An advanced laser fluorescence apparatus is employed to determine the surface or translational diffusion coefficients of fluorescent-labeled probes in monomolecular films at air-water interfaces using fluorescence recovery after photobleaching. The effects of molecular packing, temperature, and various ions on the translational mobility in single and double hydrocarbon-chain surfactant systems are investigated. The fluorescence-recovery-after-photobleaching approach avoids many of the problems associated with the radiotracer technique for measuring surface diffusion. This experimental technique also permits one to extend the study of surface diffusion to interfacial systems previously inaccessible to study by the radiotracer method. In addition, one is able to distinguish between flow and diffusional transport processes occurring in monomolecular films. The procedure provides, for the first time, reliable experimental values of the surface diffusion coefficient to test a recently developed theoretical model of surface diffusion and to elucidate the nature of two- dimensional mass transfer at fluid/fluid interfaces. The results of this research have applicability to a wide range of areas, including pulp and paper processing. Reliable values of the surface diffusion coefficient are needed for use in a variety capillary waves and the stability of thin films which, in turn, are very important to evaluating and understanding foam problems in the pulp and paper industry. Surface mobility is important in the stability of foam; however, experimental determinations of this parameter are virtually nonexistent. This research on interfacial mass transfer is directed to overcoming this serious limitation; when combined with interfacial rheological measurements, it will assist in advancing the understanding of foams and the development of improved foam inhibitors and foam breakers.
