Stabilization mechanisms of nonpolar metal colloids with thin organic shells

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Nonpolar nanoparticles are an important class of colloids. Syntheses developed in the last two decades yield particles of gold, silver, copper, and other metals and semiconductors. Their cores are covered with organic ligands that render them dispersible in alkanes, aromatics, and other nonpolar solvents. These particles are uncharged and not stabilized by Coulomb repulsion. Recent results from the applicant and other research groups demonstrate, however, that certain combinations of metal core, ligand, and solvent lead to unexpectedly great colloidal stabilities. The results indicate that the stability is due to the molecular structure of the shell. This is in stark contrast to conventional aqueous colloids that are described in DLVO theory.We propose here that nonpolar metal colloids with thin organic shells are stabilized by three mechanisms that have not yet been studied systematically: the entropic contributions of disordered ligand shells; the solvation of the organic shells by solvents that strongly interact on a molecular level; and the intercalation of solute molecules in the ligand shell that provide synergistic stability.This project investigates the proposed mechanisms using gold colloids with core diameters of 1-10 nm. Systematic series are chemically synthesized and covered with strongly bound linear, branched, or kinked (unsaturated) hydrocarbon chains. The resulting colloids are carefully purified and transferred to different solvents and solvent mixtures. The structure of the particles - in particular the density of the shell and its configuration - is assessed quantitatively.The colloidal stability or the dispersions is quantified as a function of concentration using a new technique where a droplet of colloid slowly evaporates during Small-Angle X-ray Scattering. Both particle concentration and agglomeration state are monitored. Thermal stability is measured using temperature-dependent X-ray and dynamic light scattering experiments. The molecular shell structure is analyzed using a combination of thermogravimetry, Raman spectrometry, NMR, optical scattering, and X-ray scattering.All experimental results are compared to detailed Molecular Dynamic simulations that are available in the group of Dr. Asaph Widmer-Cooper in Sydney, Australia. They were refined in previous collaborations between the two groups and will now be used to test the proposed mechanisms of nanoparticle stabilization.