Classical, non-classical critical divergences and partial molar properties from adsorption measurements in near-critical mixtures

This paper presents results related to the development of methods for measuring and analyzing dilute solute thermodynamic properties in near-critical solvent mixtures. An analysis of limiting conditions at the system's critical point provides results that were used to analyze capacity factor data for a variety of organic solutes in near-critical mixtures of carbon dioxide and ethane. Both classical and non-classical perspectives were employed in the analysis, with particular focus on solvent mixtures with compositions close to that of the critical azeotrope in the carbon dioxide-ethane system. These solvent mixtures have unique critical divergence properties and the theoretical results obtained for this system are qualitatively borne out by the data taken. Partial molar volumes and enthalpies for several dilute solutes at a range of thermodynamic conditions have been presented in mixtures of 0.26, 0.30 and 0.70 mole fraction ethane in carbon dioxide. The isothermal data were taken at three temperatures (308, 328 and 343 K) and pressures ranged from 50 to 107 bar. The isobaric data were taken at 65.3 and 75.5 bar over a temperature range of 295.5-441 K. The solutes used were benzene, toluene, o-xylene, naphthalene and 2,6-dimethylnaphthalene. Because at the present time such data are not available in the published literature these data were often compared with values obtained by model calculations. Agreement between the two approaches has been fairly good and suggests that the approach taken here should contribute towards developing methods for reliable and rapid thermodynamic property estimation in mixtures.