Understanding the role of ions and water molecules in the NaCl dissolution process

Rapid screening of esophageal squamous cell carcinoma by near-infrared spectroscopy combined with aquaphotomics

Esophageal cancer (EC), the fifth most common cause of cancer-related mortality in China, poses a significant threat to public health. Among the pathological types, esophageal squamous cell carcinoma (ESCC) is predominant, comprising approximately 90 % of cases. Screening is crucial for early detection, diagnosis and treatment, thereby reducing ESCC mortality. This study aimed to develop a rapid, accurate, and cost-effective method based on near-infrared (NIR) spectroscopy combined with aquaphotomics for ESCC screening. NIR spectra were obtained from plasma samples of both healthy controls and ESCC patients. Subsequently, principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) were utilized to identify the water matrix coordinates (WAMACS), thereby delineating the water absorption spectrum pattern (WASP) and constructing an aquagram. The results showed that the PLS-DA screening test model demonstrated high accuracy and precision rates of 95.12 % and 97.10 %, respectively, along with sensitivity and specificity rates of 97.10 % and 84.62 %. The area under the curve (AUC) achieved 0.9064. Aquaphotomic analysis revealed that the WASP of the healthy group predominantly exhibited strong absorption in regions indicative of strong hydrogen bonds (1460 nm, 1480 nm, 1494 nm), while the WASP of the ESCC group showed strong absorption in regions associated with strong hydrogen bonds, weak hydrogen bonds and free water, especially the regions of weak hydrogen bonds (1434 nm) and free water (1390 nm) were significantly different from those of the healthy group. The findings indicated that the rapid screening model for ESCC, integrating NIR spectroscopy with aquaphotomics, is both effective and feasible, with the WASP presenting as a potentially valuable biomarker for ESCC screening.

Vacancy-Mediated Increases in Brine-Salt Surface Energies

Salt formations have been explored for the permanent isolation of spent nuclear fuel based on their high thermal conductivity, self-healing nature, and low hydraulic permeability to brine flow. Vacancy defect concentrations in salt complicate fracture mechanics not driven by dislocation dynamics and can influence the resulting surface structure. Classical molecular dynamic simulations were used to simulate tensile testing of salt crystals (halite) with vacancy defect concentrations of up to 0.5 defects/nm3. Increasing defect concentrations resulted in a decrease in ultimate tensile strength and fracture surface energies, driven by increased surface roughness rather than changes in the amount of surface area. Brine-salt surface energies of the fractured surfaces were 0.22 to 0.26 J/m2, significantly higher than values reported for atomically flat (100) surfaces at the same brine composition. This change in surface energy increased the brine-salt dihedral angle by ∼27°. The dihedral angle threshold for percolation in salt is 60°, and a 27° increase due to rough fracture surfaces identifies a reduction in porosity percolation and a decrease in salt permeability. Therefore, bedded salt and salt domes may be even more stable than those previously predicted from dihedral angle calculations.

Crumbling crystals: on the dissolution mechanism of NaCl in water

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.

Controlled dissolution of a single ion from a salt interface

Interactions between monatomic ions and water molecules are fundamental to understanding the hydration of complex polyatomic ions and ionic process. Among the simplest and well-established ion-related reactions is dissolution of salt in water, which is an endothermic process requiring an increase in entropy. Extensive efforts have been made to date; however, most studies at single-ion level have been limited to theoretical approaches. Here, we demonstrate the salt dissolution process by manipulating a single water molecule at an under-coordinated site of a sodium chloride film. Manipulation of molecule in a controlled manner enables us to understand ion-water interaction as well as dynamics of water molecules at NaCl interfaces, which are responsible for the selective dissolution of anions. The water dipole polarizes the anion in the NaCl ionic crystal, resulting in strong anion-water interaction and weakening of the ionic bonds. Our results provide insights into a simple but important elementary step of the single-ion chemistry, which may be useful in ion-related sciences and technologies.

Convergence of dissolving and melting at the nanoscale

Phase transitions of water and its mixtures are of fundamental importance in physical chemistry, the pharmaceutical industry, materials sciences, and atmospheric sciences. However, current understanding remains elusive to explain relevant observations, especially at the nanoscale. Here, by using molecular dynamics simulations, we investigate the dissolution of sodium chloride (NaCl) nanocrystals with volume-equivalent diameters from 0.51 to 1.75 nm. Our results show that the dissolution of NaCl in aqueous nanodroplets show a strong size dependence, and its solubility can be predicted by the Ostwald-Freundlich equation and Gibbs-Duhem equation after considering a size-dependent solid-liquid surface tension. We find that the structure of dissolved ions in the saturated aqueous nanodropplet resembles the structure of a molten NaCl nanoparticle. With decreasing nanodroplet size, this similarity grows and the average potential energy of NaCl in solution, the molten phase and the crystal phase converges.

Early osteoarthritis diagnosis based on near-infrared spectroscopy combined with aquaphotomics

Osteoarthritis (OA) is the most common joint disease and the leading cause of disability in elderly individuals. Despite rapid advances in imaging techniques, early OA diagnosis remains a clinical challenge. In the present study, the feasibility of early OA diagnosis was explored via near-infrared spectroscopy (NIRS) combined with aquaphotomics. Synovial fluid samples from 65 cases of OA categorized as mild, moderate, and severe according to theKellgrenandLawrence classification criteria were analyzed via NIRS. The 1st overtone of water (1300-1600 nm) was considered as the research object for an aquaphotomics model, and aquagrams of the mild, moderate, and severe OA cases were generated using 12 water absorption patterns for early OA diagnosis.The aquaphotomics results exhibited clear differences in the region of 1300-1500 nm, and the number of hydrogen bonds of different water species (1412,1424, 1482, and 1496 nm) evidently correlated with OA occurrence and development. With OA progression, the absorption intensity of water molecules without hydrogen bonds (1412 nm/1424 nm) became stronger, while the absorption intensity of water molecules with four hydrogen bonds (1482 nm/1496 nm) decreased.These results together reveal that the established accurate and rapid early OA diagnosis model based on NIRS combined with aquaphotomics is effective and feasible, and that the number of hydrogen bonds can be used as a biomarker for early OA diagnosis.

Ab initio study of hydrated cesium iodide dimer (CsI)2-/0(H2O)0-6 and the cation size effect on (MI)2-/0(H2O)0-6 (M = Li, Na, K, Cs)

The structures of microsolvated (CsI)₂^-/0(H₂O)_0-6 clusters were determined using ab initio calculations. Our studies show that one Cs atom at the apex was firstly separated from the pyramid-shaped (CsI)₂^- unit when the water number reaches 3, whereas CsI distances did not increase significantly from n = 0 to 6 for neutrals. Additionally, the atomic charge and reduced density gradient analyses were carried out; the results reveal that the extra electrons are almost entirely localized on terminal Cs atom and the Cs⁺-water interactions dominate in (CsI)₂^-(H₂O)_0-6. The water-water interactions show up at n = 5. The comparison of (CsI)₂^-/0(H₂O)_n with (MI)₂^-/0(H₂O)_n (M = Li, Na, K) shows that neutral (CsI)₂ is the most difficult to be separated, which matches the law of matching water affinity. As for anions, the most difficult separation occurs in the case of small size (LiI)₂^- due to the effect of extra electrons, and (MI)₂^- with larger size cation is more likely to interact with water to form a pyramid structure.

Unique emissive behavior of combustion-derived particles under illumination with femtosecond pulsed near-infrared laser light

Exposure to combustion-derived particles (CDPs), such as carbonaceous particulate matter (PM), has adverse effects on human health. Hence, selective detection of these particles in biological environments is required to understand their toxicity. The optical detection of carbonaceous PM is possible in biological samples based on white light (WL) emission under illumination with a femtosecond (fs) pulsed near-infrared (NIR) laser. However, it is unclear if common non-CDPs in ambient PM, such as silica and metal oxides, can interfere with CDP detection when illuminated using a fs-pulsed NIR laser. Here, we show that WL emission, when illuminated with a fs-pulsed NIR laser, is observed only for CDPs amongst other common air pollution particles. We report that the intense WL emission from CDPs spanning over the whole visible spectrum is not observed from non-CDPs. This observation is made for four different CDPs and five different, relevant non-CDPs, in wet and dried samples using biologically relevant imaging conditions. This investigation confirms the uniqueness of WL emission as a selective detection mechanism of CDPs using a multiphoton microscopy platform, commonly available in research laboratories. Furthermore, some relevant signatures for the non-CDPs are provided that could potentially lead to the selective monitoring of pollution related nanoparticles (NPs).

Stability of Two-Dimensional Ionic Clusters at Solid-Liquid Interfaces

The stability of two-dimensional clusters (2DCs) at the interface between ionic crystals and their solutions was investigated by molecular dynamics simulations. We found that 2DCs show a remarkable feature of odd-even alternation in stability. In NaCl and NaBr systems, the clusters containing an odd number of ions are more stable than those with an even number of ions, while in KCl systems, it is the other way round. Accordingly, the stability of water molecules in the first hydration shell of 2DCs also shows an odd-even alternation, which is consistent with the associated 2DCs. The odd-even alternation is discussed based on a competition mechanism between two factors: the Coulomb repulsion in charged 2DCs and the interaction between charges and water dipoles. Our discussion indicates that this odd-even alternation should be a universal feature in similar systems and would be important for understanding the nucleation and crystallization of solutions on ionic crystal surfaces.

Water Recovery Rate in Short-Circuited Closed-Cycle Operation of Flow-Electrode Capacitive Deionization (FCDI)

While flow-electrode CDI is a promising desalination technology that has major advantages when the electrodes are operated in the short-circuited closed-cycle (SCC) mode, little attention has been paid to the water recovery rate, which, in the SCC mode, is determined by the need for partial replacement of the saline electrolyte of the flow electrodes. Results of this study show that an extremely high water recovery rate of ∼95% can be achieved when desalting a 1000 mg NaCl L^-1 brackish influent to a potable level of 150 mg L^-1. The improved performance with regard to the electrical cost is related, at least in part, to the alleviated concentration polarization at the membrane/electrolyte interface during electrosorption. In effect, the current efficiency decreases with an increase in the water recovery rate. This finding is ascribed to inevitable co-ion leakage since the flow electrodes reject ions with the same charge. In addition, water transport across the ion exchange membranes also influences the water recovery rate. The effect of partial replacement of the saline electrolyte during (semi-)continuous operation requires particular consideration because the associated dilution of the carbon content in the flow electrodes results in a decrease in process performance.

Ion Distribution and Hydration Structure at Solid-Liquid Interface between NaCl Crystal and Its Solution

The interface structure between NaCl crystal and its solution has been investigated at the saturated concentration of 298 K by molecular dynamics simulations. We have found that there are many fine structures at this complex interface. Near the surface of crystal, most of Na⁺ only coordinate with water molecules, while almost all Cl^- coordinate with Na⁺ in addition to water molecules. An ion coordinating with more water molecules is farther away from the epitaxial position of lattice. As approaching to the interface, the first hydration shell of ions has the tendency of being ordered, while the orientation of dipole of water molecules in the first hydration shell becomes more disordered than that in the solution. Generally, the first hydration shell of Na⁺ is less affected by nearest Cl^-, whereas the first hydration shell of Cl^- is significantly affected by nearest Na⁺.

Dissolution of Sodium Halides by Confined Water on Au(111) via Langmuir-Hinshelwood Process

Salt dissolution is generally encountered in widespread phenomena in nature. As a typical case, the dissolution of NaCl has been widely investigated in aqueous environment, while the process and mechanism of the on-surface dissolution may differ from that in solution and remain to be explored. Herein, we model a NaCl dissolution process on the Au(111) surface with confined water at room temperature (RT) and above. With the assistance of adenine molecules as water reservoir and carrier, the dissolution of NaCl is achieved above RT via the Langmuir-Hinshelwood mechanism rather than the Eley-Rideal one, along with the selective formation of stable Cl^- hydrates, which desorb from the surface in the next step. To explore the generality, such a strategy has been extended to other sodium halide systems ( e. g., NaBr and NaI), and expectedly, the dissolution of sodium halides is also achieved by forming stable Br^- and I^- hydrates via the Langmuir-Hinshelwood process.

Restricting Solvation to Two Dimensions: Soft Landing of Microsolvated Ions on Inert Surfaces

In an effort to scrutinize dimensional restriction effects on finite hydrogen-bonded networks, we deposit ion-doped water clusters by computational soft landing on a chemically inert supported xenon surface. In stark contrast to the much studied metal or metal oxide surfaces, the rare gas surface interacts only rather weakly and nondirectionally with these networks. Surprisingly, the strongly bound Na⁺-doped networks undergo very significant plastic deformations, whereas the weakly bound Cl^- counterparts barely change upon surface deposition. This counterintuitive finding is traced back to the significantly less favorable water-water interactions enforced by the cation, which results in an easier adaption to geometric restrictions, whereas H-bonding stabilizes the anionic clusters.

Ion dissolution mechanism and kinetics at kink sites on NaCl surfaces

Desolvation barriers are present for solute-solvent exchange events, such as ligand binding to an enzyme active site, during protein folding, and at battery electrodes. For solution-grown crystals, desolvation at kink sites can be the rate-limiting step for growth. However, desolvation and the associated kinetic barriers are poorly understood. In this work, we use rare-event simulation techniques to investigate attachment/detachment events at kink sites of a NaCl crystal in water. We elucidate the desolvation mechanism and present an optimized reaction coordinate, which involves one solute collective variable and one solvent collective variable. The attachment/detachment pathways for Na⁺ and Cl^- are qualitatively similar, with quantitative differences that we attribute to different ion sizes and solvent coordination. The attachment barriers primarily result from kink site desolvation, while detachment barriers largely result from breaking ion-crystal bonds. We compute ion detachment rates from kink sites and compare with results from an independent study. We anticipate that the reaction coordinate and desolvation mechanism identified in this work may be applicable to other alkali halides.

Atomic-scale imaging of the dissolution of NaCl islands by water at low temperature

The dissolution of sodium chloride (NaCl) in water is a frequently encountered process in our daily lives. While the NaCl dissolution process in liquid water has been extensively studied, whether and how the dissolution occurs below the freezing point is still not clear. Using a low-temperature scanning tunneling microscope (STM), here we were able to directly visualize the dissolution of Au-supported NaCl (0 0 1) bilayer islands by water at atomic level. We found that the single water molecule on the STM tip can assist the extraction of single Na⁺ from the NaCl surface even at 5 K, while leaving the Cl^- intact. When covered with a full water monolayer, the NaCl islands started to dissolve from the step edges and also showed evidence of dissolution inside the terraces as the temperature was raised up to 145 K. At 155 K, the water molecules completely desorbed from the surface, which was accompanied with the decomposition and restructuring of the bilayer NaCl islands. Those results suggest that the dissolution of NaCl may occur well below the freezing point at the ice/NaCl interfaces and is mainly driven by the interaction between the water molecules and the Na⁺, which is in clear contrast with the NaCl dissolution in liquid water.

Perspective: Structure and dynamics of water at surfaces probed by scanning tunneling microscopy and spectroscopy

The detailed and precise understanding of water-solid interaction largely relies on the development of atomic-scale experimental techniques, among which scanning tunneling microscopy (STM) has proven to be a noteworthy example. In this perspective, we review the recent advances of STM techniques in imaging, spectroscopy, and manipulation of water molecules. We discuss how those newly developed techniques are applied to probe the structure and dynamics of water at solid surfaces with single-molecule and even submolecular resolution, paying particular attention to the ability of accessing the degree of freedom of hydrogen. In the end, we present an outlook on the directions of future STM studies of water-solid interfaces as well as the challenges faced by this field. Some new scanning probe techniques beyond STM are also envisaged.

Ambient atomic resolution atomic force microscopy with qPlus sensors: Part 1

Atomic force microscopy (AFM) is an enormous tool to observe nature in highest resolution and understand fundamental processes like friction and tribology on the nanoscale. Atomic resolution in highest quality was possible only in well-controlled environments like ultrahigh vacuum (UHV) or controlled buffer environments (liquid conditions) and more specified for long-term high-resolution analysis at low temperatures (∼4 K) in UHV where drift is nearly completely absent. Atomic resolution in these environments is possible and is widely used. However, in uncontrolled environments like air, with all its pollutants and aerosols, unspecified thin liquid films as thin as a single molecular water-layer of 200 pm or thicker condensation films with thicknesses up to hundred nanometer, have been a problem for highest resolution since the invention of the AFM. The goal of true atomic resolution on hydrophilic as well as hydrophobic samples was reached recently. In this manuscript we want to review the concept of ambient AFM with atomic resolution. The reader will be introduced to the phenomenology in ambient conditions and the problems will be explained and analyzed while a method for scan parameter optimization will be explained. Recently developed concepts and techniques how to reach atomic resolution in air and ultra-thin liquid films will be shown and explained in detail, using several examples. Microsc. Res. Tech. 80:50-65, 2017. © 2016 Wiley Periodicals, Inc.

Effect of salt on the H-bond symmetrization in ice

The richness of the phase diagram of water reduces drastically at very high pressures where only two molecular phases, proton-disordered ice VII and proton-ordered ice VIII, are known. Both phases transform to the centered hydrogen bond atomic phase ice X above about 60 GPa, i.e., at pressures experienced in the interior of large ice bodies in the universe, such as Saturn and Neptune, where nonmolecular ice is thought to be the most abundant phase of water. In this work, we investigate, by Raman spectroscopy up to megabar pressures and ab initio simulations, how the transformation of ice VII in ice X is affected by the presence of salt inclusions in the ice lattice. Considerable amounts of salt can be included in ice VII structure under pressure via rock-ice interaction at depth and processes occurring during planetary accretion. Our study reveals that the presence of salt hinders proton order and hydrogen bond symmetrization, and pushes ice VII to ice X transformation to higher and higher pressures as the concentration of salt is increased.

Ab initio Kinetic Monte Carlo simulations of dissolution at the NaCl-water interface

We have used ab initio molecular dynamics (AIMD) simulations to study the interaction of water with the NaCl surface. As expected, we find that water forms several ordered hydration layers, with the first hydration layer having water molecules aligned so that oxygen atoms are on average situated above Na sites. In an attempt to understand the dissolution of NaCl in water, we have then combined AIMD with constrained barrier searches, to calculate the dissolution energetics of Na(+) and Cl(-) ions from terraces, steps, corners and kinks of the (100) surface. We find that the barrier heights show a systematic reduction from the most stable flat terrace sites, through steps to the smallest barriers for corner and kink sites. Generally, the barriers for removal of Na(+) ions are slightly lower than for Cl(-) ions. Finally, we use our calculated barriers in a Kinetic Monte Carlo as a first order model of the dissolution process.

An unconventional bilayer ice structure on a NaCl(001) film

Water-solid interactions are of broad importance both in nature and technology. The hexagonal bilayer model based on the Bernal-Fowler-Pauling ice rules has been widely adopted to describe water structuring at interfaces. Using a cryogenic scanning tunnelling microscope, here we report a new type of two-dimensional ice-like bilayer structure built from cyclic water tetramers on an insulating NaCl(001) film, which is completely beyond this conventional bilayer picture. A novel bridging mechanism allows the interconnection of water tetramers to form chains, flakes and eventually a two-dimensional extended ice bilayer containing a regular array of Bjerrum D-type defects. Ab initio density functional theory calculations substantiate this bridging growth mode and reveal a striking proton-disordered ice structure. The formation of the periodic Bjerrum defects with unusually high density may have a crucial role as H donor sites in directing multilayer ice growth and in catalysing heterogeneous chemical reactions on water-coated salt surfaces.