Overlapping hydration shells in salt solutions causing non-monotonic Soret coefficients with varying concentration

Area of Science:

• Physical Chemistry
• Solution Thermodynamics
• Transport Phenomena

Background:

• Understanding thermodiffusive properties of electrolyte solutions is crucial for various chemical processes.
• Previous studies often focused on dilute solutions, leaving behavior in more concentrated regimes less explored.

Purpose of the Study:

• To investigate the thermodiffusive properties of aqueous sodium iodide, potassium iodide, and lithium iodide solutions.
• To analyze the concentration and temperature dependence of the Soret coefficient (S_T) in these salt systems.

Main Methods:

• Utilized thermal diffusion forced Rayleigh scattering (TD-FRS) for precise measurements.
• Studied solutions across a concentration range of 0.5-4 mol kg^-1 and temperatures from 15 to 45 °C.

Main Results:

• Observed non-monotonic variations in the Soret coefficient (S_T) for all three iodide salts.
• Identified a consistent minimum in S_T around 1 mol kg^-1 concentration for all systems.
• Found that length and energy scales of hydrated ions are similar across the studied salts.

Conclusions:

• The observed minimum in S_T suggests a common behavior related to the packing of fully hydrated salt molecules.
• Preliminary models indicate S_T rises as hydrated particles approach random close packing.
• The study provides insights into salt solution behavior at higher concentrations, highlighting limitations of current models.