Differentiated dissociation and distribution of species in concentrated hydrochloric acid at interface and in the bulk: Controllable separation based on specific ion recognition

Abstract

Understanding the dissociated species of concentrated hydrochloric acid and their distribution on surfaces versus in the bulk phase is a challenging aspect of chemistry separation processes. The interfacial affinity sequence for species in concentrated hydrochloric is HCl > HCl₂^- > Cl^- > CIPs > H₂Cl⁺ >H₃O⁺. Ionization of HCl molecules at the interface begins with the collision between the oxygen atom in water molecules and the hydrogen atom in HCl molecules. The covalent bonds between H and Cl in the HCl molecule transform spontaneously from the H-bond network of the water cluster to ionic bonds in H₃O⁺Cl^- contact ion pairs (CIP). The dynamic equilibrium of HCl molecule solvation and CIP de-solvation resulted in the interface being occupied by HCl, HCl₂^- and Cl^-, while H₂Cl⁺ and H₃O⁺ remained away from the interface. This inhibits the reaction of N1923 molecules with H₃O⁺, driving N1923 to perform substitution instead of ionic association.