Mass transport in LiBr-H2O solutions: Coupling between diffusion, thermodiffusion, and composition

We present an experimental investigation of mass transport phenomena in aqueous lithium bromide (LiBr-H2O) solutions, focusing on both molecular diffusion and thermodiffusion (Soret effect). Our results show a non-monotonic trend in the diffusion coefficient D with a pronounced maximum at w ≈ 0.3 kg/kg (LiBr), a feature that is consistently observed in multiple independent studies and likely reflects microstructural transitions within the solution. Using a Soret cell with porous media, we measured Soret coefficients ST over a range of mass fractions relevant for practical applications. The Soret coefficient revealed consistently negative values, indicating thermophilic behavior in which LiBr migrates toward warmer regions under a temperature gradient. Interestingly, while the absolute value of thermodiffusion coefficient |DT| decreases with concentration, the Soret ST = |DT/D| increases in magnitude due to the stronger suppression of molecular diffusion. These results highlight the importance of considering both molecular mobility and solution structure when analyzing transport behavior.