Covalent and Ionic States of Strong Acids at the Ice Surface

Abstract

The relationship between the degree of ionization and the environment of a strong acid is of basic scientific interest. Often this relationship reduces to the interdependence of iodacid hydration and proton transfer. Despite the presence of pure water, the surface of crystalline ice, particularly at cryogenic temperatures, is one of limited (controlled?) availability of water of hydration. Here, the detailed nature of the ice surface and the states of strong acids adsorbed to ice at cryogenic temperatures are examined. These subjects are of special current interest since the ability to model the complex chemistry that occurs on the surfaces of water-rich particles in the atmosphere, particularly in the stratosphere over the polar regions, requires a valid concept of the acid-ice interface. Our combined spectroscopic and simulation studies have identified the surface of free-standing ice particles as badly disordered, with a range of water-ring sizes and an increased level of H-bond saturation relative to an ordered ice surface. FT-IR results are reported for the interaction of the surface of such ice particles with submonolayer amounts of adsorbed DCI, DBr, and HNO, and for multilayer exposure to DCI. The DCI and DBr adsorbed states demonstrate behavior familiar from observations on strongly bound molecular adsorbates. Two methods have been devised for exposure of the nanocrystals to HNO,. One gives an ionic state initially, while the initial state of the other approach is molecular. In both instances, the system is observed to evolve, with time/warming, towards a common mixed molecular-ionic adsorbed state. ~ ~

Spectroscopic Study of HNO3Dissociation on Ice

“…This broad absorption is ubiquitous in the vibrational spectroscopy of aqueous acid solutions and is thus broadly recognized as the spectroscopic hallmark of the aqueous proton. The presence of aqueous protons suggests that most (some) HNO 3 molecules have dissociated in the 5–15% (20–60%) HNO 3 mole fraction cryogenic nitric acid solutions even at temperatures as low as 45 K, a significantly lower temperature than those of previous spectroscopic studies. ,− …”

Dissociative Adsorption of Nitric Acid at the Surface of Amorphous Solid Water Revealed by X-ray Absorption Spectroscopy

“…Investigating the chemical state of HNO 3 at the surface of supercooled solutions or ice therefore requires surface-sensitive spectroscopic tools and/or appropriate sample preparation methods. Only by reaching cryogenic temperatures (i.e., in the 70−135 K range by Devlin et al 14 and the 130−150 K range by Pursell et al 13 ) has molecular nitric acid been observed at the surface of ice. Unfortunately, both studies relied on FTIR spectroscopy, which lacks the surface specificity and sensitivity to detect submonolayer coverages of HNO 3 adsorbed onto ice.…”

“…Accordingly, their sample preparation methodology and their experimental conditions suggest that they observed molecular nitric acid as a result of multilayer condensation 13 or in the form of nitric acid nanocrystals at the surface of ice. 14 Recently, Krepelova et al 35 used near-edge X-ray absorption fine structure (NEXAFS) to study the adsorption of HNO 3 onto ice at 230 K, relying, however, on heterogeneous NO 2 hydrolysis as a source of HNO 3 . They reported the observation of nitrates dissolved within a 1.1 nm thick supercooled nitric acid solution layer at the ice surface, but they could not observe molecular HNO 3 under these conditions.…”

“…Given its major role in a plethora of environmental processes, the dissociative adsorption and acid–base chemistry of HNO 3 at aqueous interfaces continues to attract tremendous interest from both theoretical − and experimental perspectives. − Indeed, our understanding of several important atmospheric chemistry processes hinges on a quantitative description of the factors and parameters that control whether HNO 3 exists in its molecular or its dissociated form at the surface of atmospheric aerosols and of ice. For instance, this would impact our interpretation of the formation, stability, and reactivity of nitrates in urban particulate matter, − the atmospheric reactive nitrogen budget, − the intriguing NO x photochemical fluxes from snowpacks, ,, and the formation and lifetime of cirrus clouds. − …”

“…Unfortunately, both studies relied on FTIR spectroscopy, which lacks the surface specificity and sensitivity to detect submonolayer coverages of HNO 3 adsorbed onto ice. Accordingly, their sample preparation methodology and their experimental conditions suggest that they observed molecular nitric acid as a result of multilayer condensation or in the form of nitric acid nanocrystals at the surface of ice . Recently, Krepelova et al used near-edge X-ray absorption fine structure (NEXAFS) to study the adsorption of HNO 3 onto ice at 230 K, relying, however, on heterogeneous NO 2 hydrolysis as a source of HNO 3 .…”