Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
J Phys Chem B. 2013 Apr 25;117(16):4254-62. doi: 10.1021/jp304933k. Epub 2012 Sep 21.
The self-assembly behavior of gemini (dimeric or twin-tail) dicarboxylate disodium surfactants is studied using molecular dynamics simulations. A united atom model is employed for the surfactants with fully atomistic counterions and water. This gemini architecture, in which two single tailed surfactants are joined through a flexible hydrophobic linker, has been shown to exhibit concentration-dependent aqueous self-assembly into lyotropic phases including hexagonal, gyroid, and lamellar morphologies. Our simulations reproduce the experimentally observed phases at similar amphiphile concentrations in water, including the unusual ability of these surfactants to form gyroid phases over unprecedentedly large amphiphile concentration windows. We demonstrate quantitative agreement between the predicted and experimentally observed domain spacings of these nanostructured materials. Through careful conformation analyses of the surfactant molecules, we show that the gyroid phase is electrostatically stabilized related to the lamellar phase. By starting with a lamellar phase, we show that use of a bulkier N(CH(3))(4)(+) counterion in place of Na(+) drives the formation of a gyroid phase. Decreasing the charge on the surfactant headgroups by carboxylate protonation decreases the degree of order in the lamellar phase. Using our models, we show that the translational diffusion of water and the Na(+) counterions is decreased by several orders of magnitude over the studied concentration range, and we attribute these effects to strong correlations between the mobile species and the surfactant headgroups.
双子（二聚体或双尾）二羧酸二钠盐表面活性剂的自组装行为使用分子动力学模拟进行研究。采用全原子反离子和水的统一原子模型来模拟表面活性剂。这种双子结构，其中两个单尾表面活性剂通过柔性疏水连接体连接在一起，已经显示出在浓度依赖性水溶液中自组装成溶致相，包括六方、各向同性和层状形态。我们的模拟在类似的两亲物浓度下再现了实验观察到的相，包括这些表面活性剂在前所未有的大两亲物浓度窗口中形成各向同性相的异常能力。我们证明了预测的和实验观察到的这些纳米结构材料的畴间距之间的定量一致性。通过仔细分析表面活性剂分子的构象，我们表明各向同性相是静电稳定的，与层状相有关。从层状相开始，我们表明使用较大的 N(CH(3))(4)(+)反离子代替 Na(+)会驱动各向同性相的形成。通过羧酸质子化降低表面活性剂头基的电荷会降低层状相的有序度。使用我们的模型，我们表明在研究的浓度范围内，水和 Na(+)反离子的平移扩散降低了几个数量级，我们将这些效应归因于移动物种和表面活性剂头基之间的强相关性。
