一维聚电解质非平均场效应微观机制的研究

Properties of poly-ions play important role in understanding related phenomena in physical, chemical and biological science and applications. Abnormal behaviors of poly-ions in aqueous solutions beyond conventional mean-field Poisson-Boltzmann (PB) theory such as like-charge attraction have been revealed recently. Though consensus is that counter-ions condensed onto the poly-ions are crucial to those non-PB phenomena, the microscopic mechanism is still under debate, calling for more direct experimental exploration on the correlation between counter-ions and their contribution to the interaction between poly-ions. The commonly used one-dimensional polyions suffer from unpredictable configuration and random distribution of surface charges, causing difficulties for experimental observation. Supra-molecular one dimensional structures can overcome the above problems, providing new research opportunities to study non-PB effects. Synthetic oligomers can spontaneously assemble into highly charged stable helices with regularly distributed surface charges, facilitating occurrence and probe of non-PB effects including counter-ion condensation and like-charge attraction. Meanwhile advanced X-ray techniques also provide opportunities for experimental discoveries. As a powerful technique, anomalous X-ray scattering based on synchrotron radiation can reveal structural information at microscopic level with atomic selection, in particular spacial position of metal elements in complex systems. Also by combining small angle X-ray scattering and osmotic dialysis, hierarchical structures of the supra-molecular colloids such as the distance between the helices as well as thermodynamic properties of the system including the interaction between charged helices can be achieved. Based on the current status of research and our preliminary results, a systematic investigation on how counter-ions condense on the supra-molecular helices and how the helices interact with each other using aforementioned X-ray scattering techniques and Monte Carlo simulations is proposed as a model-system study to further understand non-PB effects between poly-electrolytes and to screen possible underlying principles. The proposed study will contribute to building a more complete physical picture of poly-electrolyte as well as to guiding related applications.

水溶液中一维聚电解质的行为是理解若干领域现象及应用的基础，最近研究发现其特殊行为如相似电荷吸引难以用传统平均场理论来解释。普遍认为凝聚在聚电解质表面的补偿离子起了关键作用，但具体的微观机制仍有分歧且缺乏直观的实验数据，因此实验探索补偿离子空间分布及其对聚电解质相互作用的影响成为理解非平均场行为的关键。常见的一维聚电解质具有构型不确定、电荷分布不规律等问题增加了实验难度，而超分子自组装体系能克服以上缺点。前期准备合成的低聚分子在水中自发组装成为结构稳定的一维螺旋，具有分布规整、较高的表面电荷密度，有利于补偿离子凝聚和相似电荷吸引的发生及检测。利用同步辐射X射线反常散射能选择性的探测补偿离子空间位置的先进技术结合蒙特卡洛模拟，本课题将系统研究自组装一维带电螺旋周围补偿离子的凝聚行为及螺旋间的相互作用；探讨一维聚电解质的非平均场效应，提供合理的微观机制，帮助完善聚电解质理论并为相关应用提供依据。

超分子聚电解质是由小分子组装形成的超分子聚合物，并且在水溶液中可以电离成聚离子和带相反电荷的反离子。具有螺旋结构的自组装聚电解质既保留了自组装的超分子特性又具有表面电荷分布以及螺旋结构的特点，提供了研究水溶液中聚电解质相互作用以及聚集行为的模型体系。此外自组装聚电解质多级结构与生物大分子比如ＤＮＡ具有相类似的结构特性，因而具有在生物医学方面的潜在应用而受到广泛关注。已有众多关于超分子聚电解质的研宄工作主要关注小分子构筑单元形成单个超分子螺旋结构的行为，即自组装的初级结构。如何让自组装一维初级结构形成更高有序的周期结构的研究仍处于起步阶段，缺乏底层的机制和影响因素等方面的研究，特别是如何利用管状自组装结构构建二维有序的溶致液晶柱状相，甚至是具有三维有序的柱状晶体相。基于以上背景和研究现状，本课题以人工合成的芳香低聚酰胺分子在水溶液中形成的自组装螺旋带电纳米管作为研宄对象，利用透射电镜、冷冻电镜和X射线小角散射等实验手段系统地研究了芳香低聚酰胺分子在稀溶液中的介观相行为和在水溶液中的多级自组装路径和动力学；更高级有序液晶结构的形成机制，结晶过程；反离子的种类、大小等性质对液晶相结构的影响以及液晶柱状相中的二维和三维有序。结果表明在超分子聚电解质体系中存在非常明显的离子特异性，离子对超分子多级结构的调控和离子特异性可以有助于理解生命科学中相关的静电相互作用。超分子多级结构中观察到的缓慢的结构形成过程和稳定的终态为溶液中制备结晶材料提供了新的思路，不仅有助于深入理解聚电解的液晶相行为而且对溶液中软物质的组装过程有广泛的指导意义。