Faculty of Chemical Technology and Biotechnology, Budapest University of Technology and Economics, Szt. Gellért tér 4, H-1111 Budapest, Hungary.
Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics, Budafoki út 8, H-1111 Budapest, Hungary.
J Phys Chem B. 2023 Jun 15;127(23):5341-5352. doi: 10.1021/acs.jpcb.3c01370. Epub 2023 Jun 5.
The surface affinity of tetramethylammonium iodide (TMAI) in aqueous solutions is investigated by surface tension measurements and molecular dynamics computer simulations. Experiments, performed in the entire composition range of solubility using the pendant drop method with two different setups, clearly reveal that TMAI is a weakly capillary active salt. Computer simulations performed with the AMBER force field reproduce the experimental data very well, while two other major force fields (i.e., CHARMM and OPLS) can still reproduce the experimental trend qualitatively; however, even qualitative reproduction of the experimental trend requires scaling down the ion charges according to the Leontyev-Stuchebrukhov correction. On the other hand, the GROMOS force field fails in reproducing the experimentally confirmed capillary activity of TMAI. Molecular dynamics simulation results show that, among the two ions, iodide has a clearly larger surface affinity than tetramethylammonium (TMA). Further, the adsorption of the I anions is strictly limited to the first molecular layer beneath the liquid-vapor interface, which is followed by several layers of their depletion. On the other hand, the net negative charge of the surface layer, caused by the excess amount of I with respect to TMA, is compensated by a diffuse layer of adsorbed TMA cations, extending to or beyond the fourth molecular layer beneath the liquid surface.
通过表面张力测量和分子动力学计算机模拟研究了四甲基碘化铵（TMAI）在水溶液中的表面亲和力。实验使用两种不同设置的悬滴法在溶解度的整个组成范围内进行，清楚地表明 TMAI 是一种弱毛细活性盐。使用 AMBER 力场进行的计算机模拟很好地再现了实验数据，而另外两个主要的力场（即 CHARMM 和 OPLS）仍然可以定性地再现实验趋势；然而，即使定性地再现实验趋势也需要根据 Leontyev-Stuchebrukhov 校正来缩小离子电荷。另一方面，GROMOS 力场无法再现实验证实的 TMAI 的毛细活性。分子动力学模拟结果表明，在这两种离子中，碘化物的表面亲和力明显大于四甲基铵（TMA）。此外，I 阴离子的吸附严格限于液体-蒸气界面下的第一层，其后是几层的耗尽。另一方面，由于 I 的量相对于 TMA 过多而导致表面层的净负电荷由吸附的 TMA 阳离子的扩散层补偿，延伸至或超出液体表面下的第四层分子。
