Tight electrostatic regulation of the OH production rate from the photolysis of hydrogen peroxide adsorbed on surfaces

The Structure of Carbon Dioxide at the Air-Water Interface and its Chemical Implications

The efficient reduction of CO2 into valuable products is a challenging task in an international context marked by the climate change crisis and the need to move away from fossil fuels. Recently, the use of water microdroplets has emerged as an interesting reaction media where many redox processes which do not occur in conventional solutions take place spontaneously. Indeed, several experimental studies in microdroplets have already been devoted to study the reduction of CO2 with promising results. The increased reactivity in microdroplets is thought to be linked to unique electrostatic solvation effects at the air-water interface. In the present work, we report a theoretical investigation on this issue for CO2 using first-principles molecular dynamics simulations. We show that CO2 is stabilized at the interface, where it can accumulate, and that compared to bulk water solution, its electron capture ability is larger. Our results suggest that reduction of CO2 might be easier in interface-rich systems such as water microdroplets, which is in line with early experimental data and indicate directions for future laboratory studies. The effect of other relevant factors which could play a role in CO2 reduction potential is discussed.

Water Dimer under Electric Fields: An Ab Initio Investigation up to Quantum Accuracy

It is well-established that strong electric fields (EFs) can align water dipoles, partially order the H-bond network of liquid water, and induce water splitting and proton transfers. To illuminate the fundamental behavior of water under external EFs, we present the first benchmark, to the best of our knowledge, of DFT calculations of the water dimer exposed to intense EFs against coupled cluster calculations. The analyses of the vibrational Stark effect and electron density provide a consistent picture of the intermolecular charge transfer effects driven along the H-bond by the increasing applied field at all theory levels. However, our findings prove that at extreme field regimes (∼1-2 V/Å) DFT calculations significantly exaggerate by ∼10-30% the field-induced strengthening of the H-bond, both within the GGA, hybrid GGA, and hybrid meta-GGA approximations. Notably, a linear correlation emerges between the vibrational Stark effect on OH stretching and H-bond strengthening: a 1 kcal mol-1 increase corresponds to an 80 cm-1 red-shift in OH stretching frequency.

Understanding the active role of water in laboratory chamber studies of reactions of the OH radical with alcohols of atmospheric relevance

In this work, we studied the reactions of three cyclic aliphatic alcohols with OH at room temperature, atmospheric pressure and different humidities in a Teflon reaction chamber. It was determined that the lower the solubility of the alcohol in water, the larger the effect of the humidity on the acceleration of the reaction. This experimental evidence allows suggesting that the acceleration is due to the reaction of the co-adsorbed reactants at the air-water interface of a thin water film deposited on the Teflon walls of the reaction chamber, instead of between co-reactants dissolved in the water film or due to gas phase catalysis as previously suggested. Therefore, formation of thin water films on different surfaces could have some implications on the tropospheric chemistry of these alcohols in the tropical regions of the planet with high humidity.

Significantly Accelerated Photosensitized Formation of Atmospheric Sulfate at the Air-Water Interface of Microdroplets

The multiphase oxidation of sulfur dioxide (SO2) to form sulfate is a complex and important process in the atmosphere. While the conventional photosensitized reaction mainly explored in the bulk medium is reported to be one of the drivers to trigger atmospheric sulfate production, how this scheme functionalizes at the air-water interface (AWI) of aerosol remains an open question. Herein, employing an advanced size-controllable microdroplet-printing device, surface-enhanced Raman scattering (SERS) analysis, nanosecond transient adsorption spectrometer, and molecular level theoretical calculations, we revealed the previously overlooked interfacial role in photosensitized oxidation of SO2 in humic-like substance (HULIS) aerosol, where a 3-4 orders of magnitude increase in sulfate formation rate was speculated in cloud and aerosol relevant-sized particles relative to the conventional bulk-phase medium. The rapid formation of a battery of reactive oxygen species (ROS) comes from the accelerated electron transfer process at the AWI, where the excited triplet state of HULIS (3HULIS*) of the incomplete solvent cage can readily capture electrons from HSO3- in a way that is more efficient than that in the bulk medium fully blocked by water molecules. This phenomenon could be explained by the significantly reduced desolvation energy barrier required for reagents residing in the AWI region with an open solvent shell.

Electrofreezing of liquid water at ambient conditions

Water is routinely exposed to external electric fields. Whether, for example, at physiological conditions, in contact with biological systems, or at the interface of polar surfaces in countless technological settings, water responds to fields on the order of a few V Å-1 in a manner that is under intense investigation. Dating back to the 19th century, the possibility of solidifying water upon applying electric fields - a process known as electrofreezing - is an alluring promise that has canalized major efforts since, with uncertain outcomes. Here, we perform long (up to 500 ps per field strength) ab initio molecular dynamics simulations of water at ambient conditions under external electric fields. We show that fields of 0.10 - 0.15 V Å-1 induce electrofreezing to a ferroelectric amorphous phase which we term f-GW (ferroelectric glassy water). The transition occurs after ~ 150 ps for a field of 0.15 V Å-1 and after ~ 200 ps for a field of 0.10 V Å-1 and is signaled by a structural and dynamic arrest and the suppression of the fluctuations of the hydrogen bond network. Our work reports evidence of electrofreezing of bulk liquid water at ambient conditions and therefore impacts several fields, from fundamental chemical physics to biology and catalysis.

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

Photochemical homolysis of hydrogen peroxide (H2O2) occurs widely in nature and is a key source of hydroxyl radicals (·OH). The kinetics of H2O2 photolysis play a pivotal role in determining the efficiency of ·OH production, which is currently mainly investigated in bulk systems. Here, we report considerably accelerated H2O2 photolysis at the air-water interface of microdroplets, with a rate 1.9 × 103 times faster than that in bulk water. Our simulations show that due to the trans quasiplanar conformational preference of H2O2 at the air-water interface compared to the bulk or gas phase, the absorption peak in the spectrum of H2O2 is significantly redshifted by 45 nm, corresponding to greater absorbance of photons in the sunlight spectrum and faster photolysis of H2O2. This discovery has great potential to solve current problems associated with ·OH-centered heterogeneous photochemical processes in aerosols. For instance, we show that accelerated H2O2 photolysis in microdroplets could lead to markedly enhanced oxidation of SO2 and volatile organic compounds.

Evaluation of relative efficiency of PDT photosensitizers in producing hydroxyl radicals and singlet oxygen in aqueous media using a UV-visible spectroscopy pNDA dosage

In order to improve the performance of PDT, it is important to develop new photosensitizers that induce the formation of both hydroxyl radicals and singlet oxygen. In this work, we developed and validated the experimental conditions and reproducibility for the evaluation of relative efficiency of hydroxyl radicals and singlet oxygen production by studying the bleaching of p-nitrosoaniline (pNDA) using a continuous flow UV-visible spectroscopy method in presence of photosensitizers in PBS media. Rapid data sampling made possible to analyze the kinetics of the bleaching by using a mathematical modeling. The pNDA dosage is specific of hydroxyl radicals' production without l-histidine and of singlet oxygen production in presence of l-histidine. A statistical approach is used to precisely evaluate the reliability of the results and to be able to compare different photosensitizers between them such as Methylene Blue and Brillant Blue G.

Electrostatics and Chemical Reactivity at the Air-Water Interface

It has been recently discovered that chemical reactions at aqueous interfaces can be orders of magnitude faster compared to conventional bulk phase reactions, but despite its wide-ranging implications, which extend from atmospheric to synthetic chemistry or technological applications, the phenomenon is still incompletely understood. The role of strong electric fields due to space asymmetry and the accumulation of ions at the interface has been claimed as a possible cause from some experiments, but the reorganization of the solvent around the reactive system should provide even greater additional electrostatic contributions that have not yet been analyzed. In this study, with the help of first-principles molecular dynamics simulations, we go deeper into this issue by a careful assessment of solvation electrostatics at the air-water interface. Our simulations confirm that electrostatic forces can indeed be a key factor in rate acceleration compared to bulk solution. Remarkably, the study reveals that the effect cannot simply be attributed to the magnitude of the local electric field and that the fluctuations of the full electrostatic potential resulting from unique dynamical behavior of the solvation shells at the interface must be accounted for. This finding paves the way for future applications of the phenomenon in organic synthesis, especially for charge transfer or redox reactions in thin films and microdroplets.

Experimental Confirmation of H2O2 Adsorption at the Water-Air Interface

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 H2O2 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 H2O2 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.