Confined Effects on Structural Isomers in the MCM-41-SIL Matrix as Seen by Extrinsic Probes via PALS and ESR: n-Butanol vs tert-Butanol

Abstract

The structural–dynamic states of n-butanol (n-BuOH) and tert-butanol (t-BuOH) isomers as representatives of a linear or globular protic polar medium in the bulk and confined states in the Mobile Composition Matter-41 (MCM-41) matrix obtained from the free volume and phase behavior using positron annihilation lifetime spectroscopy (PALS) or differential scanning calorimetry (DSC), respectively, together with the spectral properties and related mobility and interaction of the spin probe (2,2,6,6-tetramethylpiperidin-1-yl)­oxyl (TEMPO) using electron spin resonance (ESR), are presented. In the bulk t-BuOH, the typical τ3 vs T response for strongly crystallizing globular organics with a stepwise effect in the vicinity of T m is found. On the other hand, the bulk n-BuOH exhibits a complicated course depending on thermal cycling due to the distinct crystallization ability of the linear constituents due to their intermolecular H-bonding. Under confinement, both n-BuOH and t-BuOH media in the MCM-41-SIL matrix were amorphized and heterogenized with larger mean free volume sizes and strong broadening of their dispersion with respect to the corresponding bulk states. In addition, very distinct temperature dependences in τ3 vs T/T g plots with some anomalous effect in the subplateau region in the linear isomer correlating with the DSC response are observed. In the ESR experiment, a drastic difference in the most pronounced characteristic ESR temperature marking a transition from the slow-to-fast-motion regime with the following relations was found: T 50G(b) ≅ T m DSC(b) for n-BuOH against T 50G(b) ≪ T m DSC(b) for t-BuOH, where T m DSC(b) is the respective melting point. Further, several changes in the TEMPO mobility in both BuOH media are related to the numerous dynamic and thermodynamic transitions as obtained from measured DSC, PALS, and literary viscosity data. Finally, the changes in hyperfine splitting constants of TEMPO sensitively reflect the altered structural–dynamic relationships in both confined BuOH isomers with close coincidences between all three characteristic PALS and ESR temperatures, indicating the same origin of the underlying processes behind the changes in spin probe mobility or free volume expansion.