Surface charge and interfacial potential of titanium dioxide nanoparticles: Experimental and theoretical investigations Jenny Perez Holmberg, Elisabet Ahlberg, Johan Bergenholtz, Martin Hassellöv, Zareen Abbas ⇑ Department of Chemistry and Molecular Biology, University of Gothenburg, SE 412 96 Gothenburg, Sweden article info Article history: Received 26 March 2013 Accepted 6 June 2013 Available online 25 June 2013 Keywords: P25 Anatase Rutile Electrophoretic mobility Zeta potential Corrected Debye–Hückel Surface complexation Salt titrations Electrokinetic charge abstract Size dependent surface charging and interfacial potential of titanium dioxide (TiO 2 ) nanoparticles are investigated by experimental and theoretical methods. Commercially available TiO 2 (P25) nanoparticles were used for surface charge determinations by potentiometric titrations. Anatase particles, 10 and 22 nm in diameter, were synthesized by controlled hydrolysis of TiCl 4 , and electrophoretic mobilities were determined at a fixed pH but at increasing salt concentrations. Corrected Debye–Hückel theory of surface complexation (CDH-SC) was modified to model the size dependent surface charging behavior of TiO 2 nanoparticles. Experimentally determined surface charge densities of rutile and P25 nanoparticles in different electrolytes were accurately modeled by the CDH-SC theory. Stern layer capacitances calculated by the CDH-SC theory were in good agreement with the values found by the classical surface complexation approach, and the interaction of protons with OH groups is found to be less exothermic than for iron oxide surfaces. Moreover, the CDH-SC theory predicts that the surface charge density of TiO 2 nanoparticles of diameter <10 nm is considerably higher than for larger particles, and pH at the point of zero charge (pH PZC ) shifts to higher pH values as the particle size decreases. The importance of includ- ing the particle size in calculating the zeta potentials from mobilities is demonstrated. Smoluchowski theory showed that 10 nm particles had lower zeta potential than 22 nm particles, whereas a reverse trend was seen when zeta potentials were calculated by Ohshima’s theory in which particle size is included. Electrokinetic charge densities calculated from zeta potentials were found to be only one third of the true surface charge densities. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Surface and interfacial properties of nanoparticles such as sur- face charge density and zeta potential are extremely important in determining their reactivity, the stability in complex media, self- assembly properties, capacity for assembling nanoarchitectures by bottom-up approaches, interaction with cells, and adsorption of proteins [1]. Most of the surface chemical properties of colloidal particles are treated as dependent only on the material and inde- pendent of size. Despite the obvious importance of particle size dependent surface charge density and zeta potential, relatively lit- tle information is available in the literature. There are some exper- imental studies in which size dependent surface charging has been investigated for hematite (a-Fe 2 O 3 ) [2], magnetite (Fe 3 O 4 ) [3], ana- tase (TiO 2 ) [4], and silica (SiO 2 ) [5–9] nanoparticles of diameter less than 50 nm. However, contradictory results have been reported. In some studies, enhanced surface charge densities were reported for 30 and 8 nm silica particles [6,7], whereas in another study, similar charge densities were found for 30, 50, and 80 nm silica [14]. Recently, Brown et al. [8,9] have shown by using the X-ray absorp- tion near edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) that the surface charge density of 7–9 nm silica particles is higher than that for 22–25 nm particles. Size dependent zeta potentials of nanoparticles have also been reported in a num- ber of studies [2,3,10–15]. As in the case of size dependent surface charging, there is no clear trend observed from these experimental studies. For example, results of Blute et al. [5] indicate that 6 nm silica particles have lower zeta potential in 0.01 M NaCl than 15 nm particles. On the other hand, Madden et al. [2] found higher zeta potential for 7 nm a-Fe 2 O 3 compared with 25 nm particles. Monte Carlo simulations and theories based on integral equations predict that at constant surface charge and salt concentration, the zeta potential of nanoparticles decreases as the particle size de- creases [16–18]. There are a number of practical difficulties that might be the cause of the contradictory experimental results reported for parti- cle size dependent surface charging and zeta potential. Nanoparti- cles aggregate strongly and once particles are aggregated, their surfaces will behave very differently compared to mono-dispersed particles. Consequently, their surface charge and zeta potential will be more like large micrometer sized particles [19]. Another 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.06.015 ⇑ Corresponding author. E-mail address: zareen@chem.gu.se (Z. Abbas). Journal of Colloid and Interface Science 407 (2013) 168–176 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis