Positronium formation in aqueous micellar solutions of sodium dodecylsulfate

Abstract

Positron annihilation lifetime measurements on aqueous solutions of sodium dodecylsulfate (NaLS) were performed over a wide range of temperatures and surfactant concentrations. The sensitivity of the positronium formation process toward changes in physical properties accompanying micellization is demonstrated. The critical micelle concentration (CMC) is associated with a sharp decrease in positronium formation. The CMC of 8.4±O.5 mM at 20"C in this system is in agreement with literature values. At higher temperatures the CMC is shifted to slightly higher surfactant concentrations (iO.4±O.7 mM at 90 "C). The formation probability for thermal positronium as expressed by 1 2 , the intensity of the long· lived component in the lifetime spectra, gradually decreases above the CMC with increasing surfactant concentration reaching a limiting value. A model is suggested involving the reactions of "hot" positrons or positronium atoms with micellar aggregates leading to the observed reduction of thermalized positronium atoms.

Size of Sodium Dodecyl Sulfate Micelles in Aqueous Solutions as Studied by Positron Annihilation Lifetime Spectroscopy

“…This phenomenon was not noted in the previous studies on SDS based on LS measurements. [5][6][7] The …”

Application of positron annihilation lifetime spectroscopy (PALS) to study the nanostructure in amphiphile self-assembly materials: phytantriol cubosomes and hexosomes

“…We have previously determined with the data analysis using PAScual that the oPs does not annihilate in the water phase, 34 so it was deduced that oPs annihilation occurs in the hydrophobic domain, consistent with previous observations. 25,59 The PALS signature for dispersions of self-assembled colloidal materials has only recently been reported for the lamellar phase (liposomes); 45,60-62 with very few studies of Type II 1 inverse architectures such as Q 2 and H 2 phases, which are the subject of increasing interest for nanotechnology and biomedical applications 4,20,63 The current work therefore constitutes the first report of PALS for the characterisation of dispersed lyotropic liquid crystalline particles with two-and three-dimensional internal order. These authors also propose that oPs diffuses to, and annihilates in the hydrocarbon domains, which has implications for macroscopic properties such as viscosity and diffusion.…”

“…Previously much of this work has focussed upon the measurement of micellar growth and geometry in the Type I 1 normal phases for charged amphiphiles 29,35 such as CTAB, 27,58 and SDS. 25,59 The PALS signature for dispersions of self-assembled colloidal materials has only recently been reported for the lamellar phase (liposomes); 45,[60][61][62] with very few studies of Type II 1 inverse architectures such as Q 2 and H 2 phases, which are the subject of increasing interest for nanotechnology and biomedical applications 4,20,63 The current work therefore constitutes the first report of PALS for the characterisation of dispersed lyotropic liquid crystalline particles with two-and three-dimensional internal order. For many applications dispersions such as cubosomes and hexosome are preferred since these possess the same nanostructure as the parent phase, though have much lower viscosity (by a factor of 10 8 ) and higher surface area; to enable ease of formulation for increased payload.…”

Positron annihilation lifetime spectroscopy (PALS): a probe for molecular organisation in self-assembled biomimetic systems

“…32,35,36 In the latter scenario, this difficulty is translated into solutions that depend upon an initial guess and is therefore biased by the user, especially when complex systems are studied. 11 Another common variation in the analysis of raw PALS data is the number of components into which the data are deconvoluted. Several PALS data analysis programs exist with CONTIN, 33 MELT, 34 and RESOLUTION 37 used to transform the experimental data into a continuous lifetime contribution function (deconvolution) whilst PositronFit, 35 PALSfit, 36 LT, 32 and PAScual 38,39 employ mathematical modelling (convolution).…”

“…3 are present in biomembranes, for which parallel structures have been identified. 15,16 The most prolific area of research utilising PALS for self-assembled systems is in micellar 4,6,7,8,10,11,[58][59][60][61][62][63] and microemulsion systems. 24,[52][53][54] Although the exact mechanisms for the formation of such morphologies are unknown, they are widely associated with membrane re-arrangements resulting from cellular stress and are implicated for inter/intra cellular trafficking.…”