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J. Phys. Chem. C 2009, 113, 12806–12810
Control of the Zeta Potential in Semiconcentrated Dispersions of Titania in Polar Organic Solvents Marek Kosmulski,* Piotr Prochniak, and Jarl B. Rosenholm Department of Physical Chemistry, Åbo Akademi UniVersity, Åbo, Finland, and Department of Electrochemistry, Lublin UniVersity of Technology, Lublin, Poland ReceiVed: April 27, 2009; ReVised Manuscript ReceiVed: May 29, 2009
The electrokinetic potential of commercial titania powder (chiefly anatase) in its 1-10 mass % dispersions in water, lower aliphatic alcohols, and dimethyl sulfoxide (DMSO) is positive, and rather insensitive to the nature of the solvent. The sign of the electrokinetic potential can be reversed to negative by addition of phosphoric acid, or of combination of phosphoric acid with triethylamine or with alkali hydroxide. The critical concentration of surface-active compounds, which induces a sign reversal, depends on the nature of the solvent, and it is higher in organic solvents than in water. The critical surface concentration of surface-active compounds for given organic solvent is rather insensitive to the solid-to-liquid ratio, and typically it is in the range of a few micromoles per square meter. 1. Introduction The methods of control of the surface charge and ζ-potential in aqueous dispersions by addition of surface-active solutes are well-known. The pH-dependent surface charging of metal oxides can serve as a classical example. The problems of charge control in nonaqueous solvents are much less well-known. Early studies of the ζ-potential of solid particles dispersed in nonaqueous media have been carried out by electrophoresis. This method requires a low solid-to liquid ratio. The fact that the studied dispersion consists of a large amount of the nonaqueous solvent and of a small amount of solid particles defines the origin of the surface-active substances present in the dispersion. The surface-active substances present in nonaqueous solvents as impurities play a leading role, since their amount in the system per unit of the surface area is high.1 The values of ζ-potentials of certain powder in various “pure” solvents reported in the literature2 are scattered, and they may even have opposite signs, because the nature and concentration of impurities varies from one solvent to another, and from one batch of certain solvent to another. The surface-active impurities occluded in the powder play a minor role, because their amount in the system per unit of volume of the solvent is low. The ζ-potentials of solid particles dispersed in nonaqueous media can be controlled by addition of various solutes to the dispersion. In polar solvents, the chemistry is similar as in water; for example, positive surface charge can be induced by addition of strong inorganic acids (protons are adsorbed) or by addition of catonic surfactants (nonpolar cations are adsorbed), and negative surface charge can be induced by addition of strong inorganic bases (hydroxyl ions are adsorbed) or by addition of anionic surfactants (nonpolar anions are adsorbed).3,4 In less polar solvents, the availability of ionic species is limited, their nature and properties are less well-known, and the effects of particular solutes on the ζ-potential are more difficult to predict.5,6 The commercial instruments based on the electroacoustic method enable measurements at high solid-to-liquid ratios, also in nonaqueous solvents.7,8 Reliable measurements can only be carried out at high concentrations of solid particles, because the electroacoustic signal is roughly proportional to the solid
load. One mass percent is sufficient for most common powders to obtain sufficient signal-to-noise ratio at an absolute value of ζ-potential higher than 10 mV. As discussed above for electrophoresis, the surface-active compounds originating from the solvent (as impurities) and from the powder (occluded) also undergo redistribution between the bulk solvent and the interfacial region. However, the significances of sources of surface-active species in concentrated dispersions are different than those in dilute dispersions, which are used in electrophoresis. In electroacoustics, the surface-active substances present in nonaqueous solvents as impurities play a minor role, since their amount per unit of the surface area is low. On the other hand, the surface-active impurities occluded in the powder play a dominant role, since their amount in the system per unit of volume of the solvent is high. Thus, the values of ζ-potentials of a certain powder in its concentrated dispersions in various “pure” solvents are more predictable and more consistent than in very dilute dispersions, and differences in the impurity level from one batch of the solvent to another have a minor effect on ζ-potentials. We argue that concentrated dispersions are more suitable for fundamental studies of solid particles in nonaqueous solvents than dilute dispersions, because the nature and concentration of ionic species in the system (other than deliberately added solutes) are more consistent and predictable than those in dilute dispersions. In this paper, we demonstrate that the dispersions of titania in polar organic solvents, which are originally positively charged, can be overcharged to negative in a controlled way by addition of surface-active substances. This possibility can be used to control the colloid stability, that is, to stabilize or destabilize the dispersion by addition of solutes and to induce or avoid homo- or heterocoagulation of colloidal particles and/ or their adhesion to certain surfaces. The ζ-potential is correlated with the rheological properties of dispersions, for example, with their viscosity, and this correlation is widely used in the formulation of paints. Thus, the present findings may also have technological applications. We selected a few reagents, which are well-known to show a high surface activity in aqueous dispersions, and tested their behavior in nonaqueous solvents.
10.1021/jp903845e CCC: $40.75 2009 American Chemical Society Published on Web 06/17/2009
Control of ζ-Potential in Dispersions of Titania 2. Experimental Section 2.1. Reagents. Aeroxide (P-25) from Degussa was used as obtained. It is a relatively pure anatase (with admixture of rutile), it consists of particles 30 nm in diameter, the specific surface area is 50 m2/g, and its pristine isoelectric point (IEP) is at pH about 6.5.9 Aeroxide forms aerosols, and an antidust mask should be worn during operations with dry powder. Water was freshly obtained from a Milli-Q device. The following solvents methanol (>99.8%, 10 µmol/m2) produced a random mixture of slightly positive and slightly negative ζ-potentials but no clear sign reversal, and in 1-propanol, 2-propanol, and DMSO, the ζ-potential of titania was clearly positive in the entire course of base titrations, that is, up to about 30 µmol of triethylamine/ m2. A limited number of NaOH and KOH titrations produced similar results as triethylamine titrations. We conclude that addition of base is not an efficient method to control the sign of ζ-potential of titania in polar nonaqueous solvents. 3.4. Organic Dispersions: Titration with Phosphoric Acid. In the present section, the effect of phosphoric acid on the ζ-potential of titania in its organic dispersions will be discussed. It should be emphasized that alkali and ammonium phosphates show a low solubility (
