Division of Environmental and Ecological Engineering, Purdue University, Potter Engineering Center, 500 Central Drive, West Lafayette, Indiana 47907, United States.
Davidson School of Chemical Engineering, Purdue University, Forney Hall of Chemical Engineering 1060, 480 Stadium Mall Drive, West Lafayette, Indiana 47907, United States.
J Phys Chem B. 2021 Dec 23;125(50):13789-13803. doi: 10.1021/acs.jpcb.1c07136. Epub 2021 Dec 13.
In water, the surfactant dioctyl sulfosuccinate (Aerosol-OT or AOT) exhibits diverse aggregate structures, ranging from micelles to lamella. An atomic-level understanding, however, of the formation and structure of these aggregates is lacking. Herein, using atomistic molecular dynamics (MD) with microsecond-long simulations, self-assembly of AOT in water is studied for concentrations of 1, 7.2, and 20 wt % at 293 K and for 7.2 wt % at 353 K. Assembly proceeds through stepwise association and dissociation of single AOT molecules, and the fusion and fission of AOT clusters. At 293 K, AOT self-assembles into either (i) spherical micelles (1 wt %), (ii) biphasic systems consisting of rod-like and prolate spheroidal micelles (7.2 wt %), or (iii) bilayers (20 wt %). We hypothesize that the observed rod-like structure is a precursor to lamellar microdomains found experimentally in biphasic dispersions. Increasing temperature to 353 K at 7.2 wt % results in a system consisting of prolate micelles but no rod-like micelles. Simulated phase behavior agrees with previously published experimental observations. Individual aggregates formed during self-assembly are identified using graph theory. Structural metrics of these aggregates like the radius of gyration, shape anisotropy, and prolateness are presented. Trends in structural metrics quantitatively reflect how shapes and sizes of AOT aggregates vary with surfactant concentration and temperature. These simulations provide deeper insight into open questions in the scientific community and demonstrate a method to generate physics-based micelle structures that can be used to rationalize experimental observations.
在水中，表面活性剂二辛基琥珀酸磺酸钠（气溶胶 OT 或 AOT）表现出多种聚集结构，从胶束到层状。然而，对于这些聚集物的形成和结构，人们缺乏原子级别的理解。在此，使用具有微秒级长模拟的原子级分子动力学 (MD)，研究了在 293 K 下浓度为 1、7.2 和 20wt%以及在 353 K 下浓度为 7.2wt%的 AOT 在水中的自组装。组装通过单 AOT 分子的逐步缔合和离解、AOT 簇的融合和裂变进行。在 293 K 下，AOT 自组装成（i）球形胶束（1wt%）、（ii）由棒状和长椭球胶束组成的两相体系（7.2wt%）或（iii）双层（20wt%）。我们假设观察到的棒状结构是实验中在两相分散体中发现的层状微域的前体。将温度升高至 7.2wt%的 353 K 导致系统由长椭球胶束组成，但没有棒状胶束。模拟的相行为与先前发表的实验观察结果一致。使用图论识别自组装过程中形成的单个聚集体。提出了这些聚集体的结构度量，如转动半径、形状各向异性和长椭球度。结构度量的趋势定量反映了 AOT 聚集体的形状和大小如何随表面活性剂浓度和温度而变化。这些模拟为科学界的悬而未决问题提供了更深入的见解，并展示了一种生成基于物理的胶束结构的方法，可用于合理化实验观察。
