Analysis of the second harmonic generation signal from a liquid/air and liquid/liquid interface

International audienceThree different liquid interfaces, water/air, thiophene/air, and water/thiophene, were probed using the second harmonic generation (SHG) technique. Thiophene and water have been chosen because the hyperpolarizability of these molecules has already been measured or calculated and the different values can be found in literature. We have studied the microscopic structure of these interfaces by comparing the components of the second order susceptibility tensor determined from the SHG polarization curve analysis with those determined via a molecular dynamics (MD) simulation of these interfaces. We have indeed computed the structure and orientation of water and thiophene molecules at the liquid/air and liquid/liquid (L/L) interfaces as a function of the distance from the interface. The integrated susceptibility values calculated by MD simulations agree well with SHG results and validate the choice of force fields that should permit to quantify more complex L/L interfaces

Why Local and Non-local Terms are Essential for Second Harmonic Generation Simulation?

International audienceSecond Harmonic Generation (SHG) today represents one of the most powerful techniques to selectively probe all types of interfaces. However, the origin of the SHG signal at a molecular level is still debated since the local dipole contribution, which is strongly correlated to the molecular orientation can be counterbalanced by non-local quadrupole contributions. Here, we propose a method to simulate the SHG signal of a model water/air interface from the molecular response of each contribution. This method includes both local and non-local terms, which are represented, respectively, by the dependency of the polarisability and hyperpolarisability upon the chemical environment of the molecule and by the bulk quadrupole response. The importance of both terms for the sound simulation of the SHG signals and their interpretation is assessed. We demonstrate that the sole dipole term is unable to simulate a SHG signal, even if the dependency of the hyperpolarisability on the local environment is considered. The inclusion of the bulk quadrupole contribution, which largely dominates the dipole contribution, is essential to predict the SHG response, although the accuracy of the prediction is increased when the dependency upon the local environment is considered

Deciphering Second Harmonic Generation Signals

International audienceSecond harmonic generation (SHG) has emerged as one of the most powerful techniques used to selectively monitor surface dynamics and reactions for all types of interfaces as well as for imaging non-centrosymmetric structures, although the molecular origin of the SHG signal is still poorly understood. Here, we present a breakthrough approach to predict and interpret the SHG signal at the atomic level, which is freed from the hyperpolarisability concept and self-consistently considers the non-locality and the coupling with the environment. The direct ab initio method developed here shows that a bulk quadrupole contribution significantly overwhelms the interface dipole term in the purely interfacial induced second-order polarisation for water/air interfaces. The obtained simulated SHG responses are in unprecedented agreement with the experimental signal. This work not only paves the road for the prediction of SHG response from more complex interfaces of all types, but also suggests new insights in the interpretation of the SHG signal at a molecular level. In particular, it highlights the modest influence of the molecular orientation and the high significance of the bulk quadrupole contribution, which does not depend on the interface, in the total experimental response

Synthesis and Characterization of a Chromo-Extractant to the Probe Liquid–Liquid Interface in a Solvent Extraction Process

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Surface activity and molecular organization of metallacarboranes at the air/water interface revealed by nonlinear optics

International audienceOwing to their amphiphilic structure, surfactants adsorb at the water/air interface with their hydrophobic tails pointing out of the water and their polar heads plunging into the water phase. Unlike classical surfactants, metallabisdicarbollides (MCs), nanometer-sized inorganic anions composed of two carborane semi-cages sandwiching a metal ion, do not have a well-defined amphiphilic structure. However, MCs have been shown to share many properties with surfactants, such as self-assembly in water (formation of micelles and vesicles), formation of lamellar lyotropic phases and surface activity. We have shown here, by combining Second Harmonic Generation (SHG) and surface tension measurement, that cobaltabis(dicarbollide) anion ([(C2B9H11)2Co]-) also named [COSAN]- with H+ as counterion, the most representative metallacarborane, adsorbs vertically at the water surface by forming a monolayer. This vertical molecular orientation facilitates the formation of intermolecular di-hydrogen bonds such as B-Hδ-…δ+H-C that has recently been proved to be at the origin of the self-assembly process of MCs in water. Therefore it appears here that lateral di-hydrogen bonds are also involved in the surface activity of MCs

SiC-TiC nanocomposite for bulk solar absorbers applications: Effect of density and surface roughness on the optical properties

International audienceIn this study, the potential of SiC-TiC nano-composites as solar absorbers has been studied. For solar thermal applications, materials with high solar absorptance and low emittance are ideally sought for (spectral selectivity). A semi-molecular sol-gel synthesis route leading to nanometric homogenous composites was described. The resulting SiC-TiC nanocomposite powder was sintered at different temperatures to produce samples with various relative densities (from 57% to 96%). The samples morphology and composition were characterized by several techniques including Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray Spectroscopy (EDX), X-Ray Diffraction (XRD), carbon and oxygen elemental analyses. The link between the surface roughness and the relative density was precised and the effects on the optical properties (0.25–25 µm wavelength range) were studied. Comparisons were made with pure SiC and pure TiC samples with various relative densities.Overall, the sample emittance was found to strongly decrease with the increase in the relative density, leading to a great increase in the spectral selectivity, despite a little decrease in the solar absorptance. The TiC-SiC composite has an intermediate reflectance compared to the pure SiC and the pure TiC samples. With an absorptance of 0.76, an emittance of 0.44 and a selectivity of 1.74, the denser SiC-TiC could be a good candidate for bulk solar applications

Effect of TiC incorporation on the optical properties and oxidation resistance of SiC ceramics

International audienceIn concentrating solar power (CSP) technologies, the absorber material should be spectrally selective under severe operating conditions, i.e. ~1000°C under air atmosphere. SiC-TiC nanocomposites could be good candidates for this application due to their spectral selectivity at room temperature, but their behavior under CSP operating conditions is unknown. Therefore, the spectral selectivity and oxidation resistance of TiC-SiC composites were studied up to 500 °C, under air, for various compositions. During heating, the emittance increased with the temperature. After heating, the TiC grains at the surface were oxidized into TiO2, while SiC grains showed good behavior. However, despite little oxidation, the spectral selectivity of the nanocomposites remained interesting after heating. Specific oxidation tests up to 9 days at 1000°C under air showed an increase of the oxidation resistance with the amount of SiC. This result is the consequence of the formation of a protective layer of SiO2 preventing the oxidation of the TiC grains. The composition 30 at% TiC-70 at% SiC seems to be a good compromise between low oxidation and good selectivity