Supercritical fluids boiling curve

Supercritical fluids represent a different type of alternative solvent to the others discussed in this book since they are not in the liquid state. A SCF is defined as a substance above its critical temperature (Tc) and pressure (Pc)1, but below the pressure required for condensation to a solid, see Figure 6.1 [1], The last requirement is often omitted since the pressure needed for condensation to occur is usually unpractically high. The critical point represents the highest temperature and pressure at which the substance can exist as a vapour and liquid in equilibrium. Hence, in a closed system, as the boiling point curve is ascended, increasing both temperature and pressure, the liquid becomes less dense due to thermal expansion and the gas becomes denser as the pressure rises. The densities of both phases thus converge until they become identical at the critical point. At this point, the two phases become indistinguishable and a SCF is obtained. 

The red curve is the vapor-pressure curve of the liquid, representing equilibrium between the liquid and gas phases. The point on this curve where the vapor pressure is 1 atm is the normal boiling point of the substance. The vapor-pressure curve ends at the critical point (C), which corresponds to the critical temperature and critical pressure of the substance. Beyond the critical point, the liquid and gas phases are indistinguishable from each other, and the substance is a supercritical fluid. 

Carbon dioxide, a nontoxic fluid with a relatively low critical point (Tc = 31 C, Pc = 7.376 MPa) is the most widely used in the supercritical fluid field. Supercritical Uquid CO2 is found in the triangular region formed by the melting curve, the boiling curve and the Une that defines the critical pressure. Carbon dioxide is known to swell and significantly plasticize many amorphous polymers, such as poly(methyl methacrylate), polystyrene, polycarbonate and... 

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