Light at interfaces

The molecular organization of liquids, in bulk or at interfaces such as gas-liquid and liquid-liquid boundaries, is fundamental to many physical, chemical, and biological processes. At interfaces, the asymmetry of forces modifies the local composition, structure, dielectric response, and transport properties, while driving key reactions such as extraction, catalysis, membrane transport, and drug delivery. Molecular organization of interfaces play also a key role in food engineering and cosmetics. Tuning the molecular nature or the concentration of surfactants can modify drastically foam or bubbly flow properties: bubble velocity, coalescence, foam rheology or stability. In bulk, solvent organization governs hydration, solvation, ion dissolution, and protein folding through ion-dipole interactions that remodel the liquid's dipole network.

Light offers a powerful and non-invasive way to study these phenomena at the molecular level. When it interacts with liquids and their interfaces, it carries direct signatures of their microscopic organization. Advanced optical techniques such as spectroscopic ellipsometry, nonlinear optics (second harmonic generation), and Raman spectroscopy can thus be used to reveal the organization of solvents, interfacial molecular arrangements, or the influence of ions on their environment.

Highlights

Probing the Molecular Origin of Polarization-Resolved SHG at Aqueous Interfaces

Second Harmonic Generation (SHG) has been applied to study air/water interface. By analyzing both local dipole and non-local quadrupole contributions, one can access key interfacial properties such as molecular orientation and charge distribution. A major challenge, however, lies in establishing a clear link between the measured SHG intensity and the underlying molecular details.

In this work, we present a numerical approach to model polarization-resolved SHG intensities at a model vapor/liquid water interface. Our study accounts for the influence of the interfacial environment on molecular hyperpolarizability, offering new insights into the microscopic origins of SHG signals at aqueous interfaces.

Microscopic view on the polarization-resolved S-SHG intensity of the vapor/liquid interface of pure water, Guillaume Le Breton, et al., Journal of Chemical Physics (2024).

Nanoscale organization of liquid water

In order to probe the structuration of water at the nanoscale, we have developped a new approach to model the second harmonic scattering (SHS) intensity of liquid water, which helps to quantify different contributions to the signal. The study finds that molecular hyperpolarizability fluctuations and correlations up to the third solvation layer play a significant role in increasing and reshaping the scattering intensity. The developed approach can be applied to other pure liquids to provide a quantitative interpretation of SHS intensities and thus probe the structuration of these liquids at the nanoscale.

Liquid Water: When Hyperpolarizability Fluctuations Boost and Reshape the Second Harmonic Scattering Intensities, Guillaume Le Breton, et al., Journal of Physical Chemistry Letters (2023).

Understanding the solidification of plaster foam thanks to Raman spectroscopy

Plaster foams are solid porous materials obtained from liquid foams. We followed their evolution along the solidification process, by combining images of the foam bubbles and Raman spectroscopy. Thanks to the Raman signal, we have been able to probe in situ the solidification reaction (hydration of plaster into gypsum) and showed that the reaction timescale controls the pore size of the final material, through the arrest of bubble coarsening.

Monitoring Gypsum Plaster Setting in a Foam through Raman Spectroscopy, Joachim Trosseille, et al., Physical Review Applied (2022).

See the organization of molecules in an ionic liquid

Ionic liquids (ILs) are a new class of green solvents composed solely of ionic species (cations and anions). They are molten salts that are liquid at room temperature. ILs have attracted a lot of interest for a multitude of applications ranging from catalysis to extraction processes as well as the development of batteries and supercapacitors. Indeed, their ionic components can be chosen according to specific needs, giving them unique and adjustable properties. In this article, the bulk structuration of ionic liquids has been studied by second harmonic scattering (SHS). Orientational correlations of cations were observed at the nanometer scale and a radial distribution of these cations was evidenced. Beyond this organization in an IL, the authors have shown that the SHS technique provides structural observation in liquids at scales well-below the diffraction limit and these findings open interesting new perspectives in domains such as understanding extraction mechanisms in the waste recycling process.

Nonlinear Optical Signature of Nanostructural Transition in Ionic Liquids, Antonin Pardon, et al., Journal of Molecular Liquids (2021).