Hydrophilicity controls thermodiffusion in alkylammonium chlorides

In this study, we examine the Soret effect of ammonium chloride ( ${\text{NH}}_4$ Cl) and its alkyl-substituted derivatives: dimethylammonium chloride (DMACl), ethylammonium chloride (EACl), and trimethylammonium chloride (TMACl) in aqueous solution using infrared thermal diffusion forced Rayleigh scattering. The Soret coefficient, $S_T$ , increases systematically with alkyl substitution, following the trend ${\text{NH}}_4$ Cl $\ll$ DMACl < EACl $\ll$ TMACl, while hydrophilicity decreases correspondingly. Across the investigated temperature range ( $15-{45}^{\circ }$ C) and concentrations (1-4 mol/kg), $S_T$ increases with both temperature and the degree of alkyl substitution. However, the concentration dependence varies among the salts. DMACl, EACl, and TMACl exhibit decreasing $S_T$ with increasing concentration and are predominantly thermophobic; TMACl remains thermophobic under all conditions. In contrast, ${\text{NH}}_4$ Cl shows a non-monotonic concentration dependence above $35_{}^{\circ }$ C and is largely thermophilic. We discuss the origin of this minimum at elevated temperatures in relation to other aqueous salt systems that exhibit non-monotonic behavior of $S_T$ with respect to concentration. Overall, each additional alkyl substitution decreases the temperature sensitivity of the Soret coefficient, $\Delta S_T\left(\Delta T\right)$ , consistent with reduced solute hydrophilicity. Furthermore, we observe a clear correlation between the thermal diffusion coefficient and the thermal expansion coefficient in these aqueous electrolyte solutions. This is consistent with the trends reported for nonpolar organic mixtures and aqueous solutions of non-ionic solutes. These findings highlight thermodiffusion as a sensitive probe for understanding how hydrophilicity and ion-specific interactions govern molecular transport in aqueous environments.