Adsorption of Ions on Small Spheres at Low Ionic Strengths

The adsorption isotherm of ions cm small spherical particles at low ionic strength is evaluated by assuming that the reactions between ioins and particles are reversible. The electrostatic repuLsion between adsorbed ions is taken into account by using the Born equation. The evaluated express,ion is applicable to systems showing properties different to those of ideall

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

Influence of Potential Determining Ions on the Stability of Silver Iodide Colloidal System

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Determination of Electrostatic Contribution to the Enthalpy of Charging at a Metal Oxide/Electrolyte Interface

Interpretation of calorimetric measurements concerning the charging of metal oxide/aquous electrolyte interface was developed. The interpretation is based on the surface complexation model and involves both the 1-pK and 2-pK concepts. The resulting relationships enable determination of standard reaction enthalpies of each assumed surface reaction, and also evaluation of the dependency of surface potential on pH