PhD defense of Rawan ABOUHAIDAR

ABOUHAIDAR  Rawan, PhLAM Laboratory- UMR8523 - Team PCMT

Title: Molecular Scale Modeling of Heterogeneous Reactivity on the Surface of Particles of Atmospheric Interest

Jury: C. TOUBIN (PhLAM, encadrante), D. DENIS (PhLAM, encadrant), N. PRISLE (Center for Atmospheric Research, Rapporteur), D. LAAGE (ENS, département de chimie, Rapporteur), C. CLAVAGUERA (ICP Université Paris Saclay, membre), M.-P. GAIGEOT (Université d'Evry Val d'Essonne, membre), D. PETITPREZ (PC2A, Université de Lille, membre), B. WYSLOUZIL (The Ohio State University, membre), M. AMMANN (PSI, Center for Energy and Environmental Sciences, membre)

Abstract:

The atmosphere is a complex component of the Earth system, comprising various chemical species. Many of these atmospheric compounds are surface-active, with enhanced concentrations at surfaces of aqueous droplets. The partitioning of these compounds between surface and bulk phases can significantly influence heterogeneous chemistry and other properties of aerosol and cloud droplets. Therefore, a molecular-level understanding of the structure of liquid interfaces is essential for unraveling their roles in various chemical processes.

Ozonolysis of unsaturated hydrocarbons is particularly important for the atmosphere, as it generates diverse oxygen-containing compounds such as aldehydes, ketones, and carboxylic acids. In this study, we investigate the multiphase ozonolysis reaction of maleic acid in water droplets using classical and quantum theoretical methods (QM/QM’) along with Ab Initio Molecular Dynamics (AIMD) simulations to explore the formation of primary ozonide and its subsequent breakdown into Criegee intermediate within a water cluster. Kinetic calculations reveal that water molecules significantly enhance the reaction rate compared to the gas phase, with a larger rate coefficient observed in the bulk phase than at the interface. Additionally, water facilitates hydrogen atom transfer through dynamic loop structures, promoting a collective proton exchange mechanism.

Another critical aspect of cloud formation is heterogeneous ice nucleation, which is influenced by the surface properties of aerosol particles, particularly those with chemical groups capable of hydrogen bonding with water. Short-chained alcohols, such as 1-pentanol and 3-hexanol, which readily accumulate at the air-water interface, are of particular interest due to their potential impact on ice nucleation, but their role in freezing processes remains largely underexplored. To address this gap, we utilized Molecular Dynamics (MD) simulations combined with topological graph analysis to investigate the onset of alcohol-water surface freezing at temperatures ranging from 283 K to 192 K. Our study analyzed surface tension, solubility, and the integration of alcohols into the 2D hydrogen-bonded network of surface water. The results indicate that 1-pentanol forms more organized, tightly packed surface layers than 3-hexanol, effectively reducing surface tension and enhancing the formation of six-membered hydrogen-bonded rings at lower temperatures. This promotes the formation of ice-like structures, with 1-pentanol being more effective in facilitating freezing, offering insights relevant to cloud formation and climate dynamics.

Finally, the multiphase cycling of halogen species in sea spray aerosol particles is linked to the abundance of halide ions at the aqueous air-water interface, which impacts both the Earth's ozone budget and radiative balance. Inspired by liquid jet X-ray photoelectron spectroscopy (XPS) experiments, we performed MD simulations to assess the impact of electrostatic interactions between monofunctional surfactants with positive (hexylammonium) and negative headgroups (propylsulfate) on the abundance of bromide and sodium ions at the interface. The MD simulations, which try to mimic experimental conditions, confirm the surface enhancement of interfacial bromide concentration in solutions containing hexylammonium and hexylamine. In contrast, the negatively charged propylsulfate leads to an increased presence of sodium ions at the interface.

In conclusion, this thesis combines various theoretical methods and explores surface chemistry at air-water interfaces, providing valuable insights into our understanding of atmospheric processes.