Departamento de Física, Comisión Nacional de Energía Atómica , Avenida Libertador 8250, 1429 Buenos Aires, Argentina , and.
Langmuir. 2013 Nov 5;29(44):13379-87. doi: 10.1021/la402683j. Epub 2013 Oct 24.
Structural and dynamical properties related to the adsorption of nonionic surfactants at vacuum/ionic liquid interfaces were studied using molecular dynamics simulations. Specifically, the surface activity of pentaethylene glycol monododecyl ether (C12E5) was investigated at the free interface of an imidazolium-based room temperature ionic liquid (RTIL), at different surface densities. At low surface coverages, the incorporation of C12E5 does not produce meaningful changes in the vacuum/RTIL interface: the C12E5 hydrophobic tails remain entangled with those of the RTIL cation groups in the outer shell, whereas the C12E5 hydrophilic heads reside at an inner layer. At high surface coverages, the structure in the substrate-in terms of the features exhibited by the local density profiles-practically vanishes; the interface becomes wider and the surfactant molecules shift toward more external positions. Information about the local structure of the interface at high surface densities can be recovered by performing a tessellation procedure. For the sake of comparison, the surface behavior of two commonly used ionic surfactants, sodium dodecyl sulfate and dodecyl trimethyl ammonium chloride, were also studied. The modifications in the width and structure of the bare vacuum/RTIL interface due to the presence of the ionic surfactants are markedly milder than those observed for the nonionic surfactant. Moreover, the RTIL seemed to behave as a better solvent for the chloride counterions than for sodium ones; which were found to remain bound to the surfactant head groups. An analysis of the dynamics at the surface was also performed. Our results indicate that the presence of increasing amounts of nonionic surfactants leads to a gradual reduction of the mobility of the RTIL species. When ionic surfactants are adsorbed, these retardations are even more severe for the surfactant head groups, where the corresponding diffusion coefficients show reductions of practically 1 order of magnitude.
使用分子动力学模拟研究了与非离子表面活性剂在真空/离子液体界面吸附相关的结构和动力学性质。具体来说，研究了在不同表面密度下，咪唑鎓基室温离子液体（RTIL）自由界面上五乙二醇单十二醚（C12E5）的表面活性。在低表面覆盖度下，C12E5 的加入不会对真空/RTIL 界面产生有意义的变化：C12E5 的疏水尾部仍然与 RTIL 阳离子基团的外壳缠绕在一起，而 C12E5 的亲水头部则位于内层。在高表面覆盖度下，基质中的结构——局部密度曲线所表现出的特征——实际上消失了；界面变宽，表面活性剂分子向更外部位置移动。通过执行细分程序，可以恢复高表面密度下界面局部结构的信息。为了进行比较，还研究了两种常用的离子表面活性剂，十二烷基硫酸钠和十二烷基三甲基氯化铵的表面行为。由于离子表面活性剂的存在，裸露的真空/RTIL 界面的宽度和结构的变化明显比非离子表面活性剂的变化温和。此外，RTIL 似乎比钠离子更适合氯离子作为反离子；氯离子被发现仍与表面活性剂的头基结合。还对表面动力学进行了分析。我们的结果表明，随着非离子表面活性剂含量的增加，RTIL 物种的迁移率逐渐降低。当吸附离子表面活性剂时，这些滞后作用对于表面活性剂的头基更为严重，其中相应的扩散系数降低了近一个数量级。
