Spectroscopic methods for determination of critical micelle concentrations of surfactants; a comprehensive review

The applications of surfactants in various fields are gaining more attention, which makes full characterization of surfactants of growing interest. It is fundamental to measure the critical micelle concentration (CMC) as a parameter for characterizing surfactants. Spectroscopic methods for determination of CMC are more common, easier to perform, and in certain applications more accurate. In this review, different spectroscopic techniques and methods used for determination of CMC are discussed. These methods include direct UV/VIS Spectroscopy, which studies liquid surface curvature in thin wells using vertical detecting light beam with the wavelength set at 900 nm. The indirect UV/VIS Spectroscopic methods include using surface plasmon resonance or surface-enhanced Raman scattering of metal nanoparticles. Direct spectrofluorometric methods measure CMC based on the intrinsic fluorescence of the tested surfactants, and it was reserved for surfactants with intrinsic fluorescence such as Triton-X100. Indirect spectrofluorometric methods include measuring the change in fluorescence intensity, spectral shape, lifetime, polarization, or the solvatochromic shift of surfactant-dye solution. Other spectroscopic methods have been reported such as X-ray diffraction, nuclear magnetic resonance spectroscopy and small-angle neutron scattering. This review article discusses the spectroscopic methods developed for CMC determination with emphasis on the principle, applications, advantages, and limitations of each method.