Acidity effects on the fluorescence properties and adsorptive behavior of rhodamine 6G molecules at the air-water interface studied with confocal fluorescence microscopy

The effects of acidity on fluorescence originated from rhodamine 6G (R6G) molecules adsorbed at the air-water interface of extremely low-concentration aqueous solutions have been studied with confocal fluorescence microscopy. Similarities and differences in the observed acidity effects between R6G molecules at the interface and those in the bulk solution have been discussed. With increasing the subphase-pH from 1 to 6, height and frequency of photon bursts as well as intensity of the interface-originated fluorescence change in two steps, while bulk fluorescence changes in one step and a little change in the number of adsorbed R6G molecules is verified with surface tension measurements. The results suggest that there is an interface-specific equilibrium among the chemical forms of R6G molecules. Chemical forms contributing to the interface-originated fluorescence above pH 5 are discussed.