The Roles of Salt Concentration and Cation Charge in Collisions of Ar and DCl with Salty Glycerol Solutions of NaI and CaI2

• Published in: Journal of physical chemistry. C 2008-02, Vol.112 (8), p.3008-3017
• Main Authors: DeZwaan, Jennifer L, Brastad, Susan M, Nathanson, Gilbert M
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp077732z

Gas−liquid scattering experiments are used to investigate the roles of ion concentration and ion charge in reactions of DCl with glycerol containing dissolved NaI and CaI2. Previous studies show that DCl molecules follow one of three pathways upon adsorption at the surface of pure glycerol:  immediate desorption of DCl back into the gas phase, near-interfacial DCl → HCl exchange, and longtime solvation and dissociation. The electrolytes NaI and CaI2 enhance immediate DCl desorption and D → H exchange at the expense of bulk solvation. We find that these enhancements rise linearly from 1.2 to 3.9 M NaI (5.5 to 1.4 glycerol molecules per ion), suggesting that the limited solvation of Na+ and I- and greater ion−ion association at higher concentrations do not abruptly change or reverse trends in these pathways. DCl desorption and D → H exchange are equally enhanced by 0.7 M CaI2 and 1.2 M NaI (nearly equal I- concentrations), but 1.3 M CaI2 is twice as effective as 2.6 M NaI. This departure may be driven by the closer proximity of Ca2+ to the surface at higher ion concentrations, especially if ion pairing between Ca2+ and surface-active I- drags the cation toward the interfacial region. Argon atom scattering and surface tension measurements provide independent evidence for the presence of ions at the surfaces of the salt solutions.