Self-Division of Macroscopic Droplets: Partitioning of Nanosized Cargo into Nanoscale Micelles
Self-Division of Macroscopic Droplets: Partitioning of Nanosized Cargo into Nanoscale Micelles
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DOI：
10.1002/anie.201002551
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Grzybowski, Bartosz A.
中科院分区：
文献类型：
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作者：
Browne, Kevin P.;Walker, David A.;Grzybowski, Bartosz A.
在液-液界面产生表面活性化合物的反应可以将大液滴分成较小的液滴(图1)。这种“自生”自我繁殖的效应最初是由Luisi和他的同事[1，2]在自划分胶束和反胶束的背景下研究的。这项开创性的工作基于两相(油-水)体系，在液-液界面上，表面活性剂通过水相中的氢氧化物离子在油相中的碱性水解酯或酸酐，[1]或通过水相中的高锰酸盐离子在液-液界面上的长链醇氧化而产生。[2]后来Luisi和他的同事[3，4]以及Szostak和他的同事[5，6]证明了在更复杂的双层甚至多层[6]小泡中可以自我繁殖。尽管有这些令人兴奋的结果，但对分裂胶束的最终大小的控制仍然有限，对“子体”实体的稳定性(包括动力学和热力学)知之甚少。在这里，我们证明了自分裂过程可以从宏观的乳液滴一直传播到纳米级的胶束(或反胶束)，其大小由溶液的pH控制。除了这些热力学稳定相外，该体系还具有动力学稳定的微乳液相，当pH降至某一临界值以下时可观察到该相。这些相的存在和稳定性以及体系在pH变化时的滞后行为由一个理论模型解释，该模型考虑了化学、曲率和静电对液滴界面自由能的贡献。可控的宏纳米分割允许将原来包含在宏观液滴中的货物(这里是纳米颗粒)以低至每胶束一到两个颗粒的精度分配到“子代”纳米胶束中。这种方法可以为具有明确大分子或纳米含量的乳液的工程设计开辟新的视角。液滴分离的过程如图1所示(另请参阅支持信息中的电影1)。将一滴含有45-50%(v/v)2-己基癸酸(2-HDA)和少量(约5mgmL1)用于显影的Calco油红染料的二氯甲烷(通常为80-100mL二氯甲烷)放入装满KOH水溶液(PH 12)的培养皿中。选择2-HDA是因为它以去质子化的形式聚集在DCM/水界面，因此可以作为表面活性剂。随着2-HDA和碱基之间的界面反应的进行，更多的去质子化的2-HDA在界面上积累，导致界面面积增加[7-9](即，液滴“拉长”)，直到液滴分裂成更小的，通常是两个子体。然后，这些液滴再次分裂，这一过程继续进行，直到液滴达到纳米尺度(例如，在pH%12时，d%30 nm；见图2)。
Reactions that produce surface-active compounds at the liquid–liquid interface can divide large droplets into smaller ones (Figure 1). This effect of “autopoietic” self-reproduction was originally studied by Luisi and co-workers [1, 2] in the context of self-dividing micelles and inverse micelles. This pioneering work was based on two-phase (oil–water) systems, in which surfactants were created at the liquid–liquid interface by either basic hydrolysis of esters or anhydrides in the oil phase by hydroxide ions in the aqueous phase,[1] or by longchain alcohol oxidation at the liquid–liquid interface by permanganate ions in the aqueous phase.[2] Later work by Luisi and co-workers [3, 4] and also by Szostak and co-workers [5, 6] demonstrated self-reproduction in more intricate bilayer and even multilamellar [6] vesicles. Such fatty acid vesicles can serve as artificial “reactors” that support the replication of nucleic acids.[4] Despite these exciting results, control over the ultimate sizes of the dividing micelles remains limited and little is known about the stability (both kinetic and thermodynamic) of the “daughter” entities. Herein we show that a self-division process can be propagated from macroscopic emulsion droplets all the way down to nanoscopic micelles (or inverse micelles), the sizes of which are controlled by the pH of the solution. In addition to these thermodynamically stable phases, the system also features a kinetically stable microemulsion phase that is observed when the pH is lowered below a certain critical value. The existence and stabilities of these phases as well as the hysteretic behavior of the system upon pH changes are explained by a theoretical model that accounts for the chemical, curvature, and electrostatic contributions to the interfacial surface free energy of the droplets. Controllable macro-to-nano division allows the partitioning of cargo (here, nanoscopic particles) originally contained in a macroscopic drop into the “progeny” nanomicelles with a precision as low as one or two particles per micelle. This method can open new perspectives for the engineering of emulsions with well-defined macromolecular or nanoscopic contents.The process of droplet division is illustrated in Figure 1 (see also Movie 1 in the Supporting Information). A drop of dichloromethane (typically 80–100 mL) that contains 45–50%(v/v) 2-hexyldecanoic acid (2-HDA) and a small amount (ca. 5 mg mLÀ1) of Calco Oil Red dye for visualization is placed in a petri dish filled with an aqueous solution of KOH (pH 12). 2-HDA was chosen because it accumulates at the DCM/water interface in its deprotonated form and can thus act as a surfactant. As the interfacial reaction between 2-HDA and the base progresses, more of the deprotonated 2-HDA accumulates at the interface to result in an increase in the interfacial area [7–9](ie, the drop “elongates”) until the drop divides into smaller, usually two, progenies. These droplets then divide again, and the process continues until the droplets reach nanoscopic dimensions (eg, d% 30 nm at pH% 12; see Figure 2).