Alvarez Fernando, Arbe Arantxa, Colmenero Juan
Departamento de Polímeros y Materiales Avanzados: Física, Química y Tecnología (UPV/EHU), Apartado 1072, E-20080 San Sebastián, Spain.
Centro de Física de Materiales (CFM) (CSIC-UPV/EHU) - Materials Physics Center (MPC), Paseo Manuel de Lardizabal 5, 20018 San Sebastián, Spain.
J Chem Phys. 2023 Oct 7;159(13). doi: 10.1063/5.0168588.
By means of massive (more than 1.2 · 106 molecules) molecular dynamics simulations at 300 K we have disentangled self- and cross-dipolar contributions to the dielectric relaxation of liquid water that cannot be experimentally resolved. We have demonstrated that cross dipolar correlations are of paramount importance. They amount for almost a 60% of the total dielectric amplitude. The corresponding relaxation function is a one-step Debye-like function with a characteristic time, τcross, of the order of the phenomenological Debye time, τD. In contrast, the relaxation function corresponding to the self-contribution is rather complex and contains a fast decay related to dipolar librations and a second relaxation step that can be well described by two exponentials: a low-amplitude fast process (τ0 = 0.31 ps) and a main slow process (τself = 5.4 ps) that fully randomizes the dipolar orientation. In addition to dipolar relaxation functions, we have also calculated scattering-like magnitudes characterizing translation and rotation of water molecules. Although these processes can be considered as "jump" processes in the short time range, at the time scale of about τD-τcross, at which the cross-dipolar correlations decay to zero, the observed behavior cannot be distinguished from that corresponding to uncoupled Brownian translational and rotational diffusion. We propose that this is the reason why the Debye model, which does not consider intermolecular dipolar interactions, seems to work at time t ≳ τD.
通过在300K下进行大规模（超过1.2·10⁶个分子）的分子动力学模拟，我们解开了液态水介电弛豫中自偶极和交叉偶极贡献，这些贡献无法通过实验分辨。我们已经证明交叉偶极相关性至关重要。它们几乎占总介电振幅的60%。相应的弛豫函数是一个具有特征时间τcross的单步德拜型函数，其数量级与唯象德拜时间τD相当。相比之下，对应自贡献的弛豫函数相当复杂，包含与偶极摆动相关的快速衰减以及可以用两个指数很好描述的第二个弛豫步骤：一个低振幅快速过程（τ0 = 0.31 ps）和一个使偶极取向完全随机化的主要缓慢过程（τself = 5.4 ps）。除了偶极弛豫函数外，我们还计算了表征水分子平移和旋转的类似散射的量。尽管这些过程在短时间范围内可被视为“跳跃”过程，但在大约τD - τcross的时间尺度上，此时交叉偶极相关性衰减至零，观察到的行为与对应于非耦合布朗平移和旋转扩散的行为无法区分。我们认为这就是不考虑分子间偶极相互作用的德拜模型在时间t≳τD时似乎有效的原因。
