Točka nul-naboja i površinska gustoća naboja TiO2 u vodenim otopinama elektrolita dobivene potenciometrijskom masenom titracijom

I Mass titration method was developed as a suitable tool for determination of the point of zero charge (p.z.c.) and surface charge density (σ0) of metal oxide colloid particles at different ionic strengths. In the course of mass titration, subsequent portions of a metal oxide powder are added to an aqueous electrolyte solution, and pH of the equilibrated dispersion is measured. The pH of the system changes gradually and approaches a constant value of pH∞, which is in the case of a metal oxide free of impurities equal to the point of zero charge pHpzc. Counterion association shifts the pHpzc either to the acidic region (preferential adsorption of cations) or to the basic region (preferential adsorption of anions). Mass titration therefore enables detection of the difference between association affinities of counterions (cations and anions), which is important information about the equilibrium within the electrical interfacial layer. Such an analysis is not possible by the conventional acid-base potentiometric titration of the dispersion, since the location of the p.z.c. is based on the common intersection point (c.i.p.) of the data obtained at different ionic strengths. It is shown that c.i.p. may be used for locating the p.z.c. only in a »symmetric case«, when affinities of the anions and cations to associate with oppositely charged surface groups are equal. Analysis of the mass titration was performed on the basis of experimental data obtained with colloidal titania dispersed in an aqueous sodium chloride solution, and also by numerical simulation based on the Surface Complexation Model and the 2-pK mechanism of surface reactions. Increase in the NaCl concentration shifted the point of zero charge to the basic region, while the isoelectric point was shifted to the acidic region, indicating higher association affinity of chloride ions compared to sodium ionsMasena titracija pokazala se pogodnom metodom za određivanje točke nul-naboja (p.z.c., point of zero charge) i površinske gustoće naboja (σ0) koloidnih čestica kovinskih oksida. Postupak se sastoji od postupnog dodavanja kovinskog oksida u otopinu elektrolita i mjerenju pH suspenzije. Svakim dodatkom, pH suspenzije se mijenja dok ne dosegne stalnu vrijednost, pH∞, koja je u slučaju čistog kovinskog oksida jednaka točki nulnaboja, pHpzc. Asocijacijom protuiona pomiče se pHpzc u kiselo područje (jača asocijacija kationa) ili u lužnato područje (jača asocijacija aniona). Rezultati masene titracije pružaju informacije o razlici u afinitetu površinske asocijacije protuiona (aniona i kationa), važnom parametru ravnoteže unutar električnog međupovršinskog sloja. Određivanje točke nul-naboja klasičnom potenciometrijskom kiselinsko-baznom titracijom suspenzije temeljeno je na zajedničkom sjecištu _0(pH) krivulja (c.i.p., common intersection point) za različite ionske jakosti. Pokazano je da se c.i.p. može koristiti za određivanje točke nul-naboja samo u slučaju kada je asocijacija aniona i kationa sa suprotno nabijenim površinskim vrstama jednaka. Analizirani su eksperimentalni rezultati masene titracije suspenzije koloidnih čestica titanijevog oksida u vodenoj otopini natrijeva klorida, te rezultati numerič kih simulacija temeljenih na 2-pK mehanizmu i modelu površinskog kompleksiranja. Povećanje koncentracije natrijeva klorida pomiče točku nul-naboja u lužnato područje, dok se izoelektrična točka pomiče u kiselo područje, što ukazuje na jaču asocijaciju kloridnih iona sa suprotno nabijenim površinskim mjestima u odnosu na natrijeve ione

Effect of Charge Distribution on the Stability of Nano-dispersions

In the course of aggregation of very small colloid particles, i.e. nano-particles, the overlap of the diffuse layers is almost complete, so that one cannot apply the common DLVO theory. Since nano-particles are small compared to the extent of the diffuse layer, the process is considered in the same way as for two interacting ions. Therefore, the Brønsted concept based on the Transition State Theory was applied. In this work the effect of charge distribution among nano-particles was examined. Results of numerical simulation show that wider charge distribution causes substantial decrease in the stability of nano-dispersions

Attenuated Total Reflection - Infrared Spectroscopy Applied to the Study of Mineral - Aqueous Electrolyte Solution Interfaces: A General Overview and a Case Study

The present chapter gives an overview of the application of Attenuated total reflection - Infrared spectroscopy (ATR-IR) to the environmentally important mineral - aqueous electrolyte interface. At these interfaces the important adsorption processes occur that limit the availability of potentially toxic solutes. These retention processes may retard for example the migration of solutes in aquifer systems or even immobilize them on the aquifer material, which is usually a natural mineral. We give an introduction to the approach and an overview of its possible applicability (and in this context its use in contributing to the understanding of the acid-base chemistry of (oxy)(hydr)oxide mineral surfaces, the adsorption of anions and cations like the uranyl-ion, and the formation of ternary surface complexes can be mentioned in general). Our contribution focuses on a review on the interaction of small organic molecules with oxidic surfaces and we highlight previous studies and point to some controversary issues in selected studies that continue to exist despite extensive research. Obviously such studies relate to other vibrational spectroscopies like Raman or sum frequency generation vibrational spectroscopies. Finally we discuss results from an experimental study on the mineral gibbsite (Al(OH)3) in the presence of 5-sulfosalicylic acid (5-SSA)

The Effects on the Response of Metal Oxide and Fluorite Single Crystal Electrodes and the Equilibration Process in the Interfacial Region

Inner surface potential, one of the most important variables affecting the interfacial equilibrium of metal oxide aqueous systems, obtained by means of single crystal electrode gives valuable information about electrical charging of the metal oxide/aqueous electrolyte solu¬tion interfaces. The influence of the potential determining ions and ionic strength on the measured electrode potential as well as time of the equilibration, direction of the titration and the effect of the magnetic stirring enables the critical examination of the processes which take place during the interfacial equilibrium. For that purpose, the selected metal oxides (hematite, ceria, sapphire, and rutile) and fluorite single crystal electrodes were examined. This work is licensed under a Creative Commons Attribution 4.0 International License

Effect of Bubbling on the Potential of Reference Electrode

The method for the examination of gas/water interface electrokinetic behavior is developed. "Bubbling potential", i.e. the difference in the potential of the indicator electrode in the absence and presence of gas bubbles was measured. The indicator electrode is a reference electrode with the porous plug exposed to gas bubbles. The charged bubble in contact with the porous plug would affect the potential of the indicator electrode. However, electrodes, whose potential depends on the reversible interfacial reaction, would be relaxed due to the interfacial ionic equilibration and their potential will not be affected by the presence of bubbles. Measured Bubbling potentials are directly related to electrokinetic - potentials. The proposed method is fast, accurate and reproducible so that it can be used for the examination of gas/water interfaces in different conditions. The isoelectric point of argon bubbles in the aqueous NaCl solution was obtained as pHiep = 3.9 and pHiep = 3.4 at ionic strength of 10–3 and 10–2 mol dm–3, respectively. (doi: 10.5562/cca2235

Equilibria in the Electrical Interfacial Layer Revisited

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, Y0 data, s0 data, z-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 65th birthday