Experimental Confirmation of H₂O₂ Adsorption at the Water–Air Interface

Abstract

Recent work has reported that hydrogen peroxide is formed at the air−water interface. Given the reduced solvation environment there, this process could give rise to enhanced production of OH from H 2 O 2 photolysis at the interface. These considerations give some importance to understanding the adsorption thermochemistry of hydrogen peroxide. Although there are two molecular dynamics studies that provide the adsorption free energy, to date there is no experimental verification that H 2 O 2 adsorbs at the air−water interface. Here we use glancing-angle Raman spectroscopy to follow the surface adsorption behavior of this molecule. Using standard states of 1 mol L −1 for each of the bulk and surface phases yields a ΔG°of −5 kJ mol −1 at 293 K, comparable to that obtained for DMSO.

Experimental and Thermodynamic Viewpoints on Claims of a Spontaneous H2O2 Formation at the Air–Water Interface

“…Hence, the only possibility that could explain the spontaneous H 2 O 2 formation from water droplets is the Gibbs free energies of H 2 O 2 and H 2 decrease when moving from bulk to the interface (Figure b). H 2 O 2 is energetically −1.2 kcal/mol more favorable at the air–water interface (as compared to its free energy in liquid water) as measured recently by glancing-angle Raman spectroscopy . MD simulations showed that H 2 O 2 and other small molecule gases such as N 2 and O 2 are about −1 kcal/mol more favorable at the air–water interface than in water .…”

“…Some interface-sensitive spectroscopies, such as sum-frequency generation (SFG), can potentially probe intermediates or H 2 O 2 product at water droplet surfaces. Recent glancing-angle Raman spectroscopy on 1 M H 2 O 2 solution confirmed the surface propensity of H 2 O 2 at the water–air interface with the standard free energy adsorption of −1.2 kcal/mol . This adsorption energy had also been predicted by molecular dynamics (MD) simulations. , However, low concentrations of these species could be a challenge for detection.…”

“…H 2 O 2 is energetically −1.2 kcal/mol more favorable at the air−water interface (as compared to its free energy in liquid water) as measured recently by glancingangle Raman spectroscopy. 37 MD simulations showed that H 2 O 2 and other small molecule gases such as N 2 and O 2 are about −1 kcal/mol more favorable at the air−water interface than in water. 38 H 2 is expected to have an energy profile with a less noticeable change.…”

Accelerated Photolysis of H2O2 at the Air–Water Interface of a Microdroplet

“…Relative to the air–water interface, the H 2 O 2 molecule in bulk water was approximately 0.7 kcal/mol higher. Our calculations are consistent with recent experimental results confirming the enrichment of H 2 O 2 molecules at the air–water interface.…”

Advances in Atmospheric Chemical and Physical Processes

“…Reactions on surfaces are examined, including a kinetic and mechanistic study of gas–solid and gas-phase ozonolysis of nitrogen-containing alkenes, and the formation of organic nitrates in the heterogeneous reaction of α-pinene with mineral surfaces . Laboratory studies of the air–water interface demonstrate surface adsorption of H 2 O 2 , as well as determining the structure of hydroxy organic acids of varying size at the interface . Additional work addresses adsorption–desorption of semivolatile VOCs on mineral surfaces, and reactive uptake of HgO and a recently discovered dimethyl sulfide oxidation product (hydroperoxymethyl thioformate) on aerosols of various compositions. , The effects of aerosol properties on ice nucleation and on the partitioning of semivolatiles, , the effects of pH on organosulfate structures in the condensed phase, and the accessibility of various nanostructures for surfactants on atmospheric aerosol proxies are also reported.…”