Kim Sung A, Jeong Kyeong-Jun, Yethiraj Arun, Mahanthappa Mahesh K
Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, MN 55455.
Department of Chemistry and Theoretical Chemistry Institute, University of Wisconsin-Madison, Madison, WI 53706.
Proc Natl Acad Sci U S A. 2017 Apr 18;114(16):4072-4077. doi: 10.1073/pnas.1701608114. Epub 2017 Apr 3.
Supramolecular self-assembly enables access to designer soft materials that typically exhibit high-symmetry packing arrangements, which optimize the interactions between their mesoscopic constituents over multiple length scales. We report the discovery of an ionic small molecule surfactant that undergoes water-induced self-assembly into spherical micelles, which pack into a previously unknown, low-symmetry lyotropic liquid crystalline Frank-Kasper σ phase. Small-angle X-ray scattering studies reveal that this complex phase is characterized by a gigantic tetragonal unit cell, in which 30 sub-2-nm quasispherical micelles of five discrete sizes are arranged into a tetrahedral close packing, with exceptional translational order over length scales exceeding 100 nm. Varying the relative concentrations of water and surfactant in these lyotropic phases also triggers formation of the related Frank-Kasper A15 sphere packing as well as a common body-centered cubic structure. Molecular dynamics simulations reveal that the symmetry breaking that drives the formation of the σ and A15 phases arises from minimization of local deviations in surfactant headgroup and counterion solvation to maintain a nearly spherical counterion atmosphere around each micelle, while maximizing counterion-mediated electrostatic cohesion among the ensemble of charged particles.
超分子自组装能够制备出特定的软材料，这些材料通常呈现出高对称性的堆积排列，从而在多个长度尺度上优化其介观组分之间的相互作用。我们报告了一种离子型小分子表面活性剂的发现，它在水的诱导下自组装成球形胶束，这些胶束堆积形成一种前所未知的低对称性溶致液晶弗兰克 - 卡斯帕σ相。小角X射线散射研究表明，这种复杂相的特征是具有一个巨大的四方晶胞，其中30个尺寸离散的亚2纳米准球形胶束排列成四面体密堆积，在超过100纳米的长度尺度上具有异常的平移有序性。改变这些溶致相中水和表面活性剂的相对浓度，还会引发相关的弗兰克 - 卡斯帕A15球体堆积以及常见的体心立方结构的形成。分子动力学模拟表明，驱动σ相和A15相形成的对称性破缺源于表面活性剂头基和抗衡离子溶剂化的局部偏差最小化，以在每个胶束周围维持近乎球形的抗衡离子气氛，同时使带电粒子集合体之间的抗衡离子介导的静电凝聚最大化。
