The Clathrate-Water Interface Is Oleophilic
The Clathrate-Water Interface Is Oleophilic
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DOI：
10.1021/acs.jpclett.8b01210
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发表时间：
2018-06-21
影响因子：
5.7
通讯作者：
Molinero, Valeria
中科院分区：
文献类型：
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作者：
Bertolazzo, Andressa A.;Naullage, Pavithra M.;Molinero, Valeria
包合物水合物成核缓慢是其在天然气储存和运输中的主要挑战。强吸附于包合物水界面的分子降低了晶体-水表面张力，降低了包合物成核的屏障。表面活性剂被广泛用于促进包合物水合物的成核和生长。有人提出，这些两亲分子通过氢键结合到笼形物表面。然而，最近的研究表明，PVCap是一种两亲性聚合物，通过疏水基团与笼形物结合。本文采用分子动力学模拟和理论研究了表面活性剂与包合物-水界面的结合模式和强度及其对成核速率的影响。我们发现表面活性剂完全通过它们的疏水尾部与笼形物水界面结合。由于烷基链的脱水熵，当它穿透水合物表面的空腔时，这种结合是很强的。烷基对笼形物表面的疏水吸引力也导致烷烃的强吸附。我们确定了表面活性剂结合的两种机制，作为它们在水合物表面密度的函数，我们解释为对应于实验中观察到的Langmuir吸附等温线的两个步骤。我们的研究结果表明，笼形物水界面的疏水性吸引是设计促进水合物成核的可溶性添加剂的关键。我们使用计算的吸附系数来估计达到与实验结果一致的甲烷水合物成核速率所需的十二烷基硫酸钠（SDS）浓度。据我们所知，本研究首次量化了表面活性剂浓度对包合物水合物成核速率的影响。
The slow nucleation of clathrate hydrates is a central challenge for their use in the storage and transportation of natural gas. Molecules that strongly adsorb to the clathrate water interface decrease the crystal-water surface tension, lowering the barrier for clathrate nucleation. Surfactants are widely used to promote the nucleation and growth of clathrate hydrates. It has been proposed that these amphiphilic molecules bind to the clathrate surface via hydrogen bonding. However, recent studies reveal that PVCap, an amphiphilic polymer, binds to clathrates through hydrophobic moieties. Here we use molecular dynamic simulations and theory to investigate the mode and strength of binding of surfactants to the clathrate-water interface and their effect on the nucleation rate. We find that the surfactants bind to the clathrate water interface exclusively through their hydrophobic tails. The binding is strong, driven by the entropy of dehydration of the alkyl chain, as it penetrates empty cavities at the hydrate surface. The hydrophobic attraction of alkyl groups to the clathrate surface also results in strong adsorption of alkanes. We identify two regimes for the binding of surfactants as a function of their density at the hydrate surface, which we interpret to correspond to the two steps of the Langmuir adsorption isotherm observed in experiments. Our results indicate that hydrophobic attraction to the clathrate water interface is key for the design of soluble additives that promote the nucleation of hydrates. We use the calculated adsorption coefficients to estimate the concentration of sodium dodecyl sulfate (SDS) required to reach nucleation rates for methane hydrate consistent with those measured in experiments. To our knowledge, this study is the first to quantify the effect of surfactant concentration in the nucleation rate of clathrate hydrates.