Microemulsions are effective media for solution-based synthesis of metallic nanoparticles where surfactants and other ionic species influence the directed assembly of the nanomaterials with specific sizes, geometries, and compositions. This study demonstrates the effects of chloride ion on the synthesis of copper nanoparticles within the sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelle system utilizing both liquid isooctane and compressed propane as the bulk solvent. Copper nanoparticle synthesis can be achieved in the presence of HCl in the micelle core, taking advantage of the buffering action of the AOT surfactant. The concentration of chloride ions influence the particle growth rate and dispersion in liquid isooctane. The presence of chloride ions during particle synthesis in compressed propane has a significant effect on the geometry and structure of the copper nanomaterials produced. Chloride ion addition to the compressed propane/CU(AOT)(2)-AOT/water reverse micelle system at 20 degrees C and 310 bar results in the formation of diamond-shaped copper nanoparticle assemblies. The copper nanoparticle assemblies exhibit unique structure and retain this structure through repeated solvent processing steps, allowing separation and recovery of the assembled diamond-shaped copper nanoparticle structures.