Reactivity of nanocolloidal particles gamma-Fe2O3 at the charged interfaces. Part 1. The approach of particles to an electrode.

We are interested here in the reactivity of magnetic nanoparticles at the electrode-electrolyte interface with the aim of the electrochemical synthesis of magnetic and conductive liquids (electronic conduction). The reactivity of charged colloidal particles occurs through a two steps process, the first being the approach toward the electrode with a possible adsorption phenomenon and the second step, the electron transfer. In this first paper we focus on the approach and the deposition of well-defined gamma-Fe(2)O(3) nanoparticles onto conductive substrates like mercury and gold under different conditions in order to vary the interactions particle/substrate especially the electrostatic interactions. The approach of the particles near the electrodes is estimated from the electrochemical currents related to the transformation of the particles. This electrochemical method is validated by coupling several techniques on gold electrodes: direct imaging by atomic force microscopy and study of kinetics by reflectometry. The results show that the electrochemical currents are always associated to adsorption of the particles, so that the electrochemical method can be used to estimate the adsorption of the particles, thus to follow the kinetics. The influence of the electrostatics on the occurrence of adsorption highly depends on the nature of the substrate and on the nature of the colloidal suspension. (ions, pH, ionic strength): whereas electrostatics governs the deposits in some cases, it is totally dominated by other interactions in other cases. Therefore, it seems difficult to predict a priori the existence of adsorption. However, when a deposit occurs, the kinetics and the maximal coverage of the substrates are controlled by the electrostatic interactions between the particles already adsorbed and those, close to the interface, in the bulk of the solution.

Reactivity of nanocolloidal particles gamma-Fe2O3 at charged interfaces. Part 2. Electrochemical conversion. Role of the electrode material.

In this paper we are interested in the reactivity of magnetic nanoparticles at the electrode involved in the electrochemical synthesis of magnetic and conductive liquids. The reactivity of charged colloidal particles occurs in two steps, first the approach toward the electrode with a possible adsorption phenomenon and secondly the electron transfer. In this paper we focus on the electrochemical behaviour of well-defined gamma-Fe(2)O(3) nanoparticles at a gold and at a mercury electrode. Particles can be electrochemically reduced at the two electrodes and can be dispersed into mercury at a highly negative potential. Here, we probe in particular the properties of nanoreactor of the particles, that is to say, the possible conservation of their size after they have undergone the electrochemical process. By correlating complementary techniques (here atomic force microscopy (AFM) observations, Raman spectroscopy and cyclic voltammetry on gold electrode) and by studying the magnetic properties of the material obtained after reduction of the particles on a mercury electrode, we are able to probe both the chemical nature and the physical state of the particles once transformed. Experimental results show that under specific conditions, the particles are individually converted into iron, which justifies their use for preparing a liquid with both magnetic properties and properties of electron conduction.

Interpretation of conductivity results from 5 to 45 degrees C on three micellar systems below and above the CMC.

Electrical conductivity has been used at different temperatures to study three micellar systems: tetradecyltrimethylammonium chloride (TTACl), dodecyltrimethylammonium chloride (DTACl), and decyltrimethylammonium chloride (DeTACl). A phenomenon of premicellization is observed for DeTACl and DTACl below the critical micellar concentration (CMC). Association constants are introduced in the MSA-transport theory to correctly reproduce experimental conductivity and also calculate the effective charge of the micelles and their degree of dissociation. Various mechanisms are considered to explain premicellization. The formation of a neutral pair followed by an association involving two monomers and a counterion appears to be the most probable first step in the premicellization process.

Determining the radius and the apparent charge of a micelle from electrical conductivity measurements by using a transport theory: explicit equations for practical use.

We propose here a procedure which combines experiments and simple analytical formulas that allows us to determine good estimations of the size and charge of ionic micelles above the critical micellar concentration (cmc). First, the conductivity of n-tetradecyltrimethylammonium bromide and chloride (TTABr and TTACl, respectively) aqueous solutions was measured at 25 degrees C, before and above their cmc. Then, an analytical expression for the concentration dependence of the conductance of an ionic mixture with three species (monomers, micelles, and counterions) was developed and applied to the analysis of the experiments. The theoretical calculations use the mean spherical approximation (MSA) to describe equilibrium properties. Here, we propose new expressions for the electrical conductivity, adapted to the case of electrolytes that are dissymmetric in size, and applicable up to a total surfactant concentration of 0.1 mol L(-1). Moreover, we show that they are good approximations of the corresponding numerical results obtained from Brownian dynamics simulations. Since the analytical formulas given in the present paper involve a small number of unknown parameters, they allow one to derive the size and charge of macroions in solution from conductivity measurements.

Study of individual Na-montmorillonite particles size, morphology, and apparent charge.

Size, morphology, and apparent charge of individual Na-montmorillonite particles of natural MX-80 sodium montmorillonite were investigated in the present study by the use of three coupling methods. In the first part of this work, natural and synthetic montmorillonite clays were studied with atomic force microscopy (AFM) and photo-correlation spectroscopy (PCS). Both techniques exhibit the presence of two clay populations with a high dispersion of the length distribution. Microscopic analysis of the system revealed that clay particles could be reasonably approximated at low concentrations to ellipsoidal tactoids about 1.2 nm high. Average dimensions of the first population were typically 320-400 nm long/250 nm wide and 200-250 nm long/120 nm wide for natural and synthetic clays, respectively. The second population exhibits smaller sizes: 65 and 50 nm long and 35 and 25 nm wide for natural and synthetic clays, respectively. The statistics obtained for natural clay were then verified by PCS experiments on sodium montmorillonite suspensions. Both techniques reveal an important length dispersion. However, the relative proportions of the two kinds of particles could not be established properly because of both lack of statistics and limitations of the employed techniques. In the following part, conductivity measurements were performed on dilute montmorillonite clay suspensions. Raw data were then interpreted with the sizes and morphological information gained in the first part of the present work. The apparent charge of the clay sheets was found to be 8% of the structural charge.

Analytical theories of transport in concentrated electrolyte solutions from the MSA.

Ion transport coefficients in electrolyte solutions (e.g., diffusion coefficients or electric conductivity) have been a subject of extensive studies for a long time. Whereas in the pioneering works of Debye, Hückel, and Onsager the ions were entirely characterized by their charge, recent theories allow specific effects of the ions (such as the ion size dependence or the pair association) to be obtained, both from simulation and from analytical theories. Such an approach, based on a combination of dynamic theories (Smoluchowski equation and mode-coupling theory) and of the mean spherical approximation (MSA) for the equilibrium pair correlation, is presented here. The various predicted equilibrium (osmotic pressure and activity coefficients) and transport coefficients (mutual diffusion, electric conductivity, self-diffusion, and transport numbers) are in good agreement with the experimental values up to high concentrations (1-2 mol L(-1)). Simple analytical expressions are obtained, and for practical use, the formula are given explicitly. We discuss the validity of such an approach which is nothing but a coarse-graining procedure.

An analytical model for probing ion dynamics in clays with broadband dielectric spectroscopy.

A simple two-state model is proposed to explicitly derive the ionic contribution to the frequency-dependent dielectric permittivity of clay. This model is based on a separation of time scales and accounts for two possible solvation modes (inner/outer-sphere complexes) for ions in the interlayer spacing and a possible chemical exchange between both forms. The influence on the permittivity of thermodynamic (distribution constant K(d)) and dynamic (diffusion coefficient, chemical relaxation rate) parameters is discussed. In turn, this model is used to analyze experimental data obtained with Na-montmorillonite for two relative humidities. The values of the parameters extracted from these measurements, and their variation with water content, show that the proposed model is at least reasonable.

Transport coefficients of aqueous dodecyltrimethylammonium bromide solutions: comparison between experiments, analytical calculations and numerical simulations.

We study dynamical properties of ionic species in aqueous solutions of dodecyltrimethylammonium bromide, for several concentrations below and above the critical micellar concentration (cmc). New experimental determinations of the electrical conductivity are given which are compared to results obtained from an analytical transport theory; transport coefficients of ions in these solutions above the cmc are also computed from Brownian dynamics simulations. Analytical calculations as well as the simulation treat the solution within the framework of the continuous solvent model. Above the cmc, three ionic species are considered: the monomer surfactant, the micelle and the counterion. The analytical transport theory describes the structural properties of the electrolyte solution within the mean spherical approximation and assumes that the dominant forces which determine the deviations of transport processes from the ideal behavior (i.e., without any interactions between ions) are hydrodynamic interactions and electrostatic relaxation forces. In the simulations, both direct interactions and hydrodynamic interactions between solutes are taken into account. The interaction potential is modeled by pairwise repulsive 1/r(12) interactions and Coulomb interactions. The input parameters of the simulation (radii and self-diffusion coefficients of ions at infinite dilution) are partially obtained from the analytical transport theory which fits the experimental determinations of the electrical conductivity. Both the electrical conductivity of the solution and the self-diffusion coefficients of each species computed from Brownian dynamics are compared to available experimental data. In every case, the influence of hydrodynamic interactions (HIs) on the transport coefficients is investigated. It is shown that HIs are crucial to obtain agreement with experiments. In particular, the self-diffusion coefficient of the micelle, which is the largest and most charged species in the present system, is enhanced when HIs are included whereas the diffusion coefficients of the monomer and the counterion are roughly not influenced by HIs.