Computing excess functions of ionic solutions: the smaller-ion shell model versus the primitive model. 1. Activity...

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Ionic Solutions

Background:

• Activity coefficients of ionic solutions are crucial for understanding electrolyte behavior.
• Traditional methods like Monte Carlo simulations of the primitive model (MC-UPM) require empirical adjustments of ion-size parameters (ISPs) for experimental agreement.
• Existing theories often struggle with accurate predictions across various electrolyte types and concentrations.

Purpose of the Study:

• To compare the predictive accuracy of the Smaller-ion Shell (SiS) treatment, a Debye-Hückel type theory (DH-SiS), with the established MC-UPM for calculating activity coefficients of binary ionic solutions.
• To evaluate the performance of DH-SiS and MC-UPM for various electrolytes, including NaCl, KCl, NaClO4, CaCl2, Ca(ClO4)2, and LaCl3 in water at 25 °C.
• To assess the agreement of theoretical single-ion activity coefficients with experimental data.

Main Methods:

• Utilized the DH-SiS theory, which considers counterions of unequal size.
• Performed parallel fitting and ISP adjustment for both DH-SiS and MC-UPM.
• Compared theoretical predictions with experimental activity coefficients for representative electrolytes.

Main Results:

• DH-SiS demonstrated significantly better fit quality and extended accuracy to higher concentrations for 1-1 electrolytes compared to MC-UPM.
• Theoretical single-ion activity coefficients from DH-SiS showed good agreement with experimental estimations.
• MC-UPM predictions were found to be inconsistent with experimental data for single-ion activity coefficients.
• DH-SiS allows for the use of crystallographic ion diameters as co-ion ISPs and simplifies the application of ISP nonadditivity for counterions.

Conclusions:

• The DH-SiS treatment offers superior accuracy and broader applicability than MC-UPM for predicting activity coefficients in ionic solutions.
• DH-SiS provides a more robust theoretical framework for understanding electrolyte behavior, particularly concerning single-ion properties.
• The advantages of DH-SiS in ISP handling suggest its potential for more reliable modeling of complex ionic systems.