A self-assembly-state-resolving decomposition of SDS infrared spectra in the absence and presence of NaCl.

In this study, infrared spectroscopy was developed from a qualitative probe of micellization into a quantitative, species-resolved method for analyzing surfactant self-assembly. Aqueous sodium dodecyl sulfate (SDS) solutions in the absence and presence of sodium chloride were investigated by transmission FTIR spectroscopy and modeled as linear superpositions of extinction coefficient spectra assigned to monomeric and micelle-bound SDS. Concentrations of the individual spectral components were obtained by least-squares fitting of the entire measured mid-infrared spectra and the resulting critical micelle concentration (CMC) values were validated by isothermal titration calorimetry (ITC). Below the CMC, the SDS spectra scaled linearly with concentration and were attributed to monomeric SDS, whereas above the CMC, redshifts and narrowing of the CHx stretching bands revealed increasing chain ordering upon micellization. The FTIR-derived CMC values decreased from 8.35 mM without NaCl to 5.55, 3.83, and 2.61 mM in the presence of 10, 25, and 50 mM NaCl, respectively, in good agreement with ITC values of 8.60, 5.43, 3.73, and 2.30 mM. At higher SDS concentrations and elevated NaCl levels, a second micellar spectral component was required to describe the spectra, consistent with a gradual shift towards more ordered and more densely packed micellar states. A fluctuation-based analysis exploiting cross-sensitivity between the related micellar components supported pseudo-phase behavior as a practically useful approximation. Beyond SDS, it was shown that the method is transferable to surfactants of different charge and molecular complexity. The results demonstrate that full-spectrum FTIR decomposition enables efficient CMC determination and quantitative monitoring of surfactant self-assembly.