I got a sample of 18nm AuNPs which did not aggregate upon addition of a Europium metal complex. I was wondering if smaller gold nanoparticles are in general more stable and why.
Aggregation depends on interparticle force, partilce size as well as medium in which they are dispersed.
Dear Fabian, the stability of colloidal nanoparticles upon addition of a chemical depends on several factors ranging from the surface functional groups and the surface charge to the type of the solvent, pH, ionic strength and the other concomitants existing in the dispersion. So, you should take all of these factors into account so you can find out what plays the major role in such systems. Based on my experience, surface charge and surface functional groups (type of capping agent) are usually the key determinants of gold nanoparticles. If you change the capping agent with a proper one, you will likely observe the aggregation.
The smaller, the stronger the tendency to agglomerate because of the absolute value of the surface energy is increasing with the increasing surface area along with the decrease of particle size.
For a more general approach on the topic of agglomeration versus dispersion I suggest: "Hiroaki Masuda, “Dry dispersion of fine particles in gaseous phase”, Advanced Powder Technology Vol. 20, p. 113-122, 2009." There is a graph that shows the equilibrium between the 2 opposing effects, influenced by agglomerative environmental effects and dispersing force.
Static particle properties rest on three pillars regarding agglomeration, microscopic single-particle characteristics, interparticle forces, and macroscopic packing characteristics. The interparticle forces which bind single particles into larger bulk of agglomerates are mainly Van der Waals forces, electrostatic forces of effective charge (attractive and surplus repelling surface charges) electrostatic image charges which are induced by an electric field, liquid bridge, and capillary forces, especially for humid environments with more than 65% water saturation, mechanical interlocking on surfaces by shape fit and mechanical friction forces between rough surfaces. At longer surface distances, electrostatic effects determine attraction and repulsion more than Van der Waals forces. For close contact, Van der Waals forces are dominant.
I suggest an investigation of your materials and environments characteristics, it is possible to deduce a theory of summarized attraction force and the likely behaviour of your material. Surface features are important, it is possible that the same chemistry acts entirely different depending on the method of how it was produced, analytical inspection might help.