Thomas Zemb

Self-assembly of regular hollow icosahedra in salt-free catanionic solutions

Self-assembled structures having a regular hollow icosahedral form (such as those observed for proteins of virus capsids) can occur as a result of biomineralization processes, but are extremely rare in mineral crystallites. Compact icosahedra made from a boron oxide have been reported, but equivalent structures made of synthetic organic components such as surfactants have not hitherto been observed. It is, however, well known that lipids, as well as mixtures of anionic and cationic single chain surfactants, can readily form bilayers that can adopt a variety of distinct geometric forms: they can fold into soft vesicles or random bilayers (the so-called sponge phase) or form ordered stacks of flat or undulating membranes. Here we show that in salt-free mixtures of anionic and cationic surfactants, such bilayers can self-assemble into hollow aggregates with a regular icosahedral shape. These aggregates are stabilized by the presence of pores located at the vertices of the icosahedra. The resulting structures have a size of about one micrometre and mass of about 1010 daltons, making them larger than any known icosahedral protein assembly or virus capsid. We expect the combination of wall rigidity and holes at vertices of these icosahedral aggregates to be of practical value for controlled drug or DNA release.

EFFECT OF NITRIC ACID EXTRACTION ON PHASE BEHAVIOR, MICROSTRUCTURE AND INTERACTIONS BETWEEN PRIMARY AGGREGATES IN THE SYSTEM DIMETHYLDIBUTYLTETRADECYLMALONAMIDE (DMDBTDMA) / n-DODECANE / WATER: A PHASE ANALYSIS AND SMALL ANGLE X-RAY SCATTERING (SAXS) CHARACTERISATION STUDY

ABSTRACT Among the different problems to be solved when designing a liquid-liquid solvent extraction process, third phase formation. i.e. the splitting of the organic phase into two layers when extracting high concentration of solutes, like nitric acid or metallic nitrates (here trivalent f ions), is one of the most important to address. In some conditions the formation of a “third phase” is observed with dimethyldibutyltetradecylmalonamide (DMDBTDMA), a potential extractant used in the DIAMEX process We have investigated the phase behavior of the system DMDBTDMA / n-dodecane / water / HNO3, in the acceptable concentration limits for the DIAMEX process.

Osmotic pressure and salt exclusion in electrostatically swollen lamellar phases

Measurements of osmotic pressure, mainly of electrostatic origin, are reported in the diluted regime of charged bilayers. The interlamellar distances (100–1000 A) are measured using small‐angle neutron scattering. The electrostatic repulsive pressure is evaluated within the framework of the Poisson–Boltzmann model, taking into account the conditions of ionic conservation and electrochemical equilibrium, and checked by Monte Carlo simulations in the grand canonical ensemble. The predicted phenomenon of salt ejection from the bilayers is evidenced by direct chemical analysis. Implications concerning the phase behavior of double‐tailed surfactants are discussed.

Facetted hollow silica vesicles made by templating catanionic surfactant vesicles

Large facetted hollow silica vesicles were synthesized by directed growth of silica on equilibrium icosahedrally facetted catanionic vesicles, made from cetyltrimethylammonium hydroxide/myristic acid surfactant bilayers. Three critical parameters were identified: (i) the surface charge of the vesicles; (ii) the rate of silica deposition by tetraethoxysilane (TEOS) hydrolysis and (iii) the rate of concomitant ethanol production. All three factors are pH-dependent. Thus, depending on the reaction pH and TEOS concentration, the silica vesicles have been shown to be either facetted, rounded or damaged, in a process controlled by the template.

Aggregation in Organic Solutions of Malonamides: Consequences for Water Extraction

The molecular organization of N,N′‐dimethyl‐N,N′‐dioctylhexylethoxymalonamide (DMDOHEMA), the current reference extractant for the DIAMEX (DIAMide EXtraction) process, is correlated with its water extraction properties from neutral media. The aggregation of DMDOHEMA in n‐heptane was investigated by vapor pressure osmometry (VPO) and the aggregate speciation characterized by combined small‐angle neutron and X‐ray scattering (SANS and SAXS, respectively). Two approaches were taken to model the aggregation of the diamide and the water extraction as a function of the diamide concentration by taking into account a single aggregation equilibrium with an average aggregation number N equal to 4.28 ± 0.05; and a competition between two types of aggregates in the organic phase, namely, aggregates of the reverse micelle type with 4 diamides per aggregate, and an oligomeric structure composed of about 10 diamide molecules which appears at high extractant concentration (>1 mol/L). In both cases, the supramolecular speciation representing the monomers/aggregates distribution was determined, and for each supramolecular organization, a solubilization parameter was calculated using the Sergievskii‐Dannus relationship. Thus, the correlation between the two types of micellization of the diamide and the extraction of water into the organic phase was demonstrated. The larger aggregates can extract about five times more water than monomers.

Structure and Solubility in Surfactant‐Free Microemulsions

Cheers! The combination of static and dynamic light scattering is demonstrated to deliver an unambiguous proof that well-defined domains of two fluids of clearly different composition can exist in surfactant-free ternary mixtures (see picture for diagram), such as Ouzo.

Cholesteryl-cyclodextrins: synthesis and insertion into phospholipid membranes

Abstract 6 I -(Cholest-5-en-3β-yloxycarbonyl)amino-6 I -deoxycyclomaltoheptaose and 6 I -(cholest-5-en-3α-ylamido)succinylamido-6 I -deoxycyclomaltoheptaose were synthesized and fully characterized by NMR spectroscopy experiments and mass spectrometry analysis. Incorporation of these monosubstituted amphiphilic cyclodextrins into phospholipid bilayers was investigated using small-angle X-ray scattering, differential scanning calorimetry and 31 P nuclear magnetic resonance. Different modes of incorporation depending on the spacer length between the cyclodextrin and cholesterol moieties were observed.

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.

Random Connected Cylinders: a New Structure in Three-Component Microemulsions

Existent models fail to explain the presence and the position of a well-defined peak often present in small-angle x-ray spectra of microemulsions. We describe the first model consistent with the observed scattering peak position and shape. This disordered connected model explains also the conductivity percolation observed in three-component ionic microemulsions at high surfactant content.

Catanionic Microcrystals: Organic Platelets, Gigadalton ‘Molecules’, or Ionic Solids?

We review known properties of catanionic bilayers—complex fluids containing anionic and cationic surfactants together—including the case of ‘true’, salt-free catanionic solutions. The properties of our interest include phase diagrams, salt effects, and chain melting that occurs in liquid crystal, as well as micron-sized aggregates, from nanodiscs to icosahedra.