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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 already taken into account by the geometrical model, neither Li+ nor Na+ occupies significant volume. Over the range of aggregation numbers NA = 50−110, values of H(25 °C) determined from measurements of A+ in LiDS micelles were found to be within ±2% of those in SDS micelles. These data support the geometric model and show that specific interactions due to Li+ or Na+ which might affect A+ are unimportant. The aggregation numbers of LiDS micelles are measured by time-resolved fluorescence quenching and are well described by NA = κ2([Li+]aq)γ with κ2 = 112 ± 2 and γ = 0.180 ± 0.005, where [Li+]aq is the concentration of lithium ions in the aqueous phase whether supplied by LiDS or by both LiDS and LiCl. Thus, LiDS micelles grow according to an empirical formula identical in form to that for SDS micelles, but at a slower rate. The aggregation numbers at the critical micelle concentration in the absence of added salt (cmc0) are the same for SDS and LiDS micelles, but above this concentration, LiDS micelles are significantly smaller than SDS micelles for a given ionic strength. By applying a simple model of a spherical hydrocarbon core surrounded by a polar shell and assuming that the spin-probe samples all portions of the shell, values of the polarity index H(25 °C) may be converted into values of N(H2O), the number of water molecules per surfactant molecule residing in the polar shell. This conversion involves no adjustable parameters because the geometrical parameters are fixed from small-angle neutron scattering measurements. As the micelles grow in the range NA = 50−110, N(H2O) decreases from 9.6 to 5.4 water molecules per surfactant molecule because the volume per surfactant molecule in the polar shell decreases allowing less water to reside within the shell. The sphere−rod transition previously observed in I for SDS at NA = 130 cannot be reproduced in LiDS, because LiCl is not sufficiently soluble to achieve an aggregation number of 130; however, interesting small, unidentified transitions appear to occur near NA = 112 and 121. A byproduct of this work is that relative aggregation numbers for LiDS micelles may be determined from measured values of A+ with a precision of about one molecule from the following: A+(NA) = (15.468 ± 0.004) − NA(3.45 ± 0.06) × 10-3 where A+(NA) is in gauss. Since a given value of NA may be prepared by choosing different combinations of SDS, LiDS, NaCl, and LiCl concentrations, neither interactions between the micelles nor the ionic strength influence the value of A+.

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