Carbon dioxide molecules, very high pressure

For the naphthalene-ethylene and biphenyl-carbon dioxide systems, the effect of the binary liquid-gas critical point is rapidly diminished as the pressure is increased above the UCEP pressure. For the naphthalene-ethylene system, where the UCEP is at a modest pressure, the solid-fluid equilibrium curve quickly attains a limiting solubility at pressures greater than the UCEP pressure. For the biphenyl-carbon dioxide system, where the UCEP pressure is more than twice that of the naphthalene-ethylene system, the solid-fluid equilibrium curve decreases sharply to lower concentrations of heavy component as the pressure is increased above the UCEP pressure. This solubility behavior is a consequence of a free volume effect that results from the large disparity in size between biphenyl and carbon dioxide (Ranee and Cussler, 1974 von Tapavicza and Prausnitz, 1976). At very high pressures, increasing the pressure further reduces the free volume between carbon dioxide molecules available to the biphenyl molecules and reduces the solubility of biphenyl. Carbon dioxide essentially squeezes out the biphenyl at these high pressures. 

When high-temperature products are in an equilibrium state, many of the constituent molecules dissociate thermally. For example, the rotational and vibrational modes of carbon dioxide are excited and their mohons become very intense. As the temperature is increased, the chemical bonds between the carbon and oxygen atoms are broken. This kind of bond breakage is called thermal dissociation. The dissociahon of H2O becomes evident at about 2000 K and produces H2, OH, O2, H, and O at 0.1 MPa. About 50% of H2O is dissociated at 3200 K, rising to 90% at 3700 K. The products H2, O2, and OH dissociate to H and O as the temperature is increased further. The fraction of thermally dissociated molecules is suppressed as the pressure is increased at constant temperature. 

Solid-Gas Equilibria Sublimation is not very familiar because solids have much lower vapor pressures than liquids. A substance sublimes rather than melts because the intermolecular attractions are not great enough to keep the molecules near each other when they leave the solid state. Some soUds do have high enough vapor pressures to sublime at ordinary conditions, including dry ice (carbon dioxide), solid iodine, and moth repellants, aU nonpolar molecules with weak intermolecular forces. 

The carbon dioxide laser is the most efficient gas laser, with a wall-plug efficiency of up to 20%. It worlcs in the IR region aromid 10 pm. In numerous apphcations a non-tmied CO2 laser is used. However, from a spectroscopic point of view, the fact that it can be fine-tuned and continuously tuned at high gas pressures is very useful. In Fig. 3.30 a level diagram and a practical arrangement for a tuned CO2 laser are shown, together with a diagram of the available fines. The CO2 molecule has three fundamental modes of... 

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