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

Area of Science:

• Physical Chemistry
• Solution Thermodynamics
• Mass Transport Phenomena

Background:

• Aqueous solutions of lithium bromide (LiBr-H2O) are crucial in various industrial applications, necessitating a deep understanding of their transport properties.
• Molecular diffusion and thermodiffusion (Soret effect) are key phenomena governing mass transport in these solutions, but their concentration dependence is not fully elucidated.

Purpose of the Study:

• To experimentally investigate mass transport phenomena, specifically molecular diffusion and thermodiffusion, in aqueous lithium bromide solutions.
• To characterize the concentration dependence of the diffusion coefficient (D) and Soret coefficient (ST).
• To analyze the interplay between molecular mobility and solution structure in determining transport behavior.

Main Methods:

• Experimental measurements of diffusion coefficients using standard techniques.
• Thermogradient measurements in a Soret cell containing porous media to determine Soret coefficients.
• Systematic variation of LiBr mass fraction (w) to study concentration effects.

Main Results:

• A non-monotonic trend in the diffusion coefficient (D) was observed, exhibiting a distinct maximum around w ≈ 0.3 kg/kg LiBr.
• Consistently negative Soret coefficients (ST) were measured across the studied concentration range, indicating thermophilic behavior (LiBr migration towards warmer regions).
• The absolute value of the thermodiffusion coefficient (|DT|) decreased with increasing concentration, while the magnitude of the Soret coefficient (|ST|) increased due to a more significant reduction in molecular diffusion.

Conclusions:

• The observed diffusion behavior suggests microstructural transitions within the LiBr-H2O solution.
• The thermophilic nature of LiBr in water is confirmed, with migration influenced by temperature gradients.
• Both molecular diffusion and the underlying solution structure are critical factors for accurately modeling mass transport in LiBr-H2O systems.