CROATICA CHEMICA ACTA CCACAA 79 (1) 95¿106 (2006) ISSN-0011-1643 CCA-3069 Original Scientific Paper Point of Zero Charge and Surface Charge Density of TiO2 in Aqueous Electrolyte Solution as Obtained by Potentiometric Mass Titration* Tajana Preo~anin** and Nikola Kallay Laboratory of Physical Chemistry, Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102 a, 10000 Zagreb, Croatia RECEIVED OCTOBER 7, 2005; REVISED NOVEMBER 22, 2005; ACCEPTED NOVEMBER 23, 2005 Mass titration method was developed as a suitable tool for determination of the point of zero s charge (p.z.c.) and surface charge density ( 0) 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 ob- tained at different ionic strengths. It is shown that c.i.p. may be used for locating the p.z.c. only Keywords in a »symmetric case«, when affinities of the anions and cations to associate with oppositely mass titration charged surface groups are equal. Analysis of the mass titration was performed on the basis of point of zero charge experimental data obtained with colloidal titania dispersed in an aqueous sodium chloride solu- surface charge density tion, and also by numerical simulation based on the Surface Complexation Model and the 2-pK counterion association mechanism of surface reactions. Increase in the NaCl concentration shifted the point of zero titan dioxide charge to the basic region, while the isoelectric point was shifted to the acidic region, indicat- sodium chloride ing higher association affinity of chloride ions compared to sodium ions. INTRODUCTION measure the pH of the dispersion. The pH of the system changes gradually and approaches a constant value pH¥. Potentiometric mass titration was originally developed In the case of a pure metal oxide powder (absence of 1 by Noh and Schwartz as a method for determination of acidic or basic contamination), pH¥ is equal to the point the point of zero charge (p.z.c.) of pure metal oxides (in of zero charge pHpzc. Mass titration was successfully ap- the absence of acidic or basic impurities). The method is plied for determination of the point of zero charge of simple. One should add subsequent portions of a metal colloidal particles2 with low3 and high specific surface oxide powder to the electrolyte solution (or water) and areas (activated carbon).4 Mass titration was also propos- * Dedicated to the memory of the late Professor Marko Branica. ** Author to whom correspondence should be addressed. (E-mail: [email protected]) 96 T. PREO^ANIN AND N. KALLAY ed for the point of zero charge determination of metal of active surface sites with the potential determining ions. oxide mixtures. The point of zero charge of a metal ox- For metal oxides, the potential determining ions are H+ ide mixture corresponds to the pH where the net surface and OH– ions. Within the Surface Complexation Model charge of the heterogeneous mixture is zero, while one (SCM), the 2-pK mechanism14,15 may be used for numeri- oxide bears a positive and the other a negative charge. cal simulation and interpretation of experimental data. Ac- For calculation, one needs to know the specific surface cordingly, surface charge is a result of the two-step pro- areas, mass fractions of components, and points of zero tonation of the surface groups produced by hydration of 5,6 charge of both metal oxides. The mass titration method the metal oxide surface was extended by Kallay and @alac7 for determination of ºMO– + ® ºMOH; o the point of zero charge of contaminated samples. The +H K1 (1) pH¥ value of contaminated dispersion is higher (basic º + ®º + o impurities) or lower (acidic impurities) compared to the MOH+H MOH2 ; K2 (2) point of zero charge. Interpretation of the mass titration o o provides information on the fraction of impurities in the where K1 and K2 are thermodynamic equilibrium con- powder and also on the point of zero charge.8 Mass titra- stants of the corresponding surface reactions. Generally, tion was found extremely suitable for determination of a thermodynamic equilibrium constant Ko is defined16 in the temperature dependency of the point of zero charge. terms of equilibrium activities (a) of species J involved One simply measures the temperature dependency of pH in the chemical reaction of a sufficiently concentrated metal oxide dispersion in the absence of impurities. The mass titration method was o n J K = Õ aJ (3) further developed for determination of surface charge den- J sity.9 The advantage of this method is that experiments For interfacial species S activity may be defined17 in can be performed at extremely low ionic strengths. terms of surface concentration G (amount of surface spe- The common method for determination of the point cies per surface area), so that of zero charge is the potentiometric acid-base titration of 10,11 G S { } a dispersion. Comparison with blank titration in the aS = yS = yS S (4) absence of the dispersed solid oxide phase yields rela- G o { } tive values of the surface charge densities. Absolute val- where yS is the activity coefficient, and S denotes the ues of the surface charge densities are then obtained by relative value of surface concentration of S with respect setting the zero value at the common intersection point to the chosen standard value. There is no recommended (c.i.p.) for different ionic strengths. However, this meth- value for the standard surface concentration, but G o = od neglects the influence of counterion association on the 1 mol m–2 seems to be a suitable choice. Consequently, point of zero charge. It assumes the same association af- {S} becomes the numerical value of surface concentra- finity for both counterions (anions and cations), which is tion of species S expressed in moles per meter square: practically never the case. One still obtains the c.i.p. but { } G –2 S = S / mol m . The activity coefficient of surface this point may be far from the p.z.c.12 For these reasons, species S (yS) is defined through the difference in chemi- potentiometric acid-base titration cannot be used for ex- cal potentials of real (mreal) and ideal (mid) states at the amination of the p.z.c. dependency on the electrolyte surface concentration. m real m id j The aim of this article is to examine the possibility RT ln yS = S – S = zSF (5) of applying mass titration for determination of the effect of electrolyte concentration on the point of zero charge where F denotes the Faraday constant, zS is the charge j in order to deduce the difference in surface association number of surface species S and is the electrostatic affinities of cations and anions. The effect of electrolyte potential affecting their state at the interface. Ideal state concentrations on the mass titration of colloidal disper- corresponds to the zero value of the electrostatic poten- tial j being equal to that in the bulk of the solution. Equa- sions of TiO2 particles was examined by numerical simu- lation and experimentally. From mass titration data, the tion (5) considers only electrostatic effects as responsi- surface charge densities and the point of zero charge at ble for the nonideal behavior. Such an approximation is different ionic strengths were obtained. The results were correct since electrostatics plays a major role. For exam- compared with »electrolyte titrations« (increase of the ple, if the potential affecting state of a positively charged ionic strength in concentrated dispersions)13 and with surface group (z = +1) changes from 180 mV to –180 mV, electrokinetic measurements. the activity coefficient at room temperature will be chang- ed due to the electrostatic effect by 6 orders of magni- 3 –3 Surface Complexation Model tudes, i.e., approximately from 10 to 10 . Equation (5) can be rewritten as: The basic process at a metal oxide surface in aqueous j environment is the surface charging due to interactions yS = exp(zSF / RT) (6) Croat. Chem. Acta 79 (1) 95¿106 (2006) POINT OF ZERO CHARGE AND SURFACE CHARGE DENSITY OF TiO2 97 In the case of the two-step protonation (2-pK mecha- Surface Charge Densities, Zero Charge Conditions nism) described by reactions (1) and (2), the activity and Surface Potential coefficients of charged interfacial species ºMO– and º + Total surface concentration of active surface sites G , MOH2 involved in surface reactions are: tot within the 2-pK model, as described by reaction equa- º – j y( MO ) = exp(–F 0 / RT) (7) tions (1), (2), (11) and (12), is equal to: y(ºMOH +) = exp(–Fj / RT) (8) 2 0 G G º G º + G º – tot = ( MOH) + ( MOH2 )+ ( MO )+ j G º + × – G º – × + where 0 is the (inner) surface potential affecting their ( MOH2 A )+ ( MO C ) (17) state at the interface.