Department of Chemistry and Theoretical Chemistry Institute, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706, United States.
Department of Chemical Engineering and Materials Science, University of Minnesota , 421 Washington Avenue, S.E., Minneapolis, Minnesota 55455, United States.
J Phys Chem B. 2017 Jan 26;121(3):565-576. doi: 10.1021/acs.jpcb.6b06882. Epub 2017 Jan 17.
Gemini surfactants comprise two single-tailed surfactants connected by a linker at or near the hydrophilic headgroup. They display a variety of water-concentration-dependent lyotropic liquid crystal morphologies that are sensitive to surfactant molecular structure and the nature of the headgroups and counterions. Recently, an interesting dependence of the aqueous-phase behavior on the length of the linker has been discovered; odd-numbered linker length surfactants exhibit characteristically different phase diagrams than even-numbered linker surfactants. In this work, we investigate this "odd/even effect" using computer simulations, focusing on experimentally studied gemini dicarboxylates with Na counterions, seven nonterminal carbon atoms in the tails, and either three, four, five, or six carbon atoms in the linker (denoted Na-73, Na-74, Na-75, and Na-76, respectively). We find that the relative electrostatic repulsion between headgroups in the different morphologies is correlated with the qualitative features of the experimental phase diagrams, predicting destabilization of hexagonal phases as the cylinders pack close together at low water content. Significant differences in the relative headgroup orientations of Na-74 and Na-76 compared to those of Na-73 and Na-75 surfactants lead to differences in linker-linker packing and long-range headgroup-headgroup electrostatic repulsion, which affects the delicate electrostatic balance between the hexagonal and gyroid phases. Much of the fundamental insight presented in this work is enabled by the ability to computationally construct and analyze metastable phases that are not observable in experiments.
双子表面活性剂由连接在亲水头部的连接基团连接的两个单尾表面活性剂组成。它们表现出多种依赖于水浓度的溶致液晶形态，这些形态对表面活性剂分子结构以及头基和抗衡离子的性质敏感。最近，人们发现水相行为与连接基团长度之间存在有趣的依赖性；奇数链接长度的表面活性剂表现出与偶数链接长度的表面活性剂明显不同的相图。在这项工作中，我们使用计算机模拟研究了这种“奇偶效应”，重点研究了具有 Na 抗衡离子、尾部有七个非末端碳原子且连接基团中分别有三个、四个、五个或六个碳原子的实验研究的双子二羧酸酯（分别表示为 Na-73、Na-74、Na-75 和 Na-76）。我们发现，不同形态之间头基的相对静电排斥与实验相图的定性特征相关，预测随着低含水量下圆柱紧密堆积，六方相的不稳定性增加。Na-74 和 Na-76 表面活性剂的相对头基取向与 Na-73 和 Na-75 表面活性剂的显著差异导致连接基团之间的空间排列和长程头基-头基静电排斥的差异，这影响了六方相和准晶相之间的微妙静电平衡。这项工作中提出的许多基本见解得益于能够计算构建和分析实验中不可观察的亚稳相的能力。
