Dispersions of Metal Oxides in the Presence of Anionic Surfactants

Abstract

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.

Investigations of mechanism of Ca2+ adsorption on silica and alumina based on Ca-ISE monitoring, potentiometric titration, electrokinetic measurements and surface complexation modeling

“…In the case of amorphous silica the negative value of the ζ-potential was reported in the almost entire pH range of 3-9. The determined IEP for alumina was equal to the PZC value of 8.8 which was within the range of the previously investigated IEP values (Izrael Živković et al 2015;Mayordomo et al 2018;Ruchomski et al 2018). Moreover, the results reported in the literature for silica (Kosmulski et al 2002) and alumina (Izrael Živković et al 2015) were quite consistent with the zeta potential curves obtained by us.…”

Influence of Dodecylsulfate Adsorption on the Stability of Cerium Oxide Nanoparticle-Based Colloidal Aqueous Dispersions

“…This hypothesis is actually highly unlikely as the experiments reported, in particular, in Figures , , and , all tend to demonstrate the adsorption of SDS on CeO 2 NPs. As already mentioned above, the negative values measured for the ζ-potential of CeO 2 NPs in the entire investigated pH range (i.e., from 2 to 12) in the presence of SDS (see red dots in Figure ) constitute a first strong indication of the adsorption of dodecylsulfate anions on CeO 2 NPs, according to ref . Moreover, in Figure , one can see a strong variation of the absorbance of the solution with time and, therefore, a variation of the concentration of CeO 2 NPs in front of the beam in an aqueous suspension corresponding to a given SDS concentration where no pH variation was expected, imposed experimentally, or observed.…”

“…20 One can also notice in Figure 1 that the variation of ζpotential of CeO 2 NPs as a function of pH is deeply perturbed in the presence of SDS at 5 × 10 −3 M. Indeed, it appears that the ζ-potential of CeO 2 NPs is always negative whatever the pH value within the investigated pH range, in good agreement with recent observations published in the literature regarding adsorption of SDS on other metal oxides (alumina, titania, and hematite). 21 3.2. UV−Visible Spectrophotometry Investigations.…”

“…The electrostatic interactions between nanoparticles depend on their surface charge that can be determined by measuring their ζ-potential. In the case of metal oxides, the latter can be altered by changing pH, ionic strength, or their concentration in the colloidal solution or by introducing molecular compounds that are able to adsorb on the surface of metal oxides . In our investigations, the first series of zetametry measurements was performed so as to determine the isoelectric point (IEP) of bare CeO 2 NPs.…”

“…In the case of metal oxides, the latter can be altered by changing pH, ionic strength, or their concentration in the colloidal solution 20 or by introducing molecular compounds that are able to adsorb on the surface of metal oxides. 21 In our investigations, the first series of zetametry measurements was performed so as to determine the isoelectric point (IEP) of bare CeO 2 NPs. For this purpose, the ζpotential of the synthesized ceria nanoparticles was measured in the absence of SDS by introducing CeO 2 NPs at a 1 g•L −1 concentration in a 10 −2 mol•L −1 NaNO 3 (Merck, France, 99% purity) aqueous solution prepared using deionized water.…”