Positronium in Normal Alkanes

Abstract

A review of data about positron annihilation lifetime in normal alkanes is presented. It was found that positronium in rigid phase locates in the interlamellar gap, while in the rotator one in free volumes inside lamellae. Interesting effects seem to be: identity of o-Ps lifetime in rotator and liquid phase in long chain alkanes; appearance of “premature” rotator phase in nonadecane with low tricosane admixture, long-term relaxation after the changes of temperature or pressure in some binary mixtures, formation of porous structure at solidification of argon saturated alkane under high pressure.

Positronium as a probe of small free volumes in crystals, polymers and porous media

“…Contrary to the rotator phase, in which for long chains the o-Ps lifetime is practically independent on temperature and equal to that in the liquid, in phase C that lifetime rises continuously from about 2.4 ns to 2.8 ns near the melting point. The step of τ 3 at melting, invisible when the preceding phase is the rotator one, for the transition C → liquid is well marked [111]. Better ordering is confirmed by Kim et al [123] who have investigated the gauche bond density m g (the number of such bonds per one molecule) by IR spectroscopy.…”

“…The X-ray scattering measurements for this alkane near room temperature prove the coexistence of two solid phases [110]. The PALS measurements at 303 and 313 K analysed by the MELT routine (Fig.40) show two o-Ps peaks, thus belonging to two phases [111]. The peak at about 1.2 ns is typical for low-temperature rigid phase, the other one, located at 1.8 ns, can belong to the other phase.…”

Investigation of PCM microcapsules at various temperatures and pressures by PALS method

“…The intensity I 4 , significantly higher than I 3 , suggests that in the mass composition of the microcapsules the filling is predominant. Stepwise changes of the τ 4 and I 4 allow us to specify the phase transitions points in MPCM, estimated as 269 K and 298 K. Above 298 K, the filling material is liquid. Below 269 K it should be in the rigid crystal phase.…”

“…The pure hydrocarbons as well as the mixtures of alkanes were successfully investigated as a function of temperature or pressure by the PALS technique [2][3][4][5]. At low temperatures (usually below 200 K) in pure n-alkanes and some of their mixtures, the o-Ps intensity increases with time by several percent with time constant  of several hours, while the o-Ps lifetime (1.1 ÷ 1.2 ns) does not change.…”

Spin probe dynamics in relation to free volume in crystalline organics from ESR and PALS: N-tridecane

“…On the other hand, only several external probe studies of nalkanes [19][20][21][22][23][24][25][26][27], especially those using atomic-sized probe PALS technique were reported [21][22][23][24][25][26][27]. In a series of n-paraffins some regularities in the o-Ps annihilation parameters as a function of even or odd type were revealed [21][22][23][24][25].…”