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 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

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

On the Reiation between Surface Charge and Sol Stability

This paper discusses . various charges distinguishable in electrical double layers, namely the electrokinetic charge oek• the Stern charge om and the surface charge 00 • The electrokinetic charge cain be calculated from the electrokinetic potential t The approximation \u27IJJct = ~ appears to work well for the AgI-system. Hence, oek - od, the diffuse double layer charge. The potential \u27IJJct is the determining quantity in colloidal stability. Concersely, from stability measurements \u27i\u27ct and hence od can be evaluated. The surface charge 0 0 can in most cases also be determined experimentally. Its relation to od, and hence to sol stability, is indirect. Because of the balance ad + om + 0 0 = 0, the Stern charge can be found from 0 0 and od by subtraction. This i·s the relatively most informative quantity. Two example are worked out for the AgI-sistem, the lyotropic sequence (om (Li+) < om (K+) < om (Rb+)) and the effect of different alcohols on om. In the latter case it is found that butanol tends to desorb cations, whereas ethylene glycol has only a neglegible effect on the cation binding

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

Biopolymer Adsorption, with Special Reference to the Serum Albumin-Polystyrene Latex System

A study has been made of the adsorption of human serum albumin (HSA) on emursifier-free, negatively charged polystyrene (PS) latices. The adsorption has been followed directly and microcalorimetrically. Important variables are: pH, temperature, csalt and the surface charge a0 of the latex. Although all adsorption isotherms have a platform, the Langmuir theory is inadequate to account for them. Distinction must be made between the initial stages of adsorption, solely determined by the HSA-PS interaction and the later stages, where lateral interaction between adsorbed HSA molecules plays also an important role. The adsorption platform as a function of pH is maximal in the isoelectric point. Here the adsorbed amount is roughly compatible with side-on native HSA molecules. Both below and above the i. e. p. adsorption proceeds in a more unfolded conformation but a spread monolayer is never reached. The adsorption is largely driven by a net entropy gain, both in the initial and later states of the process. Besides this, there is a recognizable influence of the electrostatic attraction between the latex surface and local positive excesses inside the molecule

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

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

Can a Protein Adsorb on its Own? The Thermodynamics of Ion Participation

We present a general thermodynamic method to obtain information about co-adsorption and oo-partitioniiug of low molecular weight ions upon adsorption or partitioning of proteins between two phases. Esin-Markov coefficients are central quantities. By this procedure useful additional information is obtained that, in turn, can be used in mechanistic interpretations. Applications are presented for (1) ion binding to free BSA, (2) charge adjustments in the protein and on the surface upon adsorption of BSA on silver iodide and (3) ion co-participation upon solubilization of cytoohrome-C into micro-emulsdon .droplets