Molecular Motions in Amorphous Ibuprofen As Studied by Broadband Dielectric Spectroscopy

Abstract

The molecular mobility of amorphous ibuprofen has been investigated by broadband dielectric relaxation spectroscopy (DRS) covering a temperature range of more than 200 K. Four different relaxation processes, labeled as alpha, beta, gamma, and D, were detected and characterized, and a complete relaxation map was given for the first time. The gamma-process has activation energy E a = 31 kJ.mol (-1), typical for local mobility. The weak beta-relaxation, observed in the glassy state as well as in the supercooled state was identified as the genuine Johari-Goldstein process. The temperature dependence of the relaxation time of the alpha-process (dynamic glass transition) does not obey a single VFTH law. Instead two VFTH regimes are observed separated by a crossover temperature, T B = 265 K. From the low temperature VFTH regime, a T g (diel) (tau =100 s) = 226 K was estimated, and a fragility or steepness index m = 93, was calculated showing that ibuprofen is a fragile glass former. The D-process has a Debye-like relaxation function but the temperature dependence of relaxation time also follows the VFTH behavior, with a Vogel temperature and a pre-exponential factor which seem to indicate that its dynamics is governed by the alpha-process. It has similar features as the Debye-type process observed in a variety of associating liquids, related to hydrogen bonding dynamics. The strong tendency of ibuprofen to form hydrogen bonded aggregates such as dimers and trimers either cyclic or linear which seems to control in particular the molecular mobility of ibuprofen was confirmed by IR spectroscopy, electrospray ionization mass spectrometry, and MD simulations.

Dielectric Spectroscopy in Ibuprofen-Dioxane Mixture

“…Also, the DSC scan of IBP-DXN (x w = 3%) (Where x w is the weight fraction of DXN) mixture is shown in Figure 1 and the observed T g-onset = 215.1 K is indicated. Moreover, it has been noticed that neat 1,4-dioxane exhibited a transition (at T t = 272 K) just a few degrees below its melting temperature (i.e., T m = 281.7 K), and the observed onset values are in agreement with literature data [1,7,30].…”

“…The presence of two relaxation processes in hydrogen-bonded systems in their supercooled regime is not new, similar behavior has been reported for monohydroxy alcohols [7][8][9][10][11][12][13][14][15] and also, recently observed in neat IBP [1,2]. As discussed above, in such systems, sterically hindered -OH group can limit the formation of hydrogen bond chain-like structures.…”

“…The study of vitrification transition phenomena has always been interested in hydrogen-bonded supercooled liquids composed of small molecules [1][2][3][4][5][6]. It is known for decades that several monohydroxy alcohols [7][8][9][10][11][12][13][14][15] and amides [16] hydrogen bonded glass forming systems results in intramolecular and intermolecular hydrogen bonding and behave differently.…”

“…Recent [1,2] dielectric studies of supercooled pharmaceuticals are revealing the existence of the Debye-like process in complex molecules, and its presence has been reported in ibuprofen. However, the dielectric strength of the Debye process is found to be very low.…”

Natural halloysite nanotubes as mesoporous carriers for the loading of ibuprofen

“…Vueba et al [68] used density functional theory methods to calculate the free C@O vibration at 1765 cm À1 . In fact, the stretching C@O vibration showed a downward shift to lower wavenumbers (1700-1725 cm À1 ) due to the formation of hydrogen bonds between the carboxyl groups of IBU and the generated hydrogenbonded aggregates, such as dimers and trimers with linear or cyclic forms, in condensed phases [68,69].The carbonyl band at 1721 cm À1 indicates that IBU existed as hydrogen-bonded aggregates in all of the IBU-loaded samples.…”