Solubilization of an indicator dye for optical fiber chemical sensors in supercritical CO₂ by ion pairing

Bromothymol blue (BTB), the indicator dye for optical fiber chemical sensors was lipophilized by ion pairing with quaternary ammoniums, dodceyltrimethylammonium, myristyltrimethylammonium, hexadecyltrimethyammonium and tetrahexylammonium. The dissolution of BTB and the four ion pairs in supercritical CO₂ without/with ethanol as the co-solvent were experimentally investigated. Although BTB is insoluble in supercritical carbon dioxide even with ethanol as the co-solvent, the four hydrophobic ion pairs formed can dissolve in ethanol modified supercritical carbon dioxide. It was found that pressure, temperature, the co-solvent concentration in CO₂, and the chemical structure of the quaternary ammonium, influenced the dissolution of ion pairs in supercritical CO₂. The hydrophobic ion pairing provides the possibilities for solubilization of hydrophilic indicators in supercritical CO₂ and applications of supercritical fluid techniques to chemical optical sensors.