Micelle aggregating condition of fluorocarbon-hydrocarbon hybrid surfactants in aqueous solution

The micelle aggregating condition of sodium 1-oxo-1-[(4-fluoroalkyl)phenyl]-2-alkanesulfonate (FCm-HCn; m = 4, 6, n = 2, 4, 6), hybrid surfactants containing a fluorocarbon chain and a hydrocarbon chain in the same molecule, has been investigated by means of the pyrene fluorescence probe method and Raman spectroscopy. Micropolarity of the hydrocarbon moiety for the FCm-HCn micelles is examined with the I-1/I-3 ratio in the fluorescence spectra of pyrene added as an indicator. The ratios for the FC6-HCn and FC4-HC6 solutions decrease significantly even above their critical micelle concentration (cmc), suggesting that these micelles gradually change their aggregation state above the cmc with concentration increment. The wavenumber of CH3 asymmetric stretching vibration for the FC6-HCn series in Raman spectra shifts to lower wavenumber at this concentration range, while that of FC4-HCn is almost constant. These results suggest that FC6-HCn and FC4-HC6 form a wet micelle first but another dehydrated micelle coexists above the second cmc.