Department of Chemistry, Imperial College London, SW7 2AZ, United Kingdom.
Sci Rep. 2017 Mar 21;7:44833. doi: 10.1038/srep44833.
Thermal gradients induce concentration gradients in alkali halide solutions, and the salt migrates towards hot or cold regions depending on the average temperature of the solution. This effect has been interpreted using the heat of transport, which provides a route to rationalize thermophoretic phenomena. Early theories provide estimates of the heat of transport at infinite dilution. These values are used to interpret thermodiffusion (Soret) and thermoelectric (Seebeck) effects. However, accessing heats of transport of individual ions at finite concentration remains an outstanding question both theoretically and experimentally. Here we discuss a computational approach to calculate heats of transport of aqueous solutions at finite concentrations, and apply our method to study lithium chloride solutions at concentrations >0.5 M. The heats of transport are significantly different for Li and Cl ions, unlike what is expected at infinite dilution. We find theoretical evidence for the existence of minima in the Soret coefficient of LiCl, where the magnitude of the heat of transport is maximized. The Seebeck coefficient obtained from the ionic heats of transport varies significantly with temperature and concentration. We identify thermodynamic conditions leading to a maximization of the thermoelectric response of aqueous solutions.
热梯度会在碱金属卤化物溶液中引起浓度梯度，盐会根据溶液的平均温度向热区或冷区迁移。这种效应可以用输运热来解释，这为合理化热泳现象提供了一种途径。早期的理论提供了无限稀释时输运热的估计值。这些值用于解释热扩散（索雷特）和热电（塞贝克）效应。然而，在理论和实验上，获取有限浓度下单个离子的输运热仍然是一个悬而未决的问题。在这里，我们讨论了一种计算有限浓度水溶液输运热的计算方法，并将我们的方法应用于研究浓度大于 0.5M 的氯化锂溶液。输运热对于 Li 和 Cl 离子有显著的差异，这与无限稀释时的预期不同。我们从理论上证明了 LiCl 的 Soret 系数存在最小值，其中输运热的幅度最大。从离子输运热得到的塞贝克系数随温度和浓度有很大的变化。我们确定了导致水溶液热电响应最大化的热力学条件。
