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Concept

Théorie de Mie

Résumé

En optique ondulatoire, la théorie de Mie, ou solution de Mie, est une solution particulière des équations de Maxwell décrivant la diffusion élastique – c'est-à-dire sans changement de longueur d'onde – d'une onde électromagnétique plane par une particule sphérique caractérisée par son diamètre et son indice de réfraction complexe. Elle tire son nom du physicien allemand Gustav Mie, qui la décrivit en détail en 1908. Le travail de son prédécesseur Ludvig Lorenz est aujourd'hui reconnu comme « empiriquement équivalent » et l'on parle parfois de la théorie de Lorenz-Mie. Elle reçut de nombreux apports du physicien Peter Debye dans les années qui suivirent. La théorie de Mie s'applique par exemple à des gouttes d'eau de taille macroscopique impliquées dans des phénomènes optiques météorologiques tels que la formation des arcs-en-ciel, des couronnes ou des gloires, mais on peut également l'appliquer à des particules microscopiques telles que des molécules, en remplaçant l'indice de réfrac

Source officielle

https://fr.wikipedia.org/wiki/Th%C3%A9orie_de_Mie

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Membranes, composed of a variety of lipids and other biomolecules, mediate signaling processes between cells and their aqueous environment. To fulfill this function, membranes can vary their composition leaflet-specific and thus alter their surface properties. To fully understand the impact of these processes on the molecular level, it is necessary to develop tools that can access the molecular properties of free-floating model membranes label-free. These tools are ideally surface-specific. In this thesis, we apply the nonlinear optical techniques second harmonic scattering (SHS) and vibrational sum-frequency scattering (SFS) together with electrokinetic measurements to label-free characterize the interfacial properties, hydration structure and surface potentials of liposomes in aqueous solutions. First, we generalize the nonlinear optical theory to describe the second-order surface response from interfaces with aqueous solutions independent of the ionic strength for reflection, transmission and scattering geometries. We demonstrate that interference effects from oriented water molecules in the bulk aqueous solution alter the probing depth and the expected second-order response at low ionic strengths. Then, we apply this theory to demonstrate that SHS patterns of liposomes and oil droplets contain all necessary information to extract the absolute surface potential of the respective particles without assuming a model for the interfacial structure. By analyzing scattering patterns that capture the orientational distribution of water around the particles, we find surface potentials of -38 mV for bare oil-droplets and -11 mV for zwitterionic liposomes in water. For anionic liposomes the surface potential varies between -150 mV and -23 mV in solutions containing different amounts of NaCl ranging from

\sim

0 mM to 10 mM. These values are remarkably different for solutions to the Gouy-Chapman model considering a fixed surface charge density. Next, we characterize the hydration and lipid asymmetries in binary mixed membranes using SHS and SFS. The liposomes exhibit hydration asymmetry between the inner and outer leaflet. The lipid number density between the inner and outer leaflet is the same, although geometrical packing arguments would suggest a different density. However, an asymmetric lipid distribution between the leaflets can be induced by fine tuning specific intermolecular interactions between the lipids. This is shown with dipalmitoylphosphoserine and dioleoylphosphocholine mixtures creating a membrane structure that allows intermolecular H-bonding between the phosphate and amine groups of the lipids. Finally, we quantify the surface properties of membranes composed of lipids containing phosphoserine and phosphocholine headgroups. Surprisingly, we find a very high degree of counterion condensation on anionic membranes in pure water: only 1 % of all lipids are ionized. This indicates a tightly packed layer of ions around the membrane that needs to be considered when modelling the interfacial structure around membranes.

EPFL2017

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Molecular Level Interfacial Studies of Nanoemulsions: Surface Structure, Specific Ion Effects and Stability

Evangelia Zdrali

This thesis is a study on the molecular structure of the interface of nanometer-sized oil droplets dispersed in water, a system known as oil-in-water nanoemulsion. The motivation for this research is the large significance of the interfacial nanostructures both for in industry and biology. Here we apply the nonlinear optical techniques of sum frequency scattering and second harmonic scattering to study the stabilization mechanism of nanoemulsions, as well as specific ion effects at the nanoscale. We begin with the study of the interfacial structure of oil nanodroplets stabilized with a positively charged, a negatively charged and a neutral surfactant, along with the stability of each system. We show that the surface density of charged surfactants on nanodroplets is an order of magnitude lower than on planar interfaces, due to repulsive interactions between like charges on opposing sides on the droplets surface through the oil phase, allowed by the small droplet size. Moreover, we find no experimental correlation between stability and surfactant surface density. Instead droplet stability is found to depend on the relative cooperativity between charge-charge, charge-dipole and hydrogen bonding interactions. Then, we study the effect of the inversion of the oil and the water phases on the droplet stability for nanometer-sized and micrometer-sized droplets. We employ an oil-soluble neutral surfactant, a water-soluble anionic surfactant, and the combination of the two. We find that, while microdroplets and water-in-oil nanodroplets follow the widely accepted empirical rules, and are stabilized only with a surfactant soluble in the continuous phase, nanometer-sized oil-in-water emulsions can be stabilized with an oil-soluble surfactant. Moreover, the structure of the surfactant is different when it approaches the interface from the dispersed or the continuous phase. Next, we study the interaction of four anions (SCN-, NO3-, Cl- and SO4-2) with different molecular structure with the nanointerface of droplets stabilized with a surfactant with a positively charged headgroup (trimethylammonium) for different anionic concentrations. Our results reveal a unique adsorption pattern for each anion that changes with concentration, possibly involving reorientation of the anions and adsorption to different patches of the interface. Interestingly, not only the weakly-hydrated SCN- and NO3 , but also the well-hydrated SO¬4-2 approaches the nanointerface, inducing strong interfacial water ordering. Last, we further study the interaction of SCN- with the positively charged nanointerface employing ab-initio molecular dynamics simulations. We find ion-pairing of SCN- with the interfacial trimethylammonium groups at concentrations as low as 5 millimolar. Moreover, a variety of ion species emerge at different ionic strengths, with differently oriented SCN- groups adsorbed on hydrophilic and/or hydrophobic parts of the surface. This diverse and heterogeneous chemical environment is surprisingly different from the behaviour at extended liquid planar interfaces, where ion pairing is typically detected at molar concentrations.

EPFL2019

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Soft matter probed by nonlinear scattering: self-assembly, interfaces, hydration, and long-range order

Jan Dedic

All organisms are made to a large extent of soft matter - macromolecules such as proteins and polysaccharides, or assemblies of small molecules such as lipids embedded in an aqueous environment. Understanding the role of order in soft matter presents a challenge for experimentalists because such systems are neither completely disordered nor perfectly crystalline. This is especially the case for the aqueous environment itself whose order fluctuates on femto- and pico-second time scales. In this thesis, we attempt to elucidate the presence, extent, changes, and implications of orientational order in several soft matter systems: from polyelectrolyte solutions to lipid membrane interfaces and supramolecular structures. To perform this task, we employ nonlinear optical techniques: second-harmonic and sum-frequency scattering. In the first two chapters, we explore the structure of water in aqueous polyelectrolyte solutions. Polyelectrolytes of high molecular mass are shown to induce orientational order in water that spans hundreds of nanometers. Using an adapted model of second-harmonic scattering from charged particles, we can explain the observed ordering in terms of weakly screened long-range electrostatic interactions that perturb the structure of water via charge-dipole interactions. Upon exchanging ordinary (light) water for heavy water, we observe a significant nuclear quantum effect in the induced order, indicating that bulk water-water correlations are enhanced as well by polyelectrolytes. In the following chapter, we exploit the same nuclear quantum effect in polyelectrolyte solutions to show that the polyelectrolyte-induced orientational order correlates with an anomalous increase in the reduced viscosity. We propose that the viscosity anomaly arises from enhanced order in a solvent which hinders the viscous flow. This observation represents a rare direct link between a microscopic property and a macroscopic observable. In the last two chapters, we change focus from homogeneous solutions to dispersions of liposomes which serve as models of biological membranes. In Chapter 5 we explore the binding of the protein alpha-synuclein to anionic vesicles from the perspective of interfacial water. We show that the adsorption of the protein to a vesicle decreases the order in the interfacial water by 30% but does not affect the surface nor the zeta potential. We propose that the changes in the structure of interfacial water can serve as a label-free probe of protein binding. In the last chapter, we investigate the supramolecular chemistry of inclusion complexes of methyl-Î²-cyclodextrin with various lipids. By combining multiple nonlinear scattering techniques, we show that the complexes self-assemble into micrometer-long directed fibers. Based on measurements of different lipids, we propose that the self-assembly process is driven by hydrogen bonding between the lipid headgroup and cyclodextrin. Lastly, we show that the long-range order of the self-assembled structure is transmitted into the structure of the hydrating water.

EPFL2020

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Soft matter probed by nonlinear scattering: self-assembly, interfaces, hydration, and long-range order

Jan Dedic

EPFL2020

\sim

EPFL2017

Molecular Level Interfacial Studies of Nanoemulsions: Surface Structure, Specific Ion Effects and Stability

Evangelia Zdrali

EPFL2019