On-Line and in situ optical detection of particles of organic molecules formed by rapid expansion of supercritical solutions (RESS) of CO2

The formation of particles by the rapid expansion of supercritical solutions (RESS) of four organic substances (n-undecane, naphthalene, trans-stilbene and benzoic acid) dissolved in supercritical CO2 has been investigated employing two complementary on-line in situ detection techniques. For the first time, a laser-based shadowgraphy (LABS) setup was applied, which allows the recording of the diameter, morphology, size distribution and concentration of particles down to sizes of roughly 8 mum. In addition, laser-based three-wavelength extinction measurements (3-WEM) have been employed to determine the average particle diameter, the width of the particle size distribution and the concentration. The average particle sizes determined from 3-WEM for n-undecane, naphthalene, trans-stilbene and benzoic acid are 2660, 1260, 550 and 430 nm, respectively, and the standard deviation of the (assumed) logarithmic normal distributions 0.37, 0.42, 0.78 and 0.60. The particle size distributions at large diameters from LABS compare favourably with the tails of the 3-WEM distributions. An analysis of the morphology of trans-stilbene particles and n-undecane droplets reveals an, on average, slightly elliptical shape.