Thermophobic diffusion becomes dominant in ultra-dilute alkali halide aqueous solutions

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 T₀. Previous theories imply that only thermophobic behaviour, i.e. the absence of T₀, 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 T₀ 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.