Study of the adsorptive behavior of water-soluble dye molecules (rhodamine 6G) at the air-water interface using confocal fluorescence microscope

A confocal fluorescence microscope was applied to directly study the characteristic behaviors of adsorbed molecules at the air-water interface for a water-soluble chromophore, rhodamine 6G (R6G), in its extremely low-concentration region (below 10^-10 M). Significant photon bursts were observed only from the surface, and their width, height, and frequency were found to depend on the bulk concentration, suggesting the inhomogeneous distribution of R6G molecules at the air-water interface. This property of the adsorbed molecules is different from that of the bulk one. The influence of the ionic strength on photon bursts from the interface was investigated. It was found that the addition of NaCl to the R6G solution caused a decrease of the fluorescence signal. A change in the size of the aggregate and in the fluorescence quantum yield of the adsorbed molecules was suggested to be responsible for this experimental result.