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

Behaviour of Pluronic® P84 block copolymer micelles above the gelification temperature as probed by positron annihilation lifetime spectroscopy

“…Quantitative analysis of the PALS spectra led to the determination of parameters relating both to the various positron states present in the solutions and to the P84 micelles, the latter arising from the trapping of Ps inside the micelles through a diffusion-controlled process. 1,3 Reference is made to our pre-vious works for the equations descriptive of the PALS decay spectra. [1][2][3] The annihilation parameters are the lifetimes, t i , and intensities, I i , of the various positron states.…”

“…The spectra relating to pure water and to the PEG solutions were analysed in terms of 3 exponential components, in order of increasing lifetimes: p-Ps, free e 1 and o-Ps. The other spectra were analysed as described previously, 1,3 in terms of 4 components ascribed, in order of increasing lifetimes, to p-Ps, free e 1 , o-Ps aq (in the water bulk) and o-Ps org (in the organic core of the micelles). In the following, the PALS parameters, lifetimes t i and intensities I i , will be denoted by subscripts i ¼ 1, 2, 3 and 4, in order of increasing lifetimes; t 4 (n Q ) will denote the o-Ps org lifetimes in the micellar cores containing n Q molecules of duroquinone.…”

“…Recently, new developments have been carried out in the laboratory to promote the application of positron annihilation lifetime spectroscopy (PALS) to quantitative determination of parameters related to micellar systems, by examining the behaviour of the triplet state (o-Ps) of positronium (Ps), the bound-state of an electron with a positron (e 1 ), used as a probe. [1][2][3] This became possible after it was found that in aqueous solutions Ps can be trapped inside the organic core of a micelle through a purely diffusion-controlled process. The main information regarding the micelles, average organic core radius (R core ) and aggregation number (N ag ), could thus be derived by analysing the experimental PALS decay spectra through the use of appropriate equations taking account of this trapping process.…”

“…1,4 At this level however, it was acknowledged that R mic and N ag are correlated in the relevant equations, so that reliable results could be obtained only when either of these parameters would be known beforehand, either by calculation or through previous results from other techniques. Therefore, a novel method was established, 2,3 similar to what is done in fluorescence quenching experiments. 5,6 In essence, the equivalent to the fluorescent molecule inside the micelles is our probe itself, Ps, which decays by emitting gamma rays, and the quencher is any water-insoluble chemical that can react with Ps (in our case, the efficient Ps oxidizer, duroquinone).…”

“…5,6 In essence, the equivalent to the fluorescent molecule inside the micelles is our probe itself, Ps, which decays by emitting gamma rays, and the quencher is any water-insoluble chemical that can react with Ps (in our case, the efficient Ps oxidizer, duroquinone). Reference is made to these previous works 2,3 for the detailed principles of the method and the relevant equations.…”