Application of the adsorption effect of optical fibres for the determination of critical micelle concentration

The critical micelle concentration (CMC) in surfactant solutions was detected by using an optical fiber. CMC detection is based on an adsorption effect in sample solutions consisting mainly of sodium dodecylbenzenesulfonate. An incident beam was reflected at the interface between the fiber core and the solution, passing through the sensing region along the fiber with repeating reflections. The change in adsorption condition leads to an effective change in refractive index at the surrounding surface of the core. The output signal suddenly increases at the CMC due to the reflectivity change caused by adsorption of surfactant molecules onto the surface of the fiber core. Two different types of fiber, plastic cladding silica fiber (PCS) with a silica glass core and a plastic optical fiber (POF) with a plastic core, were used for the experiments, where the cores were hydrophilic and hydrophobic, respectively. It was determined that the outputs from both PCS and POF fibers show a large change in output power at the CMC point as the surfactant concentration increases and also that adsorption onto the POF core occurs in a two-step process, that is some surfactant molecules have already started to adsorb onto the surface of the POF core even before CMC. CMC detection using an LED device is also proposed in the present paper.