Rhodamine B adsorption on anodic porous alumina in sodium dodecyl sulfate solutions

Anodic porous alumina (APA) with its positively charged surface can be dyed by electrostatic interactions with negatively charged dye molecules. In this study, an anionic surfactant, sodium dodecyl sulfate (SDS) was found to allow such use of cationic dyes as well, using rhodamine B (RB) as a test. By dipping APA in the RB-SDS solutions, RB was successfully adsorbed to the APA surface to create a uniform pink color. The amount of adsorbed RB first increased with [SDS] up to around 3 mM, and then decreased to almost zero with the corresponding color change in the APA surface. A bath pH of 6-7 was suitable for dyeing APA in the RB-SDS solutions. From the absorption and fluorescence studies, we concluded that RB is dominantly adsorbed on APA as a zwitterionic form in non-fluorescent H-dimers in the dye-rich induced micelles and SDS micelles. Surface analysis studies of the dyed APA showed that RB is entrapped in the pores of APA with the average pore diameter of 17 nm up to about 600 nm in depth.