Showing papers by "Peter Schurtenberger published in 2000"

Sol-Gel transition of concentrated colloidal suspensions

Abstract: Diffusing-wave spectroscopy (DWS) was used to follow the sol-gel transition of concentrated colloidal suspensions. We present a new technique based on a sandwich of two scattering cells aimed to overcome the problem of nonergodicity in DWS of solidlike systems. Using this technique we obtain quantitative information about the microscopic dynamics all the way from an aggregating suspension to the final gel, thereby covering the whole sol-gel transition. At the gel point a dramatic change of the particle dynamics from diffusion to a subdiffusive arrested motion is observed. A critical-power-law behavior is found for the time evolution of the maximum mean square displacement delta(2) probed by a single particle in the gel.

A Hexanol-Induced Sphere-to-Flexible Cylinder Transition in Aqueous Alkyl Polyglucoside Solutions

Abstract: Sugar surfactants have recently attracted considerable interest. Here we report a systematic light and neutron scattering investigation of aqueous solutions of the alkyl polyglucoside C8/10G1.5. The system exhibits a transition from globular to giant wormlike micelles upon the addition of hexanol. We can finely tune the extent of micellar growth and in particular reach conditions where solutions of relatively short cylinders exist, which dramatically grow into giant wormlike micelles upon the addition of small amounts of hexanol. While considerable progress has been made in the understanding of interaction effects in solutions of spheres or large polymer-like objects, limited information exists for short flexible cylinders. An important reason for this is the lack of good model systems. Here we now demonstrate that the APG system provides us with an ideal set of data for an analysis of micellar growth and interactions in solutions with cylindrical particles of moderate axial ratios. We apply theoretical e...

New controlled environment vitrification system for cryo-transmission electron microscopy: design and application to surfactant solutions.

Abstract: A newly designed controlled environment vitrification system for cryo-transmission electron microscopy of vitrified samples (cryo TEM) is presented. It significantly reduces the lag-time from blotting to vitrification by means of an optimized blotting procedure. Furthermore, a modified transfer system allows rapid transfer of the vitrified sample into the microscope and reduces the equilibration time. The practicable throughput of samples is thereby markedly increased. This new equipment is used to determine the overall size of individual polymer-like mixed lecithin–bile salt micelles, which very critically depends on the composition of the sample. The quantitative agreement with the results from independent light scattering experiments on the same sample supports the validity and performance of the presented sample preparation and handling procedures. In addition, these experiments demonstrate that cryo TEM experiments not only provide direct structural information, but furthermore permit a quantitative determination of micellar properties.

Size, flexibility, and scattering functions of semiflexible polyelectrolytes with excluded volume effects: monte carlo simulations and neutron scattering experiments

Abstract: We present a systematic Monte Carlo study of the scattering function S(q) of semiflexible polyelectrolytes at infinite dilution, in solutions with different concentrations of added salt. In the spirit of a theoretical description of polyelectrolytes in terms of the equivalent parameters, namely, persistence length and excluded volume interactions, we used a modified wormlike chain model, in which the monomers are represented by charged hard spheres placed at distance a. The electrostatic interactions are approximated by a Debye-Huckel potential. We show that the scattering function is quantitatively described by that of uncharged wormlike chains with excluded volume effects provided that an electrostatic contribution is added to the persistence length. In addition we have studied the expansion of the radius of gyration and of the end-to-end distance. The results are in agreement with the picture outlined in the Odijk-Skolnick-Fixman theory, in which the behavior of charged polymers is described only in terms of increasing local rigidity and excluded volume effects. Moreover, the Monte Carlo data are found to be in very good agreement with experimental scattering measurements with equilibrium polyelectrolytes, i.e., giant wormlike micelles formed in mixtures of nonionic and ionic surfactants in dilute aqueous solution, with added salt.

Time-resolved SANS for surfactant phase transitions

Abstract: Surfactants spontaneously self-assemble in solution to form a variety of microstructures. Our understanding of the equilibrium properties of these aggregates, such as their shape and size, has made significant progress. However, only limited information is available on the kinetics and pathways of phase transitions and on the existence of non-equilibrium or metastable states. Fortunately, this unsatisfactory situation can now be overcome owing to recent developments in small-angle neutron scattering (SANS) instrumentation which significantly improve the temporal and spatial resolution. These new, fascinating possibilities are illustrated by two examples. Firstly, time-resolved SANS reveals the structure of the intermediate in a spontaneous transition from micelles to vesicles in aqueous mixtures of lecithin and bile salt. Secondly, a microemulsion containing spherical oil droplets is temperature quenched into a two-phase situation where excess oil separates out. The nucleation and growth of the separating oil droplets is, again, followed by time-resolved SANS.

Micelles as model systems for equilibrium polyelectrolytes: a light and neutron scattering study

Abstract: We demonstrate that aqueous solutions of giant polymerlike non-ionic micelles “doped” with small amounts of ionic surfactants serve as ideal model systems for “equilibrium polyelectrolytes”. We report systematic light and neutron scattering investigations of the effect of ionic strength, total concentration and doping level on the static properties of dilute and semi-dilute micellar solutions. In dilute solutions we observe a dramatic influence of (intramicellar) electrostatic interactions on the micellar flexibility, and the results are in close agreement with Monte Carlo simulations. In the semi-dilute regime, strong long-range interactions between micelles induce liquid-like ordering.

Structure, dynamics and interactions in concentrated colloidal suspensions and gels

Abstract: The application of static and dynamic light scattering to many colloidal systems of practical interest has often be considered too complicated owing to strong multiple scattering. There are two new approaches to overcome this problem. One of them aims at suppressing contributions from multiple scattering using novel cross-correlation schemes. While this relies on the suppression of multiple scattering, the so-called diffusing wave spectroscopy (DWS) works in the limit of very strong multiple scattering. DWS can be used for the characterization of dynamic and static properties of colloidal systems on a large range of time and length scales ranging from a few Angstroms to hundreds of nanometers. We demonstrate that a wealth of information can be obtained from these methods on the structure, dynamics, interaction effects, stability, aggregation and solgel transition in colloidal dispersions.

Hexanol-induced sphere-to-flexible-coil transition in sugar surfactant solutions: a small-angle neutron and static light scattering study

Abstract: Sugar surfactants have recently attracted considerable interest. We report systematic light and neutron scattering experiments with aqueous solutions of the alkyl polyglucoside C8/10G1.5. The system exhibits a transition from globular to polymer-like micelles upon adding hexanol. We can finely tune the extent of micellar growth and reach conditions where solutions of relatively short cylinders exist; these cylinders grow dramatically into giant wormlike micelles upon the addition of small amounts of hexanol. This system provides us with an ideal data set for analyzing micellar growth and interactions in solutions with cylindrical particles of moderate axial ratios.