Supercritical fluids pressure-density behavior

Figure 1. Pressure-density behavior for a pure supercritical fluid (CO2) in terms of reduced parameters. The area below the dotted line represents the two-phase gas-liquid equilibrium region.

Figure 1. Typical pressure-density behavior for a pure supercritical fluid in terms of reduced parameters.

Important solvent properties of SC-CO2 (e.g., dielectric constant, solubility parameter, viscosity, density) can be altered via manipulation of temperature and pressure. This unique property of a supercritical fluid could be exploited to control the behavior (e.g., kinetics and selectivity) of some chemical processes. 

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. 

For a pure supercritical fluid, the relationships between pressure, temperature and density are easily estimated (except very near the critical point) with reasonable precision from equations of state and conform quite closely to that given in Figure 1. The phase behavior of binary fluid systems is highly varied and much more complex than in single-component systems and has been well-described for selected binary systems (see, for example, reference 13 and references therein). A detailed discussion of the different types of binary fluid mixtures and the phase behavior of these systems can be found elsewhere (X2). Cubic ecjuations of state have been used successfully to describe the properties and phase behavior of multicomponent systems, particularly fot hydrocarbon mixtures (14.) The use of conventional ecjuations of state to describe properties of surfactant-supercritical fluid mixtures is not appropriate since they do not account for the formation of aggregates (the micellar pseudophase) or their solubilization in a supercritical fluid phase. A complete thermodynamic description of micelle and microemulsion formation in liquids remains a challenging problem, and no attempts have been made to extend these models to supercritical fluid phases. 

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. 

Sen, Y. L., and E. Kiran. 1990. A new experimental system to study the temperature and pressure dependence of viscosity, density, and phase behavior of pure fluids and solutions. J. Supercritical Fluids 3 91-99. 

The application of supercritical processes will always be intimately coupled with the high pressure phase behavior and the physical chemistry of the system. Supercritical fluids have received much attention because the solubility of a solute is readily influenced by small variations in pressure or temperature. In both cases, the density of the supercritical fluid also changes. The solubility of a given compound may be influenced in several ways. Both vapor pressure and intermolecular forces determine... 

The simplest type of phase behavior to understand is the solubility of a solid solute, such as naphthalene, in a supercritical fluid. When the solute is a crystalline solid, the solid phase may be assumed to be pure and only the supercritical phase is a mixture. Imagine solid naphthalene in a closed vessel under one atmosphere of carbon dioxide at 40°C. The reduced temperature and reduced density of CO2 are 1.03 and 3.7x10 respectively. At this pressure, the gas phase is ideal and the naphthalene solubility is determined by its vapor pressure. As the container volume is decreased isothermally, the solubility initially decreases when the gas phase is still nearly ideal. As the pressure is increased further, however, the gas phase density becomes increasingly nonideal and approaches the mixture critical density (near the critical density of CO2 because the gas phase is still mostly CO2). The reduced density of CO2 increases rapidly near the critical region as shown in Figure 2. The solvent power of CO2 is related to the density which leads to a rapid solubility increase. A brief description of intermolecular interactions is helpful in understanding this behavior. 

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