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We studied the behavior of dilute dispersions of nanoparticles of hematite, alumina, and titania in the presence of various concentrations of very pure sodium dodecyl-, tetradecyl-, and hexadecylsulfate. The concentrations studied were up to critical micelle concentration (CMC) for sodium dodecylsulfate, and up to the solubility limit in case of sodium tetradecyl- and hexadecylsulfate. The dispersions were adjusted to different pH (3–11), and 10−3 M NaCl was used as the supporting electrolyte. The solid-to-liquid ratio was strictly controlled in all dispersions, and the behavior of fresh dispersions was compared with dispersions aged for up to eight days. The presence of very low concentrations of ionic surfactants had rather insignificant effects on the ζ potentials of the particles. At sufficient concentrations of ionic surfactants the isoelectric point (IEP) of metal oxides shifted to low pH, and the long-chain surfactants were more efficient in shifting the IEP than their shorter-chain analogues. Once the surfactant concentration reached a critical value, the ζ potentials of the particles reached a pH-independent negative value, which did not change on further increase in the surfactant concentration and/or aging of the dispersion. This critical concentration increases with the solid-to-liquid ratio, and it is rather consistent (for certain oxides and certain surfactants) when it is expressed as the amount of surfactant per unit of surface area. Surprisingly, the surfactant-stabilized dispersions always showed a substantial degree of aggregation; that is, the particle size observed in dispersions by dynamic light scattering was higher than the size of particles observed in dry powders by electron microscopy. Apparently, in spite of relatively high ζ potentials (about 60 mV in absolute value), the surfactant-stabilized dispersions consist of aggregates rather than of primary particles, and in certain dispersions the high concentration of surfactant seems to induce aggregation rather than prevent it.

In this work, nanocrystalline powders of iron-doped zinc oxide ZnO (iron content 3, 5, and 10 at.%) were prepared utilizing co-precipitation method. X-ray diffraction, scanning electron microscopy, and the Mössbauer spectroscopy were used as complementary methods to investigate the structure and hyperfine interactions of the material. It was found that Fe dopant is incorporated into the ZnO würtzite structure. As confirmed by energydispersive X-ray spectroscopy the distribution of Fe dopant in the obtained samples is homogeneous up to 5 at.%. For 10 at.% of iron, spinel ZnFe2O4 phase was registered both by X-ray diffraction and the Mössbauer techniques. Paramagnetic behavior in Fe-doped ZnO was observed in the Mössbauer spectra at room temperature. Hyperfine interactions parameters indicate the presence of Fe 3+ ions substituting Zn 2+ ions at tetrahedral sites both in the crystallite interior and near the surface of grains.

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