Kinetics of Dissolution of an Amorphous Solid

Water-Mediated Conversion of BaTiO3 Nanoparticles into BaCO3 Nanorods in Electrospun Polymer Fibers: Implications for Carbon Capture Applications

Under ambient conditions and in aqueous environments, transformations of nanoparticle-based ferroelectric components can raise important stability issues that are relevant for applications as multilayer capacitors, flexible piezoelectrics, or biomedical devices. We show that X-ray amorphous BaTiO3 nanoparticles that were grown by flame spray pyrolysis and which can be incorporated into electrospun polymer fibers undergo incongruent Ba2+ dissolution in the presence of water. At pH > 5 and in contact with air, corresponding Ba solutes spontaneously convert into crystalline BaCO3 needles to produce characteristic nano- and microstructures. We compared the reactivity of amorphous BaTiO3 nanoparticle powders with those of nanocrystals after annealing-induced crystallization. The stability of aqueous nanoparticle-polymer formulations, which are typically part of nanoparticle encapsulation in polymers and electrospinning, was included in this analysis. Nanoparticle size, crystallinity, surface area, the presence of carbonaceous surface contaminants, and the effect of surface passivation with polymers are addressed to underline the critical role of condensed water during the synthesis, storage, and processing of BaTiO3 nanoparticle-based composites.

Computation of the chemical potential and solubility of amorphous solids

Using a recently developed technique to estimate the equilibrium free energy of glassy materials, we explore if equilibrium simulation methods can be used to estimate the solubility of amorphous solids. As an illustration, we compute the chemical potentials of the constituent particles of a two-component Kob-Andersen model glass former. To compute the chemical potential for different components, we combine the calculation of the overall free energy of the glass with a calculation of the chemical potential difference of the two components. We find that the standard method to compute chemical potential differences by thermodynamic integration yields not only a wide scatter in the chemical potential values, but also, more seriously, the average of the thermodynamic integration results is well above the extrapolated value for the supercooled liquid. However, we find that if we compute the difference in the chemical potential of the components with the non-equilibrium free-energy expression proposed by Jarzynski, we obtain a good match with the extrapolated value of the supercooled liquid. The extension of the Jarzynski method that we propose opens a potentially powerful route to compute the free-energy related equilibrium properties of glasses. We find that the solubility estimate of amorphous materials obtained from direct-coexistence simulations is only in fair agreement with the solubility prediction based on the chemical potential calculations of a hypothetical "well-equilibrated glass." In direct-coexistence simulations, we find that, in qualitative agreement with experiments, the amorphous solubility decreases with time and attains a low solubility value.

Influence of the pK a Value of Cinnamic Acid and P-Hydroxycinnamic Acid on the Solubility of a Lurasidone Hydrochloride-Based Coamorphous System

Coamorphization of a poorly water-soluble active pharmaceutical ingredient (API) has been proven to be effective in improving its solubility. Generally, API can form multiple coamorphous systems with different coformers. However, it remains unclear how the pK _a value of different coformers influences the solubility of the API. In this study, structurally related cinnamic acid (CA, pK _a = 4.37) and p-hydroxycinnamic acid (pHCA, pK _a = 4.65) were chosen as coformers for the coamorphization of lurasidone hydrochloride (LH). To investigate the influence of the pK _a value of the coformers on the solubility of LH, LH-CA/pHCA coamorphous systems were prepared by the vacuum rotary evaporation method and characterized by powder X-ray diffraction and differential scanning calorimetry. Fourier-transform infrared spectroscopy, Raman spectroscopy, and molecular dynamics (MD) simulations were employed to investigate the intermolecular interaction of the coamorphous systems. It was found that the solubility of LH in the coamorphous LH-pHAC with a higher-pK _a coformer was higher than that of the coamorphous LH-CA. In addition, according to the solubility product principle-based formula derivation, we established the functional relationship between the solubility of LH and the pK _a of the coformers at different-pH buffering solution. It was found that the coformer with a larger pK _a value would be more beneficial to improve the solubility profile of LH. Collectively, the current study offers an effective strategy to improve the poor solubility of drugs by increasing the pK _a value of the coformer in coamorphous systems.

Formation of Ultrastable Glasses via Precipitation: A Modeling Study

The precipitation of a glass forming solute from solution is modeled using a lattice model previously introduced to study dissolution kinetics of amorphous materials. The model includes the enhancement of kinetics at the surface of a glass in contact with a plasticizing solvent. We demonstrate that precipitation can produce a glass substantially more stable than that produced by very long time annealing of the bulk glass former. The energy of these ultrastable amorphous precipitates is found to be dominated by residual solvent rather than high energy glass configurations.