Nanofluids: A New Class of Materials Produced from Nanoparticle Assemblies

We present evidence of a novel nanostructured fluid, a nanofluid, composed of molecular clusters of a polar organic dye and surfactant. These are not nanoparticles dispersed in a solvent; there are no solvent molecules present. These materials, which are solids under ambient conditions, are non‐reactively precipitated from a compressed CO₂ solution, resulting in a liquid‐like material, which we call a nanofluid. The precipitated dye–surfactant clusters are 1–4 nm in size. This nanofluid exhibits intense luminescent signatures, which are significantly blue‐shifted with respect to the dye powder or a solution of it. The X‐ray diffraction pattern did not show any structure in the low‐angle regime. The fluorinated surfactant is highly soluble in compressed CO₂. The polar dye does not dissolve in compressed CO₂ but is solubilized by electrostatic interactions with the surfactant head groups. We believe that the ultrafast and controlled precipitation from compressed CO₂ preserves the electrostatic coupling and promotes a structured molecular cluster. Additionally, we demonstrate the formation of organic nanoparticles using this controlled precipitation process from compressed CO₂.