Abstract EN:

The microscopic comprehension of chemical reactions that occur at the boundary between water and other media represents an essential step in chemical science for the development of high yield chemical reactions. However the experimental difficulty in discerning the specific catalytic effect originating from the specific interfacial water environment makes the role of the interface along the reaction pathways still far to be understood.The main objective of this PhD thesis has been to provide a molecular description of chemical reactions at aqueous interfaces by Density Functional Theory-based Molecular Dynamics simulation techniques (DFT-MD/AIMD) in order to rationalize the catalytic roles of interfaces, especially from the structural point of view.The first part of the manuscript is dedicated to the study of chemical reactions of relevance in prebiotic chemistry, where many routes at aqueous interfaces have been envisaged in the literature in order to rationalize the origin of the first biopolymers on the primordial Earth. In particular the first peptide bond condensation reaction is a highly debated topic. The condensation of amino acids into oligopeptides in absence of enzymes is known to be hindered by both thermodynamic and kinetic reasons in bulk water at ambient conditions. By contrast, Prof. Vaida group has experimentally observed the formation of polypeptides at room temperature in dilute conditions from amino acid esters and CuCl2 salt at the air-water interface, suggesting the surface of oceans as a suitable environment for the birth of life on the prebiotic Earth. However the specific role of the air-water interface during the peptide bond formation and the reasons for which this reaction occurs at the interface and not in bulk water are still unclear.We provide a microscopical insight into the peptide bond formation reaction at the air-water interface by DFT-MD simulations. The characterization of the interfacial water network, coupled with the comparison between energetics and reaction mechanisms observed at the air-water interface versus bulk water allows us to reveal the key factors that promote the polypeptides formation in such heterogeneous conditions. Our data point to novel catalytic roles of the air-water interface which are essential in making the reaction occur. We especially identify why the solvation properties of the reactive species and the presence of Cl- anions at the interface are of utmost importance in catalyzing the peptide bond condensation reaction.In the second part of this PhD, we have focused on the characterization of the structure and reactivity of aqueous amorphous silica interfaces by coupling DFT-MD and SFG (Sum Frequency Generation) spectroscopy. This is especially relevant for the development of new technologies in the field of materials design and heterogeneous catalysis. The main targets of our investigation is the comprehension of the silica surface reactivity in contact with liquid water as a function of pH conditions. We have followed the evolution of the silica surface structure and chemistry in response to the variation in the water pH conditions by coupling experimental SFG spectroscopy (in collaboration with Prof. Wei-Tao Liu, China) with DFT-MD simulations. Our work provides a deep insight into the controversial acid-base behaviour of the silica surface, showing that a molecular picture based only on the balance between silanol SiOH and silanolate SiO- species at the silica surface is not enough to rationalize the trends in the SFG bands as a function of pH. The emergence of a third molecular species, denoted ”Si5”, reveals a more complex acid-base surface chemistry, and strongly modulates the silica surface acidity. Our data provide a new microscopical rationalization of the acid-base bimodal behaviour of the silica surface groups observed experimentally and reveal the Si5 species as an essential key to understand the chemistry at aqueous silica surfaces.