Observation of liquid solution volume expansion during particle precipitation in the supercritical CO2 antisolvent process

An optical measurement technique, which is based on the Foerster resonant energy transfer (FRET) between two different dye molecules, has been applied successfully to observe volume expansion of a liquid solution, when it is pressurized with CO2. Rhodamine-B and Rhodamine-700 were dissolved in ethanol to form the FRET active dye solution. In a first “prove of principle” experiment, the sensitivity of the FRET efficiency towards volume expansion was demonstrated by pressurizing the liquid dye solution in a cuvette with CO2. From the rise of the meniscus of the solution inside the cuvette as a function of CO2 pressure, the simultaneously acquired FRET spectra could be correlated with the volume expansion of the dye solution. In a second experiment, the dye solution was injected into CO2 at different supercritical antisolvent operation pressures. FRET spectra were recorded 3 mm downstream of the injector nozzle, always upstream of the breakup of the injected liquid solution. At pressures below the thermodynamic mixture critical pressure (7.9 MPa @ 313 K) of the system ethanol/CO2 no liquid phase volume expansion was observed. At pressures between the thermodynamic and the dynamic mixture critical pressure (8.5 MPa @ 313 K) of the same system, volume expansion could be evidenced before the breakup of the injected liquid solution.