UV/vis spectroscopic investigations of micellisation of homologous N-alkyl betaines using the dye indicator E(T)(30)

The micellisation of homologous N-alkyl betaines of chain lengths from C-8 to C-18 in aqueous buffer solution was investigated UV/vis-spectroscopically using the dye indicator pyridinium N-phenoxide betaine E(T)(30) at a concentration C-dye less than or equal to 10(-5). Above the critical micelle concentration (CMC) the absorption energy of the longest-wavelength absorption band of the dye indicator (E(T)(3D) values) strongly decreases up to 1.3 x CMC due to the transfer of dye molecules from an aqueous into a micellar micro-environment. The determined CMC values agree with values obtained from gamma vs. log c measurements and values from the literature. At the CMC, part of the E(T)(30) molecules is already in a micellar micro-environment in accordance with the mass-action theory of micellar aggregation. Spectral simulations and calculations of the degree of micellisation below the CMC confirm these findings. The degree of micellisation is higher and the decrease of transition energies of the longest-wavelength absorption maximum of E(T)(30) below the CMC are more pronounced for short chain betaines. The E(T)(30) values in a complete micellar micro-environment correlate linearly with the mean aggregation number of the N-alkyl betaines.