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

Abstract

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, Δ_solE_p and Δ_solE_ρ, have been derived and interpreted. We summarize our findings in four main statements: Δ_solE_ρ remains almost constant within a significant density range, here approximately between 400 and 800 kg m⁻³; Δ_solE_p is drastically pressure (and density) dependent; both expressions are connected with each other via the isobaric expansion coefficient α_p of the solvent; and finally for these two systems the Δ_solE_p and Δ_solE_ρ values, respectively, are quite similar, whereas the solubilities differ by about a factor of ten.