the excess amount, keeping the total number (gas + adsorbed phases) constant. The excess quantities (capacity and energy) require a choice of a reference gaseous phase, which is defined as the adsorptive gas phase occupying the accessible volume and having a density equal to the bulk gas density. The accessible volume is defined as the mean volume space accessible to the center of mass of the adsorbate under consideration. With this choice, the excess isotherm passes through a maximum but always remains positive. This is in stark contrast to the literature where helium void volume is used (which is always greater than the accessible volume) and the resulting excess can be negative. Our definition of enthalpy change is equivalent to the difference between the partial molar enthalpy of the gas phase and the partial molar enthalpy of the adsorbed phase. There is no need to assume ideal gas or negligible molar volume of the adsorbed phase as is traditionally done in the literature. We illustrate this new approach with adsorption of argon, nitrogen, and carbon dioxide under subcritical and supercritical conditions.
我们提出了一种通过蒙特卡罗分子模拟方法计算表面或受限空间内吸附的过量等温线和微分吸附焓的新方法。该方法非常通用，最重要的是，它在应用于任何固体结构构型（晶体、石墨层或无序多孔玻璃）、任何类型的流体（简单或复杂分子）以及任何操作条件（亚临界或超临界）时都是明确无误的。使用模拟盒的偏摩尔能量来研究吸附相的行为。然而，为了表征吸附以便与实验数据进行比较，等温线最好用过量量来描述，吸附焓定义为模拟盒总焓随过量量的变化而变化，同时保持总数（气相 + 吸附相）不变。过量量（容量和能量）需要选择一个参考气相，它被定义为占据可及体积且密度等于体相气体密度的吸附性气相。可及体积被定义为所考虑的吸附质质心可及的平均体积空间。通过这种选择，过量等温线会经过一个最大值但始终保持为正。这与使用氦空穴体积（其总是大于可及体积）的文献形成鲜明对比，在文献中得到的过量量可能为负。我们对焓变的定义等同于气相偏摩尔焓与吸附相偏摩尔焓之间的差值。无需像文献中传统做法那样假设理想气体或吸附相的摩尔体积可忽略不计。我们用亚临界和超临界条件下氩、氮和二氧化碳的吸附来说明这种新方法。
