Thermodiffusion is the migration of solute molecules along a thermal gradient and it is quantified by the Soret coefficient (ST). In aqueous solutions, ST is highly sensitive to the nature of solute-water interactions and has emerged as powerful probe of molecular interactions, including protein-ligand binding. Despite its growing use, the microscopic mechanisms behind thermodiffusion remain unclear, largely due to the complexity of solvents with specific interactions and solutes containing both ionic and non-ionic groups. To disentangle these effects, we systematically investigate aqueous ammonium chloride salts with progressively increasing alkyl substitution: ammonium chloride (NH4Cl), dimethylammonium chloride (DMACl), ethylammonium chloride (EACl), and trimethylammonium chloride (TMACl). These substitutions simultaneously increase hydrophobicity while modifying molecular size, weight, and charge distribution. Using thermal diffusion forced Rayleigh scattering (TDFRS) over a temperature range of 15–45 °C and concentrations of 1–4 m, we examined how these structural variations influence thermodiffusion. Our results show that eachadditional alkyl substitution consequently decreases the temperature sensitivity of the Soret coefficient (∆ST(∆T)), which is typically related to a decrease in solute hydrophilicity. These results highlight thermodiffusion as a powerful tool for dissecting how hydrophilicity and ion-specific interactions dictate molecular transport in aqueous systems.
Rudani et al. (Thu,) studied this question.