溶液中纳米尺度疏水相互作用的X射线散射研究

Hydrophobic interactions between apolar solutes or apolar part of solutes are essential to understanding micro-structure and self assembling processe at molecular and nanometer scale. Recent advances in theory and simulation predicted a size dependence of hydrophobic effects and a crossover length from microscopic to macroscopic behavior around subnanometer，however few advances have been achieved from experimental aspects since then. Here we propose a systematic study on this issue by X-ray scattering at middle angle range in aqueous solutions of tetraalkyl-ammonium (CnH2n+1)4N+(n=1-5)cations with diameters around nanometer. X-ray scattering at middle angle about Q=1.0?-1 was found be to senstitive to cation association around nanometer scale and contains more contribution from (CnH2n+1)4N+ cations than that from water solvents; hence is capable of measuring hydrophobic interaction between (CnH2n+1)4N+ cations. Through evaluating the effects of R4N+ cations and temperature on scattering intensity in combination with calculation of scattering intensity using hypernetted chain method and effective potential between solvated ions, quantitative understanding of hydrophbic effects at nanometer scale as well as its size dependence can be achieved. Furthermore, the influence of additional alkali halide to solutions on hydrophobic interactions between R4N+ cations will be investigated in order to imitate real situation or manipulate the crossover behavior of hydrophobic interaction. This proposal is potential of achieving new information and deep understanding on nano-meter scale hydrophobic interaction in aqueous solutions and will help understand biological and chemical phenomena such as aggregation of organic moleclues and protein folding .

疏水相互作用是理解分子及纳米量级微观结构和自组织过程的关键之一。近年来理论及模拟研究表明疏水相互作用在纳米尺度表现出对疏水介质尺寸的依赖，但缺乏直接的实验支持。与其它技术相比X射线中角散射能有效探测到纳米量级的关联与相互作用。本项目拟采用这一手段研究直径约为1纳米的(CnH2n+1)4N+(n=1-5)四烷基季铵(R4N+)阳离子在溶液中的疏水相互作用。通过测量水溶液中阳离子和温度对X射线中角散射的影响，结合超网链方法定量分析来系统研究R4N+阳离子之间疏水相互作用的性质及其与离子尺寸的关系。通过比较水溶液与有机溶剂中R4N+离子之间的关联来探讨疏水相互作用对离子间相互吸引的贡献。进一步探讨溶液中碱卤化合物的存在对离子间疏水相互作用的影响，达到模拟现实或人为控制疏水相互作用的目的。本研究有望深入透彻理解溶液中纳米尺度疏水相互作用行为，并有助于对诸如有机分子聚集过程和蛋白质折叠等现象的理解。

疏水相互作用是理解分子及纳米量级微观结构和自组织过程的关键之一。近年来理论及模拟研究表明疏水相互作用在纳米尺度表现出对疏水介质尺寸的依赖，但缺乏直接的实验支持。与其它技术相比X射线中角散射能有效探测到纳米量级的关联与相互作用。本项目采用这一手段研究直径约为1纳米的(CnH2n+1)4N+(n=1-5)四烷基季铵(R4N+)阳离子（两亲离子）在溶液中的疏水相互作用。设计并加工了氦气氛围室和样品池，可以有效的降低空气背地散射，突出溶液在中角范围的散射信号。通过测量水溶液中阳离子和阴离子（Br-、Cl-）对X射线中角散射的影响，系统研究了R4N+阳离子之间疏水相互作用的性质及其与离子尺寸的关系；发现阴离子的大小也会对阳离子之间的疏水相互作用产生不可忽视的影响。通过比较水溶液与有机溶液的中角散射，发现了R4N+离子之间的关联在不同的溶剂中有不同的表现，取决于疏水相互作用是否对离子间相互吸引做出贡献。在无机盐及亲水有机盐的水溶液中也观察到了微弱的散射信号，来源自带相反电荷的阴阳离子的结合。本研究表明中角散射是研究溶液中纳米尺度离子关联的有效工具，在水溶液及有机溶液中离子间存在各种不同来源的关联，驱动力包括但不限于疏水相互作用导致的阳离子间的吸引。进一步证明了溶液中纳米尺度疏水相互作用行为受到其他离子的影响，发生临界疏水效应的阳离子尺寸会随着阴离子性质的改变而偏移。

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