Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada.
J Chem Phys. 2017 Jul 14;147(2):024503. doi: 10.1063/1.4990505.
Molecular dynamics simulations are used to investigate the behavior of aqueous tert-butanol (TBA) solutions for a range of temperatures, using the CHARMM generalized force field (CGenFF) to model TBA and the TIP4P/2005 or TIP4P-Ew water model. Simulation results for the density, isothermal compressibility, constant pressure heat capacity, and self-diffusion coefficients are in good accord with experimental measurements. Agreement with the experiment is particularly good at low TBA concentration, where experiments have revealed anomalies in a number of thermodynamic properties. Importantly, the CGenFF model does not exhibit liquid-liquid demixing at temperatures between 290 and 320 K (for systems of 32 000 molecules), in contrast with the situation for several other common TBA models [R. Gupta and G. N. Patey, J. Chem. Phys. 137, 034509 (2012)]. However, whereas real water and TBA are miscible at all temperatures where the liquid is stable, we observe some evidence of demixing at 340 K and above. To evaluate the structural properties at low concentrations, we compare with both neutron scattering and recent spectroscopic measurements. This reveals that while the CGenFF model is a definite improvement over other models that have been considered, the TBA molecules still exhibit a tendency to associate at low concentrations that is somewhat stronger than that indicated by experiments. Finally, we discuss the range and decay times of the long-range correlations, providing an indication of the system size and simulation times that are necessary in order to obtain reliable results for certain properties.
采用分子动力学模拟方法，使用 CHARMM 广义力场（CGenFF）来模拟叔丁醇（TBA），并采用 TIP4P/2005 或 TIP4P-Ew 水模型，研究了一系列温度下的水溶液的行为。模拟得到的密度、等温压缩系数、定压热容和自扩散系数与实验测量值吻合较好。在低 TBA 浓度下，模拟结果与实验吻合尤其好，因为在该浓度下，实验已经揭示了一些热力学性质的异常。重要的是，与其他几种常见的 TBA 模型相比，CGenFF 模型在 290 到 320 K 的温度范围内（对于 32000 个分子的体系）没有表现出液-液分相现象[R. Gupta 和 G. N. Patey, J. Chem. Phys. 137, 034509 (2012)]。然而，虽然真实的水和 TBA 在液体稳定的所有温度下都是混溶的，但我们在 340 K 及以上温度下观察到了一些分相的迹象。为了评估低浓度下的结构性质，我们将模拟结果与中子散射和最近的光谱测量结果进行了比较。这表明，虽然 CGenFF 模型相对于其他已经考虑过的模型有了明显的改进，但 TBA 分子在低浓度下仍然表现出一定的缔合倾向，这种倾向比实验所表明的要强一些。最后，我们讨论了长程相关的范围和衰减时间，为获得某些性质的可靠结果提供了所需的体系大小和模拟时间的指示。
