Supercritical solubility measurements

Figure 1. Schematic drawing of NMR sample cell for supercritical solubility measurements.

Keywords Supercritical carbon dioxide Solubility measurement Homogeneous catalysis Multi-phase catalysis Hydroformylation... 

Supercritical fluid extraction is a potential technique for the purification of pharmaceutical products containing residual solvents. The solubilities of three inhibitors of inflammatory activity, Ketoprofen, Piroxicam, and Nimesulide, in supercritical CO2, measured using a dynamic saturation technique, were reported at pressures between 100 bar and 220 bar and at two temperatures 312.5 K and 331.5 K. The solubilities exhibit a clear dependence on the solvent density, and this has been used to provide a simple and precise correlation of the data (Macnaughton et al., 1996). 

In this paper solubility measurements of synthetic and natural dyestuffs are presented using VIS-spectroscopy. The investigations concentrate on two different methods. I. P-carotene was measured as a function of temperature and pressure in near- and supercritical C02 (289 to 309 K, 10 to 160 MPa) and CC1F3 (297 to 326 K, 12 to 180 MPa), respectively, using a static method. II. Additionally, the solubilities of l,4-bis-(n-alkylamino)-9,10-anthraquinones (with n-alkyl = butyl, octyl) were determined with a dynamic method in temperature and pressure ranges from 310 to 340 K and 8 to 20 MPa, respectively this method permits a continuous purification from better soluble impurities as well as the measurement of solubilities at the same time. For both anthraquinone dyestuffs intersection points of the solubility isotherms were found in the plot of concentration versus pressure. This behavior can be explained by a density effect. 

Reverse micelle and microemulsion solutions are mixtures of a surfactant, a nonpolar fluid and a polar solvent (typically water) which contain organized surfactant assemblies. The properties of a micelle phase in supercritical propane and ethane have been characterized by conductivity, density, and solubility measurements. The phase behavior of surfactant-supercritical fluid solutions is shown to be dependent on pressure, in contrast to liquid systems where pressure has little or no effect. Potential applications of this new class of solvents are discussed. 

In the studies described here, we examine in more detail the properties of these surfactant aggregates solubilized in supercritical ethane and propane. We present the results of solubility measurements of AOT in pure ethane and propane and of conductance and density measurements of supercritical fluid reverse micelle solutions. The effect of temperature and pressure on phase behavior of ternary mixtures consisting of AOT/water/supercritical ethane or propane are also examined. We report that the phase behavior of these systems is dependent on fluid pressure in contrast to liquid systems where similar changes in pressure have little or no effect. We have focused our attention on the reverse micelle region where mixtures containing 80 to 100% by weight alkane were examined. The new evidence supports and extends our initial findings related to reverse micelle structures in supercritical fluids. We report properties of these systems which may be important in the field of enhanced oil recovery. 

FIGURE 6 Variable-volume view cell for use in the static/synthetic method of solubility measurement. Components follow variable volume ceU (1), screw pump (2), magnetic stirrer (3), magnetic bar (4), upper sampling line (5), lower sampling line (6), sapphire window (7), thermostated bath (8), light som-ce (9), video camera (10), and monitor (11). (From Crampton, C., Charbit, G., and Neau, E., J. Supercrit. Fluids, 16(1), 11-20, 1999, reproduced with permission from Elsevier.)... 

However, it is helpful to have data on the solubility of compounds. For example, solubility measurements on copper, yttrium, and barium acetylacetonates and hex-afluoro acetylacetonates have been carried out in supercritical CO2 at 423 and 447 K at a pressure range between 120 and 220 bar. The thermal behavior of these solutions has been studied, as well as the influence of ethanol as the added solvent... 

The flow method ts the simplest and the most straightforward. In the flow method, the solvent fluid is supplied to a compressor by a pressure cylinder. At the desired pressure, the fluid passes into the thermostatted extractor cell that contains the solute present in appropriate matrix (e.g., multiple layer of glass wool). The fluid dissolves the solute in the extractor and, on expansion through a heated metering valve, precipitates solute into a series of collection vessels to be measured gravimetrically. The volume of the decompressed fluid is totaled by a wet or dry gas test meter. Static or equilibrium solubility measurement methods are used to eliminate the need to sample the supercritical fluid solution. A high-pressure flow cell is placed in the flow circuit to monitor the dissolution process by spectrophotometry. 

The first summation is over the surface points with positive potential and the second over the points with negative potential. Vg and Vg are the positive and negative surface averages in V(r), respectively, Since the terms in eq. 10, are squared, Otot is, in contrast to fl, particularly sensitive to the extremes in V(r). The two quantities have also been found to be quite different and even been found to vary in opposite directions for some groups of molecules [106]. atot i considered to be indicative of a molecule s electrostatic interaction tendencies. For example, has been used in conjunction with measures of molecular size, i.e. surface area or volume, for correlating solubilities in supercritical fluids [101, 105]. It has been suggested that Ojot in these relationships reflect solute-solute interactions, since the supercritical solubility mainly is determined by the solute vapor pressure [105]. 

Tutna, D. Schneider G.M. High Pressure Investigations on the Solubility of Disperse Dyes in Near- and Supercritical Fluids Measurements up to 100 Mpa by a static Method" The 4 International Symposium on Supercritical Fluids 1997. 

Among the cosolvents studied, methanol and acetone have received the greatest interest (36-38). Methanol may act as either a Lewis acid or a Lewis base while acetone is a weaker Lewis base and very slightly acidic ( ). The dipole moment of acetone is 2.88 Debeye compared to 1.7 Debeye for methanol. Based on these properties, Walsh, et al., (40), interpret the data of Van Alsten (37) and Schmitt (38) and present liquid phase IR measurements which show Lewis acid-base interactions in the systems methanol/acridine and acetone/benzoic acid. Supercritical solubility data of Dobbs, et al., ( ), exhibit trends which indicate the importance of acid-base interactions. Van Alsten and Schmitt present data which show that acid-base interactions are a secondary cosolvent effect superimposed on a primary effect determined by cosolvent concentration. 

DOM Domingo, C., Vega, A., Fanovich, M.A., Elvira, C., and Subia, P., Behavior of poly(methyl methacrylate)-based systems in supercritical CO2 and CO2 plus cosolvent solubility measurements and process assessment, J. Appl. Polym. Sci., 90, 3652,2003. 

In this chapter, the possibihty of using late transition metal catalysts to synthesize polyolefins in supercritical carbon dioxide was demonstrated [43]. The multicomponent phase behavior of polyolefin systems at supercritical conditions was studied experimentally by measuring cloud-point curves as well as by modeling polymer systems at supercritical conditions. The cloud-point measurements show that CO2 acts as a strong antisolvent for the ethylene-PEP system, which implies that the polymerization concerned will involve a precipitation reaction. The model calculations prove that SAFT is able to describe the ethylene-PEP-CO2 system accurately. Solubility measurements of the Brookhart catalyst reveal that the maximum catalyst solubility is rather low (in the order of 1x10 mol L ). However, a number of strategies are given to enhance this value. 

Figure 1 Schematic diagram of a fiberoptic system for solubility measurements in supercritical fluids. (From Ref. 19). Diagrams on the right (a, b) show the structures of the short pathlength (<1 mm) and the 1-cm fiberoptic cells, respectively. (From Ref. 17.)... 

The solubilities of adamantane and diamantane in supercritical (dense) methane, ethane, and carbon dioxide gases have been measured by a number of investigators [35-37] at a few temperatures with various pressures and solvent densities. These measurements are reported in Figs. 9-12. 

A cosolvent used as a miscible additive to CO2 changed the properties of the supercritical gas phase. The addition of a cosolvent resulted in increased viscosity and density of the gas mixture and enhanced extraction of the oil compounds into the C02-rich phase. Gas phase properties were measured in an equilibrium cell with a capillary viscometer and a high-pressure densitometer. Cosolvent miscibility with CO2, brine solubility, cosolvent volatility, and relative quantity of the cosolvent partitioning into the oil phase are factors that must be considered for the successful application of cosolvents. The results indicate that lower-molecular-weight additives, such as propane, are the most effective cosolvents to increase oil recovery [1472]. 

With traditional solvents, the solvent power of a fluid phase is often related to its polarity. Compressed C02 has a fairly low dielectric constant under all conditions (e = 1.2-1.6), but this measure has increasingly been shown to be insufficiently accurate to define solvent effects in many cases [13], Based on this value however, there is a widespread (yet incorrect ) belief that scC02 behaves just like hexane . The Hildebrand solubility parameter (5) of C02 has been determined as a function of pressure, as demonstrated in Figure 8.3. It has been found that the solvent properties of a supercritical fluid depend most importantly on its bulk density, which depends in turn on the pressure and temperature. In general higher density of the SCF corresponds to stronger solvation power, whereas lower density results in a weaker solvent. 

Whilst it is obviously valuable to measure the solubility of reagents in the SCF, it is important to be aware that the solubility in a multicomponent system can be very different from that in the fluid alone. It is also important to note that the addition of reagents and catalysts can have a profound effect on the critical parameters of the mixture. Indeed, at high concentrations of reactants, the mole fraction of C02 is necessarily lower and it may not be possible to achieve a supercritical phase at the temperature of interest. Increases in pressure (i.e. further additions of C02) could yield a single liquid phase (which would have a much lower compressibility than scC02). For example, the Diels-Alder reaction (see Chapter 7) between 2-methyl-1,3-butadiene and maleic anydride has been carried out a pressure of 74.5 bar and a temperature of 50 °C, assuming that this would be under supercritical conditions as it would if it were pure C02. However, the critical parameters calculated for this system are a pressure of 77.4bar and a temperature of 123.2 °C, far in excess of those used [41]. 

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