R. Strey

University of Cologne, Köln, North Rhine-Westphalia, Germany

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Publications (224)506.93 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We follow the nucleation of n-alcohols, n-propanol through n-pentanol, in two sets of supersonic nozzles having differing linear expansion rates. Combining the data from static pressure trace measurements with small angle X-ray scattering we report the experimental nucleation rates and critical cluster sizes. For n-propanol, position resolved measurements clearly confirm that coagulation of the 2 - 10 nm size (radius) droplets occurs on the timescale of the experiment but that the effect of coagulation on the results is minimal. Under the conditions of the current experiments, our results suggest that alcohols have critical clusters that range from the dimer (n-pentanol) to the hexamer (n-propanol). We then compare the experimental results with classical nucleation theory (CNT), the Girshick-Chiu variant of CNT (GC) and the mean field kinetic nucleation theory (MKNT). Both CNT and MKNT underestimate the nucleation rates by up to 5 and 7 orders of magnitude, respectively, while GC theory predicts rates within 2 orders of magnitude. Correspondingly, the critical cluster size for all alcohols is over-predicted by factors of 2 - 9 with agreement improving with increasing chain length. An interesting byproduct of our experiments is that we find that the coagulation rate is enhanced by a factor of 3 over the value one would calculate for the free molecule regime.
    The journal of physical chemistry. B. 10/2014;
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    Chemie Ingenieur Technik 09/2014; 86(9). · 0.70 Impact Factor
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    ABSTRACT: Control over the self-assembly of magnetic nanoparticles (MNP) into superstructures due to different types of coupling is of interest in the development of "bottom-up" fabrication schemes. Here we realize a simple strategy for the systematic variation of particle interaction potential in magnetic nanoparticles. This is achieved by varying the effective surface potential by means of a co-surfactant introduced in the course of the synthesis process. As a consequence, the ability to form chain-like assemblies is affected by the resulting balance of attractive and repulsive forces. We use electron microscopy, electron diffraction, and light scattering methods to study a series of cobalt nanoparticles as a characteristic example of ferromagnetic MNP. We demonstrate experimentally and substantiate theoretically that the observed behavior results from a balance between magnetic dipole-dipole, steric, and electrostatic interactions.
    Journal of Colloid and Interface Science 08/2014; 436C:83-89. · 3.55 Impact Factor
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    ABSTRACT: CO2-microemulsions show strong pressure dependent properties. Using time-resolved SANS to investigate the kinetics of structural changes upon periodic pressure jumps of adjustable amplitude, we found that the compression-induced formation of cylinders occurs on a timescale of one second, whereas the expansion-induced disintegration into CO2 swollen spherical micelles is much faster.
    Physical Chemistry Chemical Physics 07/2014; · 4.20 Impact Factor
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    S Quester, M Dahesh, R Strey
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    ABSTRACT: We have generated closed-cell microcellular foams from gliadin, an abundantly available wheat storage protein. The extraction procedure of gliadin from wheat gluten, which involves only the natural solvents water and ethanol, respec-tively, is described with emphasis on the precipitation step of gliadin which results in a fine dispersion of mostly spherical, submicron gliadin particles composed of myriad of protein molecules. A dense packing of these particles was hydrated and subjected to an atmosphere of carbon dioxide or nitrogen in a high-pressure cell at 250 bar. Subsequent heating to temperatures close to but still below 100 °C followed by sudden expansion and simultaneous cooling resulted in closed-cell microcellular foam. The spherical gliadin tem-plates along with the resulting foam have been analyzed by scanning electron microscope (SEM) pictures. The size distri-bution of the primary particles shows diameters peaked around 0.54 μm, and the final foam cell size peaks around 1.2 μm, at a porosity of about 80 %. These are the smallest foam cell sizes ever reported for gliadin. Interestingly, the cell walls of these microcellular foams are remarkably thin with thicknesses in the lower nanometer range, thus nourishing the hope to be able to reach gliadin nanofoam.
    Colloid and polymer science. 06/2014; 292(9).
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    ABSTRACT: Nucleation rates of n-propanol were investigated in the Laminar Flow Diffusion Chamber. Nucleation temperatures between 270 and 300 K and rates between 10(0) and 10(6) cm(-3) s(-1) were achieved. Since earlier measurements of n-butanol and n‑pentanol suggest a dependence of nucleation rates on carrier gas pressure, similar conditions were adjusted for these measurements. The obtained data fit well to results available from literature. A small positive pressure effect was found which strengthen the assumption that this effect is attributed to the carbon chain length of the n-alcohol [D. Brus, A. P. Hyvärinen, J. Wedekind, Y. Viisanen, M. Kulmala, V. Ždímal, J. Smolík, and H. Lihavainen, J. Chem. Phys. 128, 134312 (2008)] and might be less intensive for substances in the homologous series with higher equilibrium vapor pressure. A comparison with the theoretical approach by Wedekind et al. [Phys. Rev. Lett. 101, 12 (2008)] shows that the effect goes in the same direction but that the intensity is much stronger in experiments than in theory.
    The Journal of Chemical Physics 05/2014; 140(17):174301. · 3.12 Impact Factor
  • Macromolecules 01/2014; 47(20):7200-7207. · 5.93 Impact Factor
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    ABSTRACT: We provide a brief overview of the extensive scientific research that has been conducted to determine the degree to which pure water can be supercooled and the rates at which the relevant phase transitions occur. Beginning with condensation, we present the homogeneous vapor - liquid nucleation rates measured over 20 orders of magnitude with a variety of experimental devices. Some of the experimental methods used to examine nucleation from the vapor phase can also be used to study freezing. In particular, using a supersonic nozzle (SSN) apparatus we have followed the condensation and subsequent freezing of pure water droplets by combining our normal pressure trace measurements (PTM) with both small angle Xray scattering (SAXS) and Fourier transform infrared (FTIR) spectroscopy. The time dependence of the fraction of frozen droplets yields the volume based nucleation rates for the freezing transition, 7.66.1023 < Jice/cm-3s-1 < 4.23.1024, confirming and extending those reported in the literature.
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    ABSTRACT: New measurements of homogeneous nucleation onset data for argon and nitrogen from the cryogenic nucleation pulse chamber have been performed. By reducing the amount of carrier gas in the gas mixture, higher onset pressures and temperatures could be reached. The new results are consistent with older data from the same apparatus and literature data. Carrier gas-free experiments with nitrogen show no associative effects. Interestingly, the classical nucleation theory predicts the onset results of nitrogen more accurately than those of argon, despite argon being the more ideal substance of the two.
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    ABSTRACT: We study the co-condensation of nonane and D2O in a supersonic flow, using small angle X-ray scattering to characterize the size and number density of the aerosol and Fourier Transform Infrared Spectroscopy to determine the composition. Under our experimental conditions, we observe D2O nucleating at a rate that is somewhat reduced by the presence of nonane. Nonane however, readily condenses on the water clusters impeding the growth of the nanodroplets by D2O.
    05/2013; 140(3):51-54.
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    ABSTRACT: Pluronic F-127 hydrogels are highly efficient microenvironments for photochemical reactions, as demonstrated for singlet oxygen reactions of monoalkenes. Nonpolar substrates are localized in the nanosized polymer compartment, which can be visualized by neutron scattering. The efficiency of (1) O(2) reactions is strongly increased for tiglate derivatives and the regioselectivity of the ene reaction of trisubstituted alkenes is completely switched in comparison with solution phase and inverted in comparison with intrazeolite photo-oxygenations.
    Chemistry - A European Journal 10/2012; · 5.93 Impact Factor
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    ABSTRACT: We present homogeneous vapor-liquid nucleation rates of the 1-alcohols (C(n)H(2n+1)OH, n = 2-4) measured in the well-established two-valve nucleation pulse chamber as well as in a novel one-piston nucleation pulse chamber at temperatures between 235 and 265 K. The nucleation rates and critical cluster sizes show a very systematic behavior with respect to the hydrocarbon chain length of the alcohol, just as their thermo-physical parameters such as surface tension, vapor pressure, and density would suggest. For all alcohols, except ethanol, predictions of classical nucleation theory lie several orders of magnitude below the experimental results and show a strong temperature-dependence typically found in nucleation experiments. The more recent Reguera-Reiss theory [J. Phys. Chem. B 108(51), 19831 (2004)] achieves reasonably good predictions for 1-propanol, 1-butanol, and 1-pentanol, and independent of the temperature. Ethanol, however, clearly shows the influence of strong association between molecules even in the vapor phase. We also scaled all experimental results with classic nucleation theory to compare our data with other data from the literature. We find the same overall temperature trend for all measurement series together but inverted and inconsistent temperature trends for individual 1-propanol and 1-butanol measurements in other devices. Overall, our data establishe a comprehensive and reliable data set that forms an ideal basis for comparison with nucleation theory.
    The Journal of Chemical Physics 08/2012; 137(5):054316. · 3.12 Impact Factor
  • Article: Surfactants
    Current Opinion in Colloid & Interface Science 08/2012; 17(4):187. · 6.63 Impact Factor
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    ABSTRACT: We present a study of the structure and local dynamics of dispersed silica nanoparticles in the liquid crystal 4′-pentyl-4-biphenylcarbonitrile (5CB). Electron microscopy and X-ray scattering data demonstrate that the nanoparticles form agglomerates with a fractal structure. Investigation of the local dynamics of the particle agglomerates using X-ray photon correlation spectroscopy reveals pronounced anisotropy in the relaxation rate and aging behaviour. We suggest that the unusual relaxation behaviour is a consequence of the anisotropy of the host solvent.
    Soft Matter 03/2012; 8(15):4062-4066. · 4.15 Impact Factor
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    ABSTRACT: We report homogeneous ice nucleation rates between 202 K and 215 K, thereby reducing the measurement gap that previously existed between 203 K and 228 K. These temperatures are significantly below the homogenous freezing limit, T(H)≈ 235 K for bulk water, and well within no-man's land. The ice nucleation rates are determined by characterizing nanodroplets with radii between 3.2 and 5.8 nm produced in a supersonic nozzle using three techniques: (1) pressure trace measurements to determine the properties of the flow as well as the temperature and velocity of the droplets, (2) small angle X-ray scattering (SAXS) to measure the size and number density of the droplets, and (3) Fourier Transform Infrared (FTIR) spectroscopy to follow the liquid to solid phase transition. Assuming that nucleation occurs throughout the droplet volume, the measured ice nucleation rates J(ice,V) are on the order of 10(23) cm(-3) s(-1), and agree well with published values near 203 K.
    Physical Chemistry Chemical Physics 03/2012; 14(13):4505-16. · 4.20 Impact Factor
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    ABSTRACT: The partial ternary phase diagram of anionic extended surfactant of alkyl polypropylene oxide sulfate C12(PO)4SO4 alone and combined with the cationic hydrotrope, tetrabutyl ammonium bromide with water and decane were determined under ambient conditions. Middle phase microemulsion was formulated using salinity scans in the dilute region of surfactant/brine/decane. Visual inspection as well as cross polarizer and optical microscopy were used to detect anisotropy. Spinning drop tensiometer was used to measure interfacial tension (IFT). The first ternary phase diagram using the extended surfactant alone showed three one phase regions, the anisotropic lamellar liquid crystalline phase, Lα and the isotropic L1 micellar liquid and L3 sponge phase. In the second ternary phase diagram using the extended surfactant combined with tetra butyl ammonium bromide, an isotropic micellar region, L1, appeared in the diluted area of the phase diagram. Meanwhile the Lα phase disappeared completely and the three phase region has a bluish transparent middle phase. Interfacial tension measurements between middle phase and brine, and between decane and brine yielded ultra low values. Calculated IFT values using the characteristic length obtained using De Gennes approximation gave almost half the measured values. The interfacial rigidity was also calculated and compared to values obtained from the literature.
    Journal of Dispersion Science and Technology 03/2012; 33(3):369-373. · 0.60 Impact Factor
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    ABSTRACT: Balanced scCO2-microemulsions contain equal volumes of water and CO2 and are a novel class of microemulsions of substantial interest for both fundamental research and technical applications. One existing feature of these systems is that the solvent quality of scCO2, and hence the overall microemulsion properties, is tuned simply by adjusting pressure, which is not possible with “classical” microemulsions containing oil instead of CO2. Motivated by this, we systematically investigated the phase behavior, the microstructure, and the dynamics of balanced microemulsion systems of the type H2O–CO2–Zonyl FSO 100/Zonyl FSN 100. In systematic phase behavior studies, we found that upon increasing pressure, CO2 and water are more efficiently solubilized. Small angle neutron scattering (SANS) experiments were conducted in order to determine the topology and the length scales of the underlying microstructure. The results obtained strongly suggest the existence of bicontinuously structured microemulsions with an adjustable characteristic length scale of up to 330 Å. From a quantitative analysis of the SANS data, we found that at a fixed microemulsion composition the stiffness of the surfactantmembrane is increased solely by increasing the pressure, whereby the renormalization corrected (i.e. bare) bending rigidity κ0,SANS rises from κ0,SANS = 0.88 kBT at 200 bar to 0.93 kBT at 300 bar. These findings were confirmed with high pressure neutron spin echo experiments.
    Soft Matter 12/2011; 8(3):797-807. · 4.15 Impact Factor
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    ABSTRACT: Microemulsions of the type H(2)O-scCO(2)-surfactant are potential candidates for novel solvent mixtures in the field of green chemistry. Furthermore, scCO(2)-microemulsions are highly interesting from a fundamental point of view since their properties such as the bending elastic constants can be strongly influenced solely by varying the pressure without changing the components. With this motivation we studied the phase behavior and the microstructure of water-rich scCO(2)-microemulsions. Such microemulsions were formulated using the technical grade non-ionic surfactants Zonyl FSO 100 and Zonyl FSN 100. At elevated pressures the temperature dependent phase behavior of these systems follows the general patterns of non-ionic microemulsions. Small angle neutron scattering experiments were conducted to determine the length scales and the topology of the microstructure of these systems. Having determined the exact scattering length densities and the composition of the respective sub-phases by a systematic contrast variation we could show that these systems consist of CO(2)-swollen microemulsion droplets that are dispersed in a continuous aqueous-phase. The scattering data were analyzed using a newly derived form factor for polydisperse, spherical core/shell particles with diffuse interfaces. The underlying analytical density profiles could be confirmed applying the model-free Generalized Indirect Fourier Transformation (GIFT) to the scattering data. Following the general patterns of non-ionic microemulsions the radius of the microemulsion droplets is found to increase almost linearly upon the addition of CO(2).
    Physical Chemistry Chemical Physics 12/2011; 13(45):20289-301. · 4.20 Impact Factor
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    ABSTRACT: Ferrohydrogels are synthesized by incorporation of magnetic CoFe2O4 nanoparticles into a polyacylamide hydrogel network during the polymerization process by utilizing different cross-linking units. Conventional cross-linked ferrohydrogels, using a molecular cross-linker, are compared to those obtained by our new approach where the magnetic particles, surface-functionalized with methacylic groups, serve as sole, multifunctional cross-linkers. Both experimental series are analyzed with regard to their swelling behavior. The novel composite network is examined with respect to the cross-linkage, the network homogeneity, and the network architecture by various experimental techniques.
    Macromolecules 03/2011; 44:2990-2999. · 5.93 Impact Factor
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    ABSTRACT: Alcohol–free microemulsions were formulated using mixtures of extended surfactant (C12–14-PO14-EO2SO4Na), sodium dodecyl benzene sulfonic acid and cationic hydrotropes with equal amounts of water and diesel. The cationic hydrotropes had short hydrocarbon or propylene oxide chain. The formulation included sodium carbonate to convert naphthenic acids in diesel to soaps. The phase behavior at ambient temperature of oil-free mixtures as a function of NaCl concentration was investigated. Visual inspection as well as cross polarizers were used to detect anisotropy. The microemulsion fish phase diagram and solubilization ratios for diesel and brine in the middle phases were determined. The minimum surfactant concentration needed to initiate middle phase formation was 0.10 wt.%.Salinity scans revealed that optimal salinity can be adjusted according to the hydrophilic/lipophilic nature of the hydrotrope used. Interfacial tension (IFT) measurements using a spinning drop tensiometer showed a minimum value of 0.0015 mN/m between middle phase microemulsion and excess brine and a value of 0.032 mN/m between diesel and brine.
    Journal of Dispersion Science and Technology 01/2011; · 0.60 Impact Factor

Publication Stats

3k Citations
506.93 Total Impact Points


  • 1997–2014
    • University of Cologne
      • • Institute of Physical Chemistry
      • • Institute for Theoretical Physics
      Köln, North Rhine-Westphalia, Germany
  • 2000–2010
    • Forschungszentrum Jülich
      • Jülich Centre for Neutron Science (JCNS)
      Düren, North Rhine-Westphalia, Germany
  • 1983–2010
    • Max Planck Institute for Biophysical Chemistry
      Göttingen, Lower Saxony, Germany
  • 2009
    • University of Barcelona
      • Departament de Física Fonamental
      Barcelona, Catalonia, Spain
  • 2006
    • Yokohama National University
      • Graduate School of Environment and Information Sciences
      Yokohama-shi, Kanagawa-ken, Japan
  • 2005
    • University College Dublin
      • School of Chemical and Bioprocess Engineering
      Dublin, L, Ireland
    • University of Minnesota Twin Cities
      • Department of Chemical Engineering and Materials Science
      Minneapolis, MN, United States
    • Worcester Polytechnic Institute
      • Department of Chemical Engineering
      Worcester, MA, United States
  • 2000–2004
    • Weizmann Institute of Science
      • Department of Materials and Interfaces
  • 2002
    • Massachusetts Institute of Technology
      • Department of Nuclear Science and Engineering
      Cambridge, MA, United States
  • 1996
    • Universität Basel
      • Department of Physical Chemistry
      Bâle, Basel-City, Switzerland