Mechanisms and Control of Self-Emulsification upon Freezing and Melting of Dispersed Alkane Drops.
Mechanisms and Control of Self-Emulsification upon Freezing and Melting of Dispersed Alkane Drops.
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DOI：
10.1021/acs.langmuir.7b02048
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发表时间：
2017-10
期刊：
影响因子：
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通讯作者：
Z. Valkova;D. Cholakova;S. Tcholakova;N. Denkov;S. Smoukov
中科院分区：
文献类型：
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作者：
Z. Valkova;D. Cholakova;S. Tcholakova;N. Denkov;S. Smoukov
乳化需要液滴破碎和在不混溶的液相之间产生大的界面面积。通常，使用产生热量并提高乳液温度的高剪切或高压乳化装置来获得具有微米和亚微米液滴的乳液。最近，我们报道了一种新的、有效的自乳化方法（Tcholakova等人，Nat.Commun.2005）。2017，8，15012），其由在粗水包油乳液中的预分散烷烃滴的一个至几个冷冻和熔化循环组成。在分散液滴的这些冻融循环中，后者自发地爆裂成成百上千个较小的液滴，而不使用任何机械搅拌。在这里，我们澄清的主要因素和机制，驱动这个自乳化过程中，通过系统地探索油和表面活性剂的类型，冷却速率，和初始液滴大小的影响。我们表明，通过这种方法产生的液滴的典型尺寸，是由在该过程的冷却-冷冻阶段形成的结构域的尺寸控制的。取决于主导机制，这些可以是在液滴自成形时形成的纤维的直径或在液滴冻结时形成的晶畴的尺寸。通常，比油分子长0-2个碳原子的表面活性剂尾部最适合观察有效的自乳化。阐明并讨论了实现不同机制的具体要求。三种不同机制的相对效率，作为液滴尺寸和冷却程序的函数，在受控实验中进行比较，为理解和进一步优化和扩大自乳化过程提供指导。
Emulsification requires drop breakage and creation of a large interfacial area between immiscible liquid phases. Usually, high-shear or high-pressure emulsification devices that generate heat and increase the emulsion temperature are used to obtain emulsions with micrometer and submicrometer droplets. Recently, we reported a new, efficient procedure of self-emulsification (Tcholakova et al. Nat. Commun. 2017, 8, 15012), which consists of one to several cycles of freezing and melting of predispersed alkane drops in a coarse oil-in-water emulsion. Within these freeze-thaw cycles of the dispersed drops, the latter burst spontaneously into hundreds and thousands of smaller droplets without using any mechanical agitation. Here, we clarify the main factors and mechanisms, which drive this self-emulsification process, by exploring systematically the effects of the oil and surfactant types, the cooling rate, and the initial drop size. We show that the typical size of the droplets, generated by this method, is controlled by the size of the structural domains formed in the cooling-freezing stage of the procedure. Depending on the leading mechanism, these could be the diameter of the fibers formed upon drop self-shaping or the size of the crystal domains formed at the moment of drop-freezing. Generally, surfactant tails that are 0-2 carbon atoms longer than the oil molecules are most appropriate to observe efficient self-emulsification. The specific requirements for the realization of different mechanisms are clarified and discussed. The relative efficiencies of the three different mechanisms, as a function of the droplet size and cooling procedure, are compared in controlled experiments to provide guidance for understanding and further optimization and scale-up of this self-emulsification process.