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Dissolution of ionic salts in water is ubiquitous, particularly for NaCl. However, an atomistic scale understanding of the process remains elusive. Simulations lend themselves conveniently to studying dissolution since they provide the spatio-temporal resolution that can be difficult to obtain experimentally. Nevertheless, the complexity of various inter- and intra-molecular interactions require careful treatment and long time scale simulations, both of which are typically hindered by computational expense. Here, we use advances in machine learning potential methodology to resolve at an ab initio level of theory the dissolution mechanism of NaCl in water. The picture that emerges is that of a steady ion-wise unwrapping of the crystal preceding its rapid disintegration, reminiscent of crumbling. The onset of crumbling can be explained by a strong increase in the ratio of the surface area to volume of the crystal. Overall, dissolution comprises a series of highly dynamical microscopic sub-processes, resulting in an inherently stochastic mechanism. These atomistic level insights contribute to the general understanding of dissolution mechanisms in other crystals, and the methodology is primed for more complex systems of recent interest such as water/salt interfaces under flow and salt crystals under confinement. [ABSTRACT FROM AUTHOR]

This article is a compilation of discussions among researchers on various scientific topics. The researchers explore the aggregation of nanoparticles in the presence of different salts, the behavior of DNA-functionalized nanoparticles, and the use of charged particles as swimmers. They also discuss the efficiency of water purification tests using micro/nano-motors and the analysis of hydrogen bond strength and density in water. The text delves into the behavior of lipid bilayers and their interactions with water, including the dynamics of lipid molecules and the formation of a metastable prepore. The researchers provide explanations and insights based on their findings, while acknowledging the need for further research in these areas. [Extracted from the article]

This document is a discussion from the journal Faraday Discussions on various topics related to the dynamics and nano-rheology of interfacial water. The participants discuss the properties of water under confinement, the use of theoretical concepts and tools to understand liquid behaviors, and the experimental confirmation of theoretical calculations. They also explore the effects of water thickness on confinement, the conductivity and selectivity of confined water, and the challenges of studying confined water under extreme conditions. The text also discusses the use of computational methods to study ion channels and their selectivity, as well as the role of conformational rigidity and fluctuations in system behavior. The authors provide insights into their research on ice nucleation and crystal growth, as well as the dissolution process of sodium chloride nanocrystals in water. The study found that factors such as ion-water interactions and surface area-to-volume ratio influence the dissolution process. [Extracted from the article]

Vibrational spectroscopy is a powerful approach to visualising interfacial phenomena. However, extracting structural and dynamical information from vibrational spectra is a challenge that requires first-principles simulations, including non-Condon and quantum nuclear effects. We address this challenge by developing a machine-learning enhanced first-principles framework to speed up predictive modelling of infrared, Raman, and sum-frequency generation spectra. Our approach uses machine learning potentials that encode quantum nuclear effects to generate quantum trajectories using simple molecular dynamics efficiently. In addition, we reformulate bulk and interfacial selection rules to express them unambiguously in terms of the derivatives of polarisation and polarisabilities of the whole system and predict these derivatives efficiently using fully-differentiable machine learning models of dielectric response tensors. We demonstrate our framework's performance by predicting the IR, Raman, and sum-frequency generation spectra of liquid water, ice and the water–air interface by achieving near quantitative agreement with experiments at nearly the same computational efficiency as pure classical methods. Finally, to aid the experimental discovery of new phases of nanoconfined water, we predict the temperature-dependent vibrational spectra of monolayer water across the solid-hexatic-liquid phases transition. [ABSTRACT FROM AUTHOR]

Nucleation in small volumes of water has garnered renewed interest due to the relevance of pore condensation and freezing under conditions of low partial pressures of water, such as in the upper troposphere. Molecular simulations can in principle provide insight on this process at the molecular scale that is challenging to achieve experimentally. However, there are discrepancies in the literature as to whether the rate in confined systems is enhanced or suppressed relative to bulk water at the same temperature and pressure. In this study, we investigate the extent to which the size of the critical nucleus and the rate at which it grows in thin films of water are affected by the thickness of the film. Our results suggest that nucleation remains bulk-like in films that are barely large enough accommodate a critical nucleus. This conclusion seems robust to the presence of physical confining boundaries. We also discuss the difficulties in unambiguously determining homogeneous nucleation rates in nanoscale systems, owing to the challenges in defining the volume. Our results suggest any impact on a film's thickness on the rate is largely inconsequential for present day experiments. [ABSTRACT FROM AUTHOR]

Gaining a fundamental understanding of crystal nucleation processes in metal alloys is crucial for the development and design of high-performance materials with targeted properties. Yet, crystallization is a complex non-equilibrium process and, despite having been studied for decades, the microscopic aspects that govern the crystallization mechanism of a material remain elusive to date. Recent evidence shows that the spatial heterogeneity in the supercooled liquid, characterised by extended regions with distinctive mobility and order, may be a key microscopic factor that determines the mechanism of crystal nucleation. These findings have advanced our view of the fundamental nature of crystallization, as most research has assumed that crystal clusters nucleate from random fluctuations in a 'homogeneous' liquid. Here, by analysing transition path sampling trajectories, we show that dynamical heterogeneity plays a key role in the mechanism of crystal nucleation in an elemental metal, nickel. Our results demonstrate that crystallization occurs preferentially in regions of low mobility in the supercooled liquid, evidencing the collective dynamical nature of crystal nucleation in Ni. In addition, our results show that low mobility regions form before and spatially overlap with pre-ordered domains that act as precursors to the crystal phase that subsequently emerges. Our results show a clear link between dynamical and structural heterogeneity in the supercooled liquid and its impact on the nucleation mechanism, revealing microscopic descriptors that could pave a novel way to control crystallization processes in metals. [ABSTRACT FROM AUTHOR]

The interfacial tension (IFT) of a fluid–fluid interface plays an important role in a wide range of applications and processes. When low IFT is desired, surface active compounds (e.g. surfactants) can be added to the system. Numerous attempts have been made to relate changes in IFT arising from such compounds to the specific nature of the interface. However, the IFT is controlled by an interplay of factors such as temperature and molecular structure of surface-active compounds, which make it difficult to predict IFT as those conditions change. In this study, we present the results from molecular dynamics simulations revealing the specific role surfactants play in IFT. We find that, in addition to reducing direct contact between the two fluids, surfactants serve to increase the disorder at the interface (related to interfacial entropy) and consequently reduce the water/oil IFT, especially when surfactants are present at high surface density. Our results suggest that surfactants that yield more disordered interfacial films (e.g. with flexible and/or unsaturated tails) reduce the water/oil IFT more effectively than surfactants which yield highly ordered interfacial films. Our results shed light on some of the factors that control IFT and could have important practical implications in industrial applications such as the design of cosmetics, food products, and detergents. [ABSTRACT FROM AUTHOR]

The corrosion of materials is an undesirable and costly process affecting many areas of technology and everyday life. As such, considerable effort has gone into understanding and preventing it. Organic molecule based coatings can in certain circumstances act as effective corrosion inhibitors. Although they have been used to great effect for more than sixty years, how they function at the atomic-level is still a matter of debate. In this work, computer simulation approaches based on density functional theory are used to investigate benzotriazole (BTAH), one of the most widely used and studied corrosion inhibitors for copper. In particular, the structures formed by protonated and deprotonated BTAH molecules on Cu(111) have been determined and linked to their inhibiting properties. It is found that hydrogen bonding, van der Waals interactions and steric repulsions all contribute in shaping how BTAH molecules adsorb, with flat-lying structures preferred at low coverage and upright configurations preferred at high coverage. The interaction of the dehydrogenated benzotriazole molecule (BTA) with the copper surface is instead dominated by strong chemisorption via the azole moiety with the aid of copper adatoms. Structures of dimers or chains are found to be the most stable structures at all coverages, in good agreement with scanning tunnelling microscopy results. Benzotriazole thus shows a complex phase behaviour in which van der Waals forces play an important role and which depends on coverage and on its protonation state and all of these factors feasibly contribute to its effectiveness as a corrosion inhibitor. [ABSTRACT FROM AUTHOR]

It is surprisingly difficult to freeze water. Almost all ice that forms under “mild” conditions (temperatures > −40 °C) requires the presence of a nucleating agent – a solid particle that facilitates the freezing process – such as clay mineral dust, soot or bacteria. In a computer simulation, the presence of such ice nucleating agents does not necessarily alleviate the difficulties associated with forming ice on accessible timescales. Nevertheless, in this work we present results from molecular dynamics simulations in which we systematically compare homogeneous and heterogeneous ice nucleation, using the atmospherically important clay mineral kaolinite as our model ice nucleating agent. From our simulations, we do indeed find that kaolinite is an excellent ice nucleating agent but that contrary to conventional thought, non-basal faces of ice can nucleate at the basal face of kaolinite. We see that in the liquid phase, the kaolinite surface has a drastic effect on the density profile of water, with water forming a dense, tightly bound first contact layer. Monitoring the time evolution of the water density reveals that changes away from the interface may play an important role in the nucleation mechanism. The findings from this work suggest that heterogeneous ice nucleating agents may not only enhance the ice nucleation rate, but also alter the macroscopic structure of the ice crystals that form. [ABSTRACT FROM AUTHOR]

Ordinary water ice forms under ambient conditions and has two polytypes, hexagonal ice (Ih) and cubic ice (Ic). From a careful comparison of proton ordering arrangements in Ih and Ic using periodic density functional theory (DFT) and diffusion Monte Carlo (DMC) approaches, we find that the most stable arrangement of water molecules in cubic ice is isoenergetic with that of the proton ordered form of hexagonal ice (known as ice XI). We denote this potential new polytype of ice XI as XIc and discuss a possible route for preparing ice XIc. [ABSTRACT FROM AUTHOR]

The initial stages of NaCl dissolution in liquid water have been examined with state-of-the-art ab initiomolecular dynamics and free energy sampling techniques. Our simulations reveal a complex multi-step process triggered by the departure of Cl ions from the lattice, with a well-defined intermediate state wherein departing ions are partially solvated but remain in contact with the crystal. The polarizability of Cl−is identified as the source of the anion's preferential initial dissolution, an effect which leads a forcefield based description of NaCl dissolution to fail to identify a preference for Cl over Na dissolution. [ABSTRACT FROM AUTHOR]

The interaction of water with solid surfaces is key to a wide variety of industrial and natural processes. However, the basic principles that dictate how stable and in which state (intact or dissociated) water will be on a given surface are not fully understood. Towards this end, we have used density functional theory to examine water monomer adsorption on the (001) surfaces of a broad range of alkaline earth oxides, alkaline earth sulfides, alkali fluorides, and alkali chlorides. Some interesting general conclusions are arrived at: (i) on all the surfaces considered only a few specific adsorption structures are favoured; (ii) water becomes more stable upon descending the oxide and fluoride series but does not vary much upon going down the chloride and sulfide series; (iii) water is stabilised both by an increase in the lattice constant, which facilitates hydrogen bonding to the substrate, and by the flexibility of the substrate. These are also factors that favour water dissociation. We hope that this study is of some value in better understanding the surface science of water in general, and in assisting in the interpretation and design of future experiments. [ABSTRACT FROM AUTHOR]

The great promise of ceria-supported gold clusters as catalysts of the future for important industrial processes, such as the water gas shift reaction, has prompted a flurry of activity aimed at understanding the molecular-level details of their operation. Much of this activity has focused on experimental and theoretical studies of the structure of perfect and defective ceria surfaces, with and without gold clusters of various sizes. The complicated electronic structure of ceria, particularly in its reduced form, means that at present it is highly challenging to carry out accurate electronic structure simulations of such systems. To overcome the challenges, the majority of recent theoretical studies have adopted a pragmatic and often controversial approach, applying the so-called DFT + Utechnique. Here we will briefly discuss some recent studies of Au on CeO2{111} that mainly use this methodology. We will show that considerable insight has been obtained into these systems, particularly with regard to Au adsorbates and Au cluster reactivity. We will also briefly discuss the need for improved electronic structure methods, which would enable more rigorous and robust studies in the future. [ABSTRACT FROM AUTHOR]

Organic molecules such as steroids or amino acids form crystals that can facilitate the formation of ice - arguably the most important phase transition on earth. However, the origin of the ice nucleating ability of organic crystals is still largely unknown. Here, we combine experiments and simulations to unravel the microscopic details of ice formation on cholesterol, a prototypical organic crystal widely used in cryopreservation. We find that cholesterol - which is also a substantial component of cell membranes - is an ice nucleating agent more potent than many inorganic substrates, including the mineral feldspar (one of the most active ice nucleating materials in the atmosphere). Scanning electron microscopy measurements reveal a variety of morphological features on the surfaces of cholesterol crystals: this suggests that the topography of the surface is key to the broad range of ice nucleating activity observed (from -4 to -20 °C). In addition, we show via molecular simulations that cholesterol crystals aid the formation of ice nuclei in a unconventional fashion. Rather than providing a template for a flat ice-like contact layer (as found in the case of many inorganic substrates), the flexibility of the cholesterol surface and its low density of hydrophilic functional groups leads to the formation of molecular cages involving both water molecules and terminal hydroxyl groups of the cholesterol surface. These cages are made of 6- and, surprisingly, 5-membered hydrogen bonded rings of water and hydroxyl groups that favour the nucleation of hexagonal as well as cubic ice (a rare occurrence). We argue that the phenomenal ice nucleating activity of steroids such as cholesterol (and potentially of many other organic crystals) is due to (i) the ability of flexible hydrophilic surfaces to form unconventional ice-templating structures and (ii) the different nucleation sites offered by the diverse topography of the crystalline surfaces. These findings clarify how exactly organic crystals promote the formation of ice, thus paving the way toward deeper understanding of ice formation in soft and biological matter - with obvious reverberations on atmospheric science and cryobiology.

Proton transport (PT) in bulk liquid water and within a thin water-filled carbon nanotube has been examined using ab initio path-integral molecular dynamics (PIMD). Barrierless proton transfer is observed in each case when quantum nuclear effects (QNEs) are accounted for. The key difference between the two systems is that in the nanotube facile PT is facilitated by a favorable pre-alignment of water molecules, whereas in bulk liquid water solvent reorganization is required prior to PT. Configurations where the quantum excess proton is delocalized over several adjacent water molecules along with continuous interconversion between different hydration states reveals that, as in liquid water, the hydrated proton under confinement is best described as a fluxional defect, rather than any individual idealized hydration state such as Zundel, Eigen, or the so-called linear H7O3(+) complex along the water chain. These findings highlight the importance of QNEs in intermediate strength hydrogen bonds (HBs) and explain why H(+) diffusion through nanochannels is impeded much less than other cations.

Investigation of methanol's surface chemistry on metals is a crucial step towards understanding the reactivity of this important chemical feedstock. Cu is a relevant metal for methanol synthesis and reforming, but due to the weak interaction of methanol with Cu, an atomic scale view of methanol's coverage-dependent ordering and self-assembly on Cu(111), the most abundant facet of most nanoparticles, has not yet been possible. Low and variable temperature scanning tunneling microscopy coupled with density functional theory reveal a coverage-dependent range of highly ordered structures stabilized by two hydrogen bonds per molecule. While extended chains that resemble the hydrogen-bonded zigzag structures reported for solid methanol are an efficient way to pack methanol at higher coverages, lower surface coverages yield isolated hexamer units. These hexamers form the same number of hydrogen bonds as the chains but appear to repel one another on the surface. Annealing treatments lead to the desorption of methanol with almost no decomposition. This data serves as a useful guide to both the preferred adsorption geometries and energies of a variety of methanol structures on Cu(111) surfaces as a function of surface coverage.

It has long been known that ice nucleation usually proceeds heterogeneously on the surface of a foreign body. However, little is known at the microscopic level about which properties of a material determine its effectiveness at nucleating ice. This work focuses on the long standing, conceptually simple, view on the role of a good crystallographic match between bulk ice and the underlying substrate. We use grand canonical Monte Carlo to generate the first overlayer of water at the surface and find that the traditional view of heterogeneous nucleation does not adequately account for the array of structures that water may form at the surface. We find that, in order to describe the structures formed, a good match between the substrate and the nearest neighbour oxygen-oxygen distance is a better descriptor than a good match to the bulk ice lattice constant.

The initial stages of NaCl dissolution in liquid water have been examined with state-of-the-art ab initio molecular dynamics and free energy sampling techniques. Our simulations reveal a complex multi-step process triggered by the departure of Cl ions from the lattice, with a well-defined intermediate state wherein departing ions are partially solvated but remain in contact with the crystal. The polarizability of Cl(-) is identified as the source of the anion's preferential initial dissolution, an effect which leads a forcefield based description of NaCl dissolution to fail to identify a preference for Cl over Na dissolution., (This journal is © the Owner Societies 2011)

The great promise of ceria-supported gold clusters as catalysts of the future for important industrial processes, such as the water gas shift reaction, has prompted a flurry of activity aimed at understanding the molecular-level details of their operation. Much of this activity has focused on experimental and theoretical studies of the structure of perfect and defective ceria surfaces, with and without gold clusters of various sizes. The complicated electronic structure of ceria, particularly in its reduced form, means that at present it is highly challenging to carry out accurate electronic structure simulations of such systems. To overcome the challenges, the majority of recent theoretical studies have adopted a pragmatic and often controversial approach, applying the so-called DFT + U technique. Here we will briefly discuss some recent studies of Au on CeO(2){111} that mainly use this methodology. We will show that considerable insight has been obtained into these systems, particularly with regard to Au adsorbates and Au cluster reactivity. We will also briefly discuss the need for improved electronic structure methods, which would enable more rigorous and robust studies in the future.

Density functional theory simulations of water on MgO(001) reveal rapid proton transfer within clusters of just two water molecules. Facile dissociation and recombination of the molecules within the dimers along with a concerted surface-mediated exchange of protons between water and hydroxyl molecules makes this possible. We suggest that surface-mediated proton transfer is in general likely to lead to proton transfer in interfacial water systems whenever the relative energies of intact and dissociated states of water are similar.