Concentrated Ag Nanoparticles in Dodecane as Phase Change Materials for Thermal Energy Storage

Silver colloids with concentrations of up to 0.1 M Ag, mass fraction = 1.3%, were prepared in dodecane and eicosane via the thermal reduction of silver neodecanoate using oleoyl sarcosine as a particle stabilizer. The dodecane colloids contained mainly spherical crystallites of fcc Ag with an average diameter of 5 nm, which despite their high concentration have remained stable against precipitation for over two years at room temperature. Images from CitoViva and transmission electron microscopies showed that the small Ag crystallites coexisted with larger particle aggregates. Colloids made in dodecane exhibited thermal conductivities that increased slightly with raising both the Ag concentration and the temperature up to 13.8% at [Ag] = 80 mM, mass fraction = 1%. These modest thermal conductivity enhancements are consistent with predictions based on classical formulations and imply that the Ag aggregates played no role at expediting heat transport. Kinetic data from the particle formation process indicated that the neodecanoate ligand served as an electron donor during the thermal reduction of Ag(I). Optical signals in the visible range generated during the initial stages of particle formation evolved into stronger absorptions upon exposure to ambient light. The optical signals persisted for days under air and seemed to originate from photochemically generated Ag clusters.