Hydrophobic films for optical detection of dry carbon dioxide based on ion pairing and an amine polymer

Abstract

Existing optical carbon dioxide sensors using ion pair materials with pH indicators need moisture to function. To sense dry carbon dioxide gas, hydrophobic films were prepared using an amine polymer (branched-polyethyleneimine) instead of water, along with an ion pair dye and an aromatic polymer. The color of the ion pair formed between cresol red sodium salt and tetra-n-octylammonium bromide was yellow when the pH was below neutral. After adding branched polyethyleneimine, the color changed to violet as the pH became basic. On exposure to carbon dioxide, the amine groups on the branched-polyethyleneimine were converted to alkyl carbamate anions and ammonium cations via reaction with the carbon dioxide. When the ion pair reacted with the resulting ammonium cations, the film changed color from violet to yellow. It was determined that the degree of color change was dependent on the carbon dioxide exposure times and the amount of branched-polyethyleneimine added to the film.