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

Ultrathin plate-like colloidal particles are effective candidates for Pickering stabilization of water-in-water emulsions, a stabilization that is complicated by the thickness and ultralow tension of the water–water interface. Plate-like particles have the advantage of blocking much of the interface while simultaneously having a low mass. Additionally, the amount of blocked interface is practically independent of the equilibrium contact angle θ at which the water–water interface contacts the nanoplates. As a result, the adsorption of nanoplates is stronger than for spheres with the same maximal cross section, except if θ = 90°.

The macroscopic phase separation of aqueous mixtures of a neutral polymer and a polyelectrolyte is well described by a modified blob model, taking into account the entropy of ideal ions under the restriction of macroscopic charge neutrality. This is demonstrated by detailed measurements on aqueous mixtures of a neutral polymer (dextran) and a polymer whose charge is adjustable via the pH (nongelling fish gelatin). The critical point of the phase diagram of demixing, the asymmetric distribution of the solvent, and the interfacial electric potential difference all depend on polyelectrolyte charge and background salt concentration in a manner that is consistent with a dominant role for ion entropy.

Upon demixing, an aqueous solution of a polyelectrolyte and an incompatible neutral polymer yields two phases separated by an interface with an ultralow tension. Here, both in theory and experiment, we study this interfacial tension in detail: how it scales with the concentrations of the polymers in the two phases and how it is affected by the interfacial difference in the electrical potential. Experiments are performed on an aqueous model system of uncharged dextran and charged nongelling gelatin. The experimental tension scales to the power ∼3 with the tie-line length in the phase diagram of demixing, in agreement with mean-field theory where space is filled with a binary mixture of polymer blobs. The interfacial electrical potential difference is experimentally found to decrease the interfacial tension in a way that is consistent with Poisson–Boltzmann theory inspired from Frenkel and Verwey–Overbeek.

Hydrogels that are pH-sensitive and partially cross-linked by cobalt ferrite nanoparticles exhibit remarkable remanent magnetization behavior. The magnetic fields measured outside our thin disks of ferrogel are weak, but in the steady state, the field dependence on the magnetic content of the gels and the measurement geometry is as expected from theory. In contrast, the time-dependent behavior is surprisingly complicated. During swelling, the remanent field first rapidly increases and then slowly decreases. We ascribe the swelling-induced field enhancement to a change in the average orientation of magnetic dipolar structures, while the subsequent field drop is due to the decreasing concentration of nanoparticles. During shrinking, the field exhibits a much weaker time dependence that does not mirror the values found during swelling. These observations provide original new evidence for the markedly different spatial profiles of the pH during swelling and shrinking of hydrogels.

•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.

The electrical impedance spectrum of simple ionic solutions is measured in a parallel plate capacitor at small applied ac voltage. The influence of the ionic strength is investigated using several electrolytes at different concentrations in solvents of different dielectric constants. The electric double layers that appear at the electrodes at low frequencies are not perfectly capacitive. At moderate ionic strength, ion transport agrees with a model based on the Poisson–Nernst–Planck (PNP) equations. At low ionic strength, double layer dynamics deviate from the PNP model, and the deviation is well described by an empirical function with only one fit parameter. This deviation from the PNP equations increases systematically with increasing Debye length, possibly caused by the long-range Coulomb interaction.

A promising approach to texturize water is by the addition of mutually incompatible polymers, leading to phase separation. Here, we demonstrate that the phase stability of aqueous polymer solutions is affected not only by chemical differences between the polymers but also by their electric charge. Direct electrochemical measurements are performed of the electric potential difference between two coexisting phases in aqueous solutions of the charged protein fish gelatin (nongelling) and the uncharged polysaccharide dextran. Charge counteracts demixing because of the entropic cost of confining the counterions to one phase, resulting in a strong shift of the critical point upon an increase of the charge on one of the polymers. Upon phase separation, the charged polymer is spatially confined, and due to the Donnan effect, an interfacial electric potential is developed. A direct proportionality is found between this Donnan potential and the difference in gelatin concentration in the two phases, for which we propose a theoretical explanation. The electrostatics may provide a new handle in the development of stable water-in-water emulsions.

The permanent electrical dipole moment of colloidal quantum dots is important for their optoelectronic properties and can be determined by dielectric spectroscopy. Until now, however, colloidal interactions have not been taken into account in the interpretation of the spectra. Here, dielectric spectra of PbSe and CdSe colloidal quantum dots dispersed in an apolar liquid are measured from 1 Hz to 10 MHz. At frequencies of 10 kHz–1 MHz, Brownian rotation of nanoparticles with a permanent electric dipole moment is detected. At the lowest concentrations (∼0.1 vol %), the nanoparticles rotate independently of each other, and their dipole moment, for both PbSe and CdSe, is on the order of 40–50 D. At higher concentrations (≥0.3 vol %), the dipolar relaxation becomes slower, indicating the presence of nanoparticle structures. A simple model is used to estimate the interaction strength, which appears to be stronger than expected from the weak dipole moment, and has possibly also contributions from electrical moments of higher order. Our results indicate that nanoparticle interactions in liquid media lead to small equilibrium structures that affect dielectric measurements of the dipole moment already at concentrations of a few tenths of a volume percent.

An important reason to prepare magnetic nanoparticles of uniform size and shape is to ensure uniform magnetic properties. However, here, we demonstrate that magnetic iron oxide crystals of 20 nm or less with a low polydispersity of the geometric size can nevertheless have a strikingly broad distribution of the magnetic dipole moment. A comparative study was performed on nanoparticles with near-perfect crystallinity, twinning defects, or a high density of dislocations. Size, shape, and crystal defects were characterized with electron microscopy and X-ray diffraction, and magnetic dipole moments were determined from magnetization curves of dilute colloidal dispersions. The largest divergence was found for spherical particles with 3.5% geometric size polydispersity and 35% magnetic size polydispersity due to crystal lattice defects that disrupt single-domain magnetic spin coupling. This is in stark contrast with the usual implicit assumption that uniform size and shape guarantee well-defined magnetic properties of the individual particles.