(496h) Understanding the Specific Conductance of NaCl Solutions in Water at Elevated Temperatures and Pressures

Supercritical water desalination is one of the alternative processes to obtain clean water and to retrieve valuable materials. The performance of the desalination process is usually evaluated in situ by measuring the specific conductance (electrical conductivity). However, the specific conductance of aqueous electrolytes at elevated temperatures and pressures are known to show a complex dependence on the process conditions, which makes it challenging to evaluate the desalination process. In order to understand how the specific conductance evolves as the process condition changes, we perform the molecular dynamics (MD) simulations of aqueous sodium chloride (NaCl) solutions at elevated temperatures and pressures (298 – 674 K at 200 bar). By applying the Green-Kubo linear response theory in conjunction with the spatial decomposition analysis based on the Voronoi network, we quantitatively examine how velocity autocorrelation and cross-correlation contributes to the specific conductance of the system. The contribution of the cross-correlation term between the oppositely charged ions, which arise from the formation of ion pairs, becomes significant above 548 K, where the experimental conductance data becomes its maximum. Together with the conductance analysis, we develop a cut-off free network scientific algorithm based on the Voronoi tessellation to classify ion pairs into solvent-separated, solvent-shared, and contact ion pairs. The classification result yields an accurate calculation of the ion association constant, which can also be used for the spatial decomposition of the specific conductance. Overall results suggest that the contact ion pair (CIP) formation is mainly responsible for the onset of the decrease in the specific conductance.