Keywords: magnetite, nanoparticles, size control, thermodynamic stability, interfacial tension, water-oxide interface, nucleation, free enthalpy, point-of-zero-charge, dispersion, adsorption, metal oxide
On the thermodynamic stability of metal oxide nanoparticles in aqueous solutions
Divided and ultra-divided systems such as colloidal and nanoparticle dispersions are generally unstable with regard to the size and number of their constituents because the solid-solution interfacial tension, acting as a driving force, leads to a reduction of the surface area to minimise the dispersion free enthalpy. Such phenomenon known as surface energy minimisation induces an increase in average particle size as a result of the decrease of the surface area at constant volume. For such a reason such dispersions are usually considered thermodynamically unstable. However, they can be thermodynamically stabilised if, by adsorption, the interfacial tension of the system becomes very low. This phenomenon, well known for microemulsions, is for the first time quantitatively modelled and demonstrated for transition metal oxide nanoparticles. When the pH of precipitation is sufficiently far from the point of zero charge and the ionic strength sufficiently high, the ripening of nanoparticles is avoided and their size can be monitored over one order of magnitude by tailoring solution pH and ionic strength. A model based on Gibbs adsorption equation leads to an analytical expression of the water-oxide interfacial tension as a function of the pH and the ionic strength of the dispersion/precipitation medium. The stability condition, defined by a 'zero' interfacial tension, corresponds to the chemical and electrostatic saturation of the water-oxide interface. In such a condition, the density of charged surface groups reaches its maximum, the interfacial tension its minimum and further adsorption forces the surface area to expand and consequently, the size of nanoparticles to decrease. An excellent agreement was found between the model prediction and the experimental results obtained from the aqueous precipitation of magnetite (Fe3O4) nanoparticles in basic medium. A general control of the metal oxide nanoparticle size when precipitated far from their point-of-zero-charge is thus expected.