H₃O⁺Cl^- ion pairs on the surface of concentrated hydrochloric acid: Reverse mass transfer of dissociated hydronium and chloride ions and the effect on separation at interface

The separation process in hydrochloric acid environments is determined by the interfacial chemistry of the acid. However, the surface species of concentrated acids are complex. The effect of dominant species on the surface of concentrated acids and their dissociation behavior on interfacial separation has been the focus of academic debate. The extraction and separation of two typical anions, PdCl42- and PtCl62-, with different hydration characters on the surface of concentrated hydrochloric acid were investigated by thin layer extraction (TLX). A new phenomenon was discovered that the effect of hydrochloric acid concentration, extractant concentration, organic film thickness, and shear flow rate on the extraction of easily hydrated PdCl42- anions is greater than that of hardly hydrated PtCl62-. However, during the conventional SX process, the phenomenon cannot be observed due to the continuous dispersion and aggregation of oil droplets. It found that HCl molecules only dissociate into solvated H3O+& mldr;Cl- ion pairs on the surface of concentrated hydrochloric acid, but not H3O+ and Cl- ions respectively. The concentration of dissociated H3O+ ions at the interface is lower than that in the bulk phase. The protonation of the A336 molecule promotes the migration of dissociated H3O+ ions in the bulk phase towards the interface. Because the PdCl42- anions are more likely to be hydrated than PtCl62- anions and combine with H3O+ ions, the orderly migration of H3O+ ions in the laminar boundary layer towards the interface drives the mass transfer of easily hydrated PdCl42- anions. The anion exchange between PdCl42- and Cl- ions associated with the A336 molecules results in a higher concentration of Cl- ions enriched at the interface than that in the bulk phase. The migration of easily hydrated Cl- ions towards the bulk phase would drag the water molecules around PtCl62- anions and therefore inhibit the mass transfer of PtCl62- anions. The orderly reverse migration of H3O+ and Cl- ions enhances the separation between easily hydrated PtCl62- and hardly hydrated PdCl42- anions. This study offers a new perspective on the dissociation of species on the surface of concentrated hydrochloric acid and its impact on separation processes.