The points of zero charge (PZC) and isoelectric points (IEP) from the recent literature are discussed. This study is an update of the previous compilation [M. Kosmulski, Surface Charging and Points of Zero Charge, CRC, Boca Raton, FL, 2009] and of its previous update [J. Colloid Interface Sci. 337 (2009) 439]. In several recent publications, the terms PZC/IEP have been used outside their usual meaning. Only the PZC/IEP obtained according to the methods recommended by the present author are reported in this paper, and the other results are ignored. PZC/IEP of albite, sepiolite, and sericite, which have not been studied before, became available over the past 2 years.

pH-dependent surface charging and points of zero charge II. Update.

One of the present challenges of the food industry is to deliver nutrition and health benefits to the consumer while keeping, or improving the taste and aroma impact. Adding active ingredients to liquid food products for fortification is in most cases not possible or not sufficient to achieve the desired goal, due to the fact that many interesting micronutrients are only hardly soluble in aqueous systems and show (i) a limited stability against chemical or physical degradation, (ii) an incompatibility between the active ingredient and the food matrix, or (iii) reveal an uncontrolled release or bioavailability. Therefore, encapsulation systems, also denoted as ‘delivery systems’, are typically used to solve these formulation problems. The task to find the appropriate delivery system is especially challenging for the food industry compared to other fields such as pharmacy, medical products or cosmetics, since only a limited amount of ingredients can be used as encapsulation and stabilization material. In the present review we will discuss the delivery systems available for (semi)-liquid foods and comment on existing advantages and limitations. The remaining technical challenges to solve in the future concern mainly the facts that (i) most of the available delivery systems for aqueous products do not yet allow to significantly stabilize degradation sensitive ‘encapsulated’ active ingredients against e.g. oxidation, (ii) the ‘encapsulation’ (solubilization) capacity of some delivery systems is still quite poor and (iii) off-taste generation is possible above certain concentrations of added delivery systems.

Delivery systems for liquid food products

Isoelectric points and points of zero charge of metal (hydr)oxides: 50years after Parks' review.

We have developed a novel flexible pH sensor based on a polymeric substrate by low-cost sol–gel fabrication process of iridium oxide (IrOx) sensing film. A pair of miniature IrOx/AgCl electrodes on a flexible substrate generated electrical potentials in solutions by electrochemical mechanisms responding to their pH levels. Our flexible IrOx pH sensors exhibited promising sensing performance with a near-Nernstian response in sensitivity repeatedly and reversibly between −51.1 mV/pH and −51.7 mV/pH in the pH range between 1.5 and 12 at 25 °C. The fabrication processes including sol–gel deposition, thermal oxidation, and AgCl electro-plating on polymeric substrates were reported. The performance and characterization of the flexible pH sensors in sensitivity, response time, stability, reversibility, repeatability and selectivity were also discussed. Our IrOx pH electrodes on a deformable substrate demonstrated their sensing capability while they were conformed to a curved surface inside a limited space with distinct responding potentials at various pH levels similar to the traditional glass-rod pH electrodes.

A flexible pH sensor based on the iridium oxide sensing film

Zeta potential of shape-controlled TiO2 nanoparticles with surfactants

Surface potential at the hematite–water interface

Reversible charging of the ice-water interface. II. Estimation of equilibrium parameters.

Electrostatic potential at the inner plane of the hematite aqueous interface, i.e., surface potential, was measured by means of a single-crystal hematite electrode. Acidic solutions were titrated with base and then back-titrated with acid. Surface potentials were evaluated from electrode potentials by setting the zero value at the isoelectric point. In the case of fast titrations the equilibration time was approximately 10 min, and significant hysteresis was obtained, more pronounced at higher electrolyte concentrations. Hysteresis disappeared in slow titration runs when the equilibration time w as extended up to 120 min, and also wh en ultrasound w as applied. Hysteresis w as observed in the pH region close to neutrality, where the concentrations of potential-determining H + and O H a ions are low. Equilibration was fast in acidic and basic regions. These results are explained on the basis of the kinetics of surface reactions, supported by the following rate of single-crystal electrode equilibration. It is concluded th at the equilibration r ate at the interface is specific for a giv en sy stem and is not a g eneral phenomenon. As sev eral sy stems may undergo fast equilibration, such data may be regarded as equilibrium data and interpreted by the surface complexation model. In other cases, one should perform kinetic tests and apply extended equilibration times. ￻ 2006 Elsevier Inc. All rights r eserved.

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Surface potential of hematite in aqueous electrolyte solution: Hysteresis and equilibration at the interface

Equilibrium at the solid-liquid interface is the subject of numerous experimental and theoretical investigations. Several different models describing the structure of the electrical interfacial layer (EIL) and the mechanism of surface reactions are suggested in the literature. At present it seems impossible to decide which of them describe the real situation best. One of the routes towards the solution of this problem is the development of new experimental methods and simultaneous interpretation of different sets of data (e.g., adsorption data, Ψ 0 data, (TO data,;-potential data, etc.). Interpretation based on such an approach may eventually enable differentiation between models and a better choice of appropriate ones. Another route is to refine or develop a new, more realistic and less approximate, theoretical concept. This article presents a review of such efforts (EIL structure, electroneutrality condition, surface potential measurements, capacitors within EIL, enthalpy of surface reactions, colloid stability, etc.), with special emphasis on the role of Nikola Kallay in this field, to whom this article is dedicated on the occasion of his 65 th birthday.

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Equilibria in the Electrical Interfacial Layer Revisited

The adsorption of salicylic acid on titanium dioxide was measured as a function of pH and interpreted using a modified adsorption isotherm based on the surface complexation model, which takes into account dissociation of the acid in the bulk of the solution and the electrostatic effect on the adsorbed ionic species. It was concluded that both singly charged and neutral species are adsorbable, and both adsorption equilibrium constants were determined.

Ana Čop

Papers

Surface potential at the hematite–water interface

Reversible charging of the ice-water interface. II. Estimation of equilibrium parameters.

Surface potential of hematite in aqueous electrolyte solution: Hysteresis and equilibration at the interface

Equilibria in the Electrical Interfacial Layer Revisited

The use of electrokinetic potential in the interpretation of adsorption phenomena. Adsorption of salicylic acid on titanium dioxide