The European Physical Journal E ( IF 1.8 ) Pub Date : 2022-04-21 , DOI: 10.1140/epje/s10189-022-00193-3 Alejandro Diaz-Marquez 1 , Guillaume Stirnemann 1
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Abstract
Thermodiffusion is the phenomenon by which molecules in a mixture present concentration gradients in response to an imposed temperature gradient. Despite decades of investigations, this effect remains poorly understood at a molecular level. A common, phenomenological approach is to individuate the molecular factors that influence the Soret coefficient, the parameter that quantifies the resulting concentration-gradient. Experimental studies, often performed on organic mixtures, as well as simulations of model particle systems have evidenced that the difference in masses between the mixture components has an important effect on the amplitude of the Soret coefficient. Here, we use molecular dynamics simulations of a thermophoretic setting to investigate the mass dependence of the Soret coefficient in dilute aqueous solutions. An advantage of simulation approaches is that they are not limited in the range of explored molecular masses, which is often limited to isotopic substitutions in the experiments. Our simulations reveal that the mass dependence of the Soret coefficient in these solutions is in agreement with previous experimental and simulation work on molecular-size systems. In particular, it is sensitive to the relative mass difference between the solute and the solvent, but not to their absolute mass. Adjusting the mass of the solvent and of the solute can turn a thermophobic solution into a thermophilic one, where solute accumulation is reversed. This demonstrates that the mass effect can indeed compensate for the other contributions to the Soret coefficient. Finally, we find that changing the molecular moments of inertia has a much more limited impact as compared to a change in the total molecular mass.
Graphical Abstract
中文翻译：
稀水溶液中热扩散的质量效应
摘要
热扩散是混合物中的分子响应于施加的温度梯度而呈现浓度梯度的现象。尽管进行了数十年的研究，但这种效应在分子水平上仍然知之甚少。一种常见的现象学方法是对影响 Soret 系数的分子因素进行个体化，Soret 系数是量化所得浓度梯度的参数。通常对有机混合物进行的实验研究以及模型粒子系统的模拟已经证明，混合物成分之间的质量差异对 Soret 系数的幅度有重要影响。在这里，我们使用热泳设置的分子动力学模拟来研究稀水溶液中 Soret 系数的质量依赖性。模拟方法的一个优点是它们不受探索的分子质量范围的限制，这通常仅限于实验中的同位素取代。我们的模拟表明，这些解决方案中 Soret 系数的质量依赖性与先前对分子大小系统的实验和模拟工作一致。特别是，它对溶质和溶剂之间的相对质量差异敏感，但对它们的绝对质量不敏感。调整溶剂和溶质的质量可以将嗜热溶液转变为嗜热溶液，其中溶质积累被逆转。这表明质量效应确实可以补偿对 Soret 系数的其他贡献。最后，
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