Dynamic percolation in water in oil microemulsions revisited

Microemulsions (dispersions of water droplets, typical radius about 10 nm, in oil) show a particular percolation pattern, a so-called dynamical percolation. Predictions of scaling theory and Monte Carlo simulations were compared with experimental static and frequency dependent conductivity data. The latter gives evidence of two different time scales of charge transport.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

On the Formation of Water/Oil-Microemulsions

The formation of a so-called water/oil (W/O)-microemulsion (AOT/isooctane/water) was followed by light scattering and ultracentrifuge measurements. With increasing weighed-in water concentrations the microemulsion is stabilized by repeated aggregational processes of micelles containing water due to a decrease of the free interfacial enthalpy. This process conforms very satisfactorily to a model describing an adsorption of surfactants at the water/hydrocarbon interface resulting from dipole-image dipole interactions.     

Clorobiocin biosynthesis in Streptomyces: identification of the halogenase and generation of structural analogs

Clorobiocin (clo) and novobiocin (nov) are potent inhibitors of bacterial DNA gyrase. The two substances differ in the substitution pattern at C-8' of the aminocoumarin ring, carrying a chlorine atom or a methyl group, respectively. By gene inactivation, clo-hal was identified as the gene of the halogenase responsible for the introduction of the chlorine atom of clorobiocin. Inactivation of cloZ did not affect clorobiocin formation, showing that this ORF is not essential for clorobiocin biosynthesis. Expression of the methyltransferase gene novO in the clo-hal(-) mutant led to the very efficient formation of a hybrid antibiotic containing a methyl group instead of a chlorine atom at C-8'. Comparison of the antibacterial activity of clorobiocin analogs with -Cl, -H, or -CH(3) at C-8' showed that chlorine leads to 8-fold higher activity than hydrogen and to 2-fold higher activity than a methyl group.     

Rheological analysis of triblock copolymer-microemulsion networks

The rheological properties of microemulsion-triblock copolymer [poly(oxyethylene)-b-polyisoprene-b-poly(oxyethylene)] networks were studied by oscillatory shear measurements. The principal viscoelastic pattern of our systems stays the same whether the surfactant is ionic or nonionic. A closer examination shows, however, the enormous influence of the respective phase behavior of the underlying microemulsions. Also, specific interactions between the surfactants and the hydrophilic poly(oxyethylene) blocks lead to a significant effect on the formation of network structures.Characteristic rheological data such as plateau moduli and relaxation times for a series of different samples were derived differing in polymer or droplet concentrations, apart from the molecular weights of the poly-(oxyethylene) and the polyisoprene blocks. Finally, we could construct polymer and droplet concentration invariant master — mastercurves using the temperature invariant mastercurves.     

Blockcopolymers in microemulsions differential heats of solution and shift of the percolation temperature in W/O microemulsions

Solutions of blockcopolymers (POE-b-PI-b-POE) in fluids of interacting aqueous nanodroplets (W/O microemulsions) are studied. The interaction strength between the (pseudo) two components is measured by the shift of the percolation temperature relative to that of the pure microemulsion. A quantitative measure of the interaction, the differential heat of solution, is thermodynamically related to the slopes of the equilibrium temperature of the system with varying monomeric nanodroplet concentration and the experimental percolation line in the presence of copolymer.     

Temperature-controlled change of charge transport mechanisms in nonionic water-in-oil microemulsions

 The temperature-controlled transition from the Stokes charge transport of aqueous nanodroplets to the intrinsic conduction of nanodroplet clusters in nonionic microemulsions was studied. Two different charge transport processes apparent from a minimum value of the conductance have been simulated based on straightforward physical models. Their predictions compare favourably with the observations. Received: 4 September 2001 Accepted: 20 September 2001     

SIMS for hydrogen quantification and structural analysis of amorphous silicon germanium compounds

Summary Hydrogen has been analysed quantitatively in a-Si_1–xGe_x:H alloys by SIMS with H variation from 1×10¹⁹ –1.3×10²² atoms/cm³ and x between 0 and 1. To quantify the absolute H concentration, SIMS measurements have been calibrated with nuclear reaction analysis, which exhibits excellent agreement with SIMS data for the total range of H and Ge variance. From abundances of the molecule ions SiH⁺ and GeH⁺ the fractions H bound to Si or Ge can be discerned and are in good accordance with quantification of SiH and GeH stretching modes in IR spectroscopic measurements. Preferential attachment of H to Si compared to Ge by a factor of 3.5 is determined for glow discharge a-SiGe:H samples; during annealing up to 900 K only small changes of this factor are observed.     

Elastically-induced shift of system-spanning cluster formation in water/oil-microemulsion networks

The influence of ABA-triblock copolymers on the temperature-induced infinite droplet cluster formation in water/oil-microemulsions is studied. The effects of the different blocks of the polymer can be well separated. With the help of an Ehrenfest relation the coupling between the elastically-induced shift of the infinite cluster formation temperature T_c and the plateau modulus of the resulting microemulsion networks is described.     

The Effect of Choleates on the Solubilization of Water in Water/AOT/Cyclohexane (W/O)-Microemulsions

Experimental investigations on the cosurfactant effect of alkali and tetraalkylammonium choleates in water/AOT/cyclohexane (W/O)-microemulsions were carried out. The results prompted considerations on a thermodynamic and molecular level. The molecular view emphasizes the significance of hydration interactions between cosurfactant and solubilized water.