Four-Component Analysis and Modeling of Positron Annihilation Lifetime Parameters in AOT/Water/Isooctane Microemulsions

Abstract

Reverse micelles are studied in sodium AOT/water/isooctane mixtures as functions of AOT concentration (C AOT ), water to AOT mole ratio (w 0 ) and temperature (T), from 294 to 333 K, using positron annihilation lifetime spectroscopy (LS). By four-component analysis of the spectra, it is possible to extract the LS parameters (intensities, I i , and lifetimes, τ i ) of the triplet positronium (o-Ps) present in the aqueous (I 3 , τ 3 ) and organic (I 4 , τ 4 ) phases. The latter lifetime is constant and corresponds with the value measured in pure isooctane, while τ 3 is remarkably lower than the value for pure water. This difference is attributed to the out-diffusion of o-Ps from the water cores to isooctane. The relevant rigorous diffusion equations imply two fitting parameters, the radius of the water aggregates (r 0 ) and a transmission factor (h). Fixation of r 0 ) 3.6 nm for C AOT ) 0.1 M, w 0 ) 20, and T ) 294 K, as known from previous work, allows the quantitative derivation of the r 0 values for all other conditions. The water spheres appear to present some permeability to o-Ps, with a transmission factor h ) 0.12 nm -1 . The sphere radius increases smoothly with C AOT and w 0 and, more importantly, with T. The changes with w 0 give r 0 ) 0.181w 0 and 0.186w 0 nm at 294 and 298 K, respectively, and are in excellent agreement with previous proposals. The sum of the intensities, I tot ) I 3 + I 4 , is much lower than the o-Ps intensity in pure isooctane. In particular, at C AOT > 0.04 M, I tot appears very close to the value found for pure water. The possibility of a strong inhibition of Ps formation due to the micelles, as proposed in previous work, is ruled out because of the negligible electron or positron scavenging ability of alkyl sulfonates. It is thus concluded that Ps formation occurs primarily in the aqueous part of the micelles, the water aggregates representing efficient traps for the positrons while they are slowing down in the solutions. IntroductionPositronium (Ps), the bound state of a positron (e + ) with an electron (e -), has been used increasingly in recent decades as an efficient probe of the physical and chemical properties of matter. 1 The most commonly used positron annihilation technique (PAT) is lifetime spectroscopy (LS), which allows us to obtain both the lifetimes (τ i ) and relative abundances (or intensities, I i ) of the various positron species. Normally, in order of increasing lifetimes, these are singlet Ps (p-Ps, i ) 1), free positron (i ) 2), and triplet Ps (o-Ps, i ) 3). Because of its distinct long lifetime, the latter appears to be the most useful probe. The o-Ps decay occurs with the emission of two γ-rays through the pick-off process, i.e., the annihilation of the positron with one of the (bound) electrons of the surrounding medium. Information is obtained through two distinct, independent processes, Ps formation and annihilation, both of which depend on various characteristics of the systems under investigation. In liquids, Ps formation may occur on a very short time ...

Phase diagram studies in two-surfactant ternary systems employing positron annihilation spectroscopy

“…The system is completely in the reverse micellar (R) phase at hexanol concentrations ranging from 25.6 to 28.4%. The high value in τ 3 in this region strongly supports the model proposed by Ferreira et al [34,35] in which o-Ps atoms formed in the aqueous cores of the reverse micelles diffuse out into the organic solvent bulk and annihilate there increasing significantly the value of τ 3. The various phase boundaries delineated by PAS in the present study are summarized in Table 1, alongside the results obtained earlier from the optical method.…”

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

“…Therefore, the decrease in τ 3 arises from the diffusion of Ps from the aqueous phase to the organic core of the micelles in a similar way to that found for the reverse micelles. 1 The corresponding kinetic scheme is thus as follows:…”

“…1 The probe used was the long-lived triplet state (o-Ps) of positronium (Ps), the bound state of a positron with an electron, whose lifetime and relative abundance, or intensity, both depend on the physicochemical properties of the medium. As expected, the LS spectra analysis showed the existence of two long-lived components physically corresponding to o-Ps present in either the organic (o-Ps org ) or the aqueous micellar (o-Ps aq ) phases.…”

Probing the size of Pluronic P84 block copolymer micelles in aqueous solution by means of positron annihilation lifetime spectroscopy

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