Molecular interaction volume model of mixing enthalpy for molten salt system: An integrated calorimetry-model case study of LaCl$_3$-(LiCl-KCl)

TL;DR

This study introduces a molecular interaction volume model (MIVM) that combines calorimetry and ab initio simulations to better understand and predict mixing enthalpies and thermodynamic properties of molten salt systems, especially for nuclear applications.

Contribution

The paper develops and applies an integrated MIVM approach that synthesizes experimental calorimetry data with molecular dynamics simulations to accurately model molten salt thermodynamics.

Findings

MIVM accurately predicts mixing enthalpies for LaCl₃-(LiCl-KCl) system.

Significant deviations occur when directly using molecular dynamics trajectories for enthalpy prediction.

The integrated approach improves understanding of solvation structures and thermodynamic properties in molten salts.

Abstract

Calorimetric determination of enthalpies of mixing ($\Delta$H$_{\rm mix}$) of multicomponent molten salts often employs empirical models that lack parameters with clear physical interpretation (e.g., coordination numbers, molar volumes, and pair potentials). Although such physics informed models are not always needed, a thermodynamic understanding of the relationships between excess energies of mixing and local to intermediate solvation structures is particularly important for pyrochemical separation, as is the case for lanthanides (Ln), which are common neutron poisons and critical industrial elements found in spent nuclear fuels. Here we implement the molecular interaction volume model (MIVM) to synthesize information from experimentally measured $\Delta$H$_{\rm mix}$ (using high temperature melt drop calorimetry) and the distribution of solvation structures from ab initio molecular dynamics (AIMD) simulations. This was demonstrated by a case study of molten salt system consisted of LaCl$_3$ mixing with a eutectic LiCl-KCl (58mol% to 42mol%) at 873 K and 1133 K. The parameters modelled from MIVM were used to extrapolate excess Gibbs energy ($\Delta$G$_{\rm mix}$), and compositional dependence of La$^{3+}$ activity in the LaCl$_3$-(LiCl-KCl) system. In contrast, by AIMD or polarizable ion model (PIM) simulations, a significant deviation regarding the predicted $\Delta$H$_{\rm mix}$ was seen if computed directly from the molecular dynamic trajectories. The integrated experimental and simulation data within the MIVM formalism are generalizable to a wide variety of molten salts and demonstrate a significant improvement over currently employed methods to study molten salts for nuclear and separations sciences.