Wang Muhan, Fang Timing, Zhong Hong, Li Jiawei, Yan Youguo, Zhang Jun
School of Materials Science and Engineering, China University of Petroleum, 266580 Qingdao, Shandong, China.
Soft Matter. 2019 Apr 17;15(16):3323-3329. doi: 10.1039/c8sm02299b.
The aggregation number is one of the most fundamental and important structural parameters for the micelle or reverse micelle (RM) system. In this work, a simple, reliable method for the determination of the aggregation number of RMs in supercritical CO2 (scCO2) was presented through a molecular dynamics simulation. The process of pulling surfactants out of the RMs one by one was performed to calculate the aggregation number. The free energies of RMs with different numbers of surfactants were calculated through this process. We found an RM with the lowest free energy, which was considered to have the optimal number of surfactants. Therefore, the optimal aggregation number of RMs was acquired. In order to explain the existence of an optimal aggregation number, detailed analyses of surfactant accumulation were conducted by combining molecular dynamics with quantum chemistry methods. The results indicated that in the RMs with the lowest free energy, the head-group and tail-terminal of the surfactants accumulated on an equipotential surface. In this case, the surfactant film could effectively separate water and CO2; thus, the lowest free energy was expected. This method determined the aggregation number of RMs by theoretical calculations that did not depend on experimental measurements. This presented approach facilitates the evaluation of the characteristics of RMs in scCO2 and can be further applied in the RM system of organic solvents or even in the micellar system.
聚集数是胶束或反胶束（RM）体系最基本、最重要的结构参数之一。在本工作中，通过分子动力学模拟提出了一种简单、可靠的测定超临界CO2（scCO2）中反胶束聚集数的方法。通过逐个将表面活性剂从反胶束中拉出的过程来计算聚集数。通过该过程计算了具有不同表面活性剂数量的反胶束的自由能。我们发现了具有最低自由能的反胶束，其被认为具有最佳数量的表面活性剂。因此，获得了反胶束的最佳聚集数。为了解释最佳聚集数的存在，结合分子动力学和量子化学方法对表面活性剂的聚集进行了详细分析。结果表明，在具有最低自由能的反胶束中，表面活性剂的头基和尾端聚集在等势面上。在这种情况下，表面活性剂膜可以有效地分离水和CO2；因此，可以预期具有最低自由能。该方法通过不依赖于实验测量的理论计算确定反胶束的聚集数。所提出的方法有助于评估scCO2中反胶束的特性，并可进一步应用于有机溶剂的反胶束体系甚至胶束体系。
