Quantification of Ostwald Ripening in Emulsions via Coarse-Grained Simulations
Quantification of Ostwald Ripening in Emulsions via Coarse-Grained Simulations
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DOI：
10.1021/acs.jctc.9b00296
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发表时间：
2019-09-01
影响因子：
5.5
通讯作者：
Striolo, Alberto
中科院分区：
文献类型：
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作者：
Khedr, Abeer;Striolo, Alberto
奥斯特瓦尔德熟化是多分散乳液中发生的扩散传质过程，通常会威胁乳液稳定性。在这项工作中，我们设计了一个模拟协议，能够在分子水平上量化奥斯特瓦尔德成熟过程。为了实现实验相关的时间尺度，实施了耗散粒子动力学（DPD）模拟协议。模拟参数被调整为代表分散在水中的两个苯滴。通过引入膜来防止两个液滴之间的聚结，膜允许苯从一个液滴扩散到另一个液滴。模拟结果根据液滴体积随时间的变化进行量化。结果与实验定性一致。当考虑苯在水中的模拟溶解度和扩散系数时，与 Lifshitz-Slyozov-Wagner 理论的一致性变得定量。还研究了两种不同表面活性剂的效果。与实验观察和理论一致，由于苯和水之间的界面张力降低，中等浓度的表面活性剂的添加降低了奥斯特瓦尔德熟化速率。随着表面活性剂膜变得致密，其他现象可能会进一步延迟奥斯特瓦尔德成熟。事实上，结果表明，在苯-水界面产生更高密度的表面活性剂可以更有效地延迟奥斯特瓦尔德熟化。胶束的形成也会影响成熟率，与实验定性一致，尽管我们的模拟对这种影响并没有结论。我们的模拟表明，粗粒度 DPD 形式能够捕获与奥斯特瓦尔德成熟相关的分子现象，并揭示有助于理解实验观察结果的分子水平特征。结果可用于预测并最终控制乳液的长期稳定性。
Ostwald ripening is a diffusional mass transfer process that occurs in polydisperse emulsions, often with the result of threatening the emulsion stability. In this work, we design a simulation protocol that is capable of quantifying the process of Ostwald ripening at the molecular level. To achieve experimentally relevant time scales, the dissipative particle dynamics (DPD) simulation protocol is implemented. The simulation parameters are tuned to represent two benzene droplets dispersed in water. The coalescence between the two droplets is prevented via the introduction of membranes, which allow diffusion of benzene from one droplet to the other. The simulation results are quantified in terms of the changes in the droplet volume as a function of time. The results are in qualitative agreement with experiments. The agreement with the Lifshitz-Slyozov-Wagner theory becomes quantitative when the simulated solubility and diffusion coefficient of benzene-in-water are considered. The effect of two different surfactants was also investigated. In agreement with both experimental observations and theory, the addition of surfactants at moderate concentrations decreased the Ostwald ripening rate because of the reduction in the interfacial tension between benzene and water; as the surfactant film becomes dense, other phenomena are likely to further delay the Ostwald ripening. In fact, the results suggest that the surfactant that yields higher density at the benzene-water interface delayed more effectively Ostwald ripening. The formation of micelles can also affect the ripening rate, in qualitative agreement with experiments, although our simulations are not conclusive on such effects. Our simulations show that the coarse-grained DPD formalism is able to capture the molecular phenomena related to Ostwald ripening and reveal molecular level features that could help to understand experimental observations. The results could be useful for predicting and eventually controlling the long-term stability of emulsions.