Nanoparticles of RDX (cyclotrimethylenetrinitramine) generated by RESS (rapid expansion of supercritical solutions) using supercritical CO2 were characterized in situ by a pulsed laser light scattering imaging technique using a gated ICCD (intensified CCD) camera. The absolute sensitivity calibration was performed using Rayleigh light scattering from air as well as light scattering from standard polystyrene spheres. The size distribution functions of the particles formed in the RESS jet were determined using the calibrated sensitivity. The diameter of RDX particles formed at the pre-expansion pressure of 180 bar was 73 nm at the maximum of the size distribution function. Assuming that the particles near the nozzle consisted mainly of CO2 and the size distribution was log-normal, the diameter of the particles near the nozzle (7.5 mm from the nozzle) at the distribution maximum was 3.3 microm at the pre-expansion pressure of 180 bar. The number densities of the particles in the RESS jet were determined by counting individual particles in the light scattering images. Based on the measured particle size distributions and the number density of particles along the RESS jet, the mechanism of particle formation in RESS is discussed. The homogeneous nucleation mechanism is rejected as it fails to explain the large particle size experimentally observed. Instead, a modified "spray-drying" mechanism is suggested.
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