Tajana Preočanin

pH and the surface tension of water.

Despite the strong adsorption of hydroxide ions, the surface tension of water is almost independent of pH between pH 1 and 13 when the pH is adjusted by addition of HCl or NaOH. This is consistent with the Gibbs adsorption isotherm which measures the surface excess of all species in the double layer, if hydronium ions and hydroxide ions are adsorbed and sodium and chloride ions are not. The surface tension becomes pH dependent around pH 7 in millimolar NaCl or KCl solutions, for now sodium ions can replace hydronium ions as counterions to the adsorbed hydroxide ions.

Electrostatic potential of specific mineral faces.

Reaction rates of environmental processes occurring at hydrated mineral surfaces are in part controlled by the electrostatic potential that develops at the interface. This potential depends on the structure of exposed crystal faces as well as the pH and the type of ions and their interactions with these faces. Despite its importance, experimental methods for determining fundamental electrostatic properties of specific crystal faces such as the point of zero charge are few. Here we show that this information may be obtained from simple, cyclic potentiometric titration using a well-characterized single-crystal electrode exposing the face of interest. The method exploits the presence of a hysteresis loop in the titration measurements that allows the extraction of key electrostatic descriptors using the Maxwell construction. The approach is demonstrated for hematite (α-Fe(2)O(3)) (001), and thermodynamic proof is provided for the resulting estimate of its point of zero charge. Insight gained from this method will aid in predicting the fate of migrating contaminants, mineral growth/dissolution processes, and mineral-microbiological interactions and in testing surface complexation theories.

Measurement of surface potential at silver chloride aqueous interface with single-crystal AgCl electrode

The surface potential at the silver chloride aqueous interface was measured by means of a single-crystal silver chloride electrode (SCr-AgCl). The measurements were conducted by titration of the KCl solution with AgNO3, and vice versa. The SCr-AgCl electrode potentials were converted to surface potentials psi(0) by setting zero at the point of zero charge at pCl = 5.2. The psi(0)(pCl) function was linear, with a slope 12% lower with respect to the Nernst equation. It was demonstrated that the surface potential at the silver halide aqueous interface could be interpreted by means of the surface complexation model, originally developed for metal oxides.

Surface and zeta-potentials of silver halide single crystals: pH-dependence in comparison to particle systems.

We have carried out surface and zeta-potential measurements on AgCl and AgBr single crystals. As for particle systems we find that, surprisingly and previously unnoted, the zeta-potential exhibits pH-dependence, while the surface potential does not. A possible interpretation of these observations is the involvement of water ions in the interfacial equilibria and in particular, stronger affinity of the hydroxide ion compared to the proton. The pH-dependence of the zeta-potential can be suppressed at sufficiently high silver concentrations, which agrees with previous measurements in particle systems where no pH-dependence was found at high halide ion concentrations. The results suggest a subtle interplay between the surface potential determining the halide and silver ion concentrations, and the water ions. Whenever the charge due to the halide and silver ions is sufficiently high, the influence of the proton/hydroxide ion on the zeta-potential vanishes. This might be related to the water structuring at the relevant interfaces which should be strongly affected by the surface potential. Another interesting observation is accentuation of the assumed water ion effect on the zeta-potential at the flat single crystal surfaces compared to the corresponding silver halide colloids. Previous generic MD simulations have indeed predicted that hydroxide ion adsorption is accentuated on flat/rigid surfaces. A thermodynamic model for AgI single crystals was developed to describe the combined effects of iodide, silver and water ions, based on two independently previously published models for AgI (that only consider constituent and background electrolyte ions) and inert surfaces (that only consider water and background electrolyte ions). The combined model correctly predicts all the experimentally observed trends.

Influence of interfacial water layer on surface properties of silver halides: Effect of pH on the isoelectric point

The hypothesis that pH dependent charge of interfacial water affects electrokinetic charge and electrokinetic potential of hydrophobic colloids, but not the (inner) surface potential was tested. It was found that isoelectric points of silver chloride, bromide and iodide shift to the higher pAg values in the acidic solutions, but that surface potential did not depend on pH. Isoelectric points of water at inert surfaces lie in the range 2<pH<4. In the neutral environment around pH≈7, the interfacial water is negatively charged due to preferential accumulation of OH(-) ions, with respect to H(+) ions, thus increasing the negative electrokinetic charge of silver halide particles. In the acidic region, the isoelectric points of silver halides were shifted to the higher pAg values by 0.5-1.0 pAg units depending on the type of silver halide, as well on the ionic strength of the solution. At pH≈3, the interfacial water is almost uncharged so that electrokinetic charge of silver halide particles is due to adsorption of silver and halide ions only. The conclusion was supported by mass titration experiments. Consequently, the electroneutrality points of silver halide surfaces correspond to values obtained in the acidic media. For silver chloride, pAg(eln)=5.3; for silver bromide, pAg(eln)=5.5; and for silver iodide, pAg(eln)=4.8.

Point of zero potential of single-crystal electrode/inert electrolyte interface.

Most of the environmentally important processes occur at the specific hydrated mineral faces. Their rates and mechanisms are in part controlled by the interfacial electrostatics, which can be quantitatively described by the point of zero potential (PZP). Unfortunately, the PZP value of specific crystal face is very difficult to be experimentally determined. Here we show that PZP can be extracted from a single-crystal electrode potentiometric titration, assuming the stable electrochemical cell resistivity and lack of specific electrolyte ions sorption. Our method is based on determining a common intersection point of the electrochemical cell electromotive force at various ionic strengths, and it is illustrated for a few selected surfaces of rutile, hematite, silver chloride, and bromide monocrystals. In the case of metal oxides, we have observed the higher PZP values than those theoretically predicted using the MultiSite Complexation Model (MUSIC), that is, 8.4 for (001) hematite (MUSIC-predicted ~6), 8.7 for (110) rutile (MUSIC-predicted ~6), and about 7 for (001) rutile (MUSIC-predicted 6.6). In the case of silver halides, the order of estimated PZP values (6.4 for AgCl<6.5 for AgBr) agrees well with sequence estimated from the silver halide solubility products; however, the halide anions (Cl(-), Br(-)) are attracted toward surface much stronger than the Ag(+) cations. The observed PZPs sequence and strong anions affinity toward silver halide surface can be correlated with ions hydration energies. Presented approach is the complementary one to the hysteresis method reported previously [P. Zarzycki, S. Chatman, T. Preočanin, K.M. Rosso, Langmuir 27 (2011) 7986-7990]. A unique experimental characterization of specific crystal faces provided by these two methods is essential in deeper understanding of environmentally important processes, including migration of heavy and radioactive ions in soils and groundwaters.

Effect of electrolyte on surface potential of haematite in aqueous electrolyte solutions

Abstract A method for determination of the surface potential Ψ0 by single crystal metal oxide electrodes is demonstrated. Theoretical analysis of metal oxides using the surface complexation model was based on the general surface reaction equation. It was shown that the slope of Ψ0(pH) function should be lower than the Nernstian slope. This reduction was found to depend on the nature of the counterions and to be more pronounced at higher electrolyte concentrations. Quantitative analysis of the surface potential data enables evaluation of the ratio of surface concentrations of positive (protonated) to negative (deprotonated) surface sites, as well as activity coefficients of charged surface species. These data, along with data obtained by other methods, enable complete characterisation of the electrical interfacial layer at the metal oxide aqueous interface.

Surface and Structural Properties of Gold Nanoparticles and Their Biofunctionalized Derivatives in Aqueous Electrolytes Solution

Gold nanoparticles reduced by sodium citrate (d ∼ 10 nm) and purchased gold colloid particles (d ∼ 500 nm) were examined and compared. The properties of both gold particles and their biofunctionalized derivatives with L-cysteine and L-glutathione were studied in the presence of sodium nitrate. The structural investigations indicated an aggregated inner structure. The isoelectric points of pure gold, citrate reduced gold, and functionalized gold were measured and compared. The low isoelectric point of pure gold/water interface was explained by considering the distribution and accumulation of H+ and OH− ions within the interfacial water layer, being more pronounced for OH− ions.

Adsorption of Oxalic Acid onto Hematite: Application of Surface Potential Measurements

The surface potential at the hematite/aqueous oxalic acid interface was measured by means of a hematite Single Crystal Electrode. This allowed the simultaneous interpretation of the surface potential, electrokinetic potential and adsorption data for the adsorption of oxalic acid onto a hematite surface. Based on the Surface Complexation Model, this interpretation suggested that the oxalate ion is bound to a metal ion at the solid surface to form a singly-charged oxalate-surface complex, with the charge being exposed to the potential at the outer Helmholtz layer. A number of equilibrium parameters describing the interfacial equilibrium were obtained. Thus, for the two protonation steps of the surface sites, the thermodynamic equilibrium constants were log K1 = 7.1 ± 0.4 and log K2 = 5.2 ± 0.4, respectively. Two assumptions were tested with regard to the adsorption equilibrium constant, i.e. the charge of the surface complex is exposed to the inner surface potential, ψ0, or to the outer surface potential, ψd. The constancy of the interfacial equilibrium constants led to the conclusion the latter assumption was the more appropriate. The observed value of the adsorption equilibrium constant was log Kads = 3.0 ± 0.8. It was shown that measurement of the surface potential by Single Crystal Electrodes is a helpful tool in elucidating the equilibrium behaviour of the complex at the interface.

Interfacial charge in mixed-oxide aqueous suspensions

Surface charge and point of zero charge (PZC) in an aqueous silica-hematite mixed system were determined as a function of pH and of mixture composition by “mass titration” and acid-base titration of the suspension. The experiments showed that charging was independent, i.e. the overall surface charge was approximately equal to the sum of the surface charges of each oxide. The PZC of metal oxide mixtures corresponds to the pH where the net (overall) surface charge is zero, while one oxide bears a positive and the other a negative charge. The PZC of mixed oxides was found to lie between the PZC of pure oxides, depending on specific surface area, mass fraction and the PZC of each oxide in the mixture. It is shown that “mass titration” could be used as a method for determination of the PZC and surface charge of pure oxides and that it also may be applied to their mixtures.