Rich self-assembly behavior from a simple amphiphile.
Rich self-assembly behavior from a simple amphiphile.
复制标题
DOI：
10.1002/cphc.201000500
复制
发表时间：
2010-10
期刊：
影响因子：
--
通讯作者：
Xuefeng Li;Yi Yang;J. Eastoe;Jinfeng Dong
中科院分区：
文献类型：
--
作者：
Xuefeng Li;Yi Yang;J. Eastoe;Jinfeng Dong
对于环境响应性两亲物，可以通过控制条件（例如pH、电解质的类型和浓度或光频率）的变化来触发不同聚集状态之间的结构转换。传统上，该领域需要设计越来越复杂的表面活性剂和官能化聚合物，通常来自多步、低产率和昂贵的合成方法。[1]在本文中，我们展示了通过引入一类剥离回表达自组装的基本特征的化合物来产生响应性表面活性剂的替代方法（C12 NC 3 N，方案1）。这种原始的表面活性剂具有一个单一的疏水碳链，耦合到一个容易调节，二胺亲水功能。尽管其分子简单，C12 NC 3 N表现出令人惊讶的丰富多样性的相结构，因为二胺具有两个可区分的pKa，提供pH和温度敏感性，允许连续控制亲水性。这些发现为两亲性分子的研究提供了一条新的途径，即设计优化的刺激响应性两亲分子。在可调自组装的情况下，胶束到囊泡的转变（MVT），特别是蠕虫状胶束到囊泡的转变（WVT），代表了有趣的情况，[2]因为它们可以引起系统性质的显着变化，包括相稳定性，光学特性增溶和流变性。囊泡和脂质双层结构之间的相似性[3]激发了人们对功能材料的MVTs的兴趣[2]，但热可逆MVTs的例子很少。[4]有趣的是，穿孔或有缺陷的囊泡已被观察到作为中间体在这些转换与一些不同的表面活性剂系统。[5]然而，这种表面瓣叶穿孔的形成机制仍不清楚。[6]本文首次报道了球形胶束可以转变为蠕虫状胶束（WLM），然后可以演变为穿孔囊泡，最终形成完全封闭的囊泡（方案2）。与之前的想法相反，这种丰富的结构转变不需要化学上复杂的两亲物，但可以发生在简单的分子中，如C12 NC 3 N。C12 NC 3 N在该应用中的重要化学特征是分子亲水部分的伯胺和仲胺基团，具有两种明显不同的水溶液pKa，分别为4.71和10.81（图S1，支持信息）。在适当的酸性、中性和碱性条件下，C12 NC 3 N的主要物种分别为1：2型阳离子表面活性剂（H2 A2+）、1：1型阳离子表面活性剂（HA+）和非离子表面活性剂（A）。浊度测量显示，溶液在pH值低于8.00时是透明的，在9.01< pH< 9.51的范围内变为浑浊相，在较高的pH值下持续存在强浊度（图1a）。作为pD的函数获得的1HNMR光谱（图S2-S5，支持信息）与两个单独胺基的质子化状态的顺序变化一致，其中化学位移在pH值增加后向高场移动，表明胺基的质子化状态变化。同时，如果溶液的pH进一步增加到9.01以上，则信号变宽，这与胶束到囊泡的转变一致。[1]在pH 9.97以下的稳定透明区域中的Zeta电位测量（图S6，支持信息）显示相对较高的正表面电荷，而在pH 9.97以上的浑浊区域中，表面电荷密度较低，可能是不稳定性的来源。电荷反转
With environmentally-responsive amphiphiles, structural transformations may be triggered between different aggregation states through controlled changes in conditions, such as pH, type and concentration of electrolyte or light frequency. Conventionally, this field has necessitated designing increasingly more sophisticated surfactants and functionalized polymers, often from multi-step, low-yield and expensive synthetic methodologies.[1] Herein we demonstrate an alternative approach to generating responsive surfactants by introducing a class of compounds stripped back to the essential features for expressing self-assembly (C12NC3N, Scheme 1). This primitive surfactant possesses a single hydrophobic carbon chain, coupled to a readily tunable, diamine hydrophilic function. Despite its molecular simplicity, C12NC3N exhibits a surprisingly rich diversity of phase structures because the diamine has two distinguishable pKas, providing pH and temperature sensitivity permitting continual control over hydrophilicity. These findings suggest a new avenue for amphiphilic molecular research, namely designing optimized stimuli-responsive amphiphiles. In the cases of tunable self-assembly, micelle-to-vesicle transitions (MVT), and especially worm-like micelle-to-vesicle transitions (WVT), represent interesting cases,[2] because they can give rise to dramatic changes in system properties, including phase stability, optical characteristics solubilization and rheology. Parallels between vesicles and lipid bilayer structures [3] have stimulated interest in MVTs for functional materials,[2] but there are only few examples of thermo-reversible MVTs.[4] Interestingly, perforated or defective vesicles have been observed as intermediates in these transformations with a number of different surfactant systems.[5] However, the formation mechanism of such surface leaflet perforations is still unclear.[6] Herein it is reported for the first time that spherical micelles can transform into worm-like micelles (WLMs), which then can evolve into perforated vesicles, finally forming fully closed vesicles (Scheme 2). Contrary to previous thinking, such rich structural transformations do not require chemically sophisticated amphiphiles, but can occur with simple molecules such as C12NC3N.The important chemical features of C12NC3N for this application are the primary and secondary amine groups in the hydrophilic part of the molecule, having two distinctly different aqueous pKas of 4.71 and 10.81 (Figure S1, Supporting Information). Under appropriate acidic, neutral and basic conditions respectively, the predominant C12NC3N species appear to be a 1: 2-type cationic surfactant (H2A2+), a 1: 1-type cationic surfactant (HA+) and a nonionic surfactant (A). Turbidity measurements show solutions to be transparent below pH 8.00, changing to turbid phases over the range 9.01< pH< 9.51, with strong turbidity persisting at higher pH values (Figure1a). 1HNMR spectra (FiguresS2–S5, Supporting information) obtained as a function of pD are consistent with sequential changes in protonation state of the two separate amine groups, where the chemical shifts move upfield after an increase in pH, suggesting the protonation state change of amine groups. At the same time, the signals become broadened if the solution’s pH is further increased to above 9.01, which is consistent with a micelle-to-vesicle transition.[1] Zetapotential measurements (Figure S6, Supporting information) in the stable transparent region below pH 9.97 show relatively high positive surface charges, whereas in the turbid zone above pH 9.97 the surface charge densities are lower, and presumably the origin of instability. The charge reversal …