Georg Ilgenfritz

Temperature-, electric field- and solute-induced percolation in water-in-oil microemulsions.

We report investigations on the percolation of the aqueous phase in water-in-oil microemulsions, comparing systems stabilized by ionic AOT and non-ionic Igepal amphiphiles. First, we briefly review the opposite effect of temperature on the two systems and compare electric conductivity with viscosity data. In the second part, we show that percolation can be induced by high electric fields resulting in a shift of the percolation curve. The electric field measurements allow to investigate the dynamics of clustering of the water droplets to form a network of percolating channels. We examine the slow build-up and the fast decay of the percolating structure, monitoring simultaneously electric conductivity and electric birefringence. In the third part we discuss the effect of some solutes on the percolation curve, especially of small molecules which act as protein denaturants and of native and denatured proteins like methemoglobin, chymotrypsin and gelatin. The spectroscopic determination of the dimerization of hemin, released from denatured hemoglobin, reflects the incorporation of the hemin monomers in the surfactant monolayer. In the gelatin system time resolved electric birefringence shows that even at low concentrations it is the macromolecule which determines the structure of the aqueous domain. In the appendix, a simple estimate of the intrinsic Kerr-constant is given for microemulsion droplets deformed in an electric field.

PHASE TRANSITIONS IN NON-IONIC DETERGENT MICELLES

Differential scanning calorimetry (DSC) studies of micellar, 60 mM solutions of the octaethyleneglycol alkylethers C14E8 and C16E8 provide evidence for a narrow endothermic transition at 41 and 32°C,respectively, characterized by an enthalpy change of 2 kJ mol−1 for both detergents. The observed thermal transition is indicative of a concerted transition of the surfactant molecules, as illustrated on the basis of a simple molecular model. The effect of co-solvents such as different alcohols on the thermal transition is investigated. Glycerol markedly lowers the transition temperature whereas the transition is absent in the presence of at least 10% ethanol. The calorimetric transition correlates with the temperature dependent increase of viscosity and static light scattering as well as with changes observed by small-angle neutron scattering (SANS). The SANS results provide clear evidence for a distinct structural change occurring at the transition temperature, which is interpreted as a sphere-to-rod transition of the detergent micelles. Moreover, the rod length increases with increasing temperature. We suggest that the process causing the thermal transition acts as the prerequisite of the growth process.

Intercluster exchange rates in AOT water-in-oil microemulsions: percolation, material transport mechanism and activation energy

The time-resolved luminescence quenching technique was employed to investigate cluster dynamics in ionic water-in-oil aerosol-OT microemulsions. Using the long living probe Tb(pyridine-2,6-dicarboxylic acid)33–(lifetime 1.9 ms) and the quencher bromophenol blue, we studied the exchange of material between clusters in AOT water-in-oil microemulsions with different alkane oils. The investigations were performed at constant amphiphile concentration and constant water: surfactant molar ratio; under these conditions the percolation temperatures are shifted to lower values with longer-chain alkane oils. In the pre-percolative regime the exchange process must be activated to break the surfactant monolayer with activation energies of 126–188 kJ mol–1. The longer the alkane-oil chain of the solvent, the higher is the activation energy. This is in contrast with expectations on the basis of literature data on the bending moduli of flat surfactant monolayers and the oil penetration concept. The results are discussed as an effect of molecular oil properties on the surfactant-monolayer compressibility. The rate constant for exchange between clusters at the percolation threshold is 3.8 × 108 dm3 mol–1 s–1 for all microemulsions studied. With the percolation transition a change in mechanism occurs from activation-controlled rate limiting to a more complex situation, in which diffusion-controlled cluster aggregation prevails. From the observed dynamics we conclude that in the percolated state the droplet structure is maintained, although the shape-restoring interfacial forces are weak. The decay kinetics are not stretched exponential, implying that the averaged droplet arrangement in a cluster on a millisecond timescale is not fractal-like.

Kinetics of quinine-deuterohemin binding.

The interaction of quinine with free hemin is of importance for the antimalarial effect of the drug in infected erythrocytes. We have investigated the kinetics of the complex formation of quinine with deuterohemin using the temperature jump relaxation method. We use ethyleneglycol-water mixtures as a solvent, where sufficient solubility for both species is provided and dimerization of the hemins, which involves mu-oxo bridges, can be controlled. Equilibrium and kinetic data for the dimerization of deuterohemin are given at 30 and 50 vol% ethyleneglycol. Binding of quinine is significantly slower than dimerization. Both processes are well separated on the time axis and yield a relaxation progress curve which is described with high accuracy by two exponential terms. The slow relaxation process is analyzed with respect to a 1:1 complex formation. This is the simplest mechanism which accounts for the present data, leading at 30 vol% ethyleneglycol, pH 7.5 to a binding constant of 10(4) M-1 and rate constants of 4.4 x 10(5) M-1 s-1 for the association and 44 s-1 for the dissociation step. However, there is evidence from the fast relaxation process that monomeric and dimeric hemin exhibit different reactivity. There is a strong decrease in rate with increasing pH. The implication of the results with respect to the proposed mechanisms of complex formation with quinoline drugs is discussed.

Electric field induced percolation in microemulsions: simulation of the electric conductivity

Structure changes can be induced by high electric fields in microemulsions which bring the system from a nonconducting state to a highly conducting state. We report conductivity and electric birefringence measurements in a microemulsion, stabilized by the nonionic surfactant Igepal CO-520 (10 wt% 0.01 M KCl/40% n-hexane, 40% c-hexane/ 10% Igepal). Based on the experimental findings we investigate two models which may be relevant for understanding the field induced percolation behavior. Computer simulations of the electric conductivity, using the random walk approach, are performed with the following heterogeneous systems: (a) statistically distributed conducting Ising chains in a nonconducting matrix, (b) nonconducting overlapping spheres in a conducting medium. Both systems are capable of modelling certain aspects of the observed percolation. The continuum percolation with overlapping spheres puts special emphasis on the Bruggeman equation of the conductivity in dispersed systems, which is found to be valid in a much wider range than might have been thought before.

Oligo- and polyethylene glycols in water-in-oil microemulsions. A SANS study

The influence of oligo- and polyethylene glycols (PEG) on ionic aerosol-OT water-in-oil (w/o) microemulsions has been investigated by small-angle neutron scattering (SANS). The scattering curves can be described with a model of polydisperse spheres, using an Ornstein–Zernicke structure factor, regardless of the PEG relative molecular mass. The results obtained at a constant temperature difference from the percolation point indicate that the droplet structure is maintained, the core radius is decreased and the polydispersity is increased with increasing PEG chain length. There is good evidence that long-chain PEGs induce aggregation of these droplets. The results support the model of polymer adsorption, which was deduced from the influence of PEGs on the percolation behaviour.

Simulation of diffusion in 2-D heterogeneous systems: comparison with effective medium and percolation theories

We report Monte Carlo simulations of the conductivity of a two-dimensional static system of non-overlapping, non- or weakly conducting discs in a conducting medium. The conductivity is evaluated using the “blind ant” diffusion algorithm. We investigate the dependence of the conductivity on (i) the size, (ii) the area fraction and (iii) the conductivity of the discs.

Ionic strength dependence of the electric dissociation field effect Investigation of 2,6-dinitrophenol and application to the acid-alkaline transition of metmyoglobin and methemoglobin

We present data on the ionic strength dependence of the dissociation field effect (2nd Wien effect) of the protolytic reaction of 2,6-dinitrophenol in aqueous solution, monitored through optical absorption changes in high electric fields. The results are in very good agreement with the Onsager-Liu theory. We then investigate the field strength dependence of the acid-alkaline transition, i.e., the hydrolysis reaction of the water coordinated to the heme iron, in metmyoglobin and methemoglobin. The true field effect of the reaction is determined from measurements in buffers which exhibit no field effect. We conclude that negative charges on the protein influence the field effect in methemoglobin but not in metmyoglobin. The Onsager-Liu theory is applied to estimate the number of charges involved.

Dynamics of structure changes in water-in-oil microemulsions: Electric birefringence and electric light scattering in percolating systems

Abstract Perturbation of a water-in-oil microemulsion system by electric field pulses and simultaneous measurement of electric current, light scattering, and birefringence on a micro- to millisecond time scale allows for structural elements and dynamical processes of these compartmentalized systems to be characterized in detail. Both optical methods probe different modes of dynamics. Whereas light scattering only indicates aggregation of larger clusters, the Kerr effect probes local structure changes as well as changes on a mesoscopic scale. At ranges below well-defined critical fields, deformation of droplets and rearrangement of droplets in clusters can be observed. Above such critical values, the field itself causes structure changes resulting in ‘electric field-induced percolation’. The Kerr effect results provides strong evidence that the droplet structure is maintained in percolating clusters. The surfactant monolayer bending constant changes by about a factor of 150 when the systems temperature passes the percolation transition. There are no principal differences in the properties of microemulsions stabilized by the ionic AOT or the non-ionic Igepal surfactant.