Phase behavior of weakly polydisperse sticky hard spheres: Perturbation theory for the Percus-Yevick solution

We study the effects of size polydispersity on the gas-liquid phase behaviour of mixtures of sticky hard spheres. To achieve this, the system of coupled quadratic equations for the contact values of the partial cavity functions of the Percus-Yevick solution is solved within a perturbation expansion in the polydispersity, i.e. the normalized width of the size distribution. This allows us to make predictions for various thermodynamic quantities which can be tested against numerical simulations and experiments. In particular, we determine the leading-order effects of size polydispersity on the cloud curve delimiting the region of two-phase coexistence and on the associated shadow curve; we also study the extent of size fractionation between the coexisting phases. Different choices for the size-dependence of the adhesion strengths are examined carefully; the Asakura-Oosawa model of a mixture of polydisperse colloids and small polymers is studied as a specific example.Comment: 43 pages, 12 figures, and 1 tabl

The Electrical Double Layer on Oxides

The properties of the electrical double layer at the interface between oxides and aqueous electrolyte solutions have been studied on the basis of apparent surface charge - pH curves. It appears that all oxides studied so far can bear very high surface charges without giving rise to particularly high electrokinetic potentials. The trend is that this charge is the higher, the more porous is the surface layer. These results are discussed in terms of a quantitative theory, based on the concept that potential-determining as well as counter ions can penetr;:ite into the solid, to an extent depending on the porosity of the surface for that ion. The theory and experiments are also applied to the glass-solution interface. The results seem to support the idea that glass-electrode potentials are diffusion potentials

The Lyotropic Sequence in the Stability of Hydrophobic Colloids

The nature of the counterion, among other parameters, has a pronounced influence on the flocculation values for hydrophobic sols. A discussion of the lyotropic sequence, i. e. of the increase of the flocculation values for a typical, negatively charged silver iodide sol, in the series Rb+, K•, Na•, and Li+, is given in terms of the Deryagin-Landau-Verwey-Overbeek theory of colloidal stability. More recent data on adsorption of alkali metal ions on the surface of a-hematite indicate a reversal of the lyotropic series. Interpretation is sought in the predominance of steric factors

Charge Inversion of Divalent Ionic Solutions in Silica Channels

Recent experiments (F.H.J. Van Der Heyden et al., PRL 96, 224502 (2006)) of streaming currents in silica nanochannels with divalent ions report charge inversion, i.e. interfacial charges attracting counterions in excess of their own nominal charge, in conflict with existing theoretical and simulation results. We reveal the mechanism of charge inversion by using all-atomic molecular dynamics simulations. Our results show excellent agreement with experiments, both qualitatively and quantitatively. We further discuss the implications of our study for the general problem of ionic correlations in solutions as well as in regards of the properties of silica-water interfaces.Comment: 5 pages, 5 figure

Electro-osmosis on anisotropic super-hydrophobic surfaces

We give a general theoretical description of electro-osmotic flow at striped super-hydrophobic surfaces in a thin double layer limit, and derive a relation between the electro-osmotic mobility and hydrodynamic slip-length tensors. Our analysis demonstrates that electro-osmotic flow shows a very rich behavior controlled by slip length and charge at the gas sectors. In case of uncharged liquid-gas interface, the flow is the same or inhibited relative to flow in homogeneous channel with zero interfacial slip. By contrast, it can be amplified by several orders of magnitude provided slip regions are uniformly charged. When gas and solid regions are oppositely charged, we predict a flow reversal, which suggests a possibility of huge electro-osmotic slip even for electro-neutral surfaces. On the basis of these observations we suggest strategies for practical microfluidic mixing devices. These results provide a framework for the rational design of super-hydrophobic surfaces.Comment: 4 pages, 4 figures; submitted to PRL Revised version: several references added, typos corrected. Supplementary file was restructured, the second part of the original EPAPS was removed and is supposed to be published as a separate pape

Electrical Double Layer on Silver Iodide and Overcharging in the Presence of Hydrolyzable Cations

Previous studies on the surface charge and electrokinetic charge on silver iodide as a function of the pAg in the presence of some monovalent and trivalent cations as the counterions were extended to include the influence of pH. The main reason for this study was to investigate the possible propensity of overcharging by adsorption of hydrolyzed counterions. It was found that for K+ as the counterion the double layer properties could be well represented in terms of a simple Gouy-Stern model, without accounting for hydrolysis. However, with Al3+ or La3+ as the counterions, adsorption of hydrolyzed species takes place when the pH is high enough, leading to electrokinetic charge reversal

First-order phase transition during displacement of amphiphilic biomacromolecules from interfaces by surfactant molecules

The adsorption of surfactants onto a hydrophobic interface, already laden with a fixed number of amphiphilic macromolecules, is studied using the self consistent field calculation method of Scheutjens and Fleer. For biopolymers having unfavourable interactions with the surfactant molecules, the adsorption isotherms show an abrupt jump at a certain value of surfactant bulk concentration. Alternatively, the same behaviour is exhibited when the number of amphiphilic chains on the interface is decreased. We show that this sudden jump is associated with a first-order phase transition, by calculating the free energy values for the stable and the metastable states at both sides of the transition point. We also observe that the transition can occur for two approaching surfaces, from a high surfactant coverage phase to a low surfactant coverage one, at sufficiently close separation distances. The consequence of this finding for the steric colloidal interactions, induced by the overlap of two biopolymer + surfactant films, is explored. In particular, a significantly different interaction, in terms of its magnitude and range, is predicted for these two phases. We also consider the relevance of the current study to problems involving the competitive displacement of proteins by surfactants in food colloid systems

Theory of electrostatically induced shape transitions in carbon nanotubes

A mechanically bistable single-walled carbon nanotube can act as a variable-shaped capacitor with a voltage-controlled transition between collapsed and inflated states. This external control parameter provides a means to tune the system so that collapsed and inflated states are degenerate, at which point the tube's susceptibility to diverse external stimuli-- temperature, voltage, trapped atoms -- diverges following a universal curve, yielding an exceptionally sensitive sensor or actuator that is characterized by a vanishing energy scale. For example, the boundary between collapsed and inflated states can shift hundreds of Angstroms in response to the presence or absence of a single gas atom in the core of the tube. Several potential nano-electromechanical devices can be based on this electrically tuned crossover between near-degenerate collapsed and inflated configurations