Phan Anh, Bui Tai, Acosta Erick, Krishnamurthy Pushkala, Striolo Alberto
Department of Chemical Engineering, University College London, WC1 E7JE London, UK.
Phys Chem Chem Phys. 2016 Sep 28;18(36):24859-71. doi: 10.1039/c6cp03296f. Epub 2016 Jul 20.
Steered and equilibrium molecular dynamics simulations were employed to study the coalescence of a sI hydrate particle and a water droplet within a hydrocarbon mixture. The size of both the hydrate particle and the water droplet is comparable to that of the aqueous core in reverse micelles. The simulations were repeated in the presence of various quaternary ammonium chloride surfactants. We investigated the effects due to different groups on the quaternary head group (e.g. methyl vs. butyl groups), as well as different hydrophobic tail lengths (e.g. n-hexadecyl vs. n-dodecyl tails) on the surfactants' ability to prevent coalescence. Visual inspection of sequences of simulation snapshots indicates that when the water droplet is not covered by surfactants it is more likely to approach the hydrate particle, penetrate the protective surfactant film, reach the hydrate surface, and coalesce with the hydrate than when surfactants are present on both surfaces. Force-distance profiles obtained from steered molecular dynamics simulations and free energy profiles obtained from umbrella sampling suggest that surfactants with butyl tripods on the quaternary head group and hydrophobic tails with size similar to the solvent molecules can act as effective anti-agglomerants. These results qualitatively agree with macroscopic experimental observations. The simulation results provide additional insights, which could be useful in flow assurance applications: the butyl tripod provides adhesion between surfactants and hydrates; when the length of the surfactant tail is compatible with that of the hydrocarbon in the liquid phase a protective film can form on the hydrate; however, once a molecularly thin chain of water molecules forms through the anti-agglomerant film, connecting the water droplet and the hydrate, water flows to the hydrate and coalescence is inevitable.
采用导向分子动力学模拟和平衡分子动力学模拟研究了含烃混合物中sI水合物颗粒与水滴的聚并过程。水合物颗粒和水滴的尺寸与反胶束中水核的尺寸相当。在各种季铵盐表面活性剂存在的情况下重复进行模拟。我们研究了季铵头基上不同基团（如甲基与丁基）以及不同疏水尾长（如正十六烷基与正十二烷基尾）对表面活性剂防止聚并能力的影响。对模拟快照序列的目视检查表明，当水滴未被表面活性剂覆盖时，与两个表面都存在表面活性剂的情况相比，它更有可能接近水合物颗粒、穿透保护性表面活性剂膜、到达水合物表面并与水合物聚并。从导向分子动力学模拟获得的力-距离曲线和从伞形采样获得的自由能曲线表明，季铵头基上带有丁基三脚架且疏水尾尺寸与溶剂分子相似的表面活性剂可作为有效的抗团聚剂。这些结果在定性上与宏观实验观察结果一致。模拟结果提供了额外的见解，这在流动保障应用中可能是有用的：丁基三脚架提供了表面活性剂与水合物之间的粘附力；当表面活性剂尾的长度与液相中的烃的长度相匹配时，可以在水合物上形成保护膜；然而，一旦通过抗团聚剂膜形成一条分子级薄的水分子链，连接水滴和水合物，水就会流向水合物，聚并就不可避免。
