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

Abstract

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, N A, 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 N A = 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 N A = κ 2([Li+]aq)γ with κ 2 = 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 N A = 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 N A = 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...

Influence of Counterion on the Interaction of Dodecyl Sulfates and Cellulose Ethers

“…At 25 mM NaDS micelles have an aggregation number of 59 monomers, in agreement with the literature (35), while LiDS micelles have 46 monomers at the same surfactant concentration. The LiDS micelles consist thus of fewer monomers than the NaDS micelles, which corresponds well with the recent results, ∼50, of Bales et al (37). The small discrepancies between Bales's values and ours can be explained by the choice of cmc value in the calculation.…”

Effect of the Nature of the Counterion on the Properties of Anionic Surfactants. 1. Cmc, Ionization Degree at the Cmc and Aggregation Number of Micelles of Sodium, Cesium, Tetramethylammonium, Tetraethylammonium, Tetrapropylammonium, and Tetrabutylammonium Dodecyl Sulfates

“…For all surfactants, at a given temperature N increases with the surfactant concentration (see Figure 2), a behavior usual for ionic surfactants. 3,21,28,31,43 2. For all surfactants, at a given concentration N decreases as the temperature is increased, a behavior also noted for ionic surfactants.…”

“…The density of the surfactant is taken equal to 1.0 g/mM. [28][29][30][31] C free is calculated after having determined the value of R but is not very sensitive to errors in this value. The overall error on the values of N is estimated to be about 5% and mostly arises from the fluorescence decay experiments and the analysis of the decay curves.…”

“…Another aim of our work was to assess the properties of the micelles as reaction medium, i.e., by studying their hydration and microviscosity and the rate of bimolecular collisions in these micelles, as in previous studies. [28][29][30][31] For the sake of comparison we have also synthesized and investigated CsDS. Indeed the ionization degree at the cmc and the aggregation number of CsDS micelles have not been reported.…”

Effect of the Nature of the Counterion on the Properties of Anionic Surfactants. 5. Self-Association Behavior and Micellar Properties of Ammonium Dodecyl Sulfate

“…In Figure 3b the lower abscissa scale uses the reduced concentration C aq /cmc 0 to show the range of validity of the scaling law eq 5. Micelles of AmDS, therefore, fit the growth law, eq 5, that has been found to describe micelle growth with increasing surfactant and/or salt concentrations in the slow-growth region for dodecyl sulfate micelles with counterions Na + , 23 Li + , 24 TMADS, TEADS, and TPADS, 2 dodecyltrimethylammonium bromide, and chloride; 16 the sodium alkyl sulfates with chain lengths 8-14; 25,26 and cetyltrimethylammonium chloride and acetate. 27 The only surfactant that we have studied that did not conform to eq 1 was TBADS, a very unusual surfactant indeed.…”