Bulk and confined n - alkanes: n - undecane in unmodified vs. modified silica gels by positron annihilation lifetime spectroscopy and electron spin resonance techniques

A joint investigation of n-undecane (n-UND) in the bulk and confined states in irregular silica gel (SG) matrices differing in their surface composition via two extrinsic probes using positron annihilation lifetime spectroscopy (PALS) and electron spin resonance (ESR) as well as DSC technique is presented. Ortho-positronium (o-Ps) exhibits a pronounced change in the annihilation behavior as detected by the o-Ps lifetime,τ3, as a function of temperature due to the enhanced amorphization of the n-UND after confinement with respect to the bulk solid n-UND. In addition, with an exception of the low-T region it exhibits rather small alterations in the confined state in the unmodified (virgin) SG100A-SIL compared to modified (silanized) SG100A - C4. On the other side, the stable free radical of nitroxide type 2,2,6,6-tetramethylpiperidinyl-1-oxy(TEMPO) shows the essential differences in the spectral properties with temperature not only in the confined n-UND with respect to the bulk sample, but especially in the confined state in the modified SG-C4 matrix compared to the unmodified SG100A-SIL one. These differences are attributed to the presence and absence of the preferential interaction of the polar spin probe TEMPO with the polar surface of the virgin SG matrix or the apolar surface of the silanized SG100A-C4 one, respectively. The coinciding relationships between the characteristic PALS and ESR temperatures reflecting the changes in annihilation and rotation dynamics of the respective extrinsic probes are related to the dynamic and phase transitions as found by wide-T DSC.