Department of Chemistry, Imperial College London, SW7 2AZ, London, UK.
J Phys Chem B. 2013 Jul 11;117(27):8209-22. doi: 10.1021/jp403862x. Epub 2013 Jun 26.
We report an extensive analysis of the non-equilibrium response of alkali halide aqueous solutions (Na(+)/K(+)-Cl(-)) to thermal gradients using state of the art non-equilibrium molecular dynamics simulations and thermal diffusion forced Rayleigh scattering experiments. The coupling between the thermal gradient and the resulting ionic salt mass flux is quantified through the Soret coefficient. We find the Soret coefficient is of the order of 10(-3) K(-1) for a wide range of concentrations. These relatively simple solutions feature a very rich behavior. The Soret coefficient decreases with concentration at high temperatures (higher than T ∼ 315 K), whereas it increases at lower temperatures. In agreement with previous experiments, we find evidence for sign inversion in the Soret coefficient of NaCl and KCl solutions. We use an atomistic non-equilibrium molecular dynamics approach to compute the Soret coefficients in a wide range of conditions and to attain further microscopic insight on the heat transport mechanism and the behavior of the Soret coefficient in aqueous solutions. The models employed in this work reproduce the magnitude of the Soret coefficient, and the general dependence of this coefficient with temperature and salt concentration. We use the computer simulations as a microscopic approach to establish a correlation between the sign and magnitude of the Soret coefficients and ionic solvation and hydrogen bond structure of the solutions. Finally, we report an analysis of heat transport in ionic solution by quantifying the solution thermal conductivity as a function of concentration. The simulations accurately reproduce the decrease of the thermal conductivity with increasing salt concentration that is observed in experiments. An explanation of this behavior is provided.
我们使用最先进的非平衡分子动力学模拟和热扩散强制瑞利散射实验，对碱金属卤化物水溶液（Na(+)/K(+)-Cl(-)）的非平衡响应进行了广泛的分析。通过 Soret 系数来量化热梯度与由此产生的离子盐质量通量之间的耦合。我们发现，在很宽的浓度范围内，Soret 系数约为 10(-3) K(-1)。这些相对简单的溶液具有非常丰富的行为。在高温（高于 T ∼ 315 K）下，Soret 系数随浓度降低，而在较低温度下则增加。与先前的实验一致，我们发现 NaCl 和 KCl 溶液的 Soret 系数存在符号反转的证据。我们使用原子非平衡分子动力学方法在广泛的条件下计算 Soret 系数，并进一步深入了解热传输机制和水溶液中 Soret 系数的行为。这项工作中采用的模型再现了 Soret 系数的大小以及该系数随温度和盐浓度的一般依赖性。我们使用计算机模拟作为微观方法，建立 Soret 系数的符号和大小与溶液离子溶剂化和氢键结构之间的相关性。最后，我们通过量化浓度对溶液热导率的影响，分析了离子溶液中的热传输。模拟准确地再现了实验中观察到的随着盐浓度增加而热导率降低的行为，并提供了对此行为的解释。
