Physical supercritical state

Adding an organic solvent (such as methanol or acetone) to the supercritical fluid can modify its solvating properties. Since the polarity of C02 in its supercritical state (at 100 atm and 35 °C) is comparable to that of hexane, it can be altered by introducing a modifier. Nonetheless, isolating analyte from the matrix requires knowledge about the solubility and the transfer rate of solute in the solvent as well as chemical and physical interactions between matrix and solvent (Fig. 20.6). 

The critical state is achieved when a substance is taken above its critical temperature and pressure (Tc, Pc). Above this point on a phase diagram, the gas and liquid phases become indistinguishable. The physical properties of the supercritical state (e.g., density, viscosity, solubility parameter, etc.) are intermediate between those of a gas and a liquid, and vary considerably as a function of temperature and pressure. 

The properties and physical chemistry of liquid and supercritical carbon dioxide have been extensively reviewed (Kiran and Brennecke, 1992), as have many fundamentals and applications for separation, chromatography, and extraction (McHugh and Krukonis, 1994). The phase diagram for pure C02 is illustrated in Figure 1.1. Due to its relatively low critical point, C02 is frequently used in the supercritical state. Other common supercritical fluids require higher temperatures and pressures, such as water with Tc = 374.2 °C and Pc = 220.5 bar, while propane (Tc = 96.7 °C and Pc = 42.5 bar) and ethane (Tc = 32.2 °C and Pc = 48.8 bar) have lower critical pressures but are flammable (McHugh and Krukonis, 1994). 

Interest in the use of SC solvents as a reaction media is founded upon recent advances in our understanding of their unique thermo-physical and chemical properties. Worthy of special note are those thermophysical properties (6) which can be manipulated as parameters to selectively direct the progress of desirable chemical reactions. These properties include the solvent s dielectric constant (7), ion product (8,9), electrolyte solvent power (10,11), transport properties"[viscosity (12), diffusion coefficients (13) and ion mobilities (14)], hydrogen bonding characteristics (15), and solute-solvent "enhancement factors" (6). All these properties are strongly influenced by the solvent s density P in the supercritical state. 

Supercritical fluids possess characteristics that make them interesting for use as polymerization media. A supercritical fluid exists at temperatures and pressures above its critical values. In the supercritical state, the fluid exhibits physical and transport properties intermediate between the gaseous and liquid state. This is illustrated in Table 2. SCFs have liquid-like densities, but gas-like diffusivities. These intermediate properties can provide advantages over liquid-based processes. In particular, the higher diffusivities of SCFs reduce mass transfer limitations in diffusion-controlled processes. Additionally, lower energy is required for processing the supercritical fluid because its viscosity is lower than that of most liquids, and because the need to vaporize large quantities of liquid is avoided. 

Of significant interest is the swelling of network structures either by ordinary solvents or by solvents in the near-critical or supercritical state. The development in this case is based on the assumption that the partition function of our system may be factored into three contributions the two factors aheady considered (one due to physical and the other due to hydrogen bonding interactions) and... 

On the contrary, the gas dissolution foaming process, and in particular the high-pressure or supercritical CO2 gas dissolution foaming, allows obtaining micro-and nanoporous polymers. In this technique CO2 is used as a physical blowing agent [39,57-59] this gas is one of the best options for this kind of process because of its excellent characteristics of diffusion in the supercritical state and the mild conditions to reach this state (31°C and 7.3 MPa). Last but not the least, carbon dioxide is a green solvent that can be removed without residue or production of any pollutant compound [60,61]. 

Yamanaka, K. Yamaguchi, T. Wakita, H. (1994) Structure of Water in the Liquid and Supercritical States by Rapid X-ray Diffractometry Using an Imaging Plate Detector, Journal of Chemical Physics 101, 9830-9836... 

Gradual reduction in pressure from the supercritical state leads into a gas phase, without any noticeable physical changes. Gradual reduction in temperature from the supercritical state leads into the liquid state, without any changes in physical appearance. 

Some physical properties of supercritical fluids are in between those of typical gases than liquids. For example, the viscosity of supercritical fluids is about an order of magnitude lower (10 vs. 10 N s/m ) and the solute diffusivity of supercritical fluids is an order of magnitude higher (10 vs. 10 cm ) than for liquid solvents (64). These properties of viscosity and solute diffusivity contribute to improved mass transfer for solutes in the supercritical state, and, therefore, speed extraction rates. The density of carbon dioxide can be increased to densities... 

Thermodynamic characteristics and physical-chemical properties of natural polymers (cellulose, starch, agar, chitin, pectin and inulin), their water mixtures and some biologically active substances extracted from vegetable substances using carbon dioxide in a supercritical state are reviewed. In addition, several aspects of practical application of thermodynamic characteristics of biologically active substances are demonstrated. 

A key feature of SFC is the supercritical nature of the CO2 and the advantageous physical properties that result. Supercritical fluids have intermediate physical properties of both gas and liquid phases and exhibit the positive chromatographic qualities of both. Low viscosity and high solute diffusivity, relative to HPLC, result in faster analysis times, shorter reequilibration times, and higher throughput without loss in efficiency. A supercritical fluid is a defined state, not an additional state of matter. A fluid is supercritical when it remains above its critical temperature (Tc) and critical pressure (Pc). For CO2, Tc is 31 °C and Pc is 74 bar. Transition from a gas to a supercritical state occurs without a phase transition and without significant changes... 

A supercritical fluid exhibits physical-chemical properties intermediate between those of liquids and gases. Mass transfer is rapid with supercritical fluids. Their dynamic viscosities are nearer to those in normal gaseous states. In the vicinity of the critical point the diffusion coefficient is more than 10 times that of a liquid. Carbon dioxide can be compressed readily to form a liquid. Under typical borehole conditions, carbon dioxide is a supercritical fluid. 

---