ESR and PALS detection of the dynamic crossover in the supercooled liquid states of short and medium-sized n-alkanes

A Combined Atomic and Molecular Probe Characterization of Aromatic Hydrocarbons via PALS and ESR: Methylbenzene

“…As for the characteristic ESR temperature T X2 fast,Azz ∼ = T X fast,τ = 215-218 K lying relatively close to T A = 220 K [55], this finding indicates that the slowing down of the spin probe TEMPO reorientation in TOL on cooling within its fast motion regime is also related to the Arrhenius to super-Arrhenius crossover of the dynamics in TOL. This is also suported by the empirical fact found by some of us for many other amorphous organics that at the characteristic ESR temperature T X fast , at which the o-Ps lifetime corresponding to the local spherical free volume, V h sphere , equals the van der Waals volume of TEMPO, V TEMPO W , τ 3 (V h sphere = V TEMPO W ) = 2.72 ns, a change in the dynamics occurs [4,[7][8][9]. The o-Ps lifetime measured in TOL at 218 K τ 3 = 2.75(8) ns and confirms the above.…”

“…Recently, joint investigations on several organic compounds of various chemical structures and morphologies using PALS and ESR were carried out to search for their characterization potential, which can be influenced by the potential interaction of the used probe with the medium's components, in comparison with the conventional techniques, where the mutual interactions between the medium's particles exist only [1][2][3][4][5][6][7][8][9][10]. A series of amorphous small molecular, oligo-as well as polymeric, organic substances with different, i.e., vdW-or H-bonding, intermolecular interactions between the constituent's entities were investigated [1][2][3][4][5][6].…”

“…In a few cases, via comparisons of the corresponding time scales, it was possible to identify the underlying dynamic processes responsible for these coinciding effects [3][4][5][6]. Subsequently, these joint studies on the morphologically simple organics have been extended towards crystalline aliphatic acyclic hydrocarbons, such as linear n-alkanes ranging from n-hexane (C6) up to n-nonadecane (C19) [7][8][9] and one of the aliphatic cyclic hydrocarbons, i.e., cyclohexane (CHX) [10]. Both the extrinsic microscopic probes mirror the main thermodynamic transitions, such as the solid to liquid (melting) phase transition at T m DSC in both the even-and odd-number members and the solid to solid (polymorphic) phase one at T ss DSC in the odd n-alkanes and CHX.…”

“…However, these basic phase transitions are reflected differently by the changes in various spin probe mobility regimes. Recently, the possible origin of this apparently surprising T 50G << T m DSC relationship has been related to the dynamic crossover at T X = T c MCT in the local disordered regions containing the TEMPO probes in the otherwise crystalline material [9,11].…”