The relaxation behavior of supercooled and glassy imidacloprid

Amorphous solid dispersions: Stability mechanism, design strategy and key production technique of hot melt extrusion

Solid dispersion (SD) system has been used as an effective formulation strategy to increase in vitro and in vivo performances of poorly water-soluble drugs, such as solubility/dissolution, stability and bioavailability. This review provides a comprehensive SD classification and identifies the most popular amorphous solid dispersions (ASDs). Meanwhile, this review further puts forward the systematic design strategy of satisfactory ASDs in terms of drug properties, carrier selection, preparation methods and stabilization mechanisms. In addition, hot melt extrusion (HME) as the continuous manufacturing technique is described including the principle and structure of HME instrument, key process parameters and production application, in order to guide the scale-up of ASDs and develop more ASD products to the market in pharmaceutical industry.

Molecular Dynamics and Diffusion in Amorphous Solid Dispersions Containing Imidacloprid

The main goal of this study is to develop an experimental toolbox to estimate the self-diffusion coefficient of active ingredients (AI) in single-phase amorphous solid dispersions (ASD) close to the glass transition of the mixture using dielectric spectroscopy (DS) and oscillatory rheology. The proposed methodology is tested for a model system containing the insecticide imidacloprid (IMI) and the copolymer copovidone (PVP/VA) prepared via hot-melt extrusion. For this purpose, reorientational and the viscoelastic structural (α-)relaxation time constants of hot-melt-extruded ASDs were obtained via DS and shear rheology, respectively. These were then utilized to extract the viscosity as well as the fragility index of the dispersions as input parameters to the fractional Stokes-Einstein (F-SE) relation. Furthermore, a modified version of Almond-West (AW) formalism, originally developed to describe charge diffusion in ionic conductors, was exercised on the present model system for the estimation of the AI diffusion coefficients based on shear modulus relaxation times. Our results revealed that, at the calorimetric glass-transition temperature (T_g), the self-diffusion coefficients of the AI in the compositional range from infinite dilution up to 60 wt % IMI content lied in the narrow range of 10^-18-10^-20 m² s^-1, while the viscosity values of the dispersions at T_g varied between 10⁸ Pa s and 10¹⁰ Pa s. In addition, the phase diagram of the IMI-PVP/VA system was determined using the melting point depression method via differential scanning calorimetry (DSC), while mid-infrared (IR) spectroscopy was employed to investigate the intermolecular interactions within the solid dispersions. In this respect, the findings of a modest variation in melting point at different compositions stayed in agreement with the observations of weak hydrogen bonding interactions between the AI and the polymer. Moreover, IR spectroscopy showed the intermolecular IMI-IMI hydrogen bonding to have been considerably suppressed, as a result of the spatial separation of the AI molecules within the ASDs. In summary, this study provides experimental approaches to study diffusivity in ASDs using DS and oscillatory rheology, in addition to contributing to an enhanced understanding of the interactions and phase behavior in these systems.

Predicting Dielectric and Shear-Rheology Properties of Glass-Forming Pharmaceutical Liquids from Each Other: Applications and Limitations

Acetaminophen, nicotine, and lidocaine hydrochloride were investigated in their deeply supercooled liquid states using oscillatory shear rheology. The mechanical spectra of these drugs are presented in modulus, compliance, as well as fluidity formats. Their frequency profiles can be described via models adapted from the field of charge transport. Inspired by the success of this approach, the Barton-Nakajima-Namikawa relation, best known from the same field, was also tested. When adapted to rheology, this approach interrelates static and dynamic characteristics of viscous flow and was found to work excellently. The temperature dependence of the characteristic shear frequencies was checked against the shoving model, which relates them to the temperature-dependent instantaneous shear modulus and acceptable agreement was found. Combined with shear mechanical literature data on ibuprofen and indomethacin, a modified version of the phenomenological model by Gemant, DiMarzio, and Bishop (GDB) was employed to successfully predict the shape and amplitude of the dielectric spectra for all studied liquids, except for lidocaine hydrochloride. For the latter, the modified GDB model is suggested to aid in mapping out the reorientational part of the dielectric response, while the experimental results are strongly superimposed by ionic conduction phenomena. The reverse transformation, the calculation of rheological spectra based on dielectric ones, is also successfully demonstrated. For the example of acetyl salicylic acid, it is shown how dielectric spectra can be used to even predict rheological ones. The limits of the central parameter governing these mutual transformations, the electroviscoelastic material constant, and indications for its correlation with the dielectric relaxation strength are discussed. For pharmaceuticals characterized by a strong dynamical decoupling of the electrical from the mechanical degrees of freedom, the modified GDB model is not expected to be applicable.

Two-Step Aging of Highly Polar Glass

Nonequilibrium processes, including physical aging, belong to the most challenging phenomena of glassy dynamics. One of the fundamental problems that needs clarification is the effect of material polarity on the time scale of the structural recovery of glass. The importance of this issue arises from practical applications and recent findings suggesting a substantial contribution of dipole-dipole interactions to the dielectric permittivity spectra of polar glass-formers. Herein, we use dielectric spectroscopy to investigate structural relaxation and aging dynamics of highly polar glass-former 4-[(4,4,5,5,5-pentafluoropentoxy)methyl]-1,3-dioxolan-2-one (FPC), a derivative of propylene carbonate with ε_s = 180 and μ = 5.1. We show that ε″(t_age) data of FPC at T_age < T_g reveal complex behavior resulting from considerable cross-correlation effects. Namely, two characteristic aging time scales, reflecting the evolution of cross-correlation mode and generic structural relaxation toward equilibrium, are obtained at a given T_age. Furthermore, a single stretched exponential behavior of ε″(t_age) has been received for weakly polar carvedilol with negligible dipole-dipole interactions.