Experimental and Theoretical Studies of the Colloidal Stability of Nanoparticles−A General Interpretation Based on Stability Maps

Abstract

The current work addresses the understanding of the stabilization of nanoparticles in suspension. Specifically, we study ZnO in ethanol for which the influence of particle size and reactant ratio as well as surface coverage on colloidal stability in dependence of the purification progress was investigated. The results revealed that the well-known ζ-potential determines not only the colloidal stability but also the surface coverage of acetate groups bound to the particle surface. The acetate groups act as molecular spacers between the nanoparticles and prevent agglomeration. Next to DLVO calculations based on the theory of Derjaguin, Landau, Verwey and Overbeek using a core-shell model we find that the stability is better understood in terms of dimensionless numbers which represent attractive forces as well as electrostatic repulsion, steric effects, transport properties, and particle concentration. Evaluating the colloidal stability in dependence of time by means of UV-vis absorption measurements a stability map for ZnO is derived. From this map it becomes clear that the dimensionless steric contribution to colloidal stability scales with a stability parameter including dimensionless repulsion and attraction as well as particle concentration and diffusivity of the particles according to a power law with an exponent of -0.5. Finally, we show that our approach is valid for other stabilizing molecules like cationic dendrons and is generally applicable for a wide range of other material systems within the limitations of vanishing van der Waals forces in refractive index matched situations, vanishing ζ-potential and systems without a stabilizing shell around the particle surface.

EPD-deposited ZnO thin films: a review

“…A wide number of authors have studied the surface reactivity of ZnO colloids synthesized following the method reported by Bahnemann et al [56][57][58][59][60]. Most of these works came to the conclusion that surface defects of the ZnO nanoparticles were passivated by acetate ions from the synthesis procedure, and that it is the presence of these acetate groups what enhances the stability of the ZnO nanoparticles in the alcoholic media, acting as spacers and quenching the attractive van der Waals forces.…”

In-Situ Versus Post-Synthetic Stabilization of Metal Oxide Nanoparticles

“…The DLVO theory has thus been applied in a number of cases to describe the behaviour of nanoparticulate systems (Schwarz & Safran, 2000;Tadmor & Klein, 2001;Reindl & Peukert, 2008;Marczak et al, 2009;Segets et al, 2011). However, some special aspects need to be taken into account when attempting to describe dispersions of nanoparticles below 20 nm in size by means of DLVO theory: On the one hand, the distance dependence of the attractive and repulsive forces is strongly dependent on particle size already in the classical DLVO concept (Verwey & Overbeek, 1948).…”

“…The application of the DLVO concept to such small particles generally leads to interaction potentials of only a few k . T, which results in only small changes between a stable and an unstable system, making the prediction of the stability of a system a challenging task (Marczak et al, 2009;Segets et al, 2011). Moreover, the minimum is also shifted in position towards smaller interparticle distances and thus, the description of the particles as solid, spherical systems with defined interface becomes inappropriate.…”

“…Moreover, it is clearly visible that the adsorbate layer formed by solvent adsorption also has a noticeable effect on the energy barrier despite the relatively large particle size of 100 nm, further increasing the stability of the system. Such core-shell models have been applied also to other systems in the meantime (Marczak et al, 2009;Segets et al, 2011). On the other hand, an extension of the DLVO theory has been proposed, leading to the socalled XDLVO concept, in the works by van Oss et al (van Oss et al, 1987(van Oss et al, , 2003.…”

“…In many cases, the precise mechanism of stabilisation is unclear, and it is generally still being disputed whether organic ligands bound to the surface in most cases bear charges that result in electrostatic stabilisation, or whether steric repulsion effects (Peyre et al, 1997;Shah et al, 2001;Arita et al, 2009) or in fact some combined effects , Segets et al, 2011 determine the stability of the system. Nonetheless, it has become clear that this concept of adsorption of small molecules, resulting in very thin organic layers (termed adlayers) around nanoparticles, has significant advantages over a conventional steric stabilisation with polymers: The packing of small particles is strongly influenced by the adlayer thickness , basically equivalent to the chain length of the surface-bound ligands, as shown in Figure 3.…”