Industrial utilization of the adsorption effect of optical fibers for the detection of critical micelle concentration

The critical micelle concentration (CMC) of surfactant solutions is detected using an optical fiber. The detection is mainly performed using a sample solution of sodium dodecylbenzenesulfonate. The principle of the detection is that an incident beam is reflected at the interface between the fiber core and the solution. Repeated reflection and passage through the sensing region occurs along the fiber. The change in adsorption condition leads to an effective change of the refractive index in the vicinity of the core surface. The output power suddenly increases at the CMC due to the change in reflectivity caused by adsorption of surfactant molecules onto the core surface. Two different kinds of fiber, plastic cladding silica fiber (PCS) with a silica core and plastic optical fiber (POF) with a plastic core, were used for the experiments. The cores are hydrophilic and hydrophobic, respectively. The output from both PCS and POF fibers shows a large change in the vicinity of the CMC point as the concentration of surfactant increases. The data from our optical method using a fiber agreed well with those adsorption isotherms by the weight method. The CMC detection using an LED is also proposed