Universidad de Vigo, Departamento de Física Aplicada, As Lagoas s/n, 32004, Ourense, Spain.
J Phys Chem B. 2011 Apr 7;115(13):3522-30. doi: 10.1021/jp110084p. Epub 2011 Mar 10.
The ability of the most popular models of methanol (H1, OPLS, L2, and L1) for the prediction of the solid-solid and the solid-fluid equilibria was analyzed in detail in this work by using molecular simulation. The three solid phases (α, β, and γ) detected experimentally as being thermodynamically stable, as well as the fluid phase, were considered for the calculations. It turns out that all the models provide similar results. The α, γ, and fluid phases were found to be thermodynamically stable for a certain range of temperatures and pressures, whereas the β phase was always metastable. The coexistence curves (α-fluid, α-γ, γ-fluid) corresponding to all the models took the same shape except for some slight differences about their locations. From a qualitative point of view, it can be considered that the four models give a reasonable prediction of the phase diagram of methanol. However, there are important quantitative discrepancies. The melting points fell in the interval 214-223 K, whereas the γ phase was predicted to be stable at pressures above 12 × 10(4) bar. These results are quite different in relation to the experiments since the melting point of methanol is 175.6 K and the γ phase is stable at 3.5 × 10(4) bar at room temperature. In addition, the values of the melting enthalpy obtained by the different models are very similar but about 50% higher than the experimental value. Therefore, it is clear that there is room for improvement. Reducing the stability of the α phase with respect to the other phases seems to be a necessary condition to construct an improved potential.
本工作通过分子模拟详细分析了最流行的甲醇模型（H1、OPLS、L2 和 L1）预测固-固和固-液平衡的能力。计算中考虑了实验中检测到的三种热力学稳定的固相（α、β 和 γ）以及液相。结果表明，所有模型都提供了相似的结果。α、γ 和液相在一定的温度和压力范围内被发现是热力学稳定的，而β相则始终是亚稳的。对应于所有模型的共存曲线（α-液、α-γ、γ-液）形状相同，只是位置略有不同。从定性的角度来看，可以认为这四个模型对甲醇的相图给出了合理的预测。然而，存在重要的定量差异。熔点落在 214-223 K 之间，而γ相在压力高于 12×10(4)巴时被预测为稳定。这些结果与实验结果有很大的不同，因为甲醇的熔点为 175.6 K，γ相在室温下在 3.5×10(4)巴下稳定。此外，不同模型得到的熔化焓值非常相似，但比实验值高约 50%。因此，显然还有改进的空间。降低相对于其他相α相的稳定性似乎是构建改进势的必要条件。
