Jean Daillant

Reduction in the surface energy of liquid interfaces at short length scales

Liquid–vapour interfaces, particularly those involving water, are common in both natural and artificial environments. They were first described as regions of continuous variation of density, caused by density fluctuations within the bulk phases. In contrast, the more recent capillary-wave model assumes a step-like local density profile across the liquid–vapour interface, whose width is the result of the propagation of thermally excited capillary waves. The model has been validated for length scales of tenths of micrometres and larger, but the structure of liquid surfaces on submicrometre length scales—where the capillary theory is expected to break down—remains poorly understood. Here we report grazing-incidence X-ray scattering experiments that allow for a complete determination of the free surface structure and surface energy for water and a range of organic liquids. We observe a large decrease of up to 75% in the surface energy of submicrometre waves that cannot be explained by capillary theory, but is in accord with the effects arising from the non-locality of attractive intermolecule interactions as predicted by a recent density functional theory. Our data, and the results of comparable measurements on liquid solutions, metallic alloys, surfactants, lipids and wetting films should thus provide a stringent test for any new theories that attempt to describe the structure of liquid interfaces with nanometre-scale resolution.

Capillary Waves and Bending Elasticity of Monolayers on Water Studied by X-Ray Reflectivity as a Function of Surface Pressure

Thermally induced capillary waves on a monolayer of behenic acid on water have been investigated by x-ray reflectivity. The corresponding roughness is measured as a function of the surface pressure. The experimental data are discussed using a theory of thermal fluctuations. A strong decrease in the roughness is observed at the transition to the solid state. This striking feature is attributed to the rigidity of the monolayer in the solid phase whose value has been determined.

High-resolution x-ray scattering measurements: I. Surfaces

X-ray scattering investigations of surfaces and interfaces in soft-condensed matter are reviewed. Both high-resolution structural determinations in the direct space and investigations of fluctuations with long-range correlations requiring a high resolution in the Fourier space are discussed. All the scattering cross-sections for diffraction or diffuse scattering are derived within a unified frame, and the experimental aspects related to their measurement are discussed in detail. The general principles are illustrated by various examples of studies of the liquid-vapour interface, Langmuir and Langmuir-Blodgett films, wetting films, polymer films, liquid crystals and liquid-liquid interfaces.

Multiple Emulsions Controlled by Stimuli‐Responsive Polymers

The phase inversion of water-toluene emulsions stabilized with a single thermo- and pH-sensitive copolymer occurs through the formation of multiple emulsions. At low pH and ambient temperature, oil in water emulsions are formed which transform into highly stable multiple emulsions at pHs immediately lower than the inversion border. At higher pHs, the emulsion turns into a water in oil one.

Reversible buckling in monolayer of gold nanoparticles on water surface

Formation of condensed films of nanoparticles having small ratio of metal-core-diameter to organic-shell-thickness is desired for several applications in nanotechnology. We report here results of a X-ray scattering study carried out to understand structure and morphology of monolayer of such nanoparticles having gold-core and thiol-shell directly on the water surface before the monolayer undergoes a continuous transition to a bilayer. Our results demonstrate buckling of the monolayer over a large surface pressure range (1 to 15 mN/m). The buckled state exhibits reversibility on decompression and can be annealed with temperature. We also show that condensed monolayer films of nanoparticles can be formed by annealing the buckled monolayer before transferring to solid substrates.

Gold Nanoparticles at the Liquid-Liquid Interface: X-ray Study and Monte Carlo Simulation

The behavior of mixed-ligand-coated gold nanoparticles at a liquid-liquid interface during compression has been investigated. The system was characterized by measuring pressure-area isotherms and by simultaneously performing in situ X-ray studies. Additionally, Monte Carlo (MC) simulations were carried out in order to interpret the experimental findings. With this dual approach it was possible to characterize and identify the different stages of compression and understand what happens microscopically: first, a compression purely in-plane, and, second, the formation of a second layer when the in-plane pressure pushes the particles out of the plane. The first stage is accompanied by the emergence of an in-plane correlation peak in the scattering signal and a strong increase of the pressure in the isotherm. The second stage is characterized by the weakening of the correlation peak and a slower increase in pressure.

Mixed-Monolayer-Protected Gold Nanoparticles for Emulsion Stabilization

Nanometer-sized gold nanoparticles have been prepared and surface-modified in order to stabilize alkane-in-water emulsions. A mixed hexane-undecanol ligand layer at the surface of the nanoparticles allowed us to tune their wettability and thus the adsorption at the oil-water interface. Oil droplets of the stable emulsions have been evidenced by confocal fluorescence microscopy, freeze-fracture transmission electron microscopy, and dynamic light scattering. Prepared emulsions were stable during performed cooling-heating cycles, in which the temperature stability of the emulsions has been studied by means of dynamic light scattering. The interfacial structure of the oil droplets was investigated by small-angle X-ray scattering. The obtained area per nanoparticle at the oil droplet interface was 30 nm(2). The investigation of the nanoparticle adsorption at the curved interface of the emulsion droplets is in agreement with our previous study at a planar oil-water interface, in which the nanoparticles started to interact with each other at about the same area per particle.

Tailoring nanostructures using copolymer nanoimprint lithography.

The generation of defect-free polymer nanostructures by nanoimprinting methods is described. Long-range nanorheology and shorter-range surface energy effects can be efficiently combined to provide alignment of copolymer lamellae over several micrometers. As an example, a perpendicular organization with respect to circular tracks is shown, demonstrating the possibility of writing ordered radial nanostructures over large distances.

Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases

Abstract:The linear compressibility of two-dimensional fatty acid mesophases has been determined by grazing incidence X-ray diffraction. The unit cell parameters of the , , , S and phases of behenic acid and of the phase of myristic acid were determined as a function of surface pressure and temperature. Surface pressure versus molecular area isotherms were reconstructed from these measurements, and the linear compressibility (relative distortion along a given direction for a two-dimensional isotropic applied stress) was determined both in the sample plane and in a plane normal to the aliphatic chain director (transverse plane). The linear compressibilities range over two orders of magnitude from 0.1 to 10 m/N and are distributed depending on their magnitude in 4 different sets which we are able to associate with different molecular mechanisms. The largest compressibilities (10 m/N) are observed in the tilted phases. They are apparently independent on the chain length and could be related to the reorganization of the headgroup hydrogen-bounded network, whose role should be revalued. Intermediate compressibilities are observed in phases with quasi long-range order (directions normal to the molecular tilt in the or phases, S phase, and could be related to the ordering of these phases. The lowest compressibilities are observed in the solid untilted phase and for one direction of the S and phases. They are similar to the compressibility of crystalline polymers and correspond to the interactions between methyl groups in the crystal. Finally, negative compressibilities are observed in the transverse plane for the and phases and can be traced to subtle reorganizations upon untilting.

A critical look at surface force measurement using a commercial atomic force microscope in the noncontact mode

The use of commercial atomic force microscopes (AFM) operating in the noncontact mode for surface force measurements is critically reviewed. Approach curves (i.e., vibration amplitude versus tip–surface distance) using standard microfabricated tips are discussed with respect to the basic theory of an equivalent harmonic oscillator. Different artifacts are addressed. In particular, we show theoretically and experimentally that the force exerted by the layer of air confined between the cantilever and the surface is a major contribution to the force on the cantilever. However, by carefully choosing the parameters (essentially the vibration amplitude) for the measurement of the approach curve, and by taking into account the damping within the confined air layer, we succeeded in measuring reliable surface force profiles with commercial AFM in the air and in describing them quantitatively by dispersion force interactions.