Dissolution and solubility of calcite-rhodochrosite solid solutions [(Ca1-xMnx)CO3] at 25 °C

A complete series of calcite-rhodochrosite solid solutions [(Ca1-xMnx)CO3] are prepared, and their dissolution processes in various water samples are experimentally investigated. The crystal morphologies of the solid solutions vary from blocky spherical crystal aggregates to smaller spheres with an increasing incorporation of Mn in the solids. Regarding dissolution in N2-degassed water, air-saturated water and CO2-saturated water at 25 °C, the aqueous Ca and Mn concentrations reach their highest values after 1240–2400 h, 6–12 h and < 1 h, respectively, and then decrease gradually to a steady state; additionally, the ion activity products (log_IAP) at the final steady state (≈ solubility products in log_Ksp) are estimated to be − 8.46 ± 0.06, − 8.44 ± 0.10 and − 8.59 ± 0.10 for calcite [CaCO3], respectively, and − 10.25 ± 0.08, − 10.26 ± 0.10 and − 10.28 ± 0.03, for rhodochrosite [MnCO3], respectively. As XMn increases, the log_IAP values decrease from − 8.44 ~ − 8.59 for calcite to − 10.25 ~ − 10.28 for rhodochrosite. The aqueous Mn concentrations increase with an increasing Mn/(Ca + Mn) molar ratio (XMn) of the (Ca1-xMnx)CO3 solid solutions, while the aqueous Ca concentrations show the highest values at XMn = 0.53–0.63. In the constructed Lippmann diagram of subregular (Ca1-xMnx)CO3 solid solutions, the solids dissolve incongruently, and the data points of the aqueous solutions move progressively up to the Lippmann solutus curve and then along the solutus curve or saturation curve of pure MnCO3 to the Mn-poor side. The microcrystalline cores of the spherical crystal aggregates are preferentially dissolved to form core hollows while simultaneously precipitating Mn-rich hexagonal prisms.

KINETIC EVOLUTION OF A 3D SPHERICAL CRYSTAL WITH MOBILE PARTICLESUSING MONTE CARLO - PART II

In this work, the migration of the three-dimensional (3D) spherical crystal in the presence of mobile particles using aMonte Carlo algorithm was studied. Different concentrations of particles (f) and different particles mobilities (Mp)were used. It was found that the grain size reaches a critical radius (Rc) which depends exclusively onf. This dependence can be written as:Rc~f^1/3. The dynamic equation of grain size evolution and its analytical solution were alsofound. The analytical solution successfully fits the simulation results. The particles fraction in the grain boundary wasalso found analytically and it fits with the computational data.

KINETIC EVOLUTION OF A 3D SPHERICAL CRYSTAL WITH MOBILE PARTICLES USING MONTE CARLO

In this work, the evolution of a tridimensional (3D) spherical crystal with mobile particles using a Monte Carlo algorithm is presented. The mean radius R of spherical crystal without particles changes according to the law: R2 = -4kt + Ro2, where Ro is the initial radius and k is a crystal constant. However, this law is modified when mobile particles are included. The effect of two types of mobile particles on the grain boundary migration of a spherical grain was also studied. One type of particle remained located in the middle of the grain boundary once it was incorporated (CT), and the other type of particle remained at the grain boundary without having any particular location (NC). It could be seen that the CT particle slowed down more the grain boundary migration than the NC particles. It was also found that the rate of reduction of the grain area is inversely proportional to the concentration of CT particles in the grain boundary for all the CT particles concentrations. Finally, it was established that the grain reaches a limit radius for CT particles which is related to the amount of particles that can be accommodated in the grain boundary.

Formulation and in vitro Evaluation of Spherical Crystal Agglomerates of Ebastine by Quasi Emulsion Solvent Diffusion Method

Ebastine (EBS) is a poorly water-soluble antihistaminic drug; it belongs to the class II group according to the biopharmaceutical classification system (BCS). The aim of the present work was to enhance the solubility, dissolution rate and micromeritic properties of the drug, by formulating it as spherical crystal agglomerates by Quasi Emulsion Solvent Diffusion (QESD) method. Spherical crystal agglomerates (SCAs) were prepared in presence of three solvents dichloromethane (DCM), water and chloroform as a good solvent, poor solvent and bridging solvent respectively. Agglomeration of EBS involved the use of some hydrophilic polymers like polyethylene glycol 4000 (PEG 4000), polyvinyl pyrrolidine K30 (PVP K30), D-?-tocopheryl polyethylene glycol 1000 succinate (TPGS) and ?. cyclodextrin. The pure drug (EBS) and its agglomerates with and without polymers were characterized for their drug content, percentage yield, solubility, in vitro drug release study and micromeritic property as well as by optical microscope, Scanning Electron Microscopy (SEM), FTIR spectroscopic studies, Differential Scanning Calorimetry (DSC) and X-ray Diffraction (XRD). The results of this work showed that there was a marketed enhancement in the solubility with improvement in dissolution rate, physiochemical properties, decrease in crystallinity and alteration in the crystal habit of the drug especially in presence of polymers. The best results were obtained with formula prepared by the combination of PEG 4000 and ?. cyclodextrin in the agglomeration process of (EBS).