Determination of the solubilities of dyestuffs in near- and supercritical fluids by a static method up to 180 MPa

Solubilities of the synthetic dyestuff 1,4-bis-(octylamino)-9,10-anthraquinone in CO2, N2O, CClF3, and SF6 and additionally of the natural dyestuff all-trans β-carotene in CO2 and N2O were measured by a static analytical set-up consisting of a high-pressure autoclave coupled to a UV/VIS-spectrophotometer. In the case of 1,4-bis-(octylamino)-9,10-anthraquinone isotherms were recorded for temperatures from 282 to 341 K and over a pressure range from 6 to 110 MPa, whereas for all-trans β-carotene experiments both on solubility and solubility kinetics were carried out in the temperature and pressure ranges from 308 to 350 K and 8 to 180 MPa, respectively. For both substances it could be found that solubility normally increases with rising temperature at a constant solvent density and equally with rising solvent densities at a given temperature. N2O proved to be the best solvent, followed by CO2. At solvent densities corresponding to normal liquids, however, the curvature of the solubility isotherms (plot concentration versus density) changed its sign and as a consequence solubility maxima were observed for the systems 1,4-bis-(octylamino)-9,10-anthraquinone+SF6 and β-carotene+CO2. Additionally, β-carotene showed a temperature dependent all-trans to cis isomerization at supercritical conditions and even thermal decomposition with further increasing temperatures.