Volume 226, Issue 7-8 - Soft Condensed Matters

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Zeitschrift für Physikalische Chemie

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Preface — An energetic life between soft matter and hard X-rays

August 6, 2012

Thomas Hellweg, Axel H. E. Müller

Page range: 543-545

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Optimizing the Sensitivity of FT-Rheology to Quantify and Differentiate for the First Time the Nonlinear Mechanical Response of Dispersed Beer Foams of Light and Dark Beer

August 13, 2012

Manfred Wilhelm, Kathrin Reinheimer, Jennifer Kübel

Page range: 547-567

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Foams are dispersions consisting of a gaseous dispersed phase and a liquid or solid dispersion medium. For the case of beer foam, only the liquid dispersion medium is of interest here. The focus of this work was closed foams that were explored for the first time using Fourier Transform Rheology (FT-Rheology). It was shown that FT-Rheology was able to resolve differences between various brands of beers. While foam rheology is of high interest especially in the food industry and for personal care products, previous investigations were typically made using steady shear or were within the linear oscillatory regime. However, the challenge facing large amplitude oscillatory shear (LAOS) measurements in foams is detection of the resulting extremely low torque related to the higher harmonics. Foams with a high volume fraction of the dispersed gaseous phase generate a LAOS response that is generally quite low, specifically torque values on the order of μN m. Therefore, the LAOS measurements have to be performed with a very sensitive and carefully optimized rheometer. To achieve the results here, the most advanced rheometer ARES G2 from TA Instruments was used because it possesses the highest sensitivity of status quo strain controlled rheometers and is able to measure torques down to 50 nN m. This requirement of high torque resolution limited previous investigations on nonlinear oscillatory shear experiments and makes the experimental results described here both unique and pioneering.
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Adsorption of Spherical Polyelectrolyte Brushes: from Interactions to Surface Patterning

August 6, 2012

Christoph Hanske, Johann Erath, Christin Kühr, Martin Trebbin, Christian Schneider, Alexander Wittemann, Andreas Fery

Page range: 569-584

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Adsorption of colloidal particles constitutes an attractive route to tailor the properties of surfaces. However, for efficient material design full control over the particle-substrate interactions is required. We investigate the interaction of spherical polyelectrolyte brushes (SPB) with charged substrates based on adsorption studies and atomic force spectroscopy. The brush layer grafted from the colloidal particles allows a precise adjustment of their adsorption behavior by varying the concentration of added salt. We find a pronounced selectivity between oppositely and like-charged surfaces for ionic strengths up to 10 mM. Near the transition from the osmotic to the salted brush regime at approximately 100 mM attractive secondary interactions become dominant. In this regime SPB adsorb even to like-charged surfaces. To determine the adhesion energy of SPB on charged surfaces directly, we synthesize micrometer-sized SPB. These particles are used in colloidal probe AFM studies. Measurements on oppositely charged surfaces show high forces of adhesion for low ionic strengths that can be attributed to an entropy gain by counterion release. Transferring our observations to charge patterned substrates, we are able to direct the deposition of SPB into two-dimensional arrays. Considering that numerous chemical modifications have been reported for SPB, our studies could open exiting avenues for the production of functional materials with a hierarchical internal organization.
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Complexation of charged colloids with polyelectrolyte stars

July 25, 2012

Arben Jusufi, Martin Konieczny, Christos N. Likos

Page range: 585-596

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We investigate the complexation of star-shaped polyelectroltyes (PEs) on oppositely charged colloids. Using Molecular Dynamics simulations, we study the chain adsorption on the colloidal surface at fixed size aspect ratio between star and colloid of around 0.6. Depending on the charge ratio between the colloid and the PE star, we observe a variety of star morphologies that were also observed in a recent work of stars on oppositely charged planar surfaces [M. Konieczny and C. N. Likos, Soft Matter 3 , 1130 (2007)]. Another important parameter is the PE star functionality that determines the crossover areas for the various star structures. The morphologies are characterized by the degree of chain adsorption and their orientation with respect to the colloid center. An orientational parameter allows us to identify the various morphologies that range from partial chain adsorption to complete adsorption for which a “starfish” configuration is identified. We present a conformational phase diagram that summarizes the three dominant structures in the parameter space built up from the star functionality and the star-colloid charge ratio.
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Charging and Aggregation of Positively Charged Colloidal Latex Particles in Presence of Multivalent Polycarboxylate Anions

July 25, 2012

Amin Sadeghpour, István Szilágyi, Michal Borkovec

Page range: 597-612

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Colloidal stability and charging behavior of amidine latex particles in the presence of multivalent oligomers of acrylic acid was investigated by electrophoresis and light scattering. The data were interpreted quantitatively with the theory of Derjaguin, Landau, Verwey and Overbeek (DLVO) whereby the surface potentials were estimated from electrophoresis. Monomer leads to slow aggregation at low concentrations and to rapid aggregation at high concentrations, as characteristic for simple salts. The oligomers induce a charge reversal of the particles. Close to the isoelectric point (IEP) aggregation is rapid while the suspension becomes stable away from this point. At high oligomer concentrations, the aggregation becomes rapid again. The agreement between DLVO theory and experiment is good close to the IEP. At higher oligomer concentrations, the theory predicts larger stabilities than observed experimentally. While inter-particle forces seem to be well described by DLVO theory near the IEP, additional attractive non-DLVO forces are becoming relevant at higher concentrations.
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Spherical Polyelectrolyte Brushes on Colloidal Poly(butadiene) Particles

August 6, 2012

Gongsheng Li, Jun Xu, Shifang Zhao, Yuanqing Zhu, Li Li, Xuhong Guo

Page range: 613-623

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A novel method to prepare spherical polyelectrolyte brushes (SPB) on the surface of colloidal poly(butadiene) (PB) core without having to use initiators with C=C double bond to attach covalently on the core surface was reported. Making use of the abundant double bonds in PB, spherical poly(acrylic acid) (PAA) bushes were grown from PB core surface by direct thermo-initiated emulsion polymerization using KPS as initiator. The thickness of SPB increased significantly upon increasing pH and decreasing the ionic strength. The grafting density of SPB with PB core was estimated from the relationship between the brush thickness and ionic strength based on the modified Daoud-Cotton model. The grafting density of SPB prepared by thermo-initiated emulsion polymerization (0.122 nm -2 ) is higher that that of SPB by photo-emulsion polymerization (0.105 nm -2 ). Using SPB with PB core as nanoreactors, nickel and silver nanoparticles were prepared by reduction of Ni 2+ and Ag + absorbed inside SPB as counterions. Relatively high catalytic activities for the reduction of p -nitrophenol by NaBH 4 were observed for SPB immobilized Ni or Ag nanoparticles.
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Repulsive Forces between Spherical Polyelectrolyte Brushes in Salt-Free Solution

August 13, 2012

Ekaterina B. Zhulina, Andrey B. Boulakh, Oleg V. Borisov

Page range: 625-643

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Interaction forces between two colloidal spherical polyelectrolyte brushes in salt-free solution are calculated within Derjaguin approximation on the basis of self-consistent field Poisson–Boltzmann theory of a planar polyelectrolyte brush (E. B. Zhulina, O. V. Borisov, J. Chem. Phys. 107 (1997) 5952). It is demonstrated that at large separations the force-distance curve has a universal form which is independent of the charge density of the brush. At small separations the repulsive force is controlled by osmotic pressure of the counterions confined inside compressed brush. The crossover between these two regimes corresponds to the distance between the surfaces comparable to (double) thickness of a brush and occurs either continuously or with abrupt variation in the magnitude of the repulsive force. The latter is the case if the interacting brushes are found in the osmotic regime, that is, most of the counterions are retained in the intra-brush volume.
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Structure-Property Relations of Liquid Crystalline Gels with ABA-Triblock Copolymers as Gelators

August 13, 2012

Robin Pettau, Thomas Müller, Maxim Khazimullin, Ingo Rehberg, Hans-Werner Schmidt

Page range: 645-664

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This paper reports on the influence of the structure of tailored ABA-triblock copolymers on physical gelation of the nematic liquid crystal 4'-n-pentyl-4-cyanobiphenyl (5CB), rheological properties, and the electro-optical response of the gels. The block copolymer gelators, comprised of two polystyrene A-blocks connected to a cyanobiphenyl-functionalized polyhydroxystyrene B-block, were synthesized by living anionic polymerization and functionalized by polymer analogous reactions. These block copolymers feature selective solubility. The B-block is soluble in the nematic and isotropic state of 5CB, whereas the polystyrene A-blocks are only soluble in the isotropic state. Consequently, upon cooling liquid crystalline gels are formed due to the controlled microphase-separation of the A-blocks. We show that the polymer composition and the different block lengths are important to vary the gel properties and the electro-optical response. It is confirmed that the gel formation correlates to the isotropic to nematic transition of 5CB for block copolymer gelators with sufficiently long A-blocks. Higher gel elasticity is obtained if gelators with short B-blocks are employed. The influence of the polymer network on the switching behavior of these liquid crystalline gels is investigated with respect to the electro-optical response in light scattering experiments. Intriguingly, these indicate a rearrangement of the nodes formed by the A-blocks under a strong electric field for block copolymers with short A-blocks.
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Direct-Imaging Cryo-SEM of Nanostructure Evolution in Didodecyldimethylammonium Bromide-Based Microemulsions

August 13, 2012

Ido Ben-Barak, Yeshayahu Talmon

Page range: 665-674

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We applied cryogenic-temperature scanning electron microscopy (cryo-SEM) to perform a first direct-imaging study of single-phase microemulsions of didodecyldimethylammonium bromide (DDAB), isooctane, and water. The structural changes from a bicontinuous network to an oil-continuous structure, observed upon the addition of water to the system, are consistent with previous indirect investigations, thus validating the model of Ninham, Evans, and coworkers, published in 1984, and demonstrating the power of this novel methodology.
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Solubilisation of Oils of Different Polarity in Aqueous Solutions of Pluronic Triblock Copolymers

August 13, 2012

Hsin-Yi Liu, Sylvain Prévost, Michael Gradzielski

Page range: 675-694

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The micellisation behaviour of Pluronic F108 was studied as a function of the addition of various more or less polar additives, such as hexanol, geraniol, toluene, methyl heptanoate, polyproylene oxide (PPO), and nitropropane. It is observed that the critical micellisation temperature (CMT) is substantially reduced upon the solubilisation of most of the additives (but not for methyl heptanoate and PPO) and at the same time the concomitant enthalpy of micellisation becomes larger. Both changes in CMT and enthalpy of micellisation are linear with respect to the concentration of additive. The pronouncedness of this effect is related to the polarity of the additive (as characterised for instance by its water solubility) and for instance similar effects are achieved for much smaller amounts of geraniol compared to hexanol. The structural evolution of the Pluronic solutions upon addition of the additives was also followed by SANS experiments and show that relatively small amounts of added solubilisate can have a very pronounced effect on the extent of the micellisation and size of the aggregates formed, i.e. , effectively lowering the CMT by rendering the amphiphilic system more hydrophobic and especially the medium chain alcohols show pronounced synergism.
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Double Responsive Hydrogels based on Tertiary Amine Methacrylate Star Block Copolymers

July 25, 2012

Alexander Schmalz, Holger Schmalz, Axel H. E. Müller

Page range: 695-709

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Double hydrophilic stimuli-responsive star block copolymers with poly(2-(dimethylamino)ethyl methacrylate) (PDMA) inner blocks and poly(2-(diethylamino)ethyl methacrylate) (PDEA) outer blocks were synthesized using ATRP. Different multifunctional initiators based on sugar scaffolds were employed in a core-first approach with sequential polymerization of both blocks yielding stars with 4 and 6 arms, respectively, and varying length of the PDEA outer block. The star block copolymers show pH- and temperature-responsive aggregation as revealed by dynamic light scattering and turbidimetry. The impact of pH, PDEA block length and arm number on the gelation behavior was investigated by tube inversion and rheology.
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Coarse-Graining of Ionic Microgels: Theory and Experiment

July 25, 2012

Jonas Riest, Priti Mohanty, Peter Schurtenberger, Christos N. Likos

Page range: 711-735

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In this work, we discuss the statistical mechanics of many-body systems consisting of electrically charged microgels, and we show that their collective behavior is determined by an interplay between the screened electrostatic and the elastic contributions to their effective interaction potential. The former is derived by means of a statistical-mechanical approach due to Denton [A. R. Denton, Phys. Rev. E 67 , 011804 (2003)], and it includes the screened electrostatic potential between penetrable spheres and the counterion entropic contribution. The latter is based on the Hertzian model of the theory of elasticity. Comparisons with experimental results demonstrate the realistic nature of the coarse-graining procedure, which makes it possible to put forward theoretical predictions on the phase diagram of ionic microgels and on the behavior of soft, neutral microgels under confinement in narrow pores.
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Non NIPAM Based Smart Microgels: Systematic Variation of the Volume Phase Transition Temperature by Copolymerization

July 25, 2012

Bastian Wedel, Michael Zeiser, Thomas Hellweg

Page range: 737-748

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Thermoresponsive copolymer microgels based on N - n -propylacrylamide (NNPAM) and N -isopropylmethacrylamide (NIPMAM) with varying compositions were synthesized via precipitation polymerization. Photon correlation spectroscopy (PCS) and turbidity measurements were used to investigate their volume phase transition. A linear correlation between the nominal composition and the volume phase transition temperature (VPTT) was observed. Furthermore, the hydrodynamic radii of the particles exhibit a linear dependency on the nominal composition. The presented system is suitable for the synthesis of thermoresponsive microgels with a well defined VPTT or size and gives access to tune these two crucial parameters in a systematic and controlled way by simply choosing the right composition of the monomer feed. Additionally, the first derivatives of the swelling curves obtained from turbidity measurements were analyzed in detail, allowing a quantitative comparison of the phase transition of different microgels.
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Temperature Controlled Activity of Lipase B from Candida Antarctica after Immobilization within p-NIPAM Microgel Particles

August 13, 2012

Kornelia Gawlitza, Changzhu Wu, Radostina Georgieva, Marion Ansorge-Schumacher, Regine von Klitzing

Page range: 749-759

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The immobilization of lipase B from Candida antarctica (CalB) within micronsized poly- N -Isopropylacrylamide (p-NIPAM) microgel particles with a crosslinker content of 5% is reported. The immobilization of the enzyme was reached by an exchange from polar to organic solvents. After determining the embedded amount of CalB within the polymer network, an enhanced specific activity in n -hexane was obtained. Due to the thermoresponsibility of the polymer particles, the activity reaction was done at 25 ºC and 50 ºC. The results presented show that the reversible collapse of the microgel leads to a decreased activity with increasing temperature. Hence, p-NIPAM microgels display a good opportunity to tailor the activity of CalB. An interesting side effect is that CalB presents a suitable probe to estimate the mesh size of the polymer network, since it penetrates in the unlabeled form but not after labeling with FITC.
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Synthesis of PEG-Stabilized Fluoro-Acrylate Particles and Study of their Glass Transition in Aqueous Dispersion

July 25, 2012

Malte Wiemann, René Schneider, Eckhard Bartsch

Page range: 761-778

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Synthesis of colloidal particles which are sterically stabilized in water and at the same time isorefractive in aqueous dispersion has been achieved by copolymerizing a fluoracrylate monomer with a PEG-macromonomer. Colloidal stability against salt addition and freeze-thawing cycles is demonstrated, indicating that the particles consist of a core of fluoroacrylate and a stabilizing surface layer of grafted PEG. To test the performance as a model system for studies of colloidal vitrification in aqueous media, the dynamics of a binary mixture of these particles with a size ratio of 0.8 and a number ratio of 1.3 (small : large particles) were studied in an isorefractive mixture of water and DMSO ( ≈ 11 mol % DMSO). The characteristic features of a system close to the glass transition were observed in the density density autocorrelation function f ( q , τ ) as measured by DLS. However, the glass transition occurred at an unexpectedly low volume fraction of 0.262. This discrepancy correlates with an unusually large difference between TEM and DLS radii and is explained by the presence of a rather extended hairy PEG surface layer.
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Probability Densities of a Forced Probe Particle in Glass: Results from Mode Coupling Theory and Simulations of Active Microrheology

August 6, 2012

Christian J. Harrer, A. M. Puertas, Th. Voigtmann, M. Fuchs

Page range: 779-795

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We investigate the displacements of a probe particle inside a glass, when a strong external force is applied to the probe (active nonlinear microrheology). Calculations within mode coupling theory are presented for glasses of hard spheres and compared to Langevin and Brownian dynamics simulations. Under not too strong forces where the probe remains trapped, the probe density distribution becomes anisotropic. It is shifted towards the direction of the force, develops an enhanced tail in that direction (signalled by a positive skewness), and exhibits different variances along and perpendicular to the force direction. A simple model of an harmonically trapped probe rationalizes the low force limit, with strong strain softening setting in at forces of the order of a few thermal energies per particle radius.
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Molecular Order and Dynamics of Tris(2-ethylhexyl)phosphate Confined in Uni-Directional Nanopores

August 13, 2012

Wycliffe Kiprop Kipnusu, Wilhelm Kossack, Ciprian Iacob, Malgorzata Jasiurkowska, Joshua Rume Sangoro, Friedrich Kremer

Page range: 797-805

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Infrared Transition Moment Orientational Analysis (IR–TMOA) and Broadband Dielectric Spectroscopy (BDS) are combined to study molecular order and dynamics of the glass-forming liquid Tris(2-ethylhexy)phosphate (TEHP) confined in uni-directional nanopores with diameters of 4, 8, and 10.4 nm. The former method enables one to determine the molecular order parameter of specific IR transition moments. It is observed that the central P = O moiety of TEHP has a weak orientational effect (molecular order parameter S z = − 0.1 ± 0.04) due the nanoporous confinement, in contrast to the terminal C–H groups. BDS traces the dynamic glass transition of the guest molecules in a broad spectral range and at widely varying temperature. An enhancement of the mobility takes place when approaching the glass transition temperature and becomes more pronounced with decreasing pore diameter. This is attributed to a slight reduction of the density of the confined liquid caused by the 2-dimensional geometrical constraint.
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The Structure of Fluids with Impurities

July 25, 2012

Markus Bier, Ludger Harnau

Page range: 807-814

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The influence of dilute impurities on the structure of a fluid solvent is investigated theoretically. General arguments, which do not rely on particular models, are used to derive an extension of the Ornstein–Zernike form for the solvent structure factor at small scattering vectors. It is shown that dilute impurities can influence the solvent structure only if they are composed of ions with significantly different sizes. Non-ionic impurities or ions of similar size are shown to not alter the solvent structure qualitatively. This picture is compatible with available experimental data. The derived form of the solvent structure factor is expected to be useful to infer information on the impurity-solvent interactions form measured scattering intensities.
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Packing of Cylindrical Keggin-Type Polyoxometalate Hybrid Micelles as a Function of Aspect Ratio

August 6, 2012

Thomas Lunkenbein, Martin Schieder, Carina Bojer, Axel H. E. Müller, Josef Breu

Page range: 815-826

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The packing of cylindrical Keggin-Type polyoxometalate hybrid micelles and its influence on specific surface area was studied as function of aspect ratio. Core-crosslinked cylindrical poly(butadiene- block -2-vinylpyidine) (PB-b-P2VP) micelles were used as templates for mesostructuring phosphomolybdic acid (H 3 PMo 12 O 40 ). Prior to hybrid formation the length of the cylindrical PB-b-P2VP micelles was adjsuted by sonication. Depending on the aspect ratio of the cylinders a remarkable and unexpected evolution of specific surface areas was observed. The changes in the specific surface area could be explained by a series of scanning electron microscopy and transmission electron microscopy images and were related to the packing of the 1D nanoobjects.
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Au-TiO₂ Yolk-Shell Particles for Photocatalysis Application

July 25, 2012

Bin Dai, Frank Polzer, Ines Häusler, Yan Lu

Page range: 827-835

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Au-TiO 2 yolk-shell particles with gold as core and mesoporous TiO 2 as shell have been synthesized by deposition of TiO 2 nanoparticles onto Au-silica core-shell particles via modified sol-gel method at room temperature followed by the removal of silica midlayer by NaOH treatment. The Au-TiO 2 yolk-shell particles exhibit a high colloidal stability, which can be applied as excellent heterogeneous photocatalysts for the photodegradation of the dye Rhodamine B under UV irradiation. The Au-TiO 2 yolk-shell particles have shown much higher photocatalytic activity than that of the hollow TiO 2 particles without gold.

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Formfactors of Hollow and Massive Rectangular Parallelepipeds at Variable Degree of Anisometry

August 13, 2012

Roman Nayuk, Klaus Huber

Page range: 837-854

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The present work is a comparative study on the scattering behaviour of hollow and filled rectangular parallelepipeds. Comparison is based on the model formfactors of rectangular parallelepipeds, which cover the entire regime of variable wall thickness. The entire regime of wall thicknesses has been made available by the present work, which completed the set of formulas by calculating the formfactor for the limit of hollow parallelepipeds with infinitely thin walls, which was still lacking. The formfactors are expressed as a function of the momentum transfer q . Discrimination between massive and hollow structures by means of the q -dependent scattering data SF(q) gets possible once the Guinier radius and the particle volume can be established at the lower limit of q or a power law of SF(q) ∼ q α can be extracted towards large q . Whereas the former requires extrapolation of the scattering data to q = 0 with high accuracy, the latter needs experiments over a very broad q -regime. If experimental data is restricted to a q -regime which includes only the first peak in a Kratky representation of q 2 SF(q) vs. q without giving way to an accurate extrapolation to q = 0 nor to a clear power law in q , discrimination between a hollow and a massive structure gets extremely difficult. Yet, the effect of anisometry is striking and enables extraction of a crude guess of the degree of anisometry already from the first Kratky peak. This could be achieved by introducing a dimensionless parameter established from the width and location of the first Kratky peak.

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Issue 11-12
Special Issue Prof. Spiess

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Memorial Prof. Kolb

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Soft Condensed Matters

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