Solubility / retention relationships in supercritical-fluid chromatography (original) (raw)
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… of Chromatography A, 1990
A thermodynamic analysis is given of the relationship between the capacity factor in high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) and the solubility of the solute in the mobile phase. The analysis shows that in certain circumstances, where particular assumptions are valid, there is a simple inverse relationship between the two quantities. In these circumstances the proportionality constant has a fixed value for a particular column, temperature and solute, and is independent of the mobile phase. Furthermore, the logarithm of the constant is approximately linearly related to the inverse of the absolute temperature over a limited temperature range. Some experimental studies are then described which show that practical conditions exist where these relationships are valid. It is shown that the same equation-of-state, with the same parameters, can be used to predict quantitatively both supercritical fluid solubilities and the temperature dependence of retention in SFC. Experimental results illustrate the inverse relationship between solubility and retention where the same column was used in both SFC and HPLC. Results are also given for the variation of the proportionality constant with temperature. The various ways .in which these relationships could be used to measure solubilities are discussed and examples of experimental measurements are given for some methods.
The Journal of Physical Chemistry, 1991
A variety of types of thermodynamic properties have been determined at infinite dilution by supercritical fluid chromatography. A key challenge is to identify clearly the retention mechanism. An experimental technique is presented for the measurement of retention due to absorption into a bulk c 1 8 liquid (stationary) phase, independently of the adsorption on the support. The important effect of the swelling of the liquid phase by the fluid phase is included. Distribution coefficients are presented for naphthalene and phenanthrene between C02 and the C,* liquid phase, and used to determine Henry's constants in the liquid phase and solute partial molar volumes and enthalpies in the fluid phase. In the highly compressible region of C02 at 35 O C , solute partial molar enthalpies have been found to reach negative values of hundreds of kJ/mol, indicating strongly exothermic solute-solvent clustering.
Industrial & Engineering Chemistry Research, 1998
The use of supercritical fluid chromatography measurements to obtain solute partial molar volumes at infinite dilution is investigated by a combination of theory, modeling, and thermodynamic connections to solubility. Though it is common to ignore contributions from solute self-interactions, theory and modeling show that these can be significant in the critical region. In particular, expressions for the first-order solute composition dependence in terms of convergent infinite-dilution quantities are used to estimate the errors introduced by ignoring solute self-interactions. These are considered in light of recent empirical work on solubilities and partial molar volumes that can be connected via thermodynamic relations. Finally, though various assumptions have been made about the solute partial molar volume in the stationary phase, none of these seem consist with solubility measurements which suggest that the stationary phase contribution should be both temperature-and density (pressure)-dependent.
Journal of Chromatography A, 2012
The retention factors of octylbenzene, octadecene, anthracene, and pyrene eluted from columns packed with neat silica and C 18 -bonded silica by pure carbon dioxide near its critical region increase with increasing temperature along low-density isopycnic lines. This behavior is markedly different from the one observed in nearby regions of the pressure-temperature diagram of CO 2 , where the retention factors of these compounds decrease with increasing temperature along high-density isopycnic lines. Several possible reasons that could explain this reversal in the behavior of the retention factors are discussed. The most plausible is related to the formation of multilayer adsorption of CO 2 on the stationary phases in a density range slightly lower than the critical density. (G. Guiochon). under low pressures and at high temperatures, on the other hand, the solvation power of CO 2 drastically reduces and the retention mechanism closely resembles the one observed in GC, where the solute volatility and the solute-stationary phase interactions are the main controlling factors. In the intermediate regions, competing retention mechanisms, involving both solute-stationary phase interactions and solute-mobile phase interactions, tend to take place, to a degree depending on the compounds involved. A survey of the literature found several reports in which the dependence of retention factors on the temperature, the pressure, or the density were investigated in this intermediate region. A brief review of these studies is provided in the following subsections, which will be useful in the analysis presented in the subsequent sections.
Solid Solubilities in Supercritical Phases : New Apparatus, Measurements and Modeling
HAL (Le Centre pour la Communication Scientifique Directe), 2000
A new apparatus for the measurement of solid solubilities in supercritical fluids is described in detail. It is based on an analytical (the composition of the phase is obtained by analyses) open flow method (the solvent flows continuously through the solid solute). Solubilities of naphthalene in supercritical CO 2 in the pressure range 8-30 MPa are reported along the isotherm 308.15 K. The solubility data are correlated using the Chrastil model.
Brazilian Journal of Chemical Engineering
Due to the importance of supercritical fluid technology (SFT) in different industries, it has been the subject of intense research in recent decades. Solubility is a key concept in SFT. In fact, obtaining knowledge about the theoretical concepts of solubility and related experimental measurement methods can be useful in developing and improving the quality of research in this field. This study reviews the fundamental knowledge of solubility in supercritical fluids and investigates the significant topics in this field, including high-pressure phase behavior, experimental measurement methods, modeling, and molecular simulation of solubility.
Chromatography theory: Application to supercritical fluid extraction
AIChE Journal, 1990
The adsorption equilibrium constants for naphthalene on alumina have been measured in the presence of subcritical and supercritical carbon dioxide by the dynamic tracer response technique. The equilibrium constants decrease with increases in density, and the isochoric temperature dependency is very small at supercritical conditions. Through chromatography theory, an expression has been developed for the density dependency of the capacity factor, a quantity related to the adsorption equilibrium constant by the porosity and density of the particle and the porosity of the bed, in terms of the partial molar volume of the solute at infinite dilution in the fluid phase. The partial molar volumes extracted from the data agree well with the published data. Investigation of the dynamics of the bed shows that a model, including axial dispersion and effective diffusion into the pores, successfully represents the data. The effective diffusivities and the axial dispersion coefficients were also extracted from the second central moments of the response curves at each condition.