Molecular relaxations in supercooled liquid and glassy states of amorphous gambogic acid: Dielectric spectroscopy, calorimetry, and theoretical approach

Abstract

The relaxation dynamics and thermodynamic properties of supercooled and glassy gambogic acid are investigated using both theory and experiment. We measure the temperature dependence of the relaxation times in three polymorphs (α−, β−, and γ− form). To gain insight into the relaxation processes, we propose a theoretical approach to quantitatively understand nature of these three relaxations. The α−relaxation captures cooperative motions of molecules while the β−process is mainly governed by local dynamics of a single molecule within the cage formed by its nearest neighbors. Based on quantitative agreement between theory and experimental data, our calculations clearly indicate that the β−process is a precursor of the structural relaxation and intramolecular motions are responsible for the γ−relaxation. Moreover, the approach is exploited to study effects of the heating process on alpha relaxation. We find that the heating rate varies logarithmically with Tg and 1000/Tg. These variations are qualitatively consistent with many prior studies. * Electronic address: anh.phanduc@phenikaa-uni.edu.vn † Electronic address: thuy.tran@inpc.vast.vn a long-range-disordered structure, which is temperature dependent. Its physical behaviors are liquid-like at high temperatures. However, at low temperatures, structural relaxation process is significantly slowed down. Physical mechanisms underlying the glassy dynamics remain mysterious.

Compression Effects on Structural Relaxation Process of Amorphous Indomethacin

“…In the ECNLE theory [18][19][20][21][22], an amorphous drug is described by an effective hard-sphere fluid having the particle diameter, σ , the particle number density, ρ, and the packing fraction, Φ = πρσ 3 /6. Structural properties of the reference system are inferred from the Percus-Yevick approximation [23].…”

“…Hence, the tagged particle is dynamically arrested within an intermolecular cage. The cage radius, r cage , is determined by the first minimum of g(r) [18][19][20][21][22]. Based on the free energy profile, one obtains three important length scales, including the localization length, r L , the barrier position, r B , and the jump distance, ∆r.…”

“…Apart from BDS measurements, one can use the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory to gain insights into the molecular mobility of amorphous drugs [18][19][20]. The ECNLE theory considers the α-relaxation as a dynamic coupling between local and collective activated events [18][19][20]. On that basis, the thermal response of τ α is determined by analyzing the bulk expansion process [18][19][20].…”

“…The ECNLE theory considers the α-relaxation as a dynamic coupling between local and collective activated events [18][19][20]. On that basis, the thermal response of τ α is determined by analyzing the bulk expansion process [18][19][20]. At ambient pressure, ECNLE calculations can provide τ α from 10 −12 to 10 2 s, which is far beyond the simulation timescale [18][19][20].…”

“…On that basis, the thermal response of τ α is determined by analyzing the bulk expansion process [18][19][20]. At ambient pressure, ECNLE calculations can provide τ α from 10 −12 to 10 2 s, which is far beyond the simulation timescale [18][19][20]. Recently, Phan et al [21,22] have modified the free energy profile to access the high-pressure regime.…”