Olivier Diat

Fuel cells: Proton channels

The structure of Nafion, the polymer electrolyte membrane used in some fuel cells, has been extensively debated over recent decades. Now, a model reveals the nanoscale arrangement that could explain the excellent transport properties of the material.

Rheology of lyotropic lamellar phases

The rheological behaviour of lyotropic lamellar phases is studied as a function of the membrane repeating distance. The steady-state rheology is described as a consequence of the so-called orientation diagram described previously. Three distinct regions of different orientations are described, which are separated by two out-of-equilibrium transitions. We show that these transitions can be either discontinuous (subcritical) or continuous. In one of these transitions, one can go continuously from one regime to the other through a bifurcation point.

Anisotropy of structure and transport properties in sulfonated polyimide membranes

An original analysis of the structure of ionomer films along the three dimensions using micro small-angle X-ray scattering (microSAXS) technique is reported. While the in-plane structure appeared as isotropic, a significant SAXS anisotropy was observed in the transverse direction, revealing a multiscale structural anisotropy from the molecular level to several tens of nanometers. Using transmission and scanning electron microscopy (TEM and SEM, respectively), a layered structure was identified up to micrometer scales. This structural anisotropy was used to interpret the conductivity data as a function of the ion content. Moreover, the diffusion coefficients of the counterions, were determined along the planar and transverse directions using pulse field gradient spin echo-NMR (PFGSE-NMR) in order to correlate structure and transport properties.

Lyotropic Lamellar Phase Formed from Monolayered θ-Shaped Carborane-Cage Amphiphiles†

Lyotropic lamellar phases are ubiquitous on earth, they occur naturally and are a key architecture for life to appear as they enable closed cell topologies to occur1, 2, 3, 4, 5 . But in addition to the latter, enabling life means that the same solvent must be on both side of the cell membrane, hence a, at least, double layered membrane structure is necessary. For this a lamellar phase must be enabled. The present contribution shows, for the first time, that the formation of lamellar phases is not exclusive to alkyl chain based surfactants with a well-defined amphiphilic structure but that it can also be obtained with metalla-carborane clusters, described previously as θ-shaped amphiphiles6 . Similarly to phospholipids cell membranes the lamellae formed here can exist both in the liquid and in the solid states depending on temperature. The determination of the 2D molecular arrangement in the lamella proved that the formation of intermolecular dihydrogen bonds, such as -C-H+…-H-B-, is the driving force in the lamella self-assembly process. Compared to the common bilayer structure that originates from the hydrophobic effect7, θ-shaped amphiphiles form lamella with a peculiar monomolecular structure reminiscent of lamellar sheets observed in inorganic layered systems8 . Nano-scale ordering of planar organic-inorganic hybrid sheets is here controlled with temperature and concentration through a self-assembly process.

Emergence of surfactant-free micelles from ternary solutions

Curious effects ranging from enzyme activity to anomalies in evaporation rates that have been known for over fifty years suggest the existence and thermodynamic stability of surfactant-free micelles. Only recently, joint X-ray, light and neutron scattering experiments have demonstrated that aggregates and bulk pseudo-phases coexist in presumably normal solutions, in which a water insoluble component is solubilized in a certain domain of concentration of a hydrotrope component like ethanol. Nevertheless, nothing is known about the molecular-level shape and structure of such aggregates. In this work we characterize mixtures of octanol, ethanol, and water by molecular dynamics simulations. For compositions in the “pre-ouzo” region (close to the single phase stability limit) we observe micelle-like aggregates that are clearly distinct from simple critical density fluctuations. We define an ethanol partition in the pseudo-phase from an integral of the van der Waals dispersion energy term. From this partition, octanol-rich aggregates swollen with ethanol appear with an emerging interface. Ethanol is present in the water pseudo-phase with an exponential decay similar to the one predicted by Marcelja and Radic forty years ago.

How to explain microemulsions formed by solvent mixtures without conventional surfactants

Significance Beginning over 40 y ago, a curious type of microemulsions—as transparent dispersion of two immiscible liquids separated by an interfacial film—at thermodynamic equilibrium has been described as “pre-Ouzo,” “detergentless,” or surfactant-free microemulsions. The experiments in ternary systems containing one hydrotropic cosolvent were ambiguous, and therefore, there was no need to come up with a general theory. Recent evidence obtained by specific deuteration in neutron scattering established the need for the extension of self-assembly theories based on Derjaguin-Landau-Verwey-Overbeck theory (DLVO), bending, or phase transfer energy. Here, we introduce a general free energy expression for weak self-assembly, where solvation effects and entropy compete without the influence of film bending, that explains all experimental results for this class of microemulsions. Ternary solutions containing one hydrotrope (such as ethanol) and two immiscible fluids, both being soluble in the hydrotrope at any proportion, show unexpected solubilization power and allow strange but yet unexplained membrane enzyme activity. We study the system ethanol-water-octanol as a simple model of such kinds of ternary solutions. The stability of “detergentless” micelles or microemulsions in such mixtures was proposed in the pioneering works of Barden and coworkers [Smith GD, Donelan CE, Barden RE (1977) J Colloid Interface Sci 60(3):488–496 and Keiser BA, Varie D, Barden RE, Holt SL (1979) J Phys Chem 83(10):1276–1281] in the 1970s and then, neglected, because no general explanation for the observations was available. Recent direct microstructural evidence by light, X-ray, and neutron scattering using contrast variation reopened the debate. We propose here a general principle for solubilization without conventional surfactants: the balance between hydration force and entropy. This balance explains the stability of microemulsions in homogeneous ternary mixtures based on cosolvents.

Elucidation of the Structure of Organic Solutions in Solvent Extraction by Combining Molecular Dynamics and X-ray Scattering†

Knowledge of the supramolecular structure of the organic phase containing amphiphilic ligand molecules is mandatory for full comprehension of ionic separation during solvent extraction. Existing structural models are based on simple geometric aggregates, but no consensus exists on the interaction potentials. Herein, we show that molecular dynamics crossed with scattering techniques offers key insight into the complex fluid involving weak interactions without any long-range ordering. Two systems containing mono- or diamide extractants in heptane and contacted with an aqueous phase were selected as examples to demonstrate the advantages of coupling the two approaches for furthering fundamental studies on solvent extraction.

Specific salt and pH effects on foam film of a pH sensitive surfactant.

Steady state foams made of a pH sensitive surfactant, nonaoxyethylene oleylether carboxylic acid, with ion complexing properties was studied using small angle neutron scattering (SANS). The effect of pH variation and salt addition on the foam film thickness was investigated and discussed in terms of the influent parameters stabilizing the foam such as surface properties and electrostatic effects determined by tensiometry and zeta potential measurements. The decrease in the film thickness by adding mono (Na(+)) and divalent (Ca(2+)) salts is classically explained by screening of the double layer in foam films (transverse interactions). On the contrary, addition of acid or complexing ion (Nd(3+)) results in an increase in the film thickness and can be analyzed in terms of cohesive forces between surfactants at the liquid/gas interface (lateral interactions). pH and specific salt effects revealed that foams produced by nonaoxyethylene oleylether carboxylic acid are of interest in the potential use of this surfactant in ion separation process.

Recycling metals by controlled transfer of ionic species between complex fluids: en route to “ienaics”

Recycling chemistry of metals and oxides relies on three steps: dissolution, separation and material reformation. We review in this work the colloidal approach of the transfer of ions between two complex fluids, i.e. the mechanism at the basis of the liquid-liquid extraction technology. This approach allows for rationalizing in a unified model transformation such as accidently splitting from two to three phases, or uncontrolled viscosity variations, as linked to the transformation in the phase diagram due to ion transfer. Moreover, differences in free energies associated to ion transfer between phases that are the origin of the selectivity need to be considered at the meso-scale beyond parameterization of an arbitrary number of competing “complexes”. Entropy and electrostatics are taken into account in relation to solvent formulation. By analogy with electronics dealing about electrons transported in conductors and semi-conductors, this “ienaic” approach deals with ions transported between nanostructures present in colloidal fluids under the influence of chemical potential gradients between nanostructures coexisting in colloidal fluids. We show in this review how this colloidal approach generalizes the multiple chemical equilibrium models used in supra-molecular chemistry. Statistical thermodynamics applied to self-assembled fluids requires only a few measurable parameters to predict liquid-liquid extraction isotherms and selectivity in multi-phase chemical systems containing at least one concentrated emulsified water in oil (w/o) or oil in water (o/w) microemulsion.

Surface Activity and Molecular Organization of Metallacarboranes at the Air–Water Interface Revealed by Nonlinear Optics

Because of 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 liquid phase. Unlike classical surfactants, metallabisdicarbollides (MCs) do not have a well-defined amphiphilic structure. They are nanometer-sized inorganic anions with an ellipsoidal shape composed of two carborane semicages sandwiching a metal ion. 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. By combining second harmonic generation and surface tension measurement, we show here that cobaltabis(dicarbollide) anion {[(C2B9H11)2Co](-) also named [COSAN](-)} with H(+) as a counterion, the most representative metallacarborane, adsorbs vertically at the water surface with its long axis normal to the surface. This vertical molecular orientation facilitates the formation of intermolecular and nonconventional dihydrogen bonds such as the B-H(δ-)···(δ+)H-C bond that has recently been proven to be at the origin of the self-assembly of MCs in water. Therefore, it appears here that lateral dihydrogen bonds are also involved in the surface activity of MCs.