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Micellar Radii and High Aggregation Numbers in a 0.6 M NaCl Sodium Dodecylsulfate Solution Determined through Positron Annihilation Lifetime Spectroscopy

Abstract : The equations related to a recently published method to measure both the micellar average radius (R_{mic}) and aggregation number (N_{ag}) using a single technique, positron annihilation lifetime spectroscopy (PALS), are extended and applied to a 0.25 M sodium dodecylsulfate (SDS) aqueous solution in the presence of large amounts of NaCl (0.6 M) as a function of temperature (T). The analysis of the PALS spectra confirms that the triplet state of positronium, o-Ps, originally formed in the aqueous subphase (o-Ps^{aq}) becomes trapped inside the organic core of the micelles (o-Ps^{org}). The equations take into account the variety of o-Ps^{org} lifetimes promoted when introducing a Ps oxidizer (duroquinone) into the solution, which distributes into the micelles according to Poisson's law. The variation with T of R_{mic} derived from PALS agrees well with two sets of values previously published, showing an important decrease upon heating, from about 5.8 nm at 305 K down to 2.3 nm at 343 K. By contrast, although all three sets of data agree on the concomitant decrease in N_{ag} with T, toward values denoting a nearly spherical shape of the micelles at high T, large differences are found at the lowest T studied, the PALS data giving much higher values than those previously published. However, our data show good agreement with the variation of N_{ag} with T, calculated from the measured R_{mic} and the volume of the SDS molecules on the assumption that the micelles display a prolate ellipsoid shape.
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http://hal.in2p3.fr/in2p3-00173805
Contributor : Béatrice Forrler <>
Submitted on : Thursday, September 20, 2007 - 4:10:25 PM
Last modification on : Friday, May 8, 2020 - 1:10:27 AM

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T. Messaoud, G. Duplatre, B. Michels, G. Waton. Micellar Radii and High Aggregation Numbers in a 0.6 M NaCl Sodium Dodecylsulfate Solution Determined through Positron Annihilation Lifetime Spectroscopy. Journal of Physical Chemistry B, American Chemical Society, 2004, 108, pp.13137-13143. ⟨10.1021/jp0371940⟩. ⟨in2p3-00173805⟩

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