Discrete stages in the solvation and ionization of hydrogen chloride adsorbed on ice particles

Ionization and dissociation reactions play a fundamental role in aqueous chemistry. A basic and well-understood example is the reaction between hydrogen chloride (HCl) and water to form chloride ions (Cl-) and hydrated protons (H3O+ or H5O2+). This acid ionization process also occurs in small water clusters1,2,3,4 and on ice surfaces5,6,7,8,9,10,11,12,13,14,15,16,17, and recent attention has focused on the mechanism of this reaction in confined-water media and the extent of solvation needed for it to proceed1,2,3,4,9,15,16,17. In fact, the transformation of HCl adsorbed on ice surfaces from a predominantly molecular form to ionic species during heating from 50 to 140 K has been observed8,13,14. But the molecular details of this process remain poorly understood. Here we report infrared transmission spectroscopic signatures of distinct stages in the solvation and ionization of HCl adsorbed on ice nanoparticles kept at progressively higher temperatures. By using Monte Carlo and ab initio simulations to interpret the spectra, we are able to identify slightly stretched HCl molecules, strongly stretched molecules on the verge of ionization, contact ion pairs comprising H3O+ and Cl-, and an ionic surface phase rich in Zundel ions, H5O2+.