[Show abstract][Hide abstract] ABSTRACT: The thermal diffusion behavior of dilute solutions of very long and thin, charged colloidal rods (fd-virus particles) is studied using a holographic grating technique. The Soret coefficient of the charged colloids is measured as a function of the Debye screening length, as well as the rod-concentration. The Soret coefficient of the fd-viruses increases monotonically with increasing Debye length, while there is a relatively weak dependence on the rod-concentration when the ionic strength is kept constant. An existing theory for thermal diffusion of charged spheres is extended to describe the thermal diffusion of long and thin charged rods, leading to an expression for the Soret coefficient in terms of the Debye length, the rod-core dimensions, and the surface charge density. The thermal diffusion coefficient of a charged colloidal rod is shown to be accurately represented, for arbitrary Debye lengths, by a superposition of spherical beads with the same diameter of the rod and the same surface charge density. The experimental Soret coefficients are compared with this and other theories, and are contrasted against the thermal diffusion behaviour of charged colloidal spheres.
[Show abstract][Hide abstract] ABSTRACT: Eiger is the next-generation single-photon-counting pixel detector following the widely used Pilatus detector. Its smaller pixel size of 75 µm × 75 µm, higher frame rate of up to 22 kHz, and practically zero dead-time (∼4 µs) between exposures will further various measurement methods at synchrotron sources. In this article Eiger's suitability for X-ray photon correlation spectroscopy (XPCS) is demonstrated. By exploiting its high frame rate, complementary small-angle X-ray scattering (SAXS) and XPCS data are collected in parallel to determine both the structure factor and collective diffusion coefficient of a nano-colloid suspension. For the first time, correlation times on the submillisecond time scale are accessible with a large-area pixel detector.
[Show abstract][Hide abstract] ABSTRACT: The electrophoretic mobility of rodlike fd viruses is measured and compared to theory, with the theoretical calculations performed according to Stigter (Stigter, D. Charged Colloidal Cylinder with a Gouy Double-Layer. J. Colloid Interface Sci.1975, 53, 296-306. Stigter, D. Electrophoresis of Highly Charged Colloidal Cylinders in Univalent Salt- Solutions. 1. Mobility in Transverse Field. J. Phys. Chem.1978, 82, 1417-1423. Stigter, D. Electrophoresis of Highly Charged Colloidal Cylinders in Univalent Salt Solutions. 2. Random Orientation in External Field and Application to Polyelectrolytes. J. Phys. Chem.1978, 82, 1424-1429. Stigter, D. Theory of Conductance of Colloidal Electrolytes in Univalent Salt Solutions. J. Phys. Chem.1979, 83, 1663-1670), who describes the electrophoretic mobility of infinite cylinders including relaxation effects. Using the dissociation constants of the ionizable groups on the surfaces of the fd viruses, we can calculate the mobility without any adjustable parameter (apart from the possible Stern layer thickness). In addition, the approximation in the theoretical description of Stigter (and others) of using a model of infinitely long cylinders, which consequently is independent of the aspect ratio, is examined by performing more elaborate numerical calculations for finite cylinders. It is shown that, although the electrophoretic mobility of cylindrical particles in the limit of low ionic strength depends on the aspect ratio much more than "end effects", at moderate and high ionic strengths the finite and infinite cylinder models differ only to a degree that can be attributed to end effects. Furthermore, the range of validity of the Stokes regime is systematically calculated.
[Show abstract][Hide abstract] ABSTRACT: We address the fundamental question: how are pair correlations and structure factors of hard-sphere fluids affected by confinement between hard planar walls at close distance? For this purpose, we combine x-ray scattering from colloid-filled nanofluidic channel arrays and first-principles inhomogeneous liquid-state theory within the anisotropic Percus-Yevick approximation. The experimental and theoretical data are in remarkable agreement at the pair-correlation level, providing the first quantitative experimental verification of the theoretically predicted confinement-induced anisotropy of the pair-correlation functions for the fluid. The description of confined fluids at this level provides, in the general case, important insights into the mechanisms of particle-particle interactions in dense fluids under confinement.
[Show abstract][Hide abstract] ABSTRACT: A dilute dispersion of charge-stabilized, monodisperse silica nanoparticles has been used to determine the instrumental resolution function parameters of a conventional small-angle neutron scattering instrument in pinhole geometry over the entire accessible range of momentum transfer q. Independent determination of the structure parameters of the colloidal silica dispersion by small-angle X-ray scattering enables refinement of both the q-independent geometric and the q-dependent (wavelength spread or polychromaticity of the neutron beam) contributions to the resolution function. The procedure described is appropriate if no further instrumental characterization is available. It is demonstrated that the sample used for refining the resolution function parameters has to exhibit sharp structural scattering features such as Bragg reflections, form factor maxima and minima, or an inter-particle correlation peak for each instrumental configuration at which the measurements have been performed to cover a q range of two orders of magnitude (0.03 < q < 3.2 nm−1).
[Show abstract][Hide abstract] ABSTRACT: There is increasing interest in the use of viruses as model systems for fundamental research and as templates for nanomaterials. In this work, the rodlike fd virus was subjected to chemical modifications targeting different solvent-exposed functional groups in order to tune its surface properties, especially reversing the surface charge from negative to positive. The carboxyl groups of fd were coupled with different kinds of organic amines by carbodiimide chemistry, resulting in modified viruses that are positively charged over a wide range of pH. Care was taken to minimize intervirus cross linking, which often occurs because of such modifications. The surface amino groups were also grafted with poly(ethylene glycol) (PEG) end-functionalized with an active succinimidyl ester in order to introduce a steric stabilization effect. By combining charge reversal with PEG grafting, a reversible attraction between positively and negatively charged PEG-grafted fd viruses could be realized, which was tuned by the ionic strength of the solution. In addition, a charge-reversed fd virus forms only a pure nematic phase in contrast to the cholesteric phase of the wild type. These modified viruses might be used as model systems in soft condensed matter physics, for example, in the study of polyelectrolyte complexes or lyotropic liquid-crystalline phase behavior.
[Show abstract][Hide abstract] ABSTRACT: A grating-based holographic X-ray diffraction technique has been developed for reconstructing density profiles of nano-scale fluids confined in channel arrays. Within this approach, the reference wave is due to diffraction from the fabricated channel array, whereas the object wave is generated by the confinement-induced ordering of the fluid. The ensemble-averaged density profile of the fluid across the confining channel, which constitutes a weak phase object, is then determined in a model-independent manner from the interference between the reference and object waves by direct Fourier inversion. The validity of the linear holographic approach and its connection to the autocorrelation function, the inclusion of channel tapering, and volume- diffraction effects are discussed in detail.
[Show abstract][Hide abstract] ABSTRACT: We have developed a unique approach for studying the ensemble-averaged nearest-neighbor coordination of confined fluids by combining small-angle X-ray scattering and phase-retrieval–based X-ray diffraction from fluid-filled nanofluidic channel arrays. We apply the method to a charge-stabilized quasi–two-dimensional colloidal fluid (particle diameter 48 nm), focusing on the structural transition from a monolayer to a bilayer with increasing fluid film thickness. In contrast to theoretical work on the paradigmatic hard-sphere fluid, we find unambiguous experimental evidence for orientational alignment of fluids in extreme confinement.
[Show abstract][Hide abstract] ABSTRACT: We study polymer depletion-driven cluster aggregation and initial phase separation in aqueous dispersions of charge-stabilized silica spheres, where the ionic strength and polymer (dextran) concentration are systematically varied, using dynamic light scattering and visual observation. Without polymers and for increasing salt and colloid content, the dispersions become increasingly unstable against irreversible cluster formation. By adding nonadsorbing polymers, a depletion-driven attraction is induced, which lowers the stabilizing Coulomb barrier and enhances the cluster growth rate. The initial growth rate increases with increasing polymer concentration and decreases with increasing polymer molar mass. These observations can be quantitatively understood by an irreversible dimer formation theory based on the classical Derjaguin, Landau, Verwey, and Overbeek pair potential, with the depletion attraction modeled by the Asakura-Oosawa-Vrij potential. At low colloid concentration, we observe an exponential cluster growth rate for all polymer concentrations considered, indicating a reaction-limited aggregation mechanism. At sufficiently high polymer and colloid concentrations, and lower salt content, a gas-liquidlike demixing is observed initially. Later on, the system separates into a gel and fluidlike phase. The experimental time-dependent state diagram is compared to the theoretical equilibrium phase diagram obtained from a generalized free-volume theory and is discussed in terms of an initial reversible phase separation process in combination with irreversible aggregation at later times.
The Journal of Chemical Physics 06/2009; 130(20):204905. DOI:10.1063/1.3141984 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have applied small angle neutron scattering (SANS), diffusing wave spectroscopy (DWS), and dynamic light scattering (DLS) to investigate the phase diagram of a sterically stabilized colloidal system consisting of octadecyl grafted silica particles dispersed in toluene. This system is known to exhibit gas-liquid phase separation and percolation, depending on temperature T, pressure P, and concentration phi. We have determined by DLS the pressure dependence of the coexistence temperature and the spinodal temperature to be dP/dT=77 bar/K. The gel line or percolation limit was measured by DWS under high pressure using the condition that the system became nonergodic when crossing it and we determined the coexistence line at higher volume fractions from the DWS limit of turbid samples. From SANS measurements we determined the stickiness parameter tau(B)(P,T,phi) of the Baxter model, characterizing a polydisperse adhesive hard sphere, using a global fit routine on all curves in the homogenous regime at various temperatures, pressures, and concentrations. The phase coexistence and percolation line as predicted from tau(B)(P,T,phi) correspond with the determinations by DWS and were used to construct an experimental phase diagram for a polydisperse sticky hard sphere model system. A comparison with theory shows good agreement especially concerning the predictions for the percolation threshold. From the analysis of the forward scattering we find a critical scaling law for the susceptibility corresponding to mean field behavior. This finding is also supported by the critical scaling properties of the collective diffusion.
The Journal of Chemical Physics 05/2009; 130(15):154903. DOI:10.1063/1.3103245 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an experimental study of short-time diffusion properties in fluidlike suspensions of monodisperse charge-stabilized silica spheres suspended in dimethylformamide. The static structure factor S(q), the short-time diffusion function D(q), and the hydrodynamic function H(q) have been probed by combining x-ray photon correlation spectroscopy experiments with static small-angle x-ray scattering. Our experiments cover the full liquid-state part of the phase diagram, including de-ionized systems right at the liquid-solid phase boundary. We show that the dynamic data can be consistently described by the renormalized density fluctuation expansion theory of Beenakker and Mazur over a wide range of concentrations and ionic strengths. In accordance with this theory and Stokesian dynamics computer simulations, the measured short-time properties cross over monotonically, with increasing salt content, from the bounding values of salt-free suspensions to those of neutral hard spheres. Moreover, we discuss an upper bound for the hydrodynamic function peak height of fluid systems based on the Hansen-Verlet freezing criterion.
The Journal of Chemical Physics 03/2009; 130(8):084503. DOI:10.1063/1.3078408 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We develop a novel method for structure determination of confined fluids using diffraction-grating-based x-ray interferometry.Within this approach, diffraction from a microfluidic array, which acts both as confinement and transmission diffraction grating, provides the reference wave, whereas the density modulations of the confined fluid, acting as a weak phase object, generate the object wave. The ensemble-averaged density profile of the fluid perpendicular to the confining channel is then unambiguously obtained from the interference between the reference and object waves by direct Fourier inversion.
[Show abstract][Hide abstract] ABSTRACT: We present a joint experimental study of small angle neutron scattering (SANS) and dynamic light scattering (DLS) under high pressures up to 1800 bar on a colloidal suspension, which consists of a core-shell system made of sterically stabilized silica particles grafted with octadecyl chains in toluene. From the analysis of SANS contrast variation under pressure, we could estimate the amount of compression in both core and shell under the action of pressure. The DLS measurements under pressure yield a diffusion coefficient which enabled us together with the SANS result to evaluate the pressure-dependent viscosity of the dilute suspension which is to a good approximation the solvent viscosity on the basis of the Stokes-Einstein relation. The excellent comparison of the so-calculated pressure-dependent viscosities of toluene with literature values demonstrates the value of our method to measure viscosities under pressure.
[Show abstract][Hide abstract] ABSTRACT: The known grafting procedures of colloidal silica particles with poly(ethylene glycol) (PEG) lead to grafting layers that detach from the silica surface and dissolve in water within a few days. We present a new grafting procedure of PEG onto silica with a significant improvement of the stability of the grafting layers in aqueous solvents. Moreover, the procedure avoids any dry states or other circumstances leading to strong aggregation of the particles. To achieve the improved water stability, Stöber silica particles are first pre-coated with a silane coupling agent (3-aminopropyl)triethoxysilane (APS) to incorporate active amine groups. The water solubility of the pre-coating layer was minimized using a combination of APS with bis-(trimethoxysilylpropyl)amine (BTMOSPA) or bis-(triethoxysilyl)ethane (BTEOSE). These pre-coated particles were then reacted with N-succinimidyl ester of mono-methoxy poly(ethylene glycol) carboxylic acid to form PEG-grafted silica particles. The particles form stable dispersions in aqueous solutions as well as several organic solvents.
Journal of Colloid and Interface Science 07/2007; 310(2):446-55. DOI:10.1016/j.jcis.2007.02.024 · 3.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: F&E-Nr: E2310208 One dimensional anisotropic inorganic nanostructures such as tubes, rods, wires, flbers, etc. are in the fo- cus of research interests due to their potential applica- tions, for example in optical, electronic and mechanical devices, sensors and catalysis(2, 3). The synthesis of these anisotropic nanostructures is a big challenge, because most inorganic materials do not form the desired struc- ture by themselves. In contrast to inorganic systems, biological and organic materials, especially supramolec- ular systems, usually have a well deflned structure down to the nano-scale. Using (bio)organic materials as a tem- plate to build up anisotropic inorganic nanostructures has therefore emerged as a highly attractive method in recent years(4). The results described in the present pa- per may serve as a basis for the further development of the synthesis of inorganic materials using biopolymers as a template. Similar routes of synthesis occur sponta- neously in nature, for instance, in the formation of the silica cell wall of the Diatom, which is formed with an amazingly perfect pattern(5). For the synthesis of nano- materials, rod-like viruses, carbon nanotubes, lipid nan- otubes, rod-like structures formed from organogelators, single DNA etc. ofier many templates to direct the for- mation of anisotropic inorganic materials with special structures and properties.
Small 03/2007; 3(3):424-8. DOI:10.1002/smll.200600357 · 8.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We studied the thermal diffusion behavior of octadecyl coated silica particles (R(h)=27 nm) in toluene between 15.0 and 50.0 degrees C in a volume fraction range of 1%-30% by means of thermal diffusion forced Rayleigh scattering. The colloidal particles behave like hard spheres at high temperatures and as sticky spheres at low temperatures. With increasing temperature, the obtained Soret coefficient S(T) of the silica particles changed sign from negative to positive, which implies that the colloidal particles move to the warm side at low temperatures, whereas they move to the cold side at high temperatures. Additionally, we observed also a sign change of the Soret coefficient from positive to negative with increasing volume fraction. This is the first colloidal system for which a sign change with temperature and volume fraction has been observed. The concentration dependence of the thermal diffusion coefficient of the colloidal spheres is related to the colloid-colloid interactions, and will be compared with an existing theoretical description for interacting spherical particles. To characterize the particle-particle interaction parameters, we performed static and dynamic light scattering experiments. The temperature dependence of the thermal diffusion coefficient is predominantly determined by single colloidal particle properties, which are related to colloid-solvent molecule interactions.
The Journal of Chemical Physics 12/2006; 125(20):204911. DOI:10.1063/1.2400860 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the phase behavior of surface-functionalized silica colloids at both the molecular and macroscopic levels. This investigation allows us to relate collective properties such as aggregation, gelation, and aging directly to molecular interfacial behavior. By using surface-specific vibrational spectroscopy, we reveal dramatic changes in the conformation of alkyl chains terminating submicrometer silica particles. In fluid suspension at high temperatures, the interfacial molecules are in a liquid-like state of conformational disorder. As the temperature is lowered, the onset of gelation is identified by macroscopic phenomena, including changes in turbidity, heat release, and diverging viscosity. At the molecular level, the onset of this transition coincides with straightening of the carbon-carbon backbones of the interfacial molecules. In later stages, their intermolecular crystalline packing improves. It is the increased density of this ordered boundary layer that increases the van der Waals attraction between particles, causing the colloidal gas to aggregate. The approach presented here can provide insights into phase transitions that occur through surface modifications in a variety of colloidal systems.
Proceedings of the National Academy of Sciences 10/2006; 103(36):13310-4. DOI:10.1073/pnas.0606116103 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper we report measurements of the form factor and the structure factor of a sterically stabilized colloidal dispersion consisting of silica spheres coated with octadecane in toluene by small angle neutron scattering (SANS). The phase diagram of this system shows the liquid-liquid coexistence line and also a jamming transition at higher concentrations, where the jamming line intersects the coexistence line roughly at the critical point. We have performed SANS experiments at a temperature well above the transition temperature and at various volume fractions phi, spanning from the very dilute regime (phi=0.2%) to the critical concentration (phi=16%) and the highly viscous regime (phi=39.2%). Except for the very dilute regime, we observe a structure factor S(q) in all other cases. We fitted our data over the whole concentration regime using a global fitting routine with a core-shell model for the form factor P(q), taking into account the structure factor, which we describe with the Robertus model for an adhesive polydisperse core-shell particle. At a volume fraction of phi=5% a SANS contrast variation experiment has been performed. From that the product of the volume of the shell and the amount of solvent within the corona of our core-shell particle could be determined. At the most probable shell thickness of 2.3 nm a solvent content of about 50% within the corona was found. Moreover we could conclude that the core is not interpenetrated by solvent molecules. From the contrast variation experiment followed that the structure factor at zero average contrast exhibits a strong q dependence, which is an effect of an inhomogeneous particle in combination with a size distribution.
The Journal of Chemical Physics 08/2006; 125(4):44715. DOI:10.1063/1.2220564 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Diffusion of a small tracer sphere (apoferritin) in isotropic and nematic networks [of fd virus] is discussed. For a tracer sphere that is smaller than the mesh size of the network, screened hydrodynamic interactions between the sphere and the network determine its diffusion coefficient. A theory is developed for such interactions as well as their relation to the long-time self-diffusion coefficient. Fluorescence correlation spectroscopy measurements on mixtures of apoferritin and fd virus are presented. The long-time self-diffusion coefficient of apoferritin is measured as a function of the fd-virus concentration, both in the isotropic and nematic state, in directions parallel and perpendicular to the nematic director. The hydrodynamic screening length of the fd-virus network as a function of fd concentration is obtained by combining these experimental data with the theory. Surprisingly, the screening length increases with increasing concentration in nematic networks. This is due to the increase in the degree of alignment, which apparently leads to a strong increase of the screening length. Hydrodynamic screening is thus strongly diminished by alignment. A self-consistent calculation of the screening length does not work at higher concentrations, probably due to the strong variation of the typical incident flow fields over the contour of a rod.
The Journal of Chemical Physics 02/2006; 124(4):044907. DOI:10.1063/1.2161204 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A systematic experimental study of dispersions of charged colloidal spheres is presented on the effect of steady shear flow on nucleation and crystal growth rates. In addition, the nonequilibrium phase diagram as it relates to the melting line is measured. Shear flow is found to strongly affect induction times, crystal growth rates, and the location of the melting line. The main findings are that (1) the crystal growth rate for a given concentration exhibits a maximum as a function of the shear rate; (2) contrary to the monotonic increase in the growth rate with increasing concentration in the absence of flow, a maximum of the crystal growth rate as a function of concentration is observed for sheared systems; and (3) the induction time for a given concentration exhibits a maximum as a function of the shear rate. These findings are partly explained on a qualitative level.