• Brás, Ana R.
• Correia, Natália T.
• Affouard, Frédéric
• Andrade, Maria Madalena Dionísio
• Fonseca, I. M.
• Schönhals, Andreas

Abstract

<p>The molecular mobility of ibuprofen confined to a mesoporous silica host (MCM-41) of 3.6 nm pore diameter is investigated by dielectric relaxation spectroscopy. It is confirmed that crystallization is suppressed; therefore, depending on the temperature, the guest exists in the glassy and supercooled state inside of the pores. A detailed relaxation map is provided where multiple processes are dynamically characterized, comprised of three processes that are also found for the bulk and two additional ones. The bulk-like processes include two secondary processes, a simple thermally activated one, a γ process and a Johari-Goldstein β_JG process, and the one associated with the dynamic glass transition of molecules located in the pore center (α process). In confinement, all of these processes display deviations in its dynamical behavior relative to the bulk, the most dramatic one undergone by the α process, which exhibits Arrhenius-like temperature dependence upon approaching the glass transition instead of Vogel/Fulcher/ Tammann/Hesse (VFTH) scaling as obeyed by the bulk. The two additional relaxations are associated with the dynamical behavior of hydrogen-bonded ibuprofen molecules lying in an interfacial layer near the pore wall, an S process for which the mobility is strongly reduced relative to the α process and a Debye-like D process for which the dynamics is closely correlated to the dynamics of the interfacial process, both exhibiting VFTH temperature dependencies. The comparison with the behavior of the same guest in the analogous host, SBA-15, with a higher pore diameter (8.6 nm) leads to the conclusion that the bulk-like mobility associated with the dynamic glass transition undergoes finite size effects being accelerated upon a decrease of the pore size with a concomitant reduction of the glass transition temperature relative to the bulk, 22 and 32 K, respectively, for the 8.6 and 3.6 nm pore diameters. The continuous decrease in the separation between the α- and β_JG-trace with pore size decrease allows one to conclude that confined ibuprofen is a suitable guest molecule to test the Coupling Model that predicts a transformation of the α process into a β_JG-mode under conditions of an extreme nanoconfinement. The overall behavior inside of pores is consistent with the existence of two distinct dynamical domains, originated by ibuprofen molecules in the core of the pore cavity and adjacent to the pore wall, from which a clear picture is given by molecular dynamics simulation.</p>

Topics

• impedance spectroscopy
• pore
• mobility
• simulation
• glass
• glass
• molecular dynamics
• Hydrogen
• glass transition temperature
• interfacial
• crystallization