Investigation of thermotropic phase transitions in a triple chain amphiphile forming hexagonal columnar and inverse micellar cubic mesophases: a positron annihilation lifetime study

Positron annihilation lifetime (PAL) measurements were performed between 25 and 100 degrees C in order to study thermotropic phase transformations in an amphiphilic liquid crystal material with three hydrophobic alkyl chains. The lifetime spectra were separated into three exponential components, the long-lived of which is attributed to ol tho-positronium (o-Ps) annihilation via pick-off processes. The lifetime of o-Ps, tau(3), and its relative intensity, I,, as well as the average positron lifetime, tau(av)proportional to I(3)tau(3), respond to variations in the structural regularities of the environment. The phase transitions, which have the sequence and temperatures (degrees C) Cr . 59 . Col(H-II)74Cub(I-II). 85 . Iso, are well reflected in abrupt changes of tau(3), I-3 and tau(av). The o-Ps lifetime in the crystalline phase of tau(3) = 2.35 ns may be interpreted as a crystalline packing coefficient of 0.56 or, alternatively, as o-Ps confined in vacancy-type crystal imperfections with a volume of 0.132 nm(3). During melting tau(3), increases to 3.05 ns, and this is attributed to the preferential formation of Ps bubbles, with a size of 0.207 nm(3), in mobile, liquid-like regions of hydrophobic alkyl chains. The bubble size remains at this Value during the H-II-I-II transition and increases slightly to 0.216 nm(3) during the transformation to the isotropic liquid phase. I-3 shows variations during all of the phase transitions, which are attributed to changes in the Ps inhibition rate due to electron/positron mobility, and solvation effects varying as a consequence of different degrees of order of the molecular dipoles.