Anthraquinone Siloxanes as Thickening Agents for Supercritical CO2

A series of aromatic amide-functionalized, low-molecular-weight polydimethylsiloxanes were synthesized, and their solubility and thickening ability in supercritical carbon dioxide (scCO2) were evaluated. Amide-terminated polydimethylsiloxane (PDMS) oligomers with simple or electron-rich aromatic groups were found to be moderate viscosity oils, indicating that the end groups do not interact strongly enough to be thickeners. Attachment of electron-deficient aromatic groups, such as 4-nitrophenyl, biphenyl, or anthraquinone, onto these amides produced solid polysiloxanes, even at relatively higher degrees of polymerization on the PDMS core (x = 40–50). PDMS derivatives with anthraquinone-2-carboxamide (AQCA) end groups were clear, somewhat rubbery solids. These compounds were found to act as thickeners and gelators of hexanes. In addition, they were able to thicken and/or gel mixtures of hexane and scCO2. The best results were obtained with an AQCA-terminated branched PDMS. Branched fluorenone-2-carboxamide was found to be a waxy solid that was very soluble and, therefore, not a thickener for hexane, and fluorenone-4-carboxamide was a viscous oil, indicating that these amide groups did not interact sufficiently to be useful. 10,10-Dioxothioxanthone-2-carboxamides were more efficient gelators of hexane than anthraquinone-2-carboxamides. However, they were not sufficiently soluble in scCO2, even in the presence of hexane co-solvent, to be useful as thickeners.