Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa, Ontario K1A 0R6, Canada.
School of Chemical, Petroleum, and Gas Engineering, Semnan University, Semnan, Iran.
J Chem Phys. 2019 Mar 21;150(11):114703. doi: 10.1063/1.5078739.
The presence of small hydrocarbons is known to reduce the interfacial tension of the gas-water interface, and this phenomenon can affect the formation of the clathrate hydrates of these gases. In this work, the interfacial behavior of the pure methane-, ethane-, and propane-water, and the ternary 90:7:3 mol. % gas mixture of (methane + ethane + propane)-water were studied with molecular dynamics simulations. The interfacial tension, γ, and z-density profiles for the gases and water from simulations of the gas-water systems were determined at the temperatures of 275.15 and 298.15 K, and pressures up to 10 MPa for methane and up to near the experimental saturation pressures of ethane and propane. The goal is to accurately calculate the interfacial tension for the hydrocarbon/water systems and to analyze the molecular behaviors at the interfaces which lead to the observed trends. At the same hydrostatic gas phase pressure, propane, ethane, and methane reduce the gas-water interfacial tension in that order. The local density of the gas molecules at the interface is enhanced relative to the bulk gas, and it was determined that about 13%-20%, 33%-40%, and 54%-59% of the gas molecules in the simulation congregated at the interfaces for the CH-, CH-, and CH-water systems, respectively, at the different simulated hydrostatic pressure ranges. For all gases in the pressure range studied, a complete monolayer of gas had not formed at the water interface. Furthermore, a dynamic equilibrium with fast exchange between molecules at the interface and in the gas phase was observed. For the gas mixture, deviations were observed between total calculated interfacial tension, γ, and the "ideal mixture" value, ∑xγ, calculated from the interfacial tensions of the pure gases, where x is the mole fraction of each substance in the simulation. Some possible implications of the results on the mechanism of clathrate hydrate formation are discussed.
已知小烃类的存在会降低气-水界面的界面张力，这种现象会影响这些气体的包合物水合物的形成。在这项工作中，我们使用分子动力学模拟研究了纯甲烷-、乙烷-、丙烷-水以及（甲烷+乙烷+丙烷）-水的三元 90:7:3mol%混合物气体的界面行为。在 275.15 和 298.15 K 的温度以及甲烷高达 10 MPa、乙烷和丙烷接近实验饱和压力的条件下，从气-水体系的模拟中确定了气体和水的界面张力 γ 和 z 密度分布。我们的目标是准确计算烃类/水体系的界面张力，并分析导致观察到的趋势的界面分子行为。在相同的静水气相压力下，丙烷、乙烷和甲烷按此顺序降低气-水界面张力。与气相相比，界面处的气体分子局部密度增强，并且确定在不同模拟静水压力范围内，CH-、CH-和 CH-水体系中分别有 13%-20%、33%-40%和 54%-59%的气体分子聚集在界面处。在所研究的压力范围内，对于所有气体，在水界面处尚未形成完整的单层气体。此外，观察到分子在界面和气相之间的快速交换达到了动态平衡。对于混合气体，观察到总计算界面张力 γ 和从纯气体界面张力计算得出的“理想混合物”值∑xγ之间存在偏差，其中 x 是模拟中每种物质的摩尔分数。讨论了结果对包合物水合物形成机制的一些可能影响。
