New Synthetic Mimics for Heteroatom Polycyclic Aromatic Hydrocarbon-Based Cosmic Dust

We report the preparation of new synthetic mimics for nitrogen-based polycyclic aromatic hydrocarbon (PANH) cosmic dust particles. From a small library of candidate molecules, we chose to study benzo[h]quinoline (mp = 51 °C). This simple PANH was processed by hot emulsification in the presence of a suitable water-soluble polymeric emulsifier: high shear homogenization at 55 °C converted the initial millimeter-sized drops into much finer molten oil droplets. The mean droplet diameter was readily controlled from 12 to 273 μm by adjusting the shear rate. Subsequent cooling to 20 °C led to crystallization and the formation of polydisperse benzo[h]quinoline microparticles. Interestingly, the nature of the polymeric emulsifier has a significant influence on the final microparticle morphology. Distinctly anisotropic microparticles were obtained using poly(vinyl alcohol), whereas the desired spherical morphology was produced when using Morwet D-425. A melting point diagram constructed for a series of binary mixtures of benzo[h]quinoline and phenanthrene indicated a eutectic composition of 65 mol % benzo[h]quinoline, with a corresponding melting point of just 37 °C. Accordingly, hot emulsification processing was again employed to produce a series of 65:35 benzo[h]quinoline/phenanthrene hybrid microparticles of 19-438 μm diameter. In this case, only the PVA emulsifier produced the desired spherical morphology. These hybrid microparticles were characterized by optical/fluorescence microscopy, laser diffraction, scanning electron microscopy, ¹H NMR spectroscopy, and Raman microscopy. Both types of microparticles were fired in turn at aluminum foil or aerogel targets at 0.9 - 1.0 km s^-1 using a light gas gun. Under such conditions, the microparticles simply rebounded from aluminum foil, with no signs of fragmentation or melting. In contrast, the 65:35 benzo[h]quinoline/phenanthrene microparticles underwent complete ablation during their aerogel capture. These new PANH synthetic mimics combined with such high-energy impact data should inform the design and calibration of cosmic dust detectors for the next generation of interplanetary unmanned spacecraft.