•Description of the repulsion between strongly overlapping electrical double layers•Intrinsic length-scale, serving as the unscreened pendant of the Debye length.•Inverse square decay for low potentialsThe Donnan equilibrium is employed to evaluate the entropic repulsion between two charged plates that feature charge regulation and are in equilibrium with a reservoir solution of monovalent salt. This approach represents the zero-field limit of the Poisson–Boltzmann equation, valid for strongly overlapping electrical double layers. We show that this scenario features an intrinsic length scale, which serves as the unscreened pendant of the Debye length for strongly overlapping double layers. In general, the scaling of the disjoining pressure with inter-plate distance is dependent on the boundary conditions (constant charge, constant potential, or charge regulation). Surprisingly, here we find for sufficiently low potentials the same inverse-square decay as for constant charge surfaces. We test the validity of the zero-field limit by comparison with self-consistent field lattice computations that invoke the full Poisson equation for finitely sized ions between two charge-regulated plates.Download high-res image (143KB)Download full-size image

The competition between surface adsorption and bulk aggregation was investigated for silica colloids dispersed in cyclohexane in contact with hydrophobized silica substrates. Central to this study is that the colloids and surfaces have the same material and surface properties. Colloid–colloid and colloid–surface interactions were controlled by addition of polymers providing depletion interaction. Bulk instability was determined by turbidity and viscosity measurements and surface adsorption by ellipsometry measurements. At increasing polymer concentration, strong surface adsorption occurred at polymer concentrations below that required for bulk phase separation. Complementary Monte Carlo simulations with the use of a new weak depletion theory support quantitatively the experimental observation of the existence of an interval of interaction strength at which aggregation in bulk is negligible while surface adsorption is substantial.

A sensor is introduced that gauges the ratio of charge z to mass m of macro-ions in liquid media. The conductivity is measured in a small volume of salt solution, separated from the macro-ions by a semipermeable membrane. The mobile counterions released by the macro-ions increase the measured salt concentration, from which z/m can be calculated without any adjustable parameter. The charge sensor constitutes a noninvasive method that probes unperturbed macro-ions in a manner that is independent of (the distribution in) macro-ion size and shape. We validate the sensor’s general applicability for three kinds of macro-ions, spanning 2 orders of magnitude in z/m, namely, dextran sulfate, bovine serum albumin, and colloidal silica. Measured z/m values comply for all macro-ion types with independent information on macro-ion surface charge.Keywords: colloids; conductivity; counterions; electric charge; membranes; nanoparticles; polymers; zeta potential

Structural transformations of superparamagnetic colloids confined within self-assembled microtubes are studied by systematically varying tube–colloid size ratios and external magnetic field directions. A magnetic field parallel to microtubes may stretch non-linear chains like zigzag chains into linear chains. Non-parallel fields induce new structures including repulsive chains of single colloids, kinked chains and repulsive dimers, which are not observed for unconfined magnetic colloids in the bulk. The formed colloidal structures are confirmed via model calculations which account for tube–colloid size ratio effects and their reconfigurability with the field direction. Furthermore, structures are formed that allow controllable switching between a helical and a non-helical state. All observed field-induced transformations in microtubes are reversible provided the microtubes are not completely filled with colloids. In addition, we demonstrate magnetic field-responsive 2D crystallization by extending control over colloidal configurations in single microtubes to multiple well-aligned microtubes.

•We present a general synthesis method for composite colloids of varying morphologies.•Positively charged zein is combined with different negatively charged nanoparticles.•A layer of silica is deposited on the zein particles to increase applicability.•Hollow silica shells containing nanoparticles can be formed.A general and reproducible heterocoagulation method is presented to prepare sub-micron sized zein protein particles, loaded with negatively charged nanoparticles. These composite carrier particles can be obtained in three different morphologies, and each morphology can be prepared using nanoparticles of various size, shape and composition. An important feature of the zein composites is their long-term stability in water even in conditions where free nanoparticles often aggregate within days. Additionally, we modify the composite particles by coating them with a thin layer of silica via condensation of sodium silicate, opening possibilities for highly specific, functionalized carrier particles. Finally, the formation of hollow silica shells containing negatively charged nanoparticles is demonstrated, using the zein composites as a template.

The shape of cubic silica colloids is exploited to form close-packed structures, whereas the hollow core of the cubes may host functional substances. A proof-of-principle of this approach is presented for iron oxide particles (hematite, α-Fe2O3) confined inside silica cubes that display the ability to accelerate the degradation of the organic dyes methylene blue and rhodamine B in the presence of hydrogen peroxide (modified Fenton reaction). The silica coating does not impede the reaction, since degradation rates similar to those of bare hematite particles are observed. Moreover, the cubic colloids are still functional when densely packed onto a substrate. The degradation reaction is greatly enhanced by illumination with visible or UV light: the degradation time reduces by two orders of magnitude. Although the silica coating is damaged during the degradation reaction, the stability of the coating is improved by heat treatment of the cubes and by illumination.

•Electrokinetics of monodisperse charged silica spheres in ethanol were studied.•Dynamic and electrophoretic mobility were compared at low and high salt.•Electroacoustics agrees with laser Doppler electrophoresis at high ionic strength.•At low ionic strength, dynamic mobility by far exceeds electrophoretic mobility.•Is the model system not as simple as it seems, is the theory inadequate, or both?Electroacoustics and laser Doppler electrophoresis were employed to measure the mobility of surface-modified silica colloids in ethanol as a function of the ionic strength. Sufficiently low volume fractions were chosen to exclude effects of interparticle interactions. At high ionic strength, the electrophoretic mobility μeμe is equal to the (electroacoustic) dynamic mobility μdμd at 3.3 MHz. However, the ratio μd/μeμd/μe increases significantly to ∼∼5 at low ionic strength. This increase may be related to the porous outer layer of the surface-modified silica spheres.

•Preparation of colloidal silica cubes with tunable coating thickness and porosity.•The thickness is governed by the amount of silica precursor.•The silica etching process for larger pores is monitored by IR spectroscopy.•A higher amount of the protective polymer PVP leads to a lower etching rate.•The molar mass of the protective polymer PVP influences the porosity of the interior.We investigate the material properties of micron-sized silica coated cubic colloids, focusing on the coating thickness and porosity. The thickness of the silica coating of core–shell α-Fe2O3@SiO2 cubes and their corresponding hollow cubes can be tuned between 20 and 80 nm, spanning the range of silica bubbles to silica boxes. The porosity of the silica cubes can be increased controllably by surface-protected etching using hot water as mild etchant and polyvinylpyrrolidone (PVP) as protecting polymer. We introduce infrared spectroscopy as a quantitative tool to monitor the extent of etching over time and to evaluate the influence of PVP on the etching process. The molar mass of PVP does not affect the etching rate, whereas an increased amount of PVP leads to enhanced protection against etching. Silica etching is found to be a two-step process, comprising a fast initial etching followed by a slower continuation. Hollow, porous silica cubes maintain their shape after extensive thermal treatment, demonstrating their mechanical stability.

Cryogenic transmission electron microscopy (cryo-TEM) is utilized to determine the second virial coefficient of osmotic pressure of PbSe quantum dots (QDs) dispersed in apolar liquid. Cryo-TEM images from vitrified samples provide snapshots of the equilibrium distribution of the particles. These snapshots yield radial distribution functions from which second virial coefficients are calculated, which agree with second virial coefficients determined with analytical centrifugation and small-angle X-ray scattering. The size dependence of the second virial coefficient points to an interparticle interaction that is proportional to the QD surface area. A plausible cause for this attraction is the interaction between the surface ions on adjacent QDs.

Langmuir’s disjoining pressure between two flat, charged planes was calculated analytically for strongly overlapping double layers in the limit of zero electric field between the planes. The resulting repulsion has a long-range algebraic decay that stems from the thermodynamic equilibrium between homogeneously distributed interplate ions and ions in the surrounding electrolyte reservoir. Together with the van der Waals attraction, the repulsion forms the zero-field pendant of the exponentially screened DLVO potential, a pendant that is always repulsive at large plate–plate distances. The experimental occurrence of algebraic repulsions can be simply predicted from surface charge density and ionic strength.

We have studied the crystallization behavior of colloidal cubes by means of tunable depletion interactions. The colloidal system consists of novel micron-sized cubic particles prepared by silica deposition on hematite templates and various non-adsorbing water-soluble polymers as depletion agents. We have found that under certain conditions the cubes self-organize into crystals with a simple cubic symmetry, which is set by the size of the depletant. The dynamic of crystal nucleation and growth is investigated, monitoring the samples in time by optical microscopy. Furthermore, by using temperature sensitive microgel particles as depletant it is possible to fine tune depletion interactions to induce crystal melting. Assisting crystallization with an alternating electric field improves the uniformity of the cubic pattern allowing the preparation of macroscopic (almost defect-free) crystals that show visible Bragg colors.

Quantum dots form equilibrium structures in liquid dispersions, due to thermodynamic forces that are often hard to quantify. Analysis of these structures, visualized using cryogenic electron microscopy, yields their formation free energy. Here we show that the nanoparticle interaction free energy can be further separated into the enthalpic and entropic contributions, using the temperature dependence of the assembled structures. Monodisperse oleic acid-capped PbSe nanoparticles dispersed in decalin were used as a model system, and the temperature-dependent equilibrium structures were imaged by cryo-TEM, after quenching from different initial temperatures. The interaction enthalpy and entropy follow from van 't Hoff's exact equation for the temperature dependence of thermodynamic equilibria, now applied to associating nanoparticles. The enthalpic component gives the magnitude of the contact interaction, which is crucial information in understanding the energetics of the self-assembly of nanoparticles into ordered structures.

Poisoning of platinum catalysts by sulphur compounds is a significant problem that prevents their application in untreated gas streams. We introduce a novel concept to circumvent the poisoning problem by encapsulating individual platinum nano-particles with silica layers that act as selective membranes. Greatly enhanced sulfur tolerance for sufficiently dense illustrates the potential of our approach to design noble metal catalysts that survive in sulphur containing gas streams.

Quantum dots form equilibrium structures in liquid dispersions, due to thermodynamic forces that are often hard to quantify. Analysis of these structures, visualized using cryogenic electron microscopy, yields their formation free energy. Here we show that the nanoparticle interaction free energy can be further separated into the enthalpic and entropic contributions, using the temperature dependence of the assembled structures. Monodisperse oleic acid-capped PbSe nanoparticles dispersed in decalin were used as a model system, and the temperature-dependent equilibrium structures were imaged by cryo-TEM, after quenching from different initial temperatures. The interaction enthalpy and entropy follow from van 't Hoff's exact equation for the temperature dependence of thermodynamic equilibria, now applied to associating nanoparticles. The enthalpic component gives the magnitude of the contact interaction, which is crucial information in understanding the energetics of the self-assembly of nanoparticles into ordered structures.

The shape of cubic silica colloids is exploited to form close-packed structures, whereas the hollow core of the cubes may host functional substances. A proof-of-principle of this approach is presented for iron oxide particles (hematite, α-Fe2O3) confined inside silica cubes that display the ability to accelerate the degradation of the organic dyes methylene blue and rhodamine B in the presence of hydrogen peroxide (modified Fenton reaction). The silica coating does not impede the reaction, since degradation rates similar to those of bare hematite particles are observed. Moreover, the cubic colloids are still functional when densely packed onto a substrate. The degradation reaction is greatly enhanced by illumination with visible or UV light: the degradation time reduces by two orders of magnitude. Although the silica coating is damaged during the degradation reaction, the stability of the coating is improved by heat treatment of the cubes and by illumination.