M. Ogita et al., Application of the adsorption effect of optical fibres for the determination of critical micelle concentration, SENS ACTU-B, 64(1-3), 2000, pp. 147-151
The critical micelle concentration (CMC) in surfactant solutions was detect
ed by using an optical fiber. CMC detection is based on an adsorption effec
t 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 repea
ting reflections. The change in adsorption condition leads to an effective
change in refractive index at the surrounding surface of the core. The outp
ut signal suddenly increases at the CMC due to the reflectivity change caus
ed 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, we
re used for the experiments, where the cores were hydrophilic and hydrophob
ic, 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 surfacta
nt concentration increases and also that adsorption onto the POF core occur
s in a two-step process, that is some surfactant molecules have already sta
rted to adsorb onto the surface of the POF core even before CMC. CMC detect
ion using an LED device is also proposed in the present paper. (C) 2000 Els
evier Science S.A. All rights reserved.