Summary of the points of zero charge (PZC) and isoelectric points (IEP) of various materials published in the recent literature and of older results overlooked in the previous compilations. The roles of experimental conditions, especially of the temperature, of the nature and concentration of supporting electrolyte, and of the type of apparatus are emphasized. The newest results are compared with the zero points reported in previous reviews. Most recent studies were carried out with materials whose pH dependent surface charging is already well-documented, and the newest results are consistent with the older literature. Isoelectric points of Gd(OH) 3 , Sm(OH) 3 , and TeO 2 have been reported for the first time in the recent literature.
As artificial nano-enzymes and effective adsorbents, CeO 2 nanomaterials have been extensively studied for their potential applications in biomedicine and environmental protection. However, the surface acid-base properties of nanoceria suspensions have not been fully studied. In this work, the ceria nanoparticles were synthesized by hydrothermal reaction and calcination . They have a BET specific surface area of 52.01 m 2 /g. FTIR and XPS showed that there are two amphoteric sites (≡Ce(IV)-OH and ≡Ce(III)-OH) and a trace acidic site (≡Ce(III)-OCOOH) which can act as a site probe. The pH range of the acid-base reaction of surface hydroxyl groups of the CeO 2 nanomaterials was determined by Gran plot method. The acidic ionization constants (log K 1 , log K 2 ), site concentration (log C) of two amphoteric surface hydroxyl sites and pH pzc in three KNO 3 supported electrolytes were simultaneously optimized by ProtoFit GUI v2.1 free software using Donnan Shell Model (DSM) and same initial parameters. Their surface species distribution was calculated by PHREEQCI 3.4.0 free software using Donnan model and database (phreeqc.dat). The results show that the proton transfer reaction occurred between ≡Ce(IV)-OH and ≡Ce(III)-OH sites. Therefore, this study reveals the surface acid-base properties, proton transfer behavior and pH pzc of the CeO 2 suspension.
The stability of nano-hydroxyapatite (nHAP) affects its fate in the environment. Few studies have compared the influence of colloids with different properties on the stability of nHAP. Fulvic acid, montmorillonite, and goethite were chosen as representative colloids. An ultraviolet–visible spectrophotometer and NanoBrook 90Plus phase analysis light scattering (PALS) were used to determine absorbance, zeta potential, and hydrodynamic diameter. Results showed that addition of fulvic acids could make nHAP more stable through electrostatic and steric effects, whereas montmorillonite affected the stability mainly by electrostatic effects. Goethite could adsorb onto nHAP particles and form goethite-nHAP heteroaggregates at pH < pH pzc,goethite (i.e., pH at point of zero charge), and its addition enhanced the electrostatic repulsion forces at pH > pH pzc,goethite . Since fulvic acid has additional steric effects, its stability enhancement was greater than that of montmorillonite and goethite. Montmorillonite colloids were stronger than goethite colloids for enhancing the stability of nHAP, because montmorillonite had a higher absolute surface potential. The order in which organic and inorganic colloids were added affects the degree of stability of nHAP. Energy barriers calculated by extended Derjaguin–Landau–Verwey–Overbeek were in good agreement with the experimental results and implied that the nHAP particles were in the stage of reaction-limited aggregation at pH 7 ± 0.1 and pH 9 ± 0.1. Our findings are important for understanding the cotransport of nanoparticles and colloids. Core Ideas Organic colloids enhance the stability of nHAP, mainly by steric effects. Organic colloids stabilize nHAP in the absence and presence of inorganic colloids. Montmorillonite and goethite stabilize nHAP particles, mainly by electrostatic forces. Energy barriers were calculated by extended Derjaguin–Landau–Verwey–Overbeek. Energy barriers showed the predominant role of steric effects.
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