Defects in the rotator and liquid phases of n-alkanes: a study using FTIR and position 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 liquid phases. These have been correlated with ortho-positronium (o-Ps) lifetimes and intensities in identically prepared samples of dodecane (C₁₂), hexadecane(C₁₆), eisosane (C₂₀), tetracosane (C₂₄), octacosane (C₂₈) and dotriacontane (C₃₂). Consideration was given to the importance of sample preparation and the presence of impurities and free oxygen in determining the correct o-Ps lifetimes in the liquid phase. It was found that o-Ps lifetimes and intensities correlate well with the appearance of conformer defects which arise when the system leaves the all-trans configuration of the crystalline phase and approaches the rotator and liquid phases. The important volume in the determination of o-Ps lifetime was found to be the defect volume per molecule, and together with the behaviour of the o-Ps intensity as a function of temperature in the liquid phase this suggests that either the positron or positronium is trapped in the conformer defects.