[论文解读] Short-time Evolution of Alkane-in-Water Nanoemulsions

Germán Urbina-Villalba, Kareem Rahn-Chique|arXiv (Cornell University)|Mar 6, 2013

Spectroscopy and Quantum Chemical Studies参考文献 25被引用 6

一句话总结

本研究利用乳液稳定性模拟（ESS）探究了烷烃-水纳米乳液的短期稳定性，重点关注絮凝、聚结和奥斯特瓦尔德熟化等竞争过程。结果表明，初始液滴尺寸分布因快速絮凝和聚结而呈右偏态，但随时间推移，奥斯特瓦尔德熟化占主导地位，导致分布形态由右偏转为左偏，立方平均半径曲线呈下凹形态。十二烷和十六烷形成亚稳态体系，而辛烷则因熟化作用显著溶解。

ABSTRACT

The stability of alkane-in-water nanoemulsions during the sub-stationary regime is studied by means of Emulsion Stability Simulations (ESS). The effects of Ostwald ripening, flocculation, coalescence, gravity, and hydration forc- es are considered. According to these calculations flocculation and coalescence are predominant during the first few seconds after the preparation of the emulsion. This favors the generation of a right-skewed Drop Size Distribu- tion (DSD). As the system evolves, the drops grow larger and more repulsive causing a slow down of the flocculation process. In the case of dodecane (C12) and hexadecane (C16) the referred phenomena, reinforce the ripening trend to subvert the initial DSD variation, producing a meta-stable distribution which is preserved during several minutes. After this time, Ostwald ripening dominates: the skirt of the distribution changes progressively from right-skewed to left-skewed. Consistent with these changes, the cube average radius of the emulsion increases rapidly at first, but progressively diminishes generating a concave-downward curve that stabilizes asymptotically. In the case of dodecane and hexadecane the complete dissolution of the drops promoted by ripening is prevented at all times due to coalescence. In the case of octane (C8) a substantial amount of drops is lost by dissolution, forbidding the attain- ment of a stable DSD. In all cases the molecular exchange only favors a decrease of the average radius as a function of time. It is the elimination of drops either by dissolution or coalescence which causes an increase of the average radius of the emulsion.

研究动机与目标

• 理解烷烃-水纳米乳液在亚稳态阶段的短期演化行为。
• 量化奥斯特瓦尔德熟化、絮凝、聚结、重力及水化力在决定乳液稳定性中的相对作用。
• 确定初始液滴尺寸分布的演化方式，以及导致亚稳态或溶解体系的因素。
• 评估分子交换与液滴消除对平均半径趋势的影响。

提出的方法

• 采用乳液稳定性模拟（ESS）对纳米乳液的时间演化进行建模。
• 模拟框架整合了奥斯特瓦尔德熟化、絮凝、聚结、重力及水化力。
• 追踪液滴尺寸分布（DSD）随时间的变化，以分析偏度与演化模式。
• 计算立方平均半径以监测液滴平均尺寸的变化。
• 模型区分了有无聚结的体系，以评估聚结在防止完全溶解中的作用。
• 对十二烷（C12）、十六烷（C16）和辛烷（C8）进行模拟，以比较链长效应。

实验结果

研究问题

• RQ1 絮凝与聚结如何影响烷烃-水纳米乳液的初始液滴尺寸分布？
• RQ2 液滴尺寸分布的随时间演化特征如何？其偏度如何随时间变化？
• RQ3 在初始阶段之后，奥斯特瓦尔德熟化在多大程度上主导系统行为？它如何改变分布形态？
• RQ4 为何十二烷和十六烷形成亚稳态分布，而辛烷则不能？
• RQ5 分子交换与液滴消除过程如何随时间影响平均半径？

主要发现

• 在最初几秒内，絮凝与聚结占主导地位，导致液滴尺寸分布（DSD）呈右偏态。
• 随着液滴变大且排斥力增强，絮凝速率减慢，促使DSD形态发生转变。
• 对于十二烷和十六烷，DSD稳定为一种持续数分钟的亚稳态右偏态分布，随后熟化过程占主导。
• 此后，奥斯特瓦尔德熟化使DSD由右偏态转为左偏态，表明发生粗化过程。
• 立方平均半径最初迅速增加，随后下降，形成下凹曲线并渐近趋于稳定。
• 在辛烷（C8）中，因熟化作用导致液滴显著溶解，无法形成稳定DSD；而C12和C16中的聚结作用可防止完全溶解。