ABSTRACT
The dissolution behaviour of an oxidic nanoparticle in a small volume of solvent was investigated. The results of thermodynamic and kinetic calculations are presented. Variations in nanoparticle size and solvent volume are considered. Two kinds of final states of the system can be formed. One state is a homogeneous monomolecular solution of the dissolved species; the other one is a system in stable equilibrium between nanoparticle and solvent. What kind of state is formed depends on the size of the nanoparticle and the amount of solvent. The concentration of the dissolved species is much higher than the saturation concentration of the bulk material in many cases. An unusual dissolution behaviour of an ensemble of nanoparticles, called kinetic size effect, follows from the calculations. A very high concentration is found of the dissolved material at the beginning of the dissolution process. The concentration decreases at longer dissolution times. As an example, the experimental results of the dissolution kinetics of an ensemble of nanoparticles of commercial titanium dioxide in water are presented. Good agreement between experimental results and theoretical calculations is found. From these data it can be deduced that a nanoparticle with a radius of 14.15 nm dissolves in a volume of water of 18 microm(3) until its radius becomes 13.77 nm and a stable system nanoparticle-dissolved substance-solvent is formed (supersaturation 24), whereas a nanoparticle of radius 11.4 nm is completely dissolved after 73 h (supersaturation 165). The effect is established for different oxides.
Subject(s)
Microchemistry , Models, Chemical , Nanoparticles/chemistry , Solvents/chemistry , Titanium/chemistry , Water/chemistry , Kinetics , Particle Size , Reproducibility of Results , Solubility , ThermodynamicsABSTRACT
The dissolution of a technical, nanodispersed gamma-alumina in water was studied at 25 degrees C in the pH range 3.0 < or = pH < or = 11.0. The obtained kinetic dissolution curves showed a distinct pH dependency, whereas only for pH > or = 4.5 the typical behavior of nanodispersed materials could be observed. X-ray powder diffraction analysis and nitrogen adsorption, as well as IR and UV-Raman spectroscopy, were used to characterize the solid material collected during and at the end of each dissolution experiment. As a result the formation of a new aluminum phase-bayerite-could be proven. The analysis of the equilibrium concentration enabled us to determine the solubility constant of the corresponding phase assuming a pH-dependent species distribution. The rate constants of the dissolution process were evaluated using the model of Gibbs free energy of cluster formation, which considers the size effect, among other things. As a result, we could show that the observed maxima in the concentration profiles are due to a size effect of the starting material having a primary particle radius of 10.1 nm.
ABSTRACT
Different types of industrially produced titanium dioxide nanoparticles and a precipitated titanium dioxide have been dissolved in aqueous NaCl solutions at temperatures of 25 and 37 degrees C. The titanium concentration in solution with regard to dependence on time has been determined up to 3000 h after starting the dissolution experiment. The effect of particle size, pH value, temperature, background electrolyte concentration, and mass concentration of titanium dioxide exposed to the liquid phase has been studied. The nanoparticles have been characterized by N2 physisorption measurements and XRD. The total dissolved titanium in solution has been determined by adsorptive stripping voltammetry (AdSV) and inductively coupled plasma mass spectrometry (ICP-MS). A new kinetic size effect has been observed. It turns out that this effect can be explained by applying an already existing phenomenological thermodynamic and kinetic model. The model describes all possible phenomena in a colloidal dispersion, nucleation, growth of particles, Ostwald ripening, and dissolution of particles using a uniform concept.
