Solubility in Carbon Dioxide

Organic solvent(s) Partition coefficients Soluble in carbon dioxide, chloroform ether Insoluble in acetone Soluble in ethanol Soluble in ethanolb Soluble in ethanol, ether... 

Figure lQis a graph of naphthalene solubility in carbon dioxide at 45 C (15 C above the critical temperature of carbon dioxide) taken from Reference 17. As is obvious from an examination of the data, the solubility of naphthalene increases dramatically when the pressure is increased beyond the critical pressure of 73 atm. The solubility (given in units of grams/liter in the reference) approaches about 10% (w/w) at a pressure level of 200 atm. 

One of the major advantages of SFC over HPLC (high-performance liquid chromatography) is its compatibility with the flame ionization detector (FID), a universal and sensitive detector for carbon compounds. Unfortunately, most modifiers used in SFC are incompatible with FID. Therefore, a search for polar modifiers that have less response to FID led to the use of water, formic acid, and formamide. These modifiers produce acceptably low background noise and enable the use of FID. Because both water and formic acid have poor solubilities in carbon dioxide, they have been used as modifiers at very low concentrations. However, the modifier effect is significant even at this low level. For example, when water or formic acid was used as modifiers, the resolution of free fatty acids was significantly improved. 

A common feature of solubility data for single solutes dissolved in a supercritical fluid is the existence of what has been termed crossover pressure (Chimowitz and Pennisi 1986). This is a pressure around which isotherms at various near-critical temperatures tend to converge. Figure 3 shows this behavior for phenanthrene solubility in carbon dioxide. Below the crossover pressure, an isobaric increase in temperature causes a solubility decrease, so that the solute is retrograde. 

Solubility in carbon dioxide-free water. The solubility of calcite in distilled water free of carbon dioxide is 14 mg/1 at 25 °C, rising to 18 mg/1 at 75 °C. That of aragonite increases from 15.3 mg/1 at 25 °C to 19.0 mg/1 at 75 °C [3.1]. However, these values are only of academic interest, as natural water contains dissolved carbon dioxide. 

From experimentation with supercritical fluid extraction it is known that neither plutonium nitrate or oxides nor americium nitrate or oxides are soluble in carbon dioxide without the aid of soluble complexing agents. Even with the aid of selected complexing agents plutonium extraction efficiency from spiked... 

Some general qualitative rules for solubility in carbon dioxide have been known for quite some time ... 

---