Time-Resolved Fluorescence Quenching and Electron Paramagnetic Resonance Studies of the Hydration of Lithium Dodecyl Sulfate Micelles

A spin-probe method to study the surface hydration of sodium dodecyl sulfate (SDS) micelles (Bales, B. L.; Messina, L.; Vidal, A.; Peric, M.; Nascimento, O. R. J. Phys. Chem. 1998, 102, 10347; referred to as I) is applied to lithium dodecyl sulfate (LiDS) micelles in order to test both the method and a model of micelle hydration. The method is based on the fact that the hyperfine spacing between the low- and center-field resonance lines, A+, varies linearly with a polarity index, H(25 °C), which is the volume fraction occupied by water in a solvent mixture that contains only water as a source of OH dipoles. The model successfully employed in I predicts that H(25 °C) is determined only by the geometry of the micelle; the amount of water associated with the micelle is determined by the volume available to house the water. Thus, SDS and LiDS micelles of the same aggregation number, NA, ought to yield the same value of H(25 °C) (and, therefore A+) because, apart from their waters of hydration which are alread...