Thermodynamic scaling of molecular dynamics in supercooled ibuprofen.

It was shown recently that ibuprofen revealed a strong tendency to form hydrogen bonded aggregates such as dimers and trimers of either cyclic or linear geometry, which somehow seems to control molecular mobility of that drug [Brás et al. J. Phys. Chem. B2008, 112 (35), 11 087-11 099]. For such hydrogen-bonded liquids, superpositioning of dynamics under various temperature T, pressure P, and volume V conditions, when plotted versus the scaling function of T(-1)V(-γ) (where γ is a material constant), may not always be satisfying. In the present work, we have tested the validity of this scaling for supercooled ibuprofen. In order to do that, pressure-volume-temperature (PVT) measurements combined with isobaric and isothermal dielectric relaxation studies (pressure up to 310 MPa) were carried out. The scaling properties of the examined drug were derived from the fitting of the τ(α)(T,V) dependences to the modified Avramov equation and by analyzing in double logarithmic scale the T(g)(V(g)) dependences, where the glass transition temperature T(g) and volume V(g) were defined for various relaxation times. In view of the obtained results, we conjecture that for ibuprofen the thermodynamic scaling idea works but not perfectly. The slight departure from the scaling behavior is discussed in the context of the hydrogen bonding abilities of the examined system and compared with the results reported for other strongly associated liquids.