Particularly relevant in the context of polymorphism is understanding how structural, thermodynamic, and kinetic factors dictate the stability domains of polymorphs, their tendency to interconvert through phase transitions, or their possibility to exist in metastable states. These three aspects were investigated here for two 4′-hydroxyacetophenone (HAP) polymorphs, differing in crystal system, space group, and number and conformation of molecules in the asymmetric unit. The results led to a Δf G m°-T phase diagram highlighting the enantiotropic nature of the system and the fact that the Z′ = 1 polymorph is not necessarily more stable than its Z′ = 2 counterpart. It was also shown that the form II → form I transition is entropy driven and is likely to occur through a nucleation and growth mechanism, which does not involve intermediate phases, and is characterized by a high activation energy. Finally, although it has been noted that conflicts between hydrogen bond formation and close packing are usually behind exceptions from the hypothesis of Z′ = 1 forms being more stable than their higher Z′ analogues, in this case, the HAP polymorph with stronger hydrogen bonds (Z′ = 2) is also the one with higher density.

Abstract:Theoretical and experimental studies indicate that crystal nucleation can take more complex pathways than expected on the ground of the classical nucleation theory. Among these pathways are the formation of prenucleation clusters and amorphous precursor phases. A direct in situ observation of the different pathways of nucleation from solution is challenging since the paths can be influenced by heterogeneous nucleation sites, such as container walls. Here, we provide insights into the crystallization process using the in situ combination of an acoustic levitator, Raman spectroscopy, and X-ray scattering. The contactless sample holder enables the observation of homogeneous crystallization processes and the detection of intermediates and final crystalline forms. We provide evidence for the existence of multiple pathways of nucleation based on the investigation of the crystallization of organic molecules from different solvents. Starting from a diluted solution, a supersaturation is reached during the experiment due to the evaporation of the solvent. The highly supersaturated solution reveals different pathways of crystallization. Depending on the degree of supersaturation either the thermodynamically stable or the metastable crystal form is observed.

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Copper nanodisks (Cu NDs) of 50 nm were prepared on square-inch anodic aluminum oxide substrates by sputtering method. The samples were annealed at 450oC, then the walls of AAO substrates were lift off in a solution of acid phosphoric. The 2D arrays of Cu NDs were fabricated in high quality. Morphology of the substrates were observed by scanning electron microscopy. Surface plasmon resonance absorption was observed with different peaks in the range of 400-1400nm wavelengths that means the substrate is promising for SERS application. To demonstrate, we observed Raman spectrum of Rhodamine 6G using the enhancement effect of the substrate.

The paper presents the effects of NaH2PO2 concentrations in electrodeposition solution on phosphorus contents and corrosion resistance of Ni-P layer. The results of SEM/EDX show that when the concentration of NaH2PO2 in solution increases, the content of phosphorus and corrosion resistance of the layer also increase and the alloy's state changes from coarse granules to a smooth surface. The paper also describes the electrodeposition method to make black Ni-P multilayer coating. The corrosion resistance of black multilayer Ni-P alloys including Ni-P (high phosphorus)/Ni-P (low phosphorus)/black Ni-P-Zn is 6 times higher than that of blackened steel when testing using salt spray methods.