Molecular simulations of thermodiffusion and thermoelectric effects in aqueous solutions

Thermal gradients applied across aqueous solutions give rise to several fascinating coupling phenomena, such as the Ludwig-Soret (thermodiffusion or thermophoresis) and the Seebeck (thermoelectricity) effects, in which temperature gradients induce concentration gradients or electric fields, respectively. Experimental studies of alkali halide aqueous solutions reported changes in the sign of the Soret coefficient, as well as minima which appear at specific concentration and temperature. We perform non-equilibrium molecular dynamics (NEMD) simulations to investigate the thermophoretic response of those solutions. The existence of a minimum in the Soret coefficient of LiCl solutions is verified, showing that the hydration shell of Li+ has a significant impact on the minimum. The role of the hydration of solute in determining thermodiffusion of aqueous solutions is further highlighted, studying the thermal response of formamide and urea solutions, which is extremely sensitive to the solute-solvent interactions. Additionally, we describe a NEMD approach to quantify the Seebeck effect in aqueous solutions and its dependence on temperature and concentration. Our simulations provide microscopic insight into the origin of this phenomenon, highlighting the importance of the recently discovered thermal orientation of water in determining thermoelectricity. The Soret and Seebeck effects have been interpreted through the heat of transport. Although there are early estimations of this quantity, compute the heat of transport of individual ions remains an outstanding question both experimentally and theoretically. A new computational approach to calculate the heat of transport of aqueous solutions is provided. The heat of transport is significantly different from the theoretical estimates at infinite dilution. Innovative applications of thermal coupling phenomena involve nanofluidics devices. NEMD simulations are used to investigate thermophoresis in salt solutions and thermal orientation of water molecules confined in amorphous silica nanopores and carbon nanotubes, respectively. Our results show that confinements influence the thermal molecular orientation and make solutions more thermophilic.Open Acces