ESRF Highlights 2007

The synthesis of ordered, homogeneous porous grains is an expanding area of materials research. One strategy for their formation is to dry the spray of a complex mixture containing nanoparticles and templating agents [1]. In this process, a continuous flow of micrometric droplets, made from the initial dilute solution, is dried along a hot tube in order to evaporate the solvent. Self-organisation of the constituents takes place during the evaporation. Organic moieties can even be removed via further calcination. The local structure of the final grains strongly depends on the initial compositions. Small-angle X-ray scattering can be used to investigate the ordered structural features of the final spray-dried grains at the nanometric scale [2]. However, the morphology of the grains at a larger scale depends critically on the kinetics of drying.

Two different regimes may be distinguished for the solvent evaporation from a complex nanoparticles solution confined in a droplet. Firstly, when the evaporation front moves faster than the time required for a particle to diffuse on the length scale of the drop, the grains may be heterogeneous. The final grains can be doughnut-like or even core-shell with an empty space inside, which has sometimes been observed by scanning electron microscopy (SEM). Secondly, when the drying is slower than the characteristic diffusion time of the nanoparticles, the evaporation occurs in a quasi equilibrium distribution of the nanoparticles inside the droplet and the formation of a dense spherical grain is anticipated.

For a solution containing a mixture of 5 nm silica nanoparticles with 50 nm polybromostyrene sulfonate we found that a large proportion of doughnuts were obtained, even when a slow evaporation rate was used (Peclet number Pe = _dif/_evap = 0.01). One of the doughnuts is shown Figure 59. Together with the sphere-doughnut transition, the inner homogeneity of this type of material remains a crucial question for their further applications.

ESRF Highlights 2007

Langmuir films of long chain amphiphiles at the air-water interface present different phases depending on temperature and surface pressure (the difference between the surface tension of pure water and the actual surface tension in presence of the film = _H20– ). Their phase transitions were first identified by isotherm measurements (surface pressure as a function of molecular area for a fixed temperature). The first grazing-incidence X-ray diffraction (GIXD) experiments carried out directly on monolayers at the air-water interface were reported in 1987 [1]. GIXD has now become the primary technique used to determine the structure of amphiphilic monolayers on water, allowing the determination of unit cell parameters, molecular tilt angle and azimuth of tilt direction. It is however generally considered that going beyond this simple unit cell characterisation is impossible.

ESRF Highlights 2007

The study of aqueous salt solutions continues to attract various research groups because of their fundamental importance in various physicochemical, biological and atmospheric processes. The air/water interface plays a crucial role in such processes and differs to a large extent when compared to bulk. To further understand the role of the interface, direct access to the surface excess or the knowledge of the concentration profiles of ions will not only improve our present understanding but also help to predict the properties associated with it. Ions, though of the same valency, tend to interact differently with proteins (salting in or salting out as predicted by Hofmeister) or differ in their degree of adsorption at the air-water interface. In recent times, there have been considerable efforts by various research groups using different sophisticated surface sensitive probes to understand the organisation of the ions and its impact on the solvent features and also through molecular dynamic simulation.