Measurement and correlation of solubility of Acid Red 57 in supercritical carbon dioxide by ion-pairing with hexadecyltrimethylammonium bromide

Solubility of C.I. Acid Red 57 (AR57) in supercritical carbon dioxide was measured by ion-pairing with hexadecyltrimethylammonium (HDTMA) bromide. The solubility measurements of AR57 and AR57-HDTMA in supercritical carbon dioxide without/with methanol as a modifier solvent were carried out at the temperature range from 35 to 75 °C and for pressures from 250 to 325 bar. The solubility of AR57 and AR57-HDTMA was examined in terms of pressure and temperature of supercritical carbon dioxide. Even though Acid Red 57 is insoluble both in supercritical carbon dioxide and methanol-modified supercritical carbon dioxide, AR57-HDTMA can easily dissolve in methanol-modified supercritical carbon dioxide. The hydrophobic ion-pairing of HDTMA provides a possibility to dissolve a hydrophilic dye in supercritical carbon dioxide. A semi-empirical equation was used to correlate the obtained experimental solubilities of AR57-HDTMA by means of the density of carbon dioxide in methanol-modified supercritical carbon dioxide.