Form selection of concomitant polymorphs: A case study informed by crystallization kinetics modeling

Molecular mechanisms and process kinetics of crystallizing concomitant polymorphs remain poorly understood. Solvent-mediated phase transformation and concomitant crystallization are difficult to be distinguished in practice, as multiple forms can be detected at the same time. Herein, we developed a population balance model to simulate a concomitant crystallization process of two polymorphs of tolfenamic acid. Our kinetic modeling aims to understand concomitant crystallization and help guide form selection of such a molecular system. Crystallization kinetics of ethanolic solutions were uncovered from induction time measurements, as well as seeded and unseeded crystallization experiments. Experimental and simulation results demonstrate that the stable form I crystallizes concomitantly with the metastable form II. The faster growing form II results in an intermediate decline in the composition of form I in crystallized samples, a characteristic feature of the concomitantly crystallized system. A four-quadrant scheme of attainable polymorph outcome was simulated under various crystallization conditions.     

New kinetic models for the crystallization of anatase nanoparticles from amorphous titania (TiO2)

In this article, amorphous Titania (TiO₂) nanoparticles were crystallized both in a solution environment and by a dry heat treatment using new kinetic models with the capabilities of more accurately predicting the polymorphic transformation behavior of TiO₂. In these models, both the nucleation and growth processes were simultaneously taken into account. The results were indicative of the Surface Nucleation (SN) of anatase occurring in a hydrothermal treatment process through the phase transformation. Also, both the surface and interface nucleation processes were found to play significant roles in the phase transformation kinetics when dealing with dry heat treatments at low temperatures. The proposed models were advantageous to any other published models, in which the nucleation mechanisms had been incorporated with suitable growth expressions. In other words, no experimental data of a particle size were required to investigate the phase transformation kinetics of TiO₂ nanoparticles in the models presented in this article.     

Hydroxyl Group Separation Distances in Anti-Freeze Compounds and Their Effects on Ice Nucleation

Since the discovery of biological antifreeze glycoproteins (AFGPs), which can inhibit ice nucleation, there has been considerable interest in understanding their mechanisms and mimicking them in synthetic polymers. In this study, we used molecular dynamics simulations of modified polyvinyl alcohol (PVA) compounds to show that the hydroxyl (OH) group distance is a key factor in whether certain compounds promote or inhibit ice nucleation. A hydroxyl distance smaller than ~2.8 Å but greater than ~7.1 Å in modified PVA (MPVA) compounds was associated with the promotion of ice nucleation, while a hydroxyl group separation distance of approximately ~5.0 Å was correlated with a delay in ice nucleation, owing to changes in the energy of the system. Thus, these results may help explain some of the mechanisms of current known anti-freeze compounds and may have implications for designing new anti-freeze compounds in the future.     

In Situ Liquid-Cell TEM Observation of Multiphase Classical and Nonclassical Nucleation of Calcium Oxalate

Calcium oxalate (CaOx) is the major phase in kidney stones and the primary calcium storage medium in plants. CaOx can form crystals with different lattice types, water contents, and crystal structures. However, the conditions and mechanisms leading to nucleation of particular CaOx crystals are unclear. Here, liquid-cell transmission electron microscopy and atomistic molecular dynamics simulations are used to study in situ CaOx nucleation at different conditions. The observations reveal that rhombohedral CaOx monohydrate (COM) can nucleate via a classical pathway, while square COM can nucleate via a non-classical multiphase pathway. Citrate, a kidney stone inhibitor, increases the solubility of calcium by forming calcium-citrate complexes and blocks oxalate ions from approaching calcium. The presence of multiple hydrated ionic species draws additional water molecules into nucleating CaOx dihydrate crystals. These findings reveal that by controlling the nucleation pathways one can determine the macroscale crystal structure, hydration state, and morphology of CaOx.     

Er2Ti2O7烧绿石基玻璃陶瓷固化体的制备工艺研究

为获得含单一钛酸盐烧绿石的玻璃陶瓷固化体，本文以Er₂Ti₂O₇作为钛酸盐烧绿石代表，在传统的烧结法制备工艺基础上，通过热雾喷解和晶核掺入的技术改进，分别研究了烧结温度、烧结时间、玻璃与陶瓷晶核质量配比等参数对Er₂Ti₂O₇基玻璃陶瓷固化体物相及结构的影响规律。XRD结果表明：在不同的烧结温度、烧结时间和质量配比下，均能获得含单一Er₂Ti₂O₇烧绿石的玻璃陶瓷固化体，Er₂Ti₂O₇前驱体在玻璃中的相稳定性好，未发生相分解，玻璃组分亦未受陶瓷相的影响而析出第二相；玻璃组分对陶瓷颗粒存在张应力作用，玻璃含量越高，Er₂Ti₂O₇的晶胞常数越大；提高烧结温度和烧结时间均有利于增强固化体中Er₂Ti₂O₇的结构有序性，但烧结温度效果强于烧结时间。SEM结果表明：Er₂Ti₂O₇在玻璃基体上呈四方形生长，与玻璃界面清晰，相容性好。固化体的物性测试结果表明，玻璃组分占比将直接影响固化体的表观孔隙率和致密度。根据以上结果可知，新工艺制备的样品具有相纯度高、烧绿石相与玻璃相容性好、两相比例可调等优点，较好地解决了钛酸盐烧绿石在玻璃基体中易发生相分解的问题。     

Rapid crystallization of poly(ethylene 2,6-naphthalate) via aryl amide derivatives

ABSTRACT

A very small amount of aryl amide derivative (TMB-5) was used for nucleating Poly(ethylene 2,6-naphathalate) (PEN) by melt blending. The crystalline temperature, crystallinity, and nucleation efficiency of the composites were increased as a result of the addition of TMB-5. The half-time of crystallization decreased upon the addition of TMB-5 and the work required in folding polymer chains were reduced simultaneously in the polymer mixtures. The crystal size of the PEN/TMB-5 blends became smaller due to the increase of nucleation sites. The TMB-5 had no effect on the PEN crystal type. TMB-5 slightly increased the impact strength of PEN.     

Validation of Models Predicting Nucleation Rates from Induction Times and Metastable Zone Widths

Three approaches for estimation of nucleation rates from induction time and metastable zone width (MSZW) were validated based on directly measured nucleation rates for paracetamol in ethanol. To quantitatively predict nucleation kinetics using Kubota's methods it is necessary to know the minimum detectable number concentration of nuclei. This was found by determination of light transmission of a series of diluted suspensions of newly nucleated crystals where the size had already been assessed by optical reflection measurement (ORM). The measured nucleation rates strongly depended on both temperature and supersaturation. The Nyvlt method predicted nucleation rates in this system reasonably well; however, it gave slightly low estimates for all temperatures. The methods of Kubota provided nucleation rates that were low by an order of magnitude.     

十二烷基硫酸钠（SDS）对铜电沉积行为的影响

在酸性体系中采用阴极扫描伏安和计时电流等电化学测试方法，研究添加十二烷基硫酸钠（SDS）对铜电沉积过程的影响机理。结果表明：SDS的添加使沉积电位正移，降低了阴极极化。SDS浓度低于临界胶束浓度1g/L时，铜的成核弛豫时间延长，形核速率降低。SDS浓度高于临界胶束浓度时，形成SDS球状胶束，铜的成核弛豫时间减少，形核速率加快。铜晶核形成过程符合Scharitker?Hill 三维成核/生长机制，当SDS为1g/L时，在-0.2V的低过电位区，铜结晶按渐进成核方式进行，在-0.23--0.28V的高过电位区，铜结晶按瞬时成核方式进行。而当SDS为0.5g/L时，铜在-0.2V--0.25V的低电位区和在-0.28V的高电位区均符合渐进成核。     

Nanoparticle evolution in flame spray pyrolysis—Process design via experimental and computational analysis

In flame spray pyrolysis (FSP), the evolution of metal oxide nanoparticles relies on quite a number of droplet (liquid) and vapor phase related physical mechanism as for instance precursor evaporation, oxidation, nucleation via gas-to-particle conversion mechanism, and subsequent particle (solid) growth mechanisms based on coagulation, sintering/coalescence, and agglomeration. The liquid precursor and dispersion oxygen feed rates are relevant control parameters of the FSP process for tailoring the nanoparticle size (diameter) and structure as well as the atomizer nozzle configuration. Sophisticated nonintrusive, laser-based in situ and ex situ diagnostics with multiscale spatial resolution (micrometer to meter range) are applied for analyzing droplet formation and size, gas velocity, temperature, species concentration, as well as primary and agglomerate diameters along the flow direction. Computational fluid dynamics (CFD) are coupled with population balance modeling (PBM) to elucidate the nanoparticle dynamics within the reactive spray. It is found that the CFD-PBM approach allows estimations of primary and agglomerate nanoparticle diameters within 80 and 75% accuracy compared to experimental data, suggesting that the methods presented could pave the way for designing next-generations of flame reactors.