Temperature, pressure and density dependencies of the solubilities of low-volatility organic compounds in compressed gases Part 1. Solution energies from solubility data for disperse dyestuffs up to 20 MPa

A method is described to calculate molar solution energies from solubility data of a binary low-volatility organic compound + compressed carbon dioxide system. It has been applied to equilibrium measurements of the two anthraquinone derivatives 1,4-bis-(methylamino)-9,10-anthraquinone and 1,4-bis-(1-methylethylamino)-9,10-anthraquinone in CO2 within a p,T-range of 6.79–18.80 MPa and 299–345 K, respectively. Since solubility data do not exceed 0.135 g dm−3, the fluid phase can be regarded as infinitely dilute. The data have been analyzed in a way that from the two “derivative” plots logarithm of the solubility vs. the reciprocal temperature at constant pressure as well as constant density, two expressions for the molar solution energy, ΔsolEp and ΔsolEρ, have been derived and interpreted. We summarize our findings in four main statements: ΔsolEρ remains almost constant within a significant density range, here approximately between 400 and 800 kg m−3; ΔsolEp is drastically pressure (and density) dependent; both expressions are connected with each other via the isobaric expansion coefficient αp of the solvent; and finally for these two systems the ΔsolEp and ΔsolEρ values, respectively, are quite similar, whereas the solubilities differ by about a factor of ten.