Foam Separation of Dyes Using Anionic, Cationic, and Amphoteric Surfactants.

Foam separation can selectively remove a target substance from a solution via adsorption of the substance with the surfactant at the surface of the bubble. A cationic dye, methylene blue, and an anionic dye, Fast Green FCF, were prepared as substances to be removed via foam separation. Anionic (sodium dodecyl sulfate, SDS), cationic (dodecyltrimethylammonium chloride, DTAC), and amphoteric (3-(dodecyldimethylammonio)propane-1-sulfonate, SB-12) surfactants were used in the foam separation process. The effectiveness of the surfactants for removing the cationic methylene blue increased as follows: DTAC < SB-12 < SDS. On the other hand, the effectiveness of the surfactants for removing the anionic Fast Green FCF was in the opposite order. The dyes were effectively adsorbed by the foams via electrostatic interactions between the oppositely charged surfactant and the dye molecules. Since amphoteric surfactants have both anionic and cationic charges in a molecule, they could effectively remove both dyes in the foam separation process. Therefore, it was found that the amphoteric surfactant was highly versatile. Analysis of the kinetics of the removal rate showed that the aqueous solutions of monomers could remove the dyes more effectively than micellar solutions in foam separation.

Removal of Zwitterionic Rhodamine B Using Foam Separation.

Foam separation promotes the removal of dissolved materials from solutions by adsorbing the molecules onto a surfactant. The zwitterion of rhodamine B was removed by using both anionic (sodium dodecyl sulfate: SDS) and cationic (dodecyltrimethylammonium chloride: DTAC) surfactants through foam separation. However, rhodamine B could not be removed from a strongly acidic DTAC solution (pH 2), because the molecular form changes from the zwitterion to cation. Moreover, the cationic dye of rhodamine 6G could not be removed from the DTAC solution. Therefore, these results demonstrate that the electrostatic interaction between a surfactant and target ion is an important factor in foam separation.