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Molecular Theory and Simulation of Water-Oil Contacts

机译：水-油接触的分子理论与模拟

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摘要

The statistical mechanical theory of hydrophobic interactions was initiated decades ago for purely repulsive hydrophobic species, in fact, originally for hard-sphere solutes in liquid water. Systems which treat only repulsive solute-water interactions obviously differ from the real world situation. The issue of the changes to be expected from inclusion of realistic attractive solute-water interactions has been of specific interest also for decades. We consider the local molecular field (LMF) theory for the effects of solute attractive forces on hydrophobic interactions. The principal result of LMF theory is outlined, then tested by obtaining radial distribution functions (rdfs) for Ar atoms in water, with and without attractive interactions distinguished by the Weeks-Chandler-Andersen (WCA) separation. Change from purely repulsive atomic solute interactions to include realistic attractive interactions substantially diminishes the strength of hydrophobic bonds. Since attractions make a big contribution to hydrophobic interactions, Pratt-Chandler theory, which did not include attractions, should not be naively compared to computer simulation results with general physical interactions, including attractions. Lack of general appreciation of this point has lead to mistaken comparisons throughout the history of this subject. The rdfs permit evaluation of osmotic second virial coefficients B2. Those B 2 are consistent with the conclusion that incorporation of attractive interactions leads to more positive (repulsive) values. In all cases here, B2 becomes more attractive with increasing temperature below T = 360K, the so-call inverse temperature behavior.;In 2010, the Gulf of Mexico Macondo well (Deepwater Horizon) oil spill focused the attention of the world on water-oil phase equilibrium. In response to the disaster, chemical dispersants were applied to break oil slicks into droplets and thus to avoid large-scale fouling of beaches and to speed up biodegradation. Eventually the dispersant COREXIT 9500 was used predominantly in responding to this accident. The formulation of COREXIT dispersants is somewhat complicated and the various constituents (and their interactions) deserve exhaustive study. Here we focus on sorbitan monooleate (SPAN80), one important component of COREXIT 9500, and we investigate its behavior in oil-water-surfactant systems. Extensive all-atom molecular dynamics calculations on the water-squalane interface for nine different loadings with SPAN80, at T = 300K, are analyzed for the surface tension equation of state, desorption free energy profiles as they depend on loading, and to evaluate escape times for absorbed SPAN80 into the bulk phases. These results suggest that loading only weakly affects accommodation of a SPAN80 molecule by this squalane-water interface. Specifically, the surface tension equation of state is simple from conditions of low loading (high tension) to high loading (lower tension) studied, and the desorption free energy profiles are weakly dependent on loading here. The perpendicular motion of the centroid of the SPAN80 head-group ring is well-described by a diffusional model near the minimum of the desorption free energy profile. Lateral diffusional motion is weakly dependent on loading. Escape times evaluated on the basis of a diffusional model and the desorption free energies are 0.07~s (into the squalane) and 300~h (into the water). The latter value is consistent with irreversible absorption observed by related experimental work.
机译：疏水相互作用的统计力学理论是几十年前针对纯斥斥性疏水物质提出的，实际上最初是针对液态水中的硬球溶质。仅处理排斥性溶质-水相互作用的系统显然与现实世界不同。几十年来，包含现实的有吸引力的溶质-水相互作用所预期的变化问题也引起了人们的特别关注。我们考虑溶质吸引力对疏水相互作用的影响的局部分子场（LMF）理论。概述了LMF理论的主要结果，然后通过获得水中Ar原子的径向分布函数（rdfs）（具有和不具有Weeks-Chandler-Andersen（WCA）分离）的有吸引力的相互作用来进行测试。从纯排斥性原子溶质相互作用到包括现实的吸引相互作用的变化，实质上降低了疏水键的强度。由于引力极大地促进了疏水相互作用，因此不应将不包括引力的Pratt-Chandler理论与具有引力的一般物理相互作用的计算机模拟结果相比较。缺乏对这一点的普遍认识导致整个学科历史上的错误比较。 rdfs允许评估渗透第二维里系数B2。那些B 2与结论是一致的，即有吸引力的相互作用的结合会导致更正的（排斥）值。在这里的所有情况下，B2随着温度升高到低于T = 360K（即所谓的逆温度行为）而变得更具吸引力。； 2010年，墨西哥湾Macondo井（深水地平线）漏油事件将世界的注意力集中在水上-油相平衡。为了应对灾难，使用了化学分散剂将浮油分解成小滴，从而避免了海滩的大规模结垢并加速了生物降解。最终，主要使用COREXIT 9500分散剂来应对这一事故。 COREXIT分散剂的配方有些复杂，各种成分（及其相互作用）值得详尽研究。在这里，我们重点介绍山梨聚糖单油酸酯（SPAN80），它是COREXIT 9500的重要组成部分，并研究了其在油水表面活性剂系统中的行为。在水=角鲨烷界面上，对于T = 300K的九种不同载荷，在SPAN-80上进行了全面的全原子分子动力学计算，分析了表面张力状态方程，取决于载荷的解吸自由能曲线，并评估了逸出时间用于将SPAN80吸收到本体相中。这些结果表明，通过该角鲨烷-水界面，负载仅对SPAN80分子的调节产生微弱的影响。具体地，从研究的低负荷（高张力）到高负荷（低张力）的条件，表面张力状态方程很简单，并且解吸自由能分布在此弱依赖于负荷。 SPAN80头环的质心的垂直运动通过解吸自由能分布的最小值附近的扩散模型很好地描述了。横向扩散运动几乎不依赖于载荷。在扩散模型和脱附自由能的基础上估算的逸出时间为0.07〜s（进入角鲨烷）和300〜h（进入水中）。后者值与相关实验工作观察到的不可逆吸收一致。

著录项

  • 作者

    Tan, Liang.;

  • 作者单位

    Tulane University School of Science and Engineering.;

  • 授予单位 Tulane University School of Science and Engineering.;
  • 学科 Chemical engineering.;Computational physics.;Statistical physics.
  • 学位 Ph.D.
  • 年度 2017
  • 页码 129 p.
  • 总页数 129
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 物理化学（理论化学）、化学物理学;
  • 关键词

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