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

Abstract

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.