Influence of the volume fraction on the electrokinetic properties of maghemite nanoparticles in suspension

Special issue in Honour of Pierre TURQInternational audienceWe used several complementary experimental and theoretical tools to characterise the charge properties of well-definedmaghemite nanoparticles in solution as a function of the volume fraction. The radius of the nanoparticles is equal to 6 nm.The structural charge was measured from chemical titration and was found high enough to expect some counterions tobe electrostatically attracted to the surface, decreasing the apparent charge of the nanoparticle. Direct-current conductivitymeasurements were interpreted by an analytical transport theory to deduce the value of this apparent charge, denoted here by‘dynamic effective charge’. This dynamic effective charge is found to decrease strongly with the volume fraction. In contrast,the ‘static’ effective charge, defined thanks to the Bjerrum criterion and computed from Monte Carlo simulations turns outto be almost independent of the volume fraction. In the range of Debye screening length and volume fraction investigatedhere, double layers around nanoparticles actually interact with each other. This strong interaction between nanocolloidalmaghemite particles is probably responsible for the experimental dependence of the electrokinetic properties with the volumefraction

Onsager's reciprocal relations for electroacoustic and sedimentation: Application to (concentrated) colloidal suspensions

In this article, the relations for electroacoustic phenomena, such as sedimentation potential, sedimentation intensity, colloid vibration potential, colloid vibration intensity/current, or electric sonic amplitude, are given, on the basis of irreversible thermodynamics. This formalism allows in particular to discuss the different expressions for concentrated suspensions found by various authors, which are of great practical interest. It was found that some existing expressions have to be corrected. Relations between the electrophoretic mobilities assessed by the different experiments are derived.Hydraulic EngineeringCivil Engineering and Geoscience

Onsager’s reciprocal relations in electrolyte solutions: II. Effect of ionic interactions on electroacoustic

In electrolyte solutions, an electric potential difference, called the Ionic Vibration Potential (IVP), related to the ionic vibration intensity, is generated by the application of an acoustic wave. Several theories based on a mechanical framework have been proposed over the years to predict the IVP for high ionic strengths, in the case where interactions between ions have to be accounted for. In this paper, it is demonstrated that most of these theories are not consistent with Onsager’s reciprocal relations. A new expression for the IVP will be presented that does fulfill the Onsager’s reciprocal relations. We obtained this expression by deriving general expressions of the corrective forces describing non-ideal effects in electrolyte solutions.Hydraulic EngineeringCivil Engineering and Geoscience

Onsager’s reciprocal relations in electrolyte solutions: I. Sedimentation and electroacoustics

In the framework of irreversible thermodynamics, we show that the sedimentation current in electrolyte solutions is mathematically equivalent to the low frequency limit of the ionic vibration current, appearing in the presence of an acoustic wave. This non-trivial result is obtained thanks to a careful choice of the reference frame used to express the mass fluxes in the context of electroacoustics. Coupled transport phenomena in electrolyte solutions can also be investigated in a mechanical framework, with a set of Newtonian equations for the dynamics of charged solutes. Both in the context of sedimentation and of electroacoustics, we show that the results obtained in the mechanical framework, in the ideal case (i.e., without interactions between ions), do satisfy the Onsager’s reciprocal relations. We also derive the general relation between corrective forces accounting for ionic interactions which must be fulfilled so that the Onsager’s reciprocal relations are verified. Finally, we show that no additional diffusion term needs to be taken into account in the flux of solutes (far from the walls), even if local concentration gradients exist, contrarily to what was done previously in the literature.Hydraulic EngineeringCivil Engineering and Geoscience