Soluble surfactant spreading: How the amphiphilicity sets the Marangoni hydrodynamics

Amphiphiles are molecules combining hydrophilic and hydrophobic parts. The way they arrange in bulk and at interfaces is related to the balance between these two parts, and can be quantified by introducing the critical micellar concentration (cmc). Amphiphiles (also named "surfactants") are also at the origin of dynamical effects: local gradients of interfacial concentrations create the so-called Marangoni flows. Here we study the coupling between the molecule amphiphilicity and these Marangoni flows. We investigate in detail a spreading configuration, where a local excess of surfactants is locally sustained, and follow how these surfactants spread at the interface and diffuse in bulk. We have measured the features of this flow (maximal distance and maximal speed), for different types of surfactant, and as a function of all experimentally available parameters, as well as for two different configurations. In parallel, we propose a detailed hydrodynamical model. For all the measured quantities, we have found a good agreement between the data and the model, evidencing that we have captured the key mechanisms under these spreading experiments. In particular, the cmc turns out to be-as for the static picture of a surfactant-a key element even under dynamical conditions, allowing us to connect the molecule amphiphilicity to its ability to create Marangoni flows.

Origin of structural analogies and differences between the atomic structures of GeSe4 and GeS4 glasses: A first principles study

First-principles molecular dynamics simulations based on density functional theory are employed for a comparative study of structural and bonding properties of two stoichiometrically identical chalcogenide glasses, GeSe4 and GeS4. Two periodic cells of 120 and 480 atoms are adopted. Both glasses feature a coexistence of Ge-centered tetrahedra and Se(S) homopolar connections. Results obtained for N = 480 indicate substantial differences at the level of the Se(S) environment, since Ge-Se-Se connections are more frequent than the corresponding Ge-S-S ones. The presence of a more prominent first sharp diffraction peak in the total neutron structure factor of glassy GeS4 is rationalized in terms of a higher number of large size rings, accounting for extended Ge-Se correlations. Both the electronic density of states and appropriate electronic localization tools provide evidence of a higher ionic character of Ge-S bonds when compared to Ge-Se bonds. An interesting byproduct of these investigations is the occurrence of discernible size effects that affect structural motifs involving next nearest neighbor distances, when 120 or 480 atoms are used.

Metal-organic molecule-metal nano-junctions: a close contact between first-principles simulations and experiments

The realization of metal-molecule junctions for future electronic devices relies on our ability to assemble these heterogeneous objects at a molecular level and understand their structure and the behavior of the electronic states at the interface. Delocalized interface states near the metal Fermi level are a key ingredient for tailoring charge injection, and such a delocalization depends on a large number of chemical, structural and morphological parameters, all influencing the spatial extension of the electron wavefunctions. Our large-scale dynamical simulations, combined with experiments, show that a double-decker organometallic compound (ferrocene) can be deposited on a Cu(111) surface, providing an ideal system to investigate the adsorption, the interface states and localized spin states at a metal-organometallic interface. Adsorbed ferrocene is shown to have a peculiar pattern and realizes a 2D-like interface state strongly resembling Shockley's surface state of Cu. By a subsequent deposition of single metal atoms on the adsorbed ferrocene, we analyze the sensitivity of the interface state to local modifications of the interface potential. This provides an insight into adsorption, spin configuration and charge redistribution processes, showing how to tune the electron behavior at a metal-molecule interface.

Three-dimensional structure of multicomponent (Na₂O)0.₃₅ [(P₂O₅)₁- x(B₂O₃)x]0.₆₅ glasses by high-energy x-ray diffraction and constrained reverse Monte Carlo simulations

Experimental structure functions for (Na(2)O)(0.35) [(P(2)O(5))(1 - x)(B(2)O(3))(x)](0.65) glasses, where x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, have been measured by high-energy x-ray diffraction up to wavevectors of 28 Å( - 1) to obtain atomic pair distribution functions with high real space resolution. The experimental diffraction data have been used to guide constrained reverse Monte Carlo simulations of the three-dimensional structure of the glasses. The resulting models show that the glasses exhibit a very complex atomic-scale structure that evolves from an assembly of chains of corner shared P(O)(4) tetrahedra for x = 0 to a network of B(O)(4) tetrahedra and planar B(O)(3) units for x = 1. In the glasses of intermediate composition (i.e.  0 < x < 1), P, B and oxygen atoms sit on the vertices of P(O)(4), B(O)(4) and B(O)(3) units mixed in various proportions. Sodium atoms are found to fill up the cavities in between the P/B-oxygen units in a more or less random manner. The new data can provide a firm structural basis for an explanation of the mixed glass former effect where a nonlinear behavior of Na ion conductivity is observed in the (Na(2)O)(0.35) [(P(2)O(5))(1 - x)(B(2)O(3))(x)](0.65) glass system.

ISAACS– interactive structure analysis of amorphous and crystalline systems

ISAACS(interactive structure analysis of amorphous and crystalline systems) is a cross-platform program developed to analyze the structural characteristics of three-dimensional structure models built by computer simulations. The models may have any degree of periodicity (i.e.crystallinity) and local symmetry. The following structural information is computed from the models: total and partial radial distribution functions and structure factors for X-ray or neutron scattering, coordination numbers, bond-angle and near atomic neighbor distributions, bond-valence sums, ring statistics, and spherical harmonics invariants. The information may be visualized conveniently and stored for further use.

Behaviour of metals following intensive pig slurry applications to a natural field treatment process in Brittany (France)

It is well-known that heavy metals brought by intensive pig slurry applications accumulate in the soil; because of their potential impact on the environment, studying their behaviour is of utmost interest. The Solepur process has been developed in Brittany (France) where pig farming causes serious environmental problems; it enables the soil to be used as a treatment medium to process pig slurry. Numerous pig slurry applications were performed from 1991 to 1995 on an experimental hydrologically-isolated field specially equipped to recover all the leachate. These applications approximately corresponded to the amount which might have been spread over one century. The Solepur process can be considered as a simulation model for what happens to metals in pig slurry under normal field conditions over a long period of time. This work measured the concentrations of Cu, Zn, Mn, Co, Fe and Cd in pig slurry, in soil and in drainage water, and the results are discussed. Total concentrations and speciation data within the three different compartments were analysed and showed that Cu and Zn accumulate in the surface layer, whereas other metals, such as Mn and Co, dissolve and are washed away in the leachate. The increase in the bioavailability and toxicity of these metals as well as their impact on the environment are discussed.