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We analyze potentiometric and conductimetric measurements simultaneously performed on Electric Double-Layer Magnetic Fluid based on cobalt ferrite nanoparticles, in order to obtain the pHdependence of the particle surface charge density. We propose a mechanism for the charging of the particle surface. This model considers the ferrofluid solution as a mixture of strong and weak diprotic acids. We show how an exact analytical treatment involving proton transfer between the particle surface and the bulk solution allows the construction of a speciation diagram of the charged superficial sites. The saturation value of the superficial density of charge is found to be equal to 0.326 ± 0.065 C m −2 .

The temperature dependence of the Soret coefficient S(T)(T) in electrostatically charged magnetic colloids is investigated. Two different ferrofluids, with different particles' mean dimensions, are studied. In both cases we obtain a thermophilic behavior of the Soret effect. The temperature dependence of the Soret coefficient is described assuming that the nanoparticles migrate along the ionic thermoelectric field created by the thermal gradient. A model based on the contributions from the thermoelectrophoresis and variation of the double-layer energy, without fitting parameters, is used to describe the experimental results of the colloid with the bigger particles. To do so, independent measurements of the ζ potential, mass diffusion coefficient, and Seebeck coefficient are performed. The agreement of the theory and the experimental results is rather good. In the case of the ferrofluid with smaller particles, it is not possible to get experimentally reliable values of the ζ potential and the model described is used to evaluate this parameter and its temperature dependence.

We focus here on the relationship between the physicochemical properties of electrostatically stabilized colloidal dispersions and the nanoparticles/solvent interface. Dispersions of maghemite (γ-Fe2O3) in polar solvents, here water and dimethyl sulfoxide (DMSO), are prepared with a new process that enables tuning easily this interface. Departing from the point of zero charge (PZC), the nanoparticles (NPs) are charged in a controlled way by adding acid or base. This pathway enables control of the surface state of the nanoparticles, i.e., the NP’s charge and the nature of the counterions, as well as the amount of free electrolyte in the dispersion. Stable dispersions are obtained because of electrostatic repulsion, in water and in DMSO, with electrolyte concentrations up to 20–40 mM. Small angle X-ray scattering (SAXS) and dynamic light scattering (DLS) techniques are here applied to concentrated dispersions in order to understand the nanostructure and quantify the interparticle interactions. Specific ionic effects are evident in both solvents. They depend on the nature of the solvent, with a remarkable effect on the Ludwig–Soret coefficient.

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In this work we focus on the surface charging properties of core shell ferrite nanoparticles dispersed in water, namely magnetic nanocolloids. This structural charge results from the Brönsted acid-base behavior of the particles surface sites and is achieved through hydrolysis reactions. It can be modeled by considering identical charged sites behaving as weak diprotic acids. Then, electrochemical techniques could be implemented to study the acid-base equilibrium between the particle surface and the colloid bulk solution. Simultaneous potentio-conductimetric titrations are therefore performed to determine the thermodynamical constants of the p H-dependent reactions and to obtain the p H variations of the surface charge density. The results reveal that the saturation value of the structural charge strongly depends on the nanoparticle mean size. For large particles, the surface tends to be fully ionized whereas for smaller particles the saturated structural charge decreases drastically. This surface charge reduction is attributed to the existence in smaller particles of metallic surface sites, which cannot be accessible to the proton charge. The existence of such dead sites would be related to hydroxo-bonded sites with very low acidity combined with a quantum size effect, which would affect the charging/discharging process at the surface of the semiconductor ferrite quantum dot.

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