Structure and dynamics of concentrated hydrochloric acid solutions

Hydrochloric acid solutions are widely used in industrial and chemical processes, yet the behavior of excess protons in concentrated solutions remains poorly understood. Prior research has shown that hydrated protons can form metastable contact ion pairs, but the mechanisms behind this phenomenon are not fully resolved. The amphiphilic nature of hydrated protons suggests a unique interaction pattern that may differ from typical ion-pairing behavior. While general knowledge exists about proton solvation in dilute solutions, the structural and dynamic properties in concentrated HCl have not been extensively modeled. Earlier studies proposed that hydronium oxygen lone pairs align in ion pairs, but the extent of this behavior in higher concentrations is unclear. No prior work has systematically explored the effects of salt addition on proton stability and diffusion. This gap motivated further investigation into how concentration and salt content influence proton dynamics and ion-pair stability. Understanding these interactions could refine models of proton transport in concentrated electrolytes.

This study aimed to investigate the structural and dynamic properties of hydrated protons in concentrated hydrochloric acid solutions. The focus was on how proton concentration affects ion-pair formation and stability. Researchers sought to determine if the amphiphilic nature of hydrated protons influences their interactions with chloride counterions. The study also examined how salt addition impacts proton diffusion and contact ion pair lifetime. By simulating multiple HCl concentrations, the authors aimed to identify trends in proton behavior. They tested whether higher acid concentrations reduce proton pair lifetimes as previously suggested. The inclusion of KCl and NaCl allowed an assessment of salt effects on proton dynamics. The ultimate goal was to provide a detailed molecular-level view of proton interactions in concentrated acid solutions.

The researchers employed self-consistent iterative multistate empirical valence bond (SCI-MS-EVB) simulations to model proton behavior. They simulated four HCl concentrations: 0.43, 0.85, 1.68, and 3.26 M. The simulations focused on hydrated proton configurations and ion-pair formation. They analyzed the spatial arrangement of hydronium oxygen lone pairs relative to chloride ions. The study tracked proton diffusion rates and contact ion pair lifetimes. Salt addition was modeled by introducing KCl and NaCl into the acid solutions. The simulations captured proton-solvent and ion-ion interactions over time. The results were compared to earlier findings to confirm or challenge prior assumptions about proton behavior.

The simulations confirmed that hydrated protons form metastable contact ion pairs with chloride ions. The hydronium oxygen lone pairs align toward each other, as observed in prior work. This behavior is attributed to the amphiphilic nature of hydrated protons. At 3.26 M HCl, proton solvation structures remained largely unchanged. However, the average lifetime of proton ion pairs decreased with higher concentration. Adding KCl or NaCl increased the stability of proton pairs. Salt presence also reduced proton diffusion rates in the solution. These findings suggest that salt content influences proton mobility and pair stability.

The study demonstrates that hydrated protons in concentrated HCl solutions form metastable contact ion pairs. The alignment of hydronium oxygen lone pairs supports earlier observations of proton amphiphilicity. Increasing acid concentration has minimal impact on proton solvation structures but reduces ion pair lifetimes. Salt addition stabilizes proton pairs and slows proton diffusion. These results align with the authors' hypothesis about proton behavior in concentrated solutions. The findings suggest that proton dynamics are sensitive to both acid concentration and salt content. The study provides a molecular-level explanation for proton interactions in HCl solutions. The results highlight the importance of considering salt effects when modeling proton transport in concentrated electrolytes.