Electric Double Layer in Water-Organic Mixed Solvents: Titania in 50% Ethylene Glycol.

Ethylene glycol (EG) and its mixtures with water are popular components of nanofluids used as heat transfer fluids. The stability of nanofluids against coagulation is correlated with their zeta potential. The electrophoretic mobility of titania nanoparticles in 50-50 w/w EG was studied as a function of the concentration of various solutes. HCl, NaOH, SDS and CTMABr at concentrations up to 0.01 M are strong electrolytes in 50% EG, that is, the conductance of their solutions is proportional to the concentration. HCl, NaOH and CTMABr were very efficient in inducing a high zeta potential for titania in 50% EG. NaOH induced a negative zeta potential in excess of 70 mV, and HCl and CTMABr induced a positive zeta potential in excess of 70 mV at concentrations below 10−4 M. Apparently, HCl, NaOH and CTMABr are also more efficient than SDS in terms of nanofluid stabilization against coagulation. An overdose of base (>1 mM) results in depression of the negative zeta potential. This result may be due to the specific adsorption of sodium on titania from 50% EG.

The Isoelectric Point of an Exotic Oxide: Tellurium (IV) Oxide.

The pH-dependent surface charging of tellurium (IV) oxide has been studied. The isoelectric point (IEP) of tellurium (IV) oxide was determined by microelectrophoresis in various 1-1 electrolytes over a concentration range of 0.001-0.1 M. In all electrolytes studied and irrespective of their concentration the zeta potential of TeO2 was negative over the pH range 3-12. In other words the IEP of TeO2 is at pH below 3 (if any). TeO2 specifically adsorbs ionic surfactants, and their presence strongly affects the zeta potential. In contrast the effect of multivalent inorganic ions on the zeta potential of TeO2 is rather insignificant (no shift in the IEP). In this respect TeO2 is very different from metal oxides.

Two types of electrokinetic behavior of solid particles in the presence of anionic surfactants.

HYPOTHESIS: The electrokinetic curves of metal oxides obtained at moderate concentrations of sodium dodecylsulfate show a maximum at pH close to the pristine IEP of the oxide (IEP in the absence of surfactant). This behavior of SDS is very different from most other anions, in which the electrokinetic curves did not show a maximum. We hypothesize that the unusual behavior of SDS is due to its micellization in solution and on the surface. EXPERIMENTS: The ζ potentials of alumina, titania, and hematite containing sodium tetradecyl- and hexadecylsulfate at different concentrations have been studied. These surfactants do not form micelles in solution. FINDINGS: In dispersions of hematite, titania, and alumina stabilized with SDS, moderate concentrations of the surfactant induced a reversal of sign of the ζ potential at acidic pH (<4) while the effect of the surfactant at neutral and basic pH (>5) was less significant. Hematite dispersions stabilized with sodium tetradecyl- and hexadecylsulfate showed the electrokinetic behavior typical for most anions (no maximum in ζ potential) rather than that characteristic for SDS. The increase in the chain length from 12 to 16 C atoms induces depression and then disappearance of a maximum in the ζ potential vs. pH curves.

Hematite and hematite-akageneite composites. XRD and electrokinetic study and interaction with ionic surfactants.

Hematite and hematite-akageneite composites were obtained by hydrolysis of FeCl3 in acidic medium. The IEP of hematite and of hematite-akageneite composites was at pH about 9. The particle radius of primary hematite particles was about 50nm and the primary particles of hematite-akageneite composites were larger and porous. Addition of SDS to dispersions containing hematite or hematite-akageneite composites resulted in substantial increase in the particle size.

The significance of the solid-to-liquid ratio in the electrokinetic studies of the effect of ionic surfactants on mineral oxides.

The effect of SDS on the electrokinetic behavior of TiO(2) and Al(2)O(3) was studied by electrophoresis at various solid-to-liquid ratios. Additionally, the effect of CTAB on electrokinetic curves of Al(2)O(3) single crystal and of Al(2)O(3) particles was studied by streaming potential. At a sufficiently low solid-to-liquid ratio, the electrokinetic potential was negative and almost pH-independent in the presence of SDS and positive and pH-independent in the presence of CTAB. Further decrease in the solid-to-liquid ratio had a limited effect on the course of the electrokinetic curves. At a sufficiently high solid-to-liquid ratio, the electrokinetic potential was not affected by the presence of the surfactant. At moderate solid-to-liquid ratios, the electrokinetic potential in the presence of SDS was negative and almost pH-independent at very high and at very low pH, and less negative or even positive electrokinetic potential (more positive at higher solid-to-liquid ratios) was observed at moderate pH with a peak 1 to 2 pH units below the pristine IEP. The inspection of the results (obtained at single solid-to-liquid ratio) from the literature confirmed the above trends, also for oxides other than TiO(2) or Al(2)O(3). The range of solid-to-liquid ratios, which can be covered by electrophoresis is limited by insufficient signal, and by insufficient transparency at low and at high solid-to-liquid ratios, respectively. The available range of solid-to-liquid ratios can be extended by using the electroacoustic method. Apparently, the significance of the solid-to-liquid ratio in the electrokinetic studies of oxide-ionic surfactant systems is underrated. To our best knowledge, this is the first systematic study of such an effect ever published, and in many publications, the solid-to-liquid ratio was not reported and probably not even controlled.

Electric conductance of dispersions of metal oxides in solutions of weak acids in mixed dioxane-water solvents.

The electric conductance of solutions of sulfuric, oxalic, benzoic, and salicylic acid (up to 0.02 M) in dioxane-water mixed solvents (90% and 93% dioxane by mass) has been studied in the presence and absence of TiO(2) and Al(2)O(3) (0.5-5% by mass). TiO(2) and Al(2)O(3) enhanced the conductance of solutions of organic acids in aqueous dioxane. The conductance is interpreted in terms of adsorption of acid in molecular form, dissolution of ceramic oxides in form of anionic complexes, and leaching of acidic impurities from ceramic oxides.

Morphology of synthetic goethite particles.

The specific surface area of synthetic goethite depends on the preparation: the Fe(III):OH ratio, the rate of base titration of Fe salt, and the temperature and time of crystallization. The crystals also have different morphologies as determined by SEM or TEM. Carbon coating is used to improve the quality of SEM images of nonconducting specimens. We show here that needle-like goethite particles become substantially thicker in the course of standard carbon coating, and the length-to-width ratio obtained for carbon-coated particles is lower than that for the original goethite particles. The morphology of the goethite particles was also studied by tapping mode AFM.

Dilatometric study of the adsorption of heavy-metal cations on goethite.

The specific volumes of adsorption of Cd, Co, Cu, Ni, Pb, and Zn on goethite determined by means of the dilatometric method are 21, 32, 32, 31, 31, and 42 cm3/mol, respectively, and are independent of pH. The effect of NaCl (up to 0.5 mol dm(-3)) on the specific volume of adsorption is rather insignificant. The specific volume of precipitation of corresponding hydroxides (determined experimentally and calculated) is about 60 cm3/mol. Apparently, the adsorbed heavy-metal cations lose half of their hydration water. The adsorption constant decreases as the pressure increases, and the effect becomes significant at pressures of > 10(7) Pa, i.e., more than 1 km of water column.

Synthesis and characterization of goethite and goethite-hematite composite: experimental study and literature survey.

Aging of synthetic goethite at 140 degrees C overnight leads to a composite material in which hematite is detectable by Mössbauer spectroscopy, but X-ray diffraction does not reveal any hematite peaks. The pristine point of zero charge (PZC) of synthetic goethite was found at pH 9.4 as the common intersection point of potentiometric titration curves at different ionic strengths and the isoelectric point (IEP). For the goethite-hematite composite, the common intersection point (pH 9.4), and the IEP (pH 8.8) do not match. The electrokinetic potential of goethite at ionic strengths up to 1 mol dm(-3) was determined. Unlike metal oxides, for which the electrokinetic potential is reversed to positive over the entire pH range at sufficiently high ionic strength, the IEP of goethite is rather insensitive to the ionic strength. A literature survey of published PZC/IEP values of iron oxides and hydroxides indicated that the average PZC/IEP does not depend on the degree of hydration (oxide or hydroxide). Our material showed a higher PZC and IEP than most published results. The present results confirm the allegation that electroacoustic measurements produce a higher IEP than the average IEP obtained by means of classical electrokinetic methods.

Electrokinetic potentials of mineral oxides and calcium carbonate in artificial seawater.

Electroacoustic method was used to measure the zeta potentials of mineral oxides and calcium carbonate in artificial sea water (pH 8), and the following values were obtained: synthetic hematite: 0.87 mV; goethite: 1.01 mV; fumed silica: -3.56 mV; quartz: -1.38 mV; calcium carbonate: 1.91 mV. The absolute values of the zeta potentials obtained in the present study are substantially lower than the zeta potentials in seawater reported by other authors. Our results indicate that the effect of electrostatic repulsion in the interactions between the particles of these materials in seawater is negligible.

Multiinstrument study of the electrophoretic mobility of fumed silica.

Although silica is one of the most common adsorbents, the position of its isoelectric point (IEP) on the pH scale remains an open question. Different values from 1 to 4 have been published, and there is even a controversy about the very existence of the IEP. The discrepancies in the literature are at least partially due to application of different instruments. Different zetameters produce different zeta potentials of silica in the acidic range and different IEP.

Multiinstrument study of the electrophoretic mobility of quartz.

The electrophoretic mobility of quartz in 0.01 mol dm(-3) NaCl and NaNO3 over the pH range 2-8 was studied using five different commercial instruments. The mobilities over the pH range 4-8 were relatively consistent, but the mobilities over the pH range 2-4 and the position of the isoelectric point IEP varied from one instrument to another. This result suggests that the discrepancies in the apparent IEP of quartz (and other silicas) reported in the literature are due to the instrument artifacts.