Jitmitsumphan Sorrasit, Sripetdee Tirayoot, Chaimueangchuen Tharathep, Tun Htet Myet, Chinkanjanarot Sorayot, Klomkliang Nikom, Srinives Sira, Jonglertjunya Woranart, Ling Tau Chuan, Phadungbut Poomiwat
Nanocomposite Engineering Laboratory (NanoCEN), Department of Chemical Engineering, Faculty of Engineering, Mahidol University, Nakhon Pathom 73170, Thailand.
National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), Pathum Thani 12120, Thailand.
Molecules. 2022 Apr 20;27(9):2656. doi: 10.3390/molecules27092656.
At temperatures below the critical temperature, discontinuities in the isotherms are one critical issue in the design and construction of separation units, affecting the level of confidence for a prediction of vapor-liquid equilibriums and phase transitions. In this work, we study the molecular mechanisms of fluids that involve the vapor-liquid phase transition in bulk and confinement, utilizing grand canonical (GCE) and meso-canonical (MCE) ensembles of the Monte Carlo simulation. Different geometries of the mesopores, including slit, cylindrical, and spherical, were studied. During phase transitions, condensation/evaporation hysteretic isotherms can be detected by GCE simulation, whereas employing MCE simulation allows us to investigate van der Waals (vdW) loop with a vapor spinodal point, intermediate states, and a liquid spinodal point in the isotherms. Depending on the system, the size of the simulation box, and the MCE method, we are able to identify three distinct groups of vdW-type isotherms for the first time: (1) a smooth S-shaped loop, (2) a stepwise S-shaped loop, and (3) a stepwise S-shaped loop with just a vertical segment. The first isotherm type is noticed in the bulk and pores having small box sizes, in which vapor and liquid phases are close and not clearly identified. The second and the third types occurred in the bulk, cylindrical, and slit mesopores with sufficiently large spaces, where vapor and liquid phases are distinctly separated. Results from our studies provide an insight analysis into vapor-liquid phase transitions, elucidating the effect of the confinement of fluid behaviors in a visual manner.
在低于临界温度时，等温线的不连续性是分离装置设计和建造中的一个关键问题，会影响气液平衡和相变预测的可信度。在这项工作中，我们利用蒙特卡罗模拟的巨正则系综（GCE）和介观正则系综（MCE），研究了涉及体相和受限体系中气液相变的流体分子机制。研究了不同几何形状的中孔，包括狭缝形、圆柱形和球形。在相变过程中，通过GCE模拟可以检测到冷凝/蒸发滞后等温线，而采用MCE模拟则使我们能够研究等温线中具有气相旋节线点、中间态和液相旋节线点的范德华（vdW）回路。根据系统、模拟盒大小和MCE方法，我们首次能够识别出三种不同类型的vdW型等温线：（1）光滑的S形回路，（2）阶梯状S形回路，（3）仅带有垂直段的阶梯状S形回路。第一种等温线类型出现在体相和盒子尺寸较小的孔中，其中气相和液相接近且难以明确区分。第二和第三种类型出现在具有足够大空间的体相、圆柱形和狭缝形中孔中，其中气相和液相明显分离。我们的研究结果为气液相变提供了深入的分析，以直观的方式阐明了受限对流体行为的影响。
