Glassy Dynamics and Translational-Rotational Coupling of an Ionically Conducting Pharmaceutical Salt-Sodium Ibuprofen

In this work, we study the structural dipolar relaxation and ionic conductivity relaxation in an ionized derived from a nonionized glass former. The latter is the salt form of a well-studied active pharmaceutical ingredient, sodium ibuprofen, and the former is ibuprofen. Quantum mechanical calculations were employed to study the variation in its molecular electrostatic potentials, and its spatial extent on its salt formation with Na⁺ ions. Measurements have been made using differential scanning calorimetry and broadband dielectric spectroscopy, and the characterization is assisted by density functional theory. The dielectric data contain information on both ionic and dipolar molecular mobility of NaIb and were extracted by representation in terms of the electric modulus and permittivity. A secondary β-conductivity relaxation coexists with the primary α-conductivity relaxation. By use of the coupling model, we show that the β-conductivity relaxation is connected to the α-conductivity relaxation and is the analogue of the relation of the Johari-Goldstein β-relaxation to the structural α-relaxation, shown valid also in ibuprofen. This remarkable result has an impact on the fundamental understanding of the dynamics of ionic conductivity. By representing the data as permittivity, a dipolar β-relaxation was found to have practically the same relaxation times as the β-conductivity relaxation in the glassy state and translational-rotational coupling is valid at a more local secondary relaxation level. However, the α-conductivity relaxation decouples from structural α-relaxation because the structural glass transition temperature is lower than the conductivity counterpart by 29 K. These are novel findings. The study elucidates the effects on the dynamics by the change in the nature of bonding and in size on introducing sodium ions to ibuprofen in the glassy and supercooled liquid states.