Determination of Equilibrium Adsorbed Morphologies of Surfactants at Metal-Water Interfaces Using a Modified Umbrella Sampling-Based Methodology
Determination of Equilibrium Adsorbed Morphologies of Surfactants at Metal-Water Interfaces Using a Modified Umbrella Sampling-Based Methodology
复制标题
使用改进的基于伞采样的方法测定金属-水界面表面活性剂的平衡吸附形态
DOI：
10.1021/acs.jctc.2c00078
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发表时间：
2022
影响因子：
5.5
通讯作者：
Sharma, Sumit
中科院分区：
文献类型：
--
作者：
Singh, Himanshu;Sharma, Sumit
表面活性剂以各种形态吸附到金属-水界面，包括自组装单分子层（SAM）、圆柱形和球形胶束或半胶束。目前的分子模拟方法是无法有效地采样这些形态的形成，因为大的扩散/充满活力的障碍。我们引入了一个修改后的伞采样为基础的方法，允许采样这些形态从任何初始配置，并提供它们之间的自由能差异。使用这种方法，我们已经研究了阳离子[季铵（季铵）的4和12个碳长的烷基尾巴]，不带电的[癸基乙基]，和阴离子[磷酸单酯]表面活性剂及其混合物在金-水界面上的吸附行为。我们发现，虽然库仑排斥带电荷的头部基团之间的季-4限制其吸附到一个稀疏的层，更强的疏水相互作用之间的烷基尾部的季-12促进吸附，导致吸附的半球形胶束坐在一个单层的顶部形态。癸硫醇分子吸附在一个密集的双层中，分子在第一层中直立在表面上，在第二层中平行于表面。阳离子和阴离子表面活性剂的混合物显示出协同吸附行为。这些结果阐明了分子特征在决定表面活性剂在金属-水界面上的吸附形态的性质中的作用。
Surfactants adsorb to metal-water interfaces in various morphologies, including self-assembled monolayers (SAMs), cylindrical and spherical micelles, or hemimicelles. Current molecular simulation methods are unable to efficiently sample the formation of these morphologies because of the large diffusive/energetic barriers. We introduce a modified umbrella sampling-based methodology that allows sampling of these morphologies from any initial configuration and provides free energy differences between them. Using this methodology, we have studied adsorption behavior of cationic [quaternary ammonium (quat) of 4 and 12 carbon long alkyl tails], uncharged [decanethiol], and anionic [phosphate monoester] surfactants and their mixtures at a gold-water interface. We find that while Coulombic repulsion between the charged head groups of quat-4 limits their adsorption to a sparse layer, stronger hydrophobic interactions between the alkyl tails of quat-12 promote adsorption resulting in a morphology with adsorbed hemispherical micelles sitting atop a monolayer. Decanethiol molecules adsorb in a densely packed bilayer with the molecules standing-up on the surface in the first layer and lying parallel to the surface in the second layer. Cationic and anionic surfactant mixtures display a synergistic adsorption behavior. These results elucidate the role of molecular characteristics in dictating the nature of adsorbed morphologies of surfactants at metal-water interfaces.