Melting-point depression by insoluble impurities: a finite size effect

Understanding melting behavior of aluminum clusters using machine learned deep neural network potential energy surfaces

Deep neural network-based deep potentials (DP), developed by Tuo et al., have been used to compute the thermodynamic properties of free aluminum clusters with accuracy close to that of density functional theory. Although Jarrold and collaborators have reported extensive experimental measurements on the melting temperatures and heat capacities of free aluminum clusters, no reports exist for finite-temperature ab initio simulations on larger clusters (N > 55 atoms). We report the heat capacities and melting temperatures for 32 clusters in the size range of 48-342 atoms, computed using the multiple histogram technique. Extensive molecular dynamics (MD) simulations at twenty four temperatures have been performed for all the clusters. Our results are in very good agreement with the experimental melting temperatures for 19 clusters. Except for a few sizes, the interesting features in the heat capacities have been reproduced. To gain insight into the striking features reported in the experiments, we used structural and dynamical descriptors such as temperature-dependent mean squared displacements and the Lindemann index. Bimodal features observed in Al116 and the weak shoulder seen in Al52 are attributed to solid-solid structural transitions. In confirmation of the earlier reports, we observe that the behavior of the heat capacities is significantly influenced by the nature of the ground state geometries. Our findings show that the sharp drop in the melting temperature of the 56-atom cluster is a consequence of the change in the geometry of Al55. Mulliken population analysis of Al55 reveals that the charge-induced local electric field is responsible for the strong bonding between core and surface atoms, leading to the higher melting temperature. Our calculations do not support the lower melting temperature observed in experimental studies of Al69. Our results indicate that Al48 is in a liquid state above 600 K and does not support the high melting temperature reported in the experiment. It turns out that the accuracy of the DP model by Tuo et al. is not reliable for MD simulations beyond 750 K. We also report low-lying equilibrium geometries and thermodynamics of 11 larger clusters (N = 147-342) that have not been previously reported, and the melting temperatures of these clusters are in good agreement with the experimental ones.

Core atoms escape from the shell: reverse segregation of Pb-Al core-shell nanoclusters via nanoscale melting

Melting is a phase transition that profoundly affects the fabrication and diverse applications of metal nanoclusters. Core-shell clusters offer distinctive properties and thus opportunities compared with other classes of nano-alloys. Molecular dynamics simulations have been employed to investigate the melting behaviour of Pb-Al core-shell clusters containing a fixed Pb147 core and varying shell thickness. Our results show that the core and shell melt separately. Surprisingly, core melting always drives the core Pb atoms to break out the shell and coat the nanoclusters in a reversed segregation process at the nanoscale. The melting point of the core increases with the shell thickness to exceed that of the bare core cluster, but the thinnest shell always supresses the core melting point. These results can be a reference for the future fabrication, manipulation, and exploitation of the core-shell nanoalloys chosen. The system chosen is ideally suited for experimental observations.

Quantitative Characterization of the Thermally Driven Alloying State in Ternary Ir-Pd-Ru Nanoparticles

Compositional and structural arrangements of constituent elements, especially those at the surface and near-surface layers, are known to greatly influence the catalytic performance of alloyed nanoparticles (NPs). Although much research effort often focuses on the ability to tailor these important aspects in the design stage, their stability under realistic operating conditions remains a major technical challenge. Here, the compositional stability and associated structural evolution of a ternary iridium-palladium-ruthenium (Ir-Pd-Ru) nanoalloy at elevated temperatures have been studied using interrupted in situ scanning transmission electron microscopy and theoretical modeling. The results are based on a combinatory approach of statistical sampling at the sub-nanometer scale for large groups of NPs as well as tracking individual NPs. We find that the solid solution Ir-Pd-Ru NPs (∼5.6 nm) evolved into a Pd-enriched shell supported on an alloyed Ir-Ru-rich core, most notably when the temperature exceeds 500 °C, concurrently with the development of expansive atomic strain in the outer surface and subsurface layers with respect to the core regions. Theoretically, we identify the weak interatomic bonds, low surface energy, and large atomic sizes associated with Pd as the key factors responsible for such observed features.

Why are Zn-rich Zn-Mg nanoalloys optimal protective coatings against corrosion? A first-principles study of the initial stages of the oxidation process

ZnMg alloys of certain compositions in the Zn-rich side of the phase diagram are particularly efficient, and widely used, as anticorrosive coatings, but a sound understanding of the physico-chemical properties behind such quality is still far from being achieved. The present work focuses on the first stage of the corrosion process, namely the initial growth of a sacrificial surface oxide layer, whose characteristics will condition the next stages of the corrosion. A comprehensive ab initio study, based on density functional theory, is carried out on ZnMg nanoalloys with 20 atoms and different compositions, which serve as model systems to simulate the complex processes that occur in extended granular surfaces. The structural and electronic properties, when progressive oxidation of the nanoalloys takes place, are analyzed in detail with the help of structural descriptors, energetic descriptors such as the oxygen adsorption energies and excess adsorption energies, as well as with electronic ones based on the topological analysis of the electron density and the electron localization function, from which a detailed analysis of the bonding patterns is extracted. We explain why small amounts of Mg create a very positive synergy between Zn and Mg that increases the reactivity to oxygen while reducing, at the same time, the stress induced on the cluster substrate, both facts working in favor of promoting the growth of the oxide crust whilst protecting the core. Moreover, we also show that stoichiometries close to the Mg₂Zn₁₁ and MgZn₂ compositions are the best candidates to optimize the protection against corrosion in Zn-Mg alloys, in agreement with the experimental observations.

Finite temperature behavior of carbon atom-doped silicon clusters: depressed thermal stabilities, coexisting isomers, reversible dynamical pathways and fragmentation channels

BOMD simulations revealed a multifarious thermo-stimuli response (from “solid-state” to reversible dynamics to fragmentation) of experimentally identified SiC mixed clusters at finite temperature.

Quantifications of Oleocolloid Matrices Made of Whey Protein and Oleogels

Consumer demand for high protein content and plant-based fat has necessitated novel approaches to healthy food products. In response to this need, oleogels (OG) (structured liquid oils) emerged as a possible means of not only replacing saturated and trans fats but also delivering food protein. Nevertheless, an in-depth view of the structure of networks made of OG and protein is deficient. Hence, the objective of this study is developing oleocolloid (OC) (whey protein and rice bran wax OG) and hydro-oleocolloid (HOC) (OC + water) matrices with varying protein content (2.5–7.5%) to characterize their structural properties. Thermal analysis of the matrices via differential scanning calorimetry (DSC) documented the effects of hydrophobic interactions on the protein structure and its stability. Whey protein denaturation temperature increased from 74.9 °C to 102.8 °C in the presence of high oleic soybean oil. The effects of vegetable oil on WPI structure was also verified by FTIR spectroscopy. Data analysis revealed slight structural changes of the WPI secondary structure in the hydrophobic oil medium and the α-helix and β-sheet proportion in the emulsion medium was significantly altered. Similar analysis was performed in OC and HOC networks to quantify possible interactions between protein and rice bran wax. Results indicated that the protein was denatured during the thermal and mechanical conditions required for the oleogelation process, while it did not affect the systems’ solid fat content (SFC) and polymorphic patterns of the oleogels. However, DSC analysis showed different onset of melting for OC and HOC samples due to colloidal interactions between the protein and the lipid phase. The role of these chemistry was confirmed by microscopy analyses where OC and HOC matrices displayed notably different microstructural properties. The observed differences in the structural properties between OC and HOC matrices indicate the different colloidal interactions mediated by oleogelation process and the liquid medium type (oil vs. emulsion).

Mean first passage times reconstruct the slowest relaxations in potential energy landscapes of nanoclusters

Relaxation modes are the collective modes in which all probability deviations from equilibrium states decay with the same relaxation rates. In contrast, a first passage time is the required time for arriving for the first time from one state to another. In this paper, we discuss how and why the slowest relaxation rates of relaxation modes are reconstructed from the first passage times. As an illustrative model, we use a continuous-time Markov state model of vacancy diffusion in KCl nanoclusters. Using this model, we reveal that all characteristics of the relaxations in KCl nanoclusters come from the fact that they are hybrids of two kinetically different regions of the fast surface and slow bulk diffusions. The origin of the different diffusivities turns out to come from the heterogeneity of the activation energies on the potential energy landscapes. We also develop a stationary population method to compute the mean first passage times as mean times required for pair annihilations of particle-hole pairs, which enables us to obtain the symmetric results of relaxation rates under the exchange of the sinks and the sources. With this symmetric method, we finally show why the slowest relaxation times can be reconstructed from the mean first passage times.

Further enhanced dissolution and oral bioavailability of docetaxel by coamorphization with a natural P-gp inhibitor myricetin

The current study aims to improve the dissolution and oral bioavailability of a BCS IV drug docetaxel (DOC) by coamorphization with a natural P-gp inhibitor myricetin (MYR). A single-phase coamorphous form of DOC with MYR in a 1:1 molar ratio was prepared by solvent-evaporation method and characterized by differential scanning calorimetry, thermogravimetric analysis and powder X-ray diffraction. In comparison to crystalline DOC, amorphous DOC showed similar equilibrium aqueous solubility, temporary improvement in the intrinsic dissolution rate (IDR) and supersaturated dissolution; while coamorphous DOC-MYR exhibited a persistent enhanced IDR and prolonged highly supersaturated dissolution. In addition, coamorphous DOC-MYR demonstrated significantly superior physical stability compared to amorphous DOC under the long-term storage condition and accelerated condition. Compared with oral administration of crystalline DOC to rats, amorphous DOC showed a significant increase in C_max (2.6-fold) and a marginal increase in AUC (1.3-fold) of DOC; but coamorphous DOC-MYR performed a 3.9-fold higher C_max and 3.1-fold higher AUC. In conclusion, coamorphization of DOC with MYR was a promising approach to enhance both dissolution and oral absorption of poorly soluble and permeable DOC.

Slowest kinetic modes revealed by metabasin renormalization

Understanding the slowest relaxations of complex systems, such as relaxation of glass-forming materials, diffusion in nanoclusters, and folding of biomolecules, is important for physics, chemistry, and biology. For a kinetic system, the relaxation modes are determined by diagonalizing its transition rate matrix. However, for realistic systems of interest, numerical diagonalization, as well as extracting physical understanding from the diagonalization results, is difficult due to the high dimensionality. Here, we develop an alternative and generally applicable method of extracting the long-time scale relaxation dynamics by combining the metabasin analysis of Okushima et al. [Phys. Rev. E 80, 036112 (2009)PLEEE81539-375510.1103/PhysRevE.80.036112] and a Jacobi method. We test the method on an illustrative model of a four-funnel model, for which we obtain a renormalized kinematic equation of much lower dimension sufficient for determining slow relaxation modes precisely. The method is successfully applied to the vacancy transport problem in ionic nanoparticles [Niiyama et al., Chem. Phys. Lett. 654, 52 (2016)CHPLBC0009-261410.1016/j.cplett.2016.04.088], allowing a clear physical interpretation that the final relaxation consists of two successive, characteristic processes.

Impurity effects on solid-solid transitions in atomic clusters

We use the harmonic superposition approach to examine how a single atom substitution affects low-temperature anomalies in the vibrational heat capacity (C_V) of model nanoclusters. Each anomaly is linked to competing solidlike "phases", where crossover of the corresponding free energies defines a solid-solid transition temperature (T_s). For selected Lennard-Jones clusters we show that T_s and the corresponding C_V peak can be tuned over a wide range by varying the relative atomic size and binding strength of the impurity, but excessive atom-size mismatch can destroy a transition and may produce another. In some tunable cases we find up to two additional C_V peaks emerging below T_s, signalling one- or two-step delocalisation of the impurity within the ground-state geometry. Results for Ni₇₄X and Au₅₄X clusters (X = Au, Ag, Al, Cu, Ni, Pd, Pt, Pb), modelled by the many-body Gupta potential, further corroborate the possibility of tuning, engineering, and suppressing finite-system analogues of a solid-solid transition in nanoalloys.

Charge-assisted intermolecular hydrogen bond formed in coamorphous system is important to relieve the pH-dependent solubility behavior of lurasidone hydrochloride

In comparison to amorphous LH, coamorphous LH-REP without evidence of intermolecular hydrogen bond, exhibited greatly improved solubility with pH-dependent behavior, significantly enhanced dissolution rate and physical stability.

Thermodynamics of nanoalloys

This article reviews recent advances in our understanding of how temperature affects the structure and the phase of multimetallic nanoparticles. Focusing on bimetallic systems, we discuss the interplay of size, shape and chemical order on the stable configurations at thermal equilibrium. Besides some considerations about experimental evidence for thermally-induced transformations, most insight is generally gained from theory and computation. The perspectives offered by mesoscopic approaches (i.e. corrected from the bulk) and atomistic simulations complement each other and often provide detailed information about the respective roles of coordination, composition and more generally surface effects to be evaluated. Order-disorder transitions and the melting phase change are strongly altered in nanoscale systems, and we describe how they possibly impact entire phase diagrams.

Reactions of CO2 on solid and liquid Al100+

The reactions of CO(2) on the Al(100)(+) cluster have been investigated as a function of cluster temperature (300-1100 K) and relative kinetic energy (0.2-10 eV). Two main products are observed at low cluster temperature: Al(100)O(+) (which is believed to result from a stripping reaction) and Al(100)CO(2)(+) from complex formation. As the cluster temperature is raised, both products dissociate by loss of Al(2)O. Al(100)O(+) forms Al(98)(+), while Al(100)CO(2)(+) forms Al(98)CO(+) and Al(96)C(+). In both cases, loss of Al(2)O turns-on above the melting temperature of Al(100)(+). This presumably occurs because the overall reaction leading to the loss of Al(2)O is significantly less endothermic for the liquid cluster than for the solid.

Thermodynamic properties of Ga27Si3 cluster using density functional molecular dynamics

Density functional molecular dynamical calculations have been carried out to explore the effect of silicon impurities on thermodynamic properties of Ga(30). We have obtained 500 distinct low energy equilibrium geometries of Ga(27)Si(3) in order to obtain reliable ground state geometry. The specific heat has been calculated using multiple histogram techniques and compared with that of Ga(30). We demonstrate that silicon impurities have a dramatic effect on the thermodynamic properties of the host cluster. In contrast to Ga(30), the specific heat of Ga(27)Si(3) shows a clear melting peak at ≈500 K, changing the character of Ga(30) from a nonmelter to a melter.

Melting and Freezing of Metal Clusters

Recent developments allow heat capacities to be measured for size-selected clusters isolated in the gas phase. For clusters with tens to hundreds of atoms, the heat capacities determined as a function of temperature usually have a single peak attributed to a melting transition. The melting temperatures and latent heats show large size-dependent fluctuations. In some cases, the melting temperatures change by hundreds of degrees with the addition of a single atom. Theory has played a critical role in understanding the origin of the size-dependent fluctuations, and in understanding the properties of the liquid-like and solid-like states. In some cases, the heat capacities have extra features (an additional peak or a dip) that reveal a more complex behavior than simple melting. In this article we provide a description of the methods used to measure the heat capacities and provide an overview of the experimental and theoretical results obtained for sodium and aluminum clusters.

Metal clusters with hidden ground states: Melting and structural transitions in Al115(+), Al116(+), and Al117(+)

Heat capacities measured as a function of temperature for Al(115)(+), Al(116)(+), and Al(117)(+) show two well-resolved peaks, at around 450 and 600 K. After being annealed to 523 K (a temperature between the two peaks) or to 773 K (well above both peaks), the high temperature peak remains unchanged but the low temperature peak disappears. After considering the possible explanations, the low temperature peak is attributed to a structural transition and the high temperature peak to the melting of the higher enthalpy structure generated by the structural transition. The annealing results show that the liquid clusters freeze exclusively into the higher enthalpy structure and that the lower enthalpy structure is not accessible from the higher enthalpy one on the timescale of the experiments. We suggest that the low enthalpy structure observed before annealing results from epitaxy, where the smaller clusters act as a nucleus and follow a growth pattern that provides access to the low enthalpy structure. The solid-to-solid transition that leads to the low temperature peak in the heat capacity does not occur under equilibrium but requires a superheated solid.