Molecular Dynamics Investigation of the Various Atomic Force Contributions to the Interfacial Tension at the Supercritical CO2鈥揥ater Interface

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• 作者：Lingling Zhao ; Shangchao Lin ; Jonathan D. Mendenhall ; Pak K. Yuet ; Daniel Blankschtein
• 刊名：Journal of Physical Chemistry B
• 出版年：2011
• 出版时间：May 19, 2011
• 年：2011
• 卷：115
• 期：19
• 页码：6076-6087
• 全文大小：1125K
• 年卷期：v.115,no.19(May 19, 2011)
• ISSN：1520-5207

文摘

Sequestration of carbon dioxide (CO₂) in deep, geological formations involves the injection of supercritical CO₂ into depleted reservoirs containing fluids such as brine or oil. The interfacial tension (IFT) between supercritical CO₂ and the reservoir fluid is an important contribution to the sequestration efficiency. In turn, the IFT is a complex function of the reservoir fluid phase composition, the molecular structure of each reservoir fluid component, and environmental conditions (i.e., temperature and pressure). Molecular dynamics simulations can be used to probe the dependence of the IFT on these factors, since the IFT can be calculated directly from the simulated atomic forces and velocities at system equilibrium using the mechanical definition of the IFT. Here, we examine the contribution of each type of atomic force to the IFT, including bonded and nonbonded forces, as quantified by the anisotropy of the atomic virial tensor. In particular, we first examine a supercritical CO₂鈥損ure liquid water interface, at typical reservoir conditions (temperature of 343 K and pressure of 20 MPa), as a reference state against which CO₂鈥揵rine systems can be compared. In this system, we note that the interactions between water molecules and between CO₂ molecules (鈥渟elf鈥?interactions) contribute positively to the IFT, while the interactions between water and CO₂ molecules (鈥渃ross鈥?interactions) contribute negatively to the IFT. We find that the magnitude of the water 鈥渟elf鈥?interactions is the dominant contribution. In terms of specific types of forces, we find that nonbonded electrostatic (QQ), bonded angle-bending, and bonded bond-stretching interactions contribute positively to the IFT, while nonbonded Lennard-Jones (LJ) interactions contribute negatively to the IFT. We also find that the balance between the LJ interactions and the bond-stretching interactions, in particular, plays a significant role in determining the magnitude of the IFT. Using orientational probability distribution functions to study molecular ordering about the interface, we find that the CO₂ molecules prefer to lie parallel to the interface at the Gibbs dividing surface (GDS) and that both the CO₂ and the water molecules are more ordered at the GDS than in the bulk. Finally, we present an initial study of a CO₂鈥揵rine system with CaCl₂ as the model salt at a concentration of 2.7 M. We quantify the effect of the salt on the molecular orientation of water, and show that this effect leads to an increase in the IFT, relative to the CO₂鈥搘ater system, which is consistent with experimental measurements.