Nanotechnology is finding applicability in the field of environmental protection and has great potential in improving air, water, and soil quality . For example, engineered nanoparticles can efficiently reduce toxic metal emissions from combustion systems and improve air quality by suppressing metal vapor nucleation and promoting metal nanoparticle condensation and coagulation [2, 3]. Many nanomaterials, such as TiO2, carbon nanotubes, and dendrimers, have been designed to degrade or absorb pollutants in water and soil systems [4–7]. These applications are often determined by the properties of nanomaterials, such as size, surface properties, crystal structures, and morphologies [8, 9]. Although nanotechnology has the potential to improve the quality of the environment, there are also concerns that it can generate a new class of hazards upon release to the environment followed by exposure of either the ecosystem or human beings that may result in potential adverse effects [1, 10, 11]. Toxicological studies with certain engineered nanoparticles (e.g., fullerenes, quantum dots, and metal oxides) have confirmed that they can be potentially harmful due to their high surface molecule/atom fraction and unique physicochemical properties [12, 13]. The emerging discipline of nanotoxicology is aiming to establish the relationship between nanoparticle properties (e.g., size, surface properties, and crystal phase) and their toxic potential [14–16].
Titanium dioxide has been widely used in environmental photocatalysis, sunscreen, and coating industry [17–19]. However, a variety of detrimental pulmonary effects in rodents and antibacterial effects have also been associated with nanosized TiO2 particle exposure [20–22]. Both the functionalities and biological effects of titania nanoparticles are controlled by its physicochemical properties. Nanomaterials that are tested are often dispersed in aqueous systems; this can potentially result in physicochemical property changes, e.g., agglomeration state and surface charge variation [15, 23, 24].
The agglomeration behavior and surface charge variation of nanoparticle dispersions can have a dramatic effect on both the reactivity of nanomaterials and their efficiency in contamination treatment [7, 25, 26]. It also affects the response of organisms upon exposure [27–30]. Therefore, accurate characterization of nanoparticle dispersions becomes very important for its environmental applications and nanotoxicology investigations. Jiang et al.  characterized the state (such as the hydrodynamic size, surface charge, and the degree of agglomeration) of titania and other nanoparticle suspensions and tested the effect of solution pH and ionic strength (IS) on dispersion properties. However, this study involved only a single value of surface area, primary particle size, and crystal phase for examined dispersion state. It has been reported that these properties of TiO2 nanoparticle can affect its photocatalytic activity [19, 31] and toxicity [16, 32–34]; however, little is known about their effect on the dispersion state and agglomeration behavior. There is evidence suggesting that the point of zero charge of hematite nanoparticle dispersion might change with varying particle size . However, systematic investigations for titania nanoparticle dispersions have not been done.
Recent developments in aerosol route synthesis of TiO2-based nanomaterials allow for greater and independent control of their physicochemical properties, such as size, crystal phase, and specific surface area [4, 36–38]. In this study, the influence of particle surface area, primary particle size, and crystal phase on titania nanoparticle dispersion properties is investigated. TiO2 samples with well-controlled properties are synthesized using flame aerosol reactors (FLAR). Six anatase TiO2 samples with different sizes (6–104 nm) are used to study the size effect. TiO2 nanoparticles of different crystal phases with the same size are used to examine the crystal phase effect. Commercially available Degussa TiO2 (P25) sample is also tested. The effect of monovalent and divalent electrolytes is examined using sodium chloride (NaCl) and magnesium chloride (MgCl2).