Effects of functional groups on the solubilities of polyhedral oligomeric silsesquioxanes (POSS) in supercritical carbon dioxide

• We investigated the phase behavior of the various POSS-CO2 binary systems.
• All the selected POSS formed homogenous solutions with CO2, except POSS with methyl groups.
• Functional groups attached to the Si–O cage affect the binary system phase behavior significantly.

The phase behavior of the binary systems containing CO2 and various hybrid polyhedral oligomeric silsesquioxanes (POSS) with different functional groups were investigated. A high-pressure view cell was used to measure the dew-points or cloud-points at temperatures between 308 and 323 K, up to 30 MPa. Among the studied POSS structures, methacryl, isooctyl and octaisobutyl POSS were observed to form homogenous solutions with CO2, while octamethyl POSS was found to be insoluble in the supercritical fluid. The solubilities of both methacryl and isooctyl POSS in scCO2 decrease with increasing temperature, while isooctyl POSS exhibits nearly an order of magnitude higher solubility than methacryl POSS at both temperatures. The octaisobutyl POSS-CO2 binary system exhibits a crossover pressure, below which the solubility of the component decreases with increasing temperature. Above this pressure, as the temperature is increased, the solubility of POSS in scCO2 increases. These results show that the functional groups of POSS affect its solubility in scCO2 and the phase behavior of the POSS-CO2 binary system significantly. The scCO2-soluble POSS can be implemented in environmentally benign material processing applications using the supercritical fluid.

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