Clusters and inverse emulsions from nanoparticle surfactants in organic solvents

A method is presented for the synthesis of self-assembling nanoparticle surfactants in nonpolar organic solvents. The method relies on the control of long-range steric repulsion imparted by grafted polystyrene and short-range attraction from short-chain thiol molecules with an alcohol or carboxylic functionality. Similar to water-based nanoparticle surfactants, these oil-dispersed materials are found to cluster in dispersion and also to stabilize oil-water interfaces to form water-in-oil emulsions. The clustering process is characterized with dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), UV-vis spectroscopy, and transmission electron microscopy (TEM). Thermogravimetric analysis (TGA) is used to quantify the surface concentration of grafted polymer, which is found to be a parameter of critical importance for the formation of stable clusters. The clustering kinetics and dispersion stability are both affected by the polymer molecular weight, surface concentration, and chemical structure of the thiol molecules that induce particle attraction. Nanometer-sized water-in-oil emulsions are formed by sonication in the presence of nanoparticle surfactants. A large broadening of the optical absorption spectrum in the NIR region is observed because of changes in the collective surface plasmon resonance of the gold particle shell.