DEFECTS IN THE ROTATOR AND LIQUID-PHASES OF N-ALKANES - A STUDY USINGFTIR AND POSITRON-ANNIHILATION

Fourier transform infra-red (FTIR) and positron lifetime spectroscopy (PLs) techniques have been used to quantify end-gauche, double-gauche and kink conformer concentrations in a number of the higher n-alkanes over a temperature range which includes crystalline, rotator and liqui d phases. These have been correlated with ortho-positronium (o-Ps) lif etimes and intensities in identically prepared samples of dodecane (C- 12), hexadecane (C-16), eicosane (C20), tetracosane (C24), octacosane (C28) and dotriacontane (C32). Consideration was given to the importan ce of sample preparation and the presence of impurities and free oxyge n in determining the correct o-Ps lifetimes in the liquid phase. It wa s found that o-Ps lifetimes and intensities correlate well with the ap pearance of conformer defects which arise when the system leaves the a ll-trans configuration of the crystalline phase and approaches the rot ator and liquid phases. The important volume in the determination of o -Ps lifetime was found to be the defect volume per molecule, and toget her with the behaviour of the o-Ps intensity as a function of temperat ure in the liquid phase this suggests that either the positron or posi tronium is trapped in the conformer defects.