A semicontinuous flow apparatus for measuring the solubility of opaque solids in supercritical solutions

A semi-continuous flow apparatus with microsampling has been developed for measuring the solubilities of solids in supercritical fluids. The apparatus is particularly appropriate for those solids for which only mg-sized quantities are available (e.g. pharmaceuticals) and which form opaque solutions when dissolved in supercritical solvents (e.g. porphyrins and dyes). The method of operation is to first obtain equilibrium in a variable-volume view cell, then use an electronic syringe pump in the constant pressure mode to deliver the contents of the cell through a sample loop at the equilibrium temperature and pressure, and finally to determine solid solubilities by analyzing the contents of the sample loop. Both an error analysis and measurements for the system carbon dioxide (CO2)+phenanthrene at 35 and 55 °C indicate that the method is accurate to better than ±5%. Subsequently, the apparatus was used to determine the solubilities of 5,10,15,20-tetrakis(3,5-bis(trifluoromethyl)phenyl)porphyrin (TBTPP), a novel fluorinated porphyrin synthesized in our laboratories, in supercritical CO2. Measurements were performed at three different temperatures (40, 70, and 100 °C) and over a pressure range of 103.4–324.1 bar. TBTPP exhibits a wide range of solubilities in CO2 (i.e. from 0.0002 to 2 wt.%) and thus is an appropriate model solute for investigating the effect of concentration on particle size and morphology during rapid expansion processing.