Abstract
Under the influence of a temperature gradient, particles diffuse to their more favourable temperature, inducing a concentration gradient, a phenomenon termed thermodiffusion. The shift between thermophobic and thermophilic transport occurs at the inversion temperature T(0). Previous theories imply that only thermophobic behaviour, i.e. the absence of T(0), occurs at infinite dilution due to the absence of ion-ion interactions, but this has yet to be confirmed. In this study, phase-shifting interferometry visualisation experiments reveal that decreasing concentration leads to a more dominant thermophobic behaviour in different alkali halide (LiCl, NaCl, KCl, NaF and NaI) aqueous solutions. However, finite T(0) can still be observed at a low concentration of 0.02 m. Free-energy perturbation (FEP) simulations with a single ion confirm that all ion types are thermophobic at ultra-dilution, except I(-); however, this is resolved when considering counterion effects in NaI. In addition, a statistical mechanics model supports that thermophobic behaviour is predominantly driven by hydration entropy ΔS(hyd). However, there are generally negligible terms that become meaningful when ΔS(hyd) is small, and counterion effects are important in understanding thermodiffusive behaviour. Focusing on the ultra-dilute concentrations allows a distinction of the ion-water interaction from the multi-ion effects and is another step towards demystifying thermodiffusion.